This is the accepted version of the following article: Andreas et al. “Responses of forest ecosystems in Europe to decreasing nitrogen deposition” in Environmental Pollution, Vol 244, (January 2019), p 980-994, which has been published in final form at: http://doi.org/10.1016/j.envpol.2018.09.101 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ 1 Responses of forest ecosystems in Europe to decreasing nitrogen deposition 2 Andreas Schmitz1, Tanja Sanders1, Andreas Bolte1, Filippo Bussotti2, Thomas Dirnböck3, Jim Johnson4, 3 Josep Peñuelas5,6, Martina Pollastrini2, Anne-Katrin Prescher1, Jordi Sardans5,6, Arne Verstraeten7, Wim 4 de Vries8,9 5 1. Thünen Institute of Forest Ecosystems, Alfred-Möller-Straße 1, Haus 41/42, 16225 Eberswalde, 6 Germany; andreas.schmitz@thuenen.de; andreas.bolte@thuenen.de; 7 anne.prescher@thuenen.de; tanja.sanders@thuenen.de 8 2. Department of Agrifood Production and Environmental Sciences (DiSPAA), University of 9 Florence, piazzale delle Cascine 28, 50144 Firenze, Italy; filippo.bussotti@unifi.it; 10 martina.pollastrini@unifi.it 11 3. Department for Ecosystem Research and Environmental Information Management, 12 Environment Agency Austria, Spittelauer Lände 5, Vienna, Austria; 13 thomas.dirnboeck@umweltbundesamt.at 14 4. School of Agriculture and Food Science, University College Dublin, Belfield, Dublin 4, Ireland; 15 jims.ucd.mail@gmail.com 16 5. CSIC, Global Ecology CREAF- CSIC-UAB, Cerdanyola del Valles, 08193 Barcelona, Catalonia, 17 Spain; j.sardans@creaf.uab.cat; josep.penuelas@uab.cat 18 6. CREAF, Cerdanyola del Valles, 08193 Barcelona, Catalonia, Spain 19 7. Research Institute for Nature and Forest (INBO), Gaverstraat 4, 9500 Geraardsbergen, 20 Belgium; arne.verstraeten@inbo.be 21 8. Wageningen University and Research, Environmental Research, PO Box 47, NL-6700 AA 22 Wageningen, The Netherlands; wim.devries@wur.nl 23 9. Wageningen University and Research, Environmental Systems Analysis Group, PO Box 47, NL- 24 6700 AA Wageningen, the Netherlands 25 26 *Corresponding author: Andreas Schmitz; Thünen Institute of Forest Ecosystems, Alfred-Möller-Straße 27 1, Haus 41/42, 16225 Eberswalde, Germany; andreas.schmitz@thuenen.de 28 29 30 Abstract 31 Average nitrogen (N) deposition across Europe has declined since the 1990’s. This resulted in 32 decreased N inputs to forests especially in Central and Western Europe where deposition is highest. 33 While the impact of atmospheric N on forests has been receiving much attention for decades, 34 ecosystem responses to the decline in N inputs have received less attention. Here, we review 35 observational studies reporting on trends in a number of indicators: soil acidification and 36 eutrophication, understory vegetation, tree nutrition (foliar element concentrations), tree vitality and 37 growth in response to decreasing N deposition across Europe. Ecosystem responses varied with limited 38 decreases in soil solution nitrate and suggested decrease in foliar N concentrations. There was no 39 large-scale response for understory vegetation, tree growth or vitality. Experimental studies support 40 the observation of a more distinct reaction of soil solution and foliar element concentrations to 41 changes in N supply compared to the three other parameters. According to the most likely scenarios, 42 further decrease of N deposition will be limited. We hypothesize that this expected decline will not 43 cause major responses of the parameters analysed in this study. Instead, future changes might be more 44 strongly controlled by the development of N pools accumulated within forest soils, affected by climate 45 change and forest management. 46 47 Keywords 48 1. Nitrogen deposition 49 2. Forest monitoring 50 3. Emission reduction 51 4. Recovery 52 5. Air pollution 53 54 Capsule 55 We find limited indication for response of Europe’s forests to declining N deposition. Reactions have 56 been reported for soil solution NO3- and potentially foliar N concentrations but not for other indicators. 57 58 Highlights 59  Europe’s forests show limited response to decreasing N deposition 60  Potential reactions have been reported for soil solution and foliage concentrations 61  Delayed or marginal responses are expected for other forest ecosystem components 62  Future decrease of N deposition to forests in Europe will likely be small 63 64 1 Introduction 65 Anthropogenic emissions have drastically altered the global nitrogen (N) cycle (Fowler et al., 2013; 66 Galloway et al., 2003; Vitousek et al., 1997), with human activities becoming the dominant contribution 67 to the annual release of reactive N to the atmosphere (Fowler et al., 2015; Galloway et al., 2004). The 68 increase in anthropogenic emissions arose from accelerated fossil fuel burning since the industrial 69 revolution, the advent of the Haber-Bosch process to create reactive N from inert atmospheric N2 at 70 the start of the 20th century as well as increases in mass transportation and livestock numbers (Engardt 71 et al., 2017; Erisman et al., 2011). Today, 18% of the global anthropogenic nitrogen fixation can be 72 attributed to combustion processes, 55% to fertilizer production and 27% to biological N fixation in 73 agriculture (Fowler et al., 2015). These activities have created benefits, such as the dependence of a 74 large part of human nutrition on mineral fertilizers (Erisman et al., 2008). On the other hand, the 75 release of reactive N causes considerable damages to human health (Van Grinsven et al., 2013) and 76 induces changes in natural and semi-natural ecosystems, such that N deposition is one of the greatest 77 threats to global plant diversity (Bobbink et al., 2010; Brink et al., 2011; Clark et al., 2013; Erisman et 78 al., 2008; Soons et al., 2017; Vitousek et al., 1997). 79 In Europe N emissions and corresponding deposition increased from pre-industrial times till the mid- 80 1980’s, followed by a decrease since the 1990s (Engardt et al., 2017). The decline in N emissions is due 81 to a combination of emission abatement policies and economic transformation (Erisman et al., 2003). 82 In Europe’s forests, N deposition has caused a variety of changes, including impacts on tree 83 productivity (De Vries et al., 2017b, 2006; Kahle, 2008), tree nutrition reflected in foliar concentrations 84 (Jonard et al., 2015; Sardans et al., 2016b; Waldner et al., 2015), sensitivity of trees to biotic and abiotic 85 stress (Bobbink and Hettelingh, 2011), the composition of understory vegetation (Dirnböck et al., 2014; 86 van Dobben and De Vries, 2017), ectomycorrhizal fungal communities (van der Linde et al., 2018), soil 87 chemistry and increased leaching of N from forest soils to surface and ground waters (Dise et al., 2009; 88 Gundersen et al., 2006). In recent decades, much discussion took place to identify the mechanisms as 89 well as the time frame by which forest ecosystems are impacted by elevated nitrogen deposition. The 90 concept of nitrogen saturation (Aber et al., 1998, 1989; Ågren and Bosatta, 1988; De Vries and Schulte- 91 Uebbing, 2018; Lovett and Goodale, 2011) suggests a set of reactions including loss of plant species 92 diversity, N losses with seepage water, soil acidification, and growth reduction. A recent perspective 93 on the stages of N saturation is depicted in figure 1. The ecological understanding is used to determine 94 critical loads of N deposition defined as 'a quantitative estimate of an exposure to one or more 95 pollutants below which significant harmful effects on specified sensitive elements of the environment 96 do not occur according to present knowledge' (Nilsson and Grennfelt, 1988). Critical loads underpin 97 emissions protocols at the European scale such as the Revised National Emissions Ceilings Directive 98 (NECD) and are also applied for example in North America (Pardo et al., 2011; Schindler and Lee, 2010) 99 and Asia (Duan et al., 2016). Exceedances of critical loads indicate risks for adverse effects on various 100 aspects of forests, such as tree nutrition and forest biodiversity (De Vries et al., 2015; Nordin et al., 101 2005; Waldner et al., 2015). 102 103 [Figure 1] 104 105 A large part of the ecological research in this context focused on the responses of forest ecosystems 106 to elevated N deposition resulting in N saturation or the exceedance of critical loads. However, much 107 less attention was paid to the potential dynamics of a “recovery” from high N loads although a decline 108 of N deposition to Europe can be observed since the 1990’s. The average deposition of inorganic N 109 across all land-use types in Europe decreased from 10.3 kg N ha-1 a-1 in 1990 to 6.6 kg N ha-1 a-1 in 2018 110 (after Engardt et al. (2017), data kindly provided by Magnuz Engardt and David Simpson). The trends 111 are distributed heterogeneously in space. While many forests in areas with higher absolute levels of N 112 deposition (e.g. in Central and Western Europe) experienced decreases of N inputs, less clear trends 113 have been reported for Northern Scandinavia and parts of Southern Europe (figure 2 and 3). Note that 114 despite these reductions, 62% of the European ecosystem area was at risk of eutrophication due to 115 the exceedance of the critical load for eutrophication in 2015 (Slootweg et al., 2015). 116 This study addresses the response of European forest ecosystems to decreasing N deposition. We 117 review published results from observational and experimental studies on well-monitored parameters: 118 soil acidification and eutrophication, foliar chemistry, ground vegetation composition, tree vitality, and 119 tree growth. This set of indictors covers a range between endpoint metrics, i.e. aspects of the 120 environment that are directly relevant to people (e.g. tree growth) and midpoint metrics, i.e. 121 parameters that are well-suited to measure progress towards desired environmental states (e.g. plant 122 tissue concentrations) (Rowe et al., 2017). While results are limited to Europe, references have also 123 been included that relate to observations and experiments the United States (US). For a detailed 124 overview of impacts of reduced N deposition in the US, we refer to Gilliam et al. (2018, in press). 125 [Figure 2] 126 127 [Figure 3] 128 2 Soil acidification and eutrophication 129 Atmospheric deposition of reactive nitrogen compounds such as nitrate (NO3-) and ammonium (NH4+) 130 contributes to acidification and eutrophication of forest soils (Driscoll et al., 2006). Soil acidification 131 involves accelerated losses of mineral nutrients (base cations, i.e. Ca2+, K+ and Mg2+) and potential for 132 the mobilization of toxic aluminium (Al), both of which can compromise tree health (Driscoll et al., 133 2006; Boudot et al., 1994; De Vries et al., 2014; De Wit et al., 2010). N deposition contributes to 134 elevated soil solution NO3- concentrations and soil N stocks (Driscoll et al., 2001). This enrichment can 135 have a variety of effects on trees and ground vegetation, covered in the subsequent chapters. NO 3- 136 concentrations in soil solution are a good indicator for the soil N status. Important determinants of 137 NO3- leaching are the forest floor C/N ratio (Gundersen et al., 1998a) and N deposition rates (Dise and 138 Wright, 1995), as well as a variety of other site and stand characteristics controlling the ecosystem N 139 cycling (Lovett and Goodale, 2011). Generally, elevated NO3- concentrations in soil solution are an 140 indication of N availability in excess of biotic demand. Spatial patterns of soil solution NO 3- are highly 141 variable but partly reflect spatial patterns in N deposition, with higher levels in the Netherlands, 142 Belgium, parts of Germany, Switzerland and Denmark and lower levels in parts of France, Norway, 143 Northern Sweden and Finland (Boxman et al., 2008; De Vries et al., 2007; Evans et al., 2001; Gundersen 144 et al., 1998a; Jonard et al., 2012; Mellert et al., 2008; Moffat et al., 2002; Pannatier et al., 2010; Pihl 145 Karlsson et al., 2011; Rothe et al., 2002; Ukonmaanaho et al., 2014; van der Heijden et al., 2011; 146 Verstraeten et al., 2012). There are relatively fewer reports of elevated NO3- in soil solution in Southern 147 and Eastern Europe, and N deposition is mostly lower in these regions (Waldner et al., 2014). 148 Observational studies 149 At the European scale, studies examining trends in soil solution N show weak or non-significant trends. 150 For example, Johnson et al. (2018) found a weakly significant (p<0.1) reduction in NO3- concentrations 151 at 40-80 cm depth corresponding to a decrease of 30% over 10 years when analysing data from 162 152 plots across Europe between 1995 and 2012. They found no significant trend in 10-20 cm depth. An 153 earlier analysis (from the early 1990s to 2006) using a similar dataset found mostly non-significant 154 trends in soil solution inorganic N concentrations (Iost et al., 2012). These studies did not focus 155 specifically on areas with high N deposition and included many sites from N limited areas of Northern 156 Europe. Within Europe, national and regional studies show variable results. For example, in the 157 Netherlands and Flanders soil solution NO3- declined in response to decreasing N deposition (Boxman 158 et al., 2008; Verstraeten et al., 2012). In contrast, an intensive study at the site Solling in Germany 159 found NO3- continued to leach from a spruce (Picea abies) stand and increased at a beech (Fagus 160 sylvatica) stand despite decreasing N deposition between 1973 and 2013, indicating a reduction of the 161 N retention capacity of the soil over time (Meesenburg et al., 2016). Other studies found no trends in 162 NO3- soil solution concentrations in connection with stable N deposition (e.g. Alewell et al., 2000.; 163 Johnson et al., 2013; Pannatier et al., 2010). At a heavily acidified forest in the Czech Republic, NO3- 164 concentrations in soil solution declined despite no decrease in N deposition. This was due to an 165 increase in N uptake by vegetation and changes in organic matter cycling as the soil became less acidic 166 (Oulehle et al., 2011). 167 Where soil solution NO3- decreased, it is generally accompanied by a decrease in base cations and total 168 Al concentrations, while soil solution pH and acid neutralizing capacity (ANC) showed no uniform 169 trends in recent decades (Iost et al., 2012; Johnson et al., 2018). In many areas, soil solution continues 170 to acidify despite the large decreases in sulphur (S), and to a lesser degree, N deposition (Johnson et 171 al., 2018). The absence of a widespread recovery of soil solution from acidification agrees with trends 172 in bulk soil chemistry. Cools and De Vos (2011) found that base saturation increased in soils with low 173 buffering capacity but decreased in soils with initially higher base saturation across Europe. A similar 174 result was found for the Netherlands between 1990 and 2015 (De Vries et al., 2017a). Table 1 175 summarizes results on trends of soil solution eutrophication and acidification status from studies 176 across Europe. 177 178 Effect Trend ↑ ↑/↔ ↔ ↔/↓ ↓ NO3- Meesenburg et al. Johnson et al. (2013) Pannatier et al. (2010) Boxman et al. (2008) (2016) (Germany) (Ireland), Löfgren and (Switzerland), Sawicka et (Netherlands), Oulehle Zetterberg (2011), Pihl al. (2016) (UK), et al. (2011) (Czech Karlsson et al. (2011) Ukonmaanaho et al. Republic), Verstraeten et (Sweden), Vanguelova et (2014) (Finland) al. (2012), Verstraeten et al. (2010) (UK) al. (2017) (Flanders) pH Akselsson et al. (2013), Vanguelova et al. (2010), Boxman et al. (2008) Löfgren et al. (2011) Sawicka et al. (2016) (Netherlands), Jonard et (Sweden), Verstraeten (UK), Fölster et al. al. (2012) (Wallonia) et al. (2016) (Flanders) (2003), Löfgren and Zetterberg (2011), Pihl Karlsson et al. (2011) (Sweden), Johnson et al. (2013) (Ireland) BC Vanguelova et al. (2010) Graf Pannatier et al. Jonard et al. (2012) (UK), Johnson et al. (2011) (Switzerland), (Wallonia), Verstraeten (2013) (Ireland) Sawicka et al. (2016) et al. (2012) (Flanders), (UK) Boxman et al. (2008) (Netherlands), Fölster et al. (2003), Akselsson et al. (2013) (Sweden) Altot Jonard et al. (2012) Sawicka et al. (2016) Vanguelova et al. (2010), Verstraeten et al. (2012) (Wallonia), Fölster et al. (UK) Löfgren et al. (2011), (Flanders), Boxman et al. (2003) (Sweden) Löfgren and Zetterberg (2008) (Netherlands) (2011), Pihl Karlsson et al. (2011) (Sweden), Johnson et al. (2013) (Ireland) BC:Altot Meesenburg et al. Graf Pannatier et al. Verstraeten et al. (2012) (2016) (Germany) (2011) (Switzerland) (Flanders) ANC Akselsson et al. (2013), Fölster et al. (2003), Löfgren et al. (2011) Löfgren and Zetterberg (Sweden), Verstraeten (2011), Pihl Karlsson et et al. (2012) (Flanders) al. (2011) (Sweden) Ionic Löfgren and Zetterberg Löfgren et al. (2011) (2011) (Sweden) (Sweden), Verstraeten et strength al. (2012) (Flanders), Vanguelova et al. (2010) (UK) 179 Table 1: Summary of trends in soil solution chemical characteristics indicative for eutrophication and 180 acidification status (concentration of NO3-, base cations (BC, i.e. Ca2+, K+ and Mg2+) and total aluminium 181 (Altot), pH, equivalent ratio of BC and Altot (BC:Altot), ANC and ionic strength) from studies across Europe. 182 Experimental studies 183 In addition to observational studies, also field experiments provide information on changes of the soil 184 chemical status under decreasing N deposition. The NITREX and EXMAN nitrogen manipulation 185 experiments at several sites in Europe are a valuable source of information (Wright and Rasmussen, 186 1998). At three NITREX sites, throughfall N deposition was brought back from high levels (36-50 kg N 187 ha-1 a-1) to 5-16 kg N ha-1 a-1 by roofing. A decline in N leaching became apparent within the first three 188 years of treatment at all three sites (Beier et al., 1998; Boxman et al., 1998; Emmett et al., 1998; 189 Gundersen et al., 1998b). A similarly fast response in N leaching has been observed from a roofing 190 experiment in southern Norway (Wright et al., 1993). These results indicate that continuous high N 191 inputs are required to sustain N leaching in most forest ecosystems, suggesting that decreasing 192 deposition quickly translates into improvements in soil water quality (Emmett et al., 1998). This, 193 however, also implies that considerable amounts of N deposited over the last decades are retained 194 and that the return of the ecosystem to the original N status is potentially slow (Gundersen et al., 195 1998b). In contrast to these findings, also unchanged or increased N leaching despite decreased 196 deposition was occasionally reported from observational (Meesenburg et al., 2016) and experimental 197 studies (Emmett et al., 1998). 198 Summary 199 Long-term monitoring data provides information on NO3- concentrations in soil solution as an indicator 200 for the soil N status. Despite considerable heterogeneity, indications for a decreasing trend in soil 201 solution NO3- concentrations at the European scale exist. Experimental studies tend to report a faster 202 and more pronounced reaction of soil solution NO3- concentrations compared to the findings from 203 large-scale observational studies. In the experiments the magnitude and speed of decrease in N supply 204 was larger compared to trends in N deposition in most parts of Europe. Furthermore, longer-term 205 changes in soil microbial activity (e.g. mineralization rates) might be reflected to a larger degree in the 206 observational studies compared to experimental studies which often focus on the time period 207 immediately after the onset of the artificial decrease of N supply. Nevertheless, both types of studies 208 report indications of a response in soil solution NO3- concentrations to decreases in N deposition. 209 210 3 Understory vegetation 211 Forests provide habitat for understory vegetation, bryophytes, lichens as well as microbial and animal 212 communities. While N is a limiting resource for many organisms (Vitousek and Howarth, 1991), the 213 efficiency with which it is used is species-specific (Chapin, 1980). As a consequence, more N causes 214 some species to thrive on the expense of others, usually causing a net loss in species diversity (Suding 215 et al., 2005). Besides this effect on interspecific competition, changes in N deposition can also modify 216 herbivory, interactions with fungi and invasibility by exotic species, thereby affecting understory 217 species composition (Gilliam, 2006). In managed forests, these mechanisms are rarely reflected in the 218 composition of the main tree species for they are typically intentionally chosen by forest managers. In 219 contrast, forest understory vegetation, bryophytes, lichens, mycorrhiza, and soil fauna can be expected 220 to be affected by N availability in addition to other environmental factors such as light availability, 221 temperature, moisture, and nutrients other than N. The responses of these groups to elevated N 222 deposition encompass changes in the abundance of species, alteration in the identity of species 223 (species composition), and pauperization of local and regional species diversity (Bobbink et al., 2010; 224 Farrer and Suding, 2016; Hautier et al., 2009; Nijssen et al., 2017). Figure 4 exemplifies effects of N 225 deposition on forest understory vegetation for lichen diversity and herb layer plant community 226 composition. 227 228 [Figure 4] 229 Observational studies 230 While there are several observational studies on the reaction of forest understory diversity to elevated 231 N deposition, to our knowledge, none of them focused specifically on the response to declining N 232 deposition. These studies confirm an increase in nitrophilic forest understory plant species on the 233 expense of oligophilic species both in European-wide (Dirnböck et al., 2014; van Dobben and De Vries, 234 2017) as well as regional approaches (Bobbink and Hettelingh, 2011 and references therein; Heinrichs 235 and Schmidt, 2016; Keith et al., 2009; Roth et al., 2015). Besides N deposition, litter quality, light 236 availability, density of large herbivores, and differences in forest management were also important 237 drivers of change in understory plant communities (Bernhardt-Römermann et al., 2015; Perring et al., 238 2017; Verheyen et al., 2012). These changes in species composition do not, however, seem to be 239 accompanied by a broad scale, synchronized decline in plant diversity in European forests (Dirnböck et 240 al., 2014; van Dobben and De Vries, 2017; Verheyen et al., 2012). 241 In contrast, elevated N deposition has clearly contributed to a dramatic diversity loss in epiphytic 242 lichens in many European forests (Bobbink and Hettelingh, 2011; Giordani et al., 2014; Hauck et al., 243 2013; Mayer et al., 2013). Similarly, major impacts in the community composition and diversity of 244 mycorrhiza were identified at the European level (Suz et al., 2014; van der Linde et al., 2018) and in 245 various regional studies (Bobbink and Hettelingh, 2011, references therein). Furthermore, diversity 246 effects of N deposition on one receptor can indirectly affect others such as soil fauna and mammals 247 because effects cascade from e.g. plants to animal species (Nijssen et al., 2017) or from soil microbes 248 to plants (Farrer and Suding, 2016). However, studies detailing the link between N deposition and 249 animal diversity in Europe’s forests are scarce, partly due to the complex dynamics of animal 250 populations and corresponding food-webs (Nijssen et al., 2017). 251 Experimental studies 252 In addition to these findings from observational studies, a limited number of N manipulation 253 experiments report on changes in understory vegetation in response to decrease of N input. 254 Strengbom et al. (2001) compared vascular plant, fungi, and bryophyte communities between control 255 and treatment plots at two experimental forested sites in Sweden where N fertilization was cancelled 256 nine and 47 years prior to the analyses, respectively. They found differences in the vascular plant 257 community at the site where treatment ended nine years ago but no longer at the site where 258 treatment was cancelled 47 years ago. Nevertheless, the fungi and bryophyte communities deviated 259 from the control plots at both sites. Sujetovienė and Stakėnas (2007) report on changes in pine forest 260 understory plant community in response to drastic emission reductions from a close-by fertilizer plant 261 in Lithuania. They found a decrease in nitrophilous species within the 16 years between two ground 262 vegetation studies (1988 and 2004). It should be noted that also light conditions and acidity status of 263 the respective forest stands changed over the same time. In one of the NITREX experiments, N- 264 indicating fern cover significantly decreased after 5 years of reduction of N deposition from 60 kg N ha- 1 -1 265 a to 5 kg N ha-1 a-1 by roofing. A recovery of other species was not recorded, however (Boxman et 266 al., 1998). 267 To a limited extent, also findings from grassland vegetation experiments might be informative for 268 forest understory vegetation response to decreasing N deposition. Stevens et al. (2012) found 269 significant differences in Ellenberg N values between control and treatment plots 15 years after 270 cessation of N fertilization in mesotrophic grassland in the UK. Shi et al. (2014) report on the vegetation 271 composition three years after cessation of N fertilization at a sandy grassland site in Northeast China. 272 They found that the vegetation at the control and the formerly treated plots still differed although 273 indications for an ongoing process of recovery were apparent. Storkey et al. (2015) report that 274 grassland biodiversity largely recovered over a period of 20 years of decreasing N deposition, based on 275 observations from the control plot of a fertilizer experiment in the UK. The pronounced recovery was 276 potentially supported by the regular export of N from the ecosystem by haying (Tilman and Isbell, 277 2015). 278 Summary 279 Recent studies based on large-scale monitoring data find shifts in understory community composition 280 in response to high levels of N deposition, but do not report on responses to decreasing N deposition. 281 Results from experimental studies suggest that while the recovery of understory vegetation from high 282 N inputs is possible, time-lags in the order of decades are to be expected. One mechanism causing 283 these delays is that in regions where high N deposition eradicated source populations, back- 284 colonization will be particularly difficult (Clark and Tilman, 2010; Dullinger et al., 2015). The complex 285 consequences of such effects have already been shown for land management legacies’ impact on 286 dispersal dynamics and subsequent community alterations (e.g. Burton et al., 2011). Strong recovery 287 delay due to dispersal limitation can be expected for epiphytic lichens because regional species 288 extinctions were particularly pronounced (Hauck et al., 2013). We hypothesize that this delay in the 289 response of understory vegetation to decreases in N deposition partly explains the absence of 290 corresponding trends in Europe-scale observational studies. In addition, changes in other 291 environmental conditions like light availability, forest management, sulphur deposition, habitat loss 292 and fragmentation, climate impact, and non-native species invasion (see e.g. Perring et al., 2017) 293 superimpose on the signal of N deposition in forest understory communities. 294 4 Tree nutrition 295 Foliar element concentrations and their ratios reflect the nutritional status of trees. Unbalanced N:P 296 ratios in foliar tissues are frequently associated with defoliation (Bontemps et al., 2011; Ferretti et al., 297 2015; Veresoglou et al., 2014; Waldner et al., 2015) and an increasing risk of attacks by parasites 298 (Flückiger and Braun, 1998) and herbivores (Pöyry et al., 2016) as well as decreasing plant capacity to 299 respond to abiotic stressors such as drought, warming, and frost (Fangmeier et al., 1994; Sardans and 300 Peñuelas, 2012). Furthermore, changes in N:P ratio in foliar tissues can have several consequences in 301 forest trophic chains (Peñuelas et al., 2013). For example, increases in foliar-litter N:P ratios have been 302 associated with shifts in community composition and decreases in species richness in soil communities 303 and understory vegetation in some European forests (Peñuelas et al., 2013). Unbalanced plant N:P 304 ratios can reduce the resistance to biotic stressors such as the competition against invasive species 305 (Sardans et al., 2016a). 306 Observational studies 307 The status and trends of tree nutrition are highly variable across Europe. At the European scale, two 308 recent studies report tendencies of decreasing foliar N concentrations for beech and oak, covering the 309 periods 1992-2009 and 2000-2015, respectively (Jonard et al., 2015; Sanders et al., 2017b). To a lesser 310 extent, decreases are also indicated for spruce, while stable or slight increasing foliar N concentrations 311 are reported for pine (Pinus sylvestris). At the same time, however, the mass per needle/leaf 312 significantly increased for spruce and beech, causing an overall increase in the c N content per 313 needle/leaf despite the decreasing concentrations (“dilution effect”, Jonard et al., 2015). At the local 314 or regional level, studies based on data from 1990 and onward report stable N concentrations or 315 moderate changes in both directions (Jonard et al., 2012; Verstraeten et al., 2017; Wellbrock et al., 316 2016). Analysis restricted to, or including data from before 1990 frequently (Duquesnay et al., 2000; 317 Hippeli and Branse, 1992; Mellert et al., 2004 for pine; Prietzel et al., 1997; Sauter, 1991) but not always 318 (Braun et al., 2010; Mellert et al., 2004) report increasing foliar N concentrations or contents across 319 Europe. Foliar P concentrations decreased continuously according to studies analyzing data from 1990 320 and onward in the important forest species in central and northern Europe, such as pine, spruce, 321 beech, and sessile oak (Quercus petraea), resulting in low or deficient foliar P status on 22% - 74% of 322 the plots depending on tree species (Ferretti et al., 2015; Jonard et al., 2015, 2012; Talkner et al., 2015). 323 For N:P, increasing ratios have been observed in several studies at European scale based on data after 324 1990 (Jonard et al., 2015; Sanders et al., 2017a; Talkner et al., 2015). Apart from N:P imbalances, also 325 trends towards increasing N:K and N:S ratios have been observed in a Europe-wide study while the 326 N:Mg ratio was decreasing (Jonard et al., 2015).N deposition can cause deficiencies in other nutrients 327 than N and nutrient imbalances due to a range of effects, including stimulation of plant growth 328 (dilution effect) and negative effects on tree nutrient acquisition by modifying mycorrhizal associations 329 (De Witte et al., 2017; Jonard et al., 2015; Peñuelas et al., 2013; Sardans et al., 2016b). Thus, the 330 decreasing tendencies in foliar concentrations of nutrients other than N and nutrient ratios suggest 331 that N availability is still high in many regions across Europe and do not imply a recovery from high N 332 deposition yet. 333 Experimental studies 334 335 Besides observational studies, a number of experiments provide indication of the reaction of foliar 336 element concentrations to decreased N supply. In one of the abovementioned NITREX roofing 337 experiments, a decrease in needle N concentrations and an improvement (reduction) of the N:Mg and 338 N:K ratio is documented after three years (Boxman et al., 1998). At the other two sites, no significant 339 reductions in foliar N concentrations were observed six years after the treatment started (Emmett et 340 al., 1998). Högberg et al. (2006) report average foliar element concentrations for the time period seven 341 to twelve years after the cessation of an N addition treatment. Foliar N concentration clearly decreased 342 and other elements showed minor increases. Twenty years after termination of the N fertilization at 343 the same site, foliar N concentration was still slightly elevated compared to the control (Högberg et 344 al., 2014). Similarly, Blaško et al. (2013) report a recovery (decrease) of foliar N concentrations based 345 on measurements 17 and 19 years after the termination of an N fertilization experiment, respectively, 346 while also still slightly exceeding the levels at the control plot. Results from grassland and moorland 347 fertilization experiments report that foliar N concentrations had decreased when measured 7-15 years 348 after cessation of the N addition (Clark et al., 2009; Edmondson et al., 2013; Stevens et al., 2012). These 349 findings from experiments indicate that decreases in N deposition can be expected to be reflected in 350 foliar N concentrations with a lag time of a several years. Further indication arises from large-scale 351 studies highlighting the relation between the spatial pattern of N deposition and foliar N 352 concentrations and contents, without, however, considering temporal trends (De Vries et al., 2003; 353 Sardans et al., 2016b). 354 Summary 355 Despite the large heterogeneity in trends in tree nutrition, studies based on large-scale long-term 356 monitoring data have reported tendencies of decreasing foliar N concentrations for beech, oak and to 357 a lesser extent for spruce. The degree to which decreasing trends in N deposition contribute to these 358 trends is not clear. (1) The decreasing tendencies of NO3- concentration in soil solution (see “Soil 359 acidification and eutrophication”), (2) findings from experimental studies and (3) large-scale studies 360 on the relation between levels of N deposition and foliar N concentrations suggest that the decrease 361 in N deposition could have affected foliar N concentrations. On the other hand, the cutback in N 362 deposition across Europe is typically much smaller compared to experimental treatments and might 363 have not yet led to a widespread decrease in N availability for tree nutrition in a relevant magnitude 364 (Braun et al., 2010; Mellert et al., 2017; Riek et al., 2016). The increase in foliar mass (dilution effect, 365 Jonard et al., 2015) likely explains a considerable proportion of the decrease in foliar N concentrations. 366 Furthermore, decreasing tendencies in other elements and N:other element ratios do not indicate 367 recovery from high N availability. Further analyses are required to gain a better understanding where 368 and to what extent changes in N deposition or other mechanisms control tree nutrition across Europe 369 and which time lags are involved. 370 5 Tree vitality 371 The concept of “vitality” of forests is linked to several inter-related aspects, including above- and 372 below-ground growth, tree nutrition as well as the susceptibility of trees to biotic (e.g. insects) and 373 abiotic (e.g. climatic extremes) stress. Tree crown condition is often interpreted as an aggregated 374 measure of tree vitality because it reflects the impacts of these different environmental drivers. It is 375 typically measured in the form of the degree of `crown defoliation` (Eichhorn et al., 2016). 376 Observational studies 377 Several studies have addressed the link between nitrogen deposition and defoliation at the European 378 scale (e.g. Ferretti et al., 2015; Klap et al., 2000), but to our knowledge none reports explicitly on the 379 effect of decreased N deposition. Existing studies focus on the relative importance of air pollution 380 among other determinants of crown condition like climate, soil, and stand age. The results reflect the 381 complexity and spatial heterogeneity of the underlying processes. For example, Ferretti et al. (2015) 382 found that N-related variables improved defoliation models based on data from 71 plots across 383 Europe. Higher N deposition led to higher percentage of defoliated trees for beech and spruce, while 384 the effect was opposite for pine. Similarly, Vitale et al. (2014) and De Marco et al. (2014) found aspects 385 of N deposition to be relevant determinants of crown condition for several species across Europe, with 386 varying direction of effect. Other studies found weak or no relation between defoliation and N 387 deposition (Hendriks et al., 2000; Klap et al., 2000; Solberg and Tørseth, 1997; Staszewski et al., 2012). 388 In a regional study, Armolaitis and Stakenas (2001) report on the response of the crown condition of a 389 pine forest to emission reductions from a close-by fertilizer plant in Lithuania. Refoliation began 6-7 390 years after the decrease of air pollution. 391 Mechanisms of N-induced effects on vitality 392 The mechanisms by which excess N supply can cause a net decrease in tree vitality can be complex, 393 interlinked and only episodically apparent, including increased susceptibility to insect attacks, 394 pathogens, frost and storm damages (Bobbink and Hettelingh, 2011), changes in mycorrhiza (Arnolds, 395 1991; Braun et al., 2010; De Witte et al., 2017; Duquesnay et al., 2000; Jaenike, 1991; van der Linde et 396 al., 2018), changes in the rooting system and aluminum toxicity to roots (Dziedek et al., 2017; Godbold 397 and Kettner, 1991; Haynes, 1982; Jonard et al., 2012; Ostonen et al., 2007), depletion of base cations 398 due to NO3- leaching (Jonard et al., 2012; Prietzel et al., 1997) or problematic P supply (Jonard et al., 399 2015; Mellert and Ewald, 2014; Neirynck et al., 1998; Ochoa-Hueso et al., 2013; Peñuelas et al., 2013; 400 Sardans et al., 2015; Sardans and Peñuelas, 2012; Thelin et al., 1998). Tree species, stand age, soil, and 401 meteorological conditions as well as other local factors co-determine these symptoms. 402 Summary 403 Tree crown condition provides an aggregated measure of tree vitality. Studies evaluating spatial and 404 temporal patterns of crown condition based on long-term monitoring data come to different 405 conclusions regarding the relative importance and direction of the effect of N deposition. To our 406 knowledge, no large-scale response to decreasing N deposition has been reported. N deposition can 407 have both a positive (fertilizing) effect on crown condition but also contribute to a range of adverse 408 mechanisms.We assume that the high complexity and spatio-temporal variability of these mechanisms 409 is partly causing the difficulty to detect signals of decreasing N deposition in tree vitality. In addition, 410 factors like stand age, drought, and frost can have strong effects on vitality independent of N 411 deposition (e.g. Eickenscheidt et al., 2016; Klap et al., 2000). 412 6 Tree growth 413 Tree growth is responsible for the primary economic benefit from most forest areas and is an 414 important process in forest CO2 budgets. Aber et al. (1998) hypothesized that net primary production 415 of trees will show an increasing and then decreasing (unimodal) response with ongoing nitrogen 416 saturation (comp. figure 1). The underlying assumption is that low to moderate levels of N deposition 417 will relieve trees from growth limitation due to originally widespread N shortage (Aber et al., 1995; De 418 Vries et al., 2009; Kahle, 2008; Schulte-Uebbing and De Vries, 2017; Solberg et al., 2009; Sutton et al., 419 2008; Vitousek and Howarth, 1991). However, when N deposition exceeds a certain level, the 420 stimulating effects diminish due to the antagonistic effects applying to overall tree vitality (see above), 421 e.g. of soil acidification, nutrient imbalances and increased susceptibility to biotic and abiotic stress 422 (Aber et al., 1998; De Vries et al., 2014; Dobbertin, 2005). For example, beneficial effects for tree 423 growth by recovery from acidification have been documented in Europe and the US (Mathias and 424 Thomas, 2018; Juknys et al., 2014). 425 There are various broad-scale and regional studies investigating the effect of N deposition on tree 426 growth, while accounting for the impacts of other drivers, such as changes in temperature and 427 precipitation (e.g. Braun et al., 2017; Kint et al., 2012; Kolář et al., 2015; Solberg et al., 2009). In these 428 studies, changes in growth patterns have rarely been explicitly linked to declining trends in nitrogen 429 deposition. In some cases, a simultaneous decrease in S and N deposition complicated the separation 430 of effects (Juknys et al., 2014; Nellemann and Thomsen, 2001). However, the results of these studies 431 can be used to derive indications for the threshold level of N deposition at which growth enhancement 432 and growth reductions can be expected (Braun et al., 2017; Kint et al., 2012). For example, field 433 monitoring data of forest growth at more than 300 plots in Europe suggest a non-linear growth 434 response to N deposition between 3 and 60 kg N ha–1yr–1 with a threshold near 35 kg N ha–1yr–1 (Solberg 435 et al., 2009). Kint et al. (2012) documented a non-linear growth response, in terms of basal area 436 increment (BAI), to increasing N availability for 180 oak and beech plots in Flanders throughout the 437 20th century (the period 1901–2008). They found positive effects of N deposition on BAI up to 20–30 438 kg N ha−1 yr−1 and declining growth above these levels. Etzold et al. (2014) found a non-linear relation 439 between NPP and N deposition, with the positive effect flattening off at sites with an N deposition 440 above 20 kg N ha-1yr-1, based on data from intensive monitoring plots in Switzerland. In experimental 441 and observational studies in forests in Switzerland, Flückiger et al. (2011) found a growth-stimulating 442 effect of N which turned into no effect or a decrease of growth with increasing duration or magnitude 443 of the N input. Anders et al. (2002, in Bobbink and Hettelingh, 2011) reported growth-reducing effects 444 of N deposition on Scots pine stands in the north-east of the German Northern Lowland in the vicinity 445 of N emission sources with deposition rates exceeding 35 kg N ha-1 a-1, while for other locations and 446 tree species, accelerated growth was observed at open field deposition rates exceeding 10 to 15 kg N 447 ha-1 a-1. 448 Further information for the growth response of trees to different levels of N deposition stems from 449 field experiments. For example, in one of the NITREX experiments, Boxman et al. (1998) report a 450 significant increase in growth after three years of artificially decreasing N deposition rates by roofing. 451 It should be noted, however, that in this experiment not only N but also S deposition decreased. 452 Högberg et al. (2006) found that very high rates of N addition (90-180 kg N ha-1 a-1) led to increases in 453 tree growth only until a cumulative amount of approximately 1 t N ha-1 while further N addition 454 lowered the gain in wood volume. In a similar experimental setup, Blaško et al. (2013) observed that a 455 strongly fertilized plot (90-180 kg N ha-1 a-1) had a lower long-term average productivity than other 456 fertilization levels (30-120 kg N ha-1 a-1) but still more than the control plot. These results support the 457 perspective that improved N supply has a positive effect on growth in case of N limitation and can act 458 negatively in case of excess N (Flückiger et al., 2011). 459 Global meta-analyses also confirm thresholds in the growth response of trees to N deposition. For 460 example, Tian et al. (2016) analysed a dataset of 44 experimental studies from wetland, grassland, 461 temperate, and boreal forest (most data are from temperate forest). They found that the effect of N 462 input on aboveground net primary production switches from increase to decrease at approximately 463 50-60 kg N ha-1 a-1. Schulte-Uebbing and de Vries (2017) found that the N-induced increase in carbon 464 sequestration was significantly lower at higher ambient N deposition rates (above 15 kg N ha-1 a-1), 465 reviewing results from forest fertilization experiments in temperate, boreal and tropical forests. Field 466 data of maximum rates of photosynthesis against N deposition for 80 forested plots over the world 467 indicated an increase in photosynthesis up to an N deposition near 10-15 kg N ha-1 a-1 followed by no 468 further change up to 35 kg N ha-1 a-1 (Fleischer et al., 2013). 469 Summary 470 We did not find indications for a large-scale response in tree growth to decreasing N deposition. 471 However, results from observational and experimental studies corroborate the concept of a unimodal 472 response of tree growth to N deposition. Estimates of thresholds above which N deposition negatively 473 affects tree growth range from as low as 15 - 20 kg N ha-1 a-1 to very high levels only relevant under 474 experimental conditions. This suggests that particularly polluted forest stands mostly located in Central 475 and Western Europe might have benefitted from declining N deposition, as decreases have been 476 strongest in the formerly most polluted regions. Few trends in N deposition have been observed in less 477 polluted areas like Northern Scandinavia, suggesting that a growth decline due to decreased N 478 deposition in these areas is less likely. 479 7 Conclusion and outlook 480 Results from observational studies across Europe for responses in soil, ground vegetation, and trees 481 (nutrition, growth and vitality) to decreasing N deposition indicate considerable spatial variability in 482 the trends published for these parameters. For soil solution NO3- concentrations and potentially also 483 for changes in foliar N concentrations, indications for a reaction to decreased nitrogen deposition exist. 484 We found several studies reporting on the effects of N deposition on understory vegetation, tree 485 growth or tree vitality, but none of them focused specifically on responses to declining N deposition. 486 For tree growth, these studies suggest a positive effect at low to moderate levels of N deposition and 487 no or adverse effects at high levels. In line with these findings from observational studies, experimental 488 studies also report more pronounced reactions of soil solution and foliar concentrations to decreased 489 nitrogen deposition compared to the other parameters. Stevens (2016) reviewed experimental and 490 observational studies in grasslands, heathlands, wetlands, and forests for information on the speed of 491 recovery from high N deposition. Mainly in line with our findings, they report a relatively rapid 492 response for mobile or plant-available forms of N in soil chemistry and for N contents in plant tissues 493 across habitats (with the exception of forests showing a slower response in foliar element 494 concentrations compared to other habitats). Similarly, Rowe et al. (2017) suggest N leaching rates and 495 (moss) tissue N concentrations as midpoint-metrics, i.e. indicators for effects-based monitoring of 496 progress towards pollution reduction targets, due to their dynamic response to changing N deposition 497 rates. 498 Linking results from observational and experimental studies is problematic due to the more controlled 499 conditions and the typically faster and stronger cutback of N supply rates in experimental settings 500 compared to real-world decreases in N deposition. A multitude of confounding factors, including the 501 joint decrease of N and S deposition (e.g. Armolaitis and Stakenas 2001) complicate the interpretation 502 of results from observational studies. Furthermore, many of the large-scale observational studies 503 reviewed in this paper are based on plots which are not distributed representatively across Europe. 504 The larger monitoring efforts in Central and Western Europe likely led to an overrepresentation of 505 plots where N deposition remained on a high level despite comparatively large decreases of N 506 deposition. 507 Future decrease of N deposition to forests in Europe and associated ecosystem responses will most 508 likely be limited (figure 3). Simpson et al. (2014) expect only minor reductions in the European 509 ecosystem area with exceedances of the critical load for nutrient nitrogen (from 64% in 2005 to 50% 510 in 2050). Under the assumption that soil solution NO3- concentrations and potentially also foliar N 511 concentrations track changes in N inputs with a delay of only a few years (see above), limited changes 512 of these parameters in response to declining N deposition would be expected for the future. For tree 513 vitality and vitality-related growth effects, time-lags in the recovery from excess N deposition might 514 be expected due to slow reversal of N-induced soil acidification and changes in mycorrhizal association. 515 For understory vegetation community composition it has to be questioned whether full recovery can 516 be expected at all since forest biodiversity is facing a number of additional “extinction debts” such as 517 habitat loss and fragmentation, climate impact, and non-native species invasion (see e.g. Perring et al., 518 2017) likely causing further decline in biodiversity (Essl et al., 2015). If at all, these recovery processes 519 will, however, only become apparent in regions with sufficient absolute magnitude of the cutback in 520 N deposition. Furthermore, responses will likely be highly heterogeneous in space controlled by site- 521 specific conditions. 522 In view of our results, a simple reversal of the stages of the classical nitrogen saturation concept (figure 523 1) does not seem to appropriately reflect the observed and expected responses to decreasing N 524 deposition. Instead, several forest ecosystem properties seem to react with varying degree of delay to 525 cutbacks in N deposition. Correspondingly, the overall forest ecosystem state develops on a different 526 trajectory during the process of N de-saturation compared to N saturation. This hysteresis behavior is 527 in line with findings from Gilliam et al. (2018, in press), who review results for soil acidification, plant 528 biodiversity, soil microbial communities, forest carbon (C) and N cycling, and surface water chemistry 529 with focus on the US. In view of the high variability of forest ecosystems, a set of “recovery types” 530 could potentially serve to roughly classify the development of major strata of forest sites under 531 decreasing N deposition. For analytic and predictive purposes, more detailed models will be required 532 to adequately represent processes of N (de-)saturation. In particular, dynamic modelling approaches 533 taking complex microbial soil N processes into account may provide insights into the developments of 534 forest ecosystem N pools accumulated over the last decades (Akselsson et al., 2016; Bonten et al., 535 2016; Dirnböck et al., 2017; Fleck et al., 2017; Rizzetto et al., 2016; Yu et al., 2016). Under the expected 536 limited future decrease in N deposition, other controlling factors like climate change and forest 537 management strategies will probably dominate the changes in N-enriched forests. 538 Acknowledgements 539 Thanks to Magnuz Engardt and David Simpson for providing the time series on reduced and oxidized 540 N deposition to Europe according to the EMEP model as published in Engardt et al., (2017). 541 Funding sources 542 Thomas Dirnböck was funded by the project ACRP7 - WOOD-N-CLIMATE, Austria [grant number 543 KR14AC7K11858]. Josep Peñuelas and Jordi Sardans would like to acknowledge the financial support 544 from the European Research Council Synergy grant ERC-SyG-2013-610028 IMBALANCE-P. 545 546 Declarations of interest 547 Declarations of interest: none 548 549 550 Literature 551 Aber, J., McDowell, W., Nadelhoffer, K., Magill, A., Berntson, G., Kamakea, M., McNulty, S., Currie, W., 552 Rustad, L., Fernandez, I., 1998. Nitrogen Saturation in Temperate Forest Ecosystems - 553 Hypotheses revisited. BioScience 48, 921–934. https://doi.org/10.2307/1313296 554 Aber, J.D., Magill, A., Mcnulty, S.G., Boone, R.D., Nadelhoffer, K.J., Downs, M., Hallett, R., 1995. Forest 555 biogeochemistry and primary production altered by nitrogen saturation. Water Air Soil Pollut 556 85, 1665–1670. https://doi.org/10.1007/BF00477219 557 Aber, J.D., Nadelhoffer, K.J., Steudler, P., Melillo, J.M., 1989. Nitrogen Saturation in Northern Forest 558 Ecosystems Excess nitrogen from fossil fuel combustion may stress the biosphere. BioScience 559 39, 378–386. https://doi.org/10.2307/1311067 560 Ågren, G.I., Bosatta, E., 1988. Nitrogen saturation of terrestrial ecosystems. Environmental Pollution 561 54, 185–197. https://doi.org/10.1016/0269-7491(88)90111-X 562 Akselsson, C., Hultberg, H., Karlsson, P.E., Pihl Karlsson, G., Hellsten, S., 2013. Acidification trends in 563 south Swedish forest soils 1986–2008 — Slow recovery and high sensitivity to sea-salt 564 episodes. Science of The Total Environment 444, 271–287. 565 https://doi.org/10.1016/j.scitotenv.2012.11.106 566 Akselsson, C., Olsson, J., Belyazid, S., Capell, R., 2016. Can increased weathering rates due to future 567 warming compensate for base cation losses following whole-tree harvesting in spruce forests? 568 Biogeochemistry 128, 89–105. https://doi.org/10.1007/s10533-016-0196-6 569 Alewell, C., Manderscheid, B., Gerstberger, P., Matzner, E., 2000. Effects of reduced atmospheric 570 deposition on soil solution chemistry and elemental contents of spruce needles in NE— 571 Bavaria, Germany. Journal of Plant Nutrition and Soil Science 163, 509–516. 572 https://doi.org/10.1002/1522-2624(200010)163:5<509::AID-JPLN509>3.0.CO;2-3 573 Anders, S., Beck, W., Bolte, A., Hofmann, G., Jenssen, M., Krakau, U., Müller, J., 2002. Ökologie und 574 Vegetation der Wälder Nordostdeutschlands. Kessel, Oberwinter. 575 Armolaitis, K., Stakenas, V., 2001. The Recovery of Damaged Pine Forests in an Area Formerly Polluted 576 by Nitrogen. The Scientific World Journal 1, 384–393. https://doi.org/10.1100/tsw.2001.451 577 Arnolds, E., 1991. Decline of ectomycorrhizal fungi in Europe. Agriculture, Ecosystems & Environment 578 35, 209–244. https://doi.org/10.1016/0167-8809(91)90052-Y 579 Beier, C., Blanck, K., Bredemeier, M., Lamersdorf, N., Rasmussen, L., Xu, Y.-J., 1998. Field-scale ‘clean 580 rain’ treatments to two Norway spruce stands within the EXMAN project—effects on soil 581 solution chemistry, foliar nutrition and tree growth. Forest Ecology and Management 101, 582 111–123. https://doi.org/10.1016/S0378-1127(97)00129-1 583 Bernhardt-Römermann, M., Baeten, L., Craven, D., De Frenne, P., Hédl, R., Lenoir, J., Bert, D., Brunet, 584 J., Chudomelová, M., Decocq, G., Dierschke, H., Dirnböck, T., Dörfler, I., Heinken, T., Hermy, 585 M., Hommel, P., Jaroszewicz, B., Keczyński, A., Kelly, D.L., Kirby, K.J., Kopecký, M., Macek, M., 586 Máliš, F., Mirtl, M., Mitchell, F.J.G., Naaf, T., Newman, M., Peterken, G., Petřík, P., Schmidt, W., 587 Standovár, T., Tóth, Z., Calster, H.V., Verstraeten, G., Vladovič, J., Vild, O., Wulf, M., Verheyen, 588 K., 2015. Drivers of temporal changes in temperate forest plant diversity vary across spatial 589 scales. Glob Change Biol 21, 3726–3737. https://doi.org/10.1111/gcb.12993 590 Blaško, R., Högberg, P., Bach, L.H., Högberg, M.N., 2013. Relations among soil microbial community 591 composition, nitrogen turnover, and tree growth in N-loaded and previously N-loaded boreal 592 spruce forest. Forest Ecology and Management 302, 319–328. 593 https://doi.org/10.1016/j.foreco.2013.02.035 594 Bobbink, R., Hettelingh, J.-P., 2011. Review and revision of empirical critical loads and dose-response 595 relationships. Coordination Centre for Effects, National Institute for Public Health and the 596 Environment (RIVM), Bilthoven. 597 Bobbink, R., Hicks, K., Galloway, J., Spranger, T., Alkemade, R., Ashmore, M., Bustamante, M., Cinderby, 598 S., Davidson, E., Dentener, F., Emmett, B., Erisman, J.-W., Fenn, M., Gilliam, F., Nordin, A., 599 Pardo, L., De Vries, W., 2010. Global assessment of nitrogen deposition effects on terrestrial 600 plant diversity: a synthesis. Ecological Applications 20, 30–59. https://doi.org/10.1890/08- 601 1140.1 602 Bontemps, J.-D., Hervé, J.-C., Leban, J.-M., Dhôte, J.-F., 2011. Nitrogen footprint in a long-term 603 observation of forest growth over the twentieth century. Trees 25, 237–251. 604 https://doi.org/10.1007/s00468-010-0501-2 605 Bonten, L.T.C., Reinds, G.J., Posch, M., 2016. A model to calculate effects of atmospheric deposition on 606 soil acidification, eutrophication and carbon sequestration. Environmental Modelling & 607 Software 79, 75–84. https://doi.org/10.1016/j.envsoft.2016.01.009 608 Boudot, J.P., Becquer, T., Merlet, D., Rouiller, J., 1994. Aluminium toxicity in declining forests: a general 609 overview with a seasonal assessment in a silver fir forest in the Vosges mountains (France). 610 Ann. For. Sci. 51, 27–51. https://doi.org/10.1051/forest:19940103 611 Boxman, A.W., Peters, R.C.J.H., Roelofs, J.G.M., 2008. Long term changes in atmospheric N and S 612 throughfall deposition and effects on soil solution chemistry in a Scots pine forest in the 613 Netherlands. Environmental Pollution 156, 1252–1259. 614 https://doi.org/10.1016/j.envpol.2008.03.017 615 Boxman, A.W., van der Ven, P.J.M., Roelofs, J.G.M., 1998. Ecosystem recovery after a decrease in 616 nitrogen input to a Scots pine stand at Ysselsteyn, the Netherlands. Forest Ecology and 617 Management 101, 155–163. https://doi.org/10.1016/S0378-1127(97)00132-1 618 Braun, S., Schindler, C., Rihm, B., 2017. Growth trends of beech and Norway spruce in Switzerland: The 619 role of nitrogen deposition, ozone, mineral nutrition and climate. Science of The Total 620 Environment 599, 637–646. https://doi.org/10.1016/j.scitotenv.2017.04.230 621 Braun, S., Thomas, V.F.D., Quiring, R., Flückiger, W., 2010. Does nitrogen deposition increase forest 622 production? The role of phosphorus. Environmental Pollution, Advances of air pollution 623 science: from forest decline to multiple-stress effects on forest ecosystem services 158, 2043– 624 2052. https://doi.org/10.1016/j.envpol.2009.11.030 625 Brink, C., van Grinsven, Hans, Jacobsen, B.H., Rabl, A., Gren, I.-M., Holland, M., Klimont, Z., Hicks, K., 626 Brouwer, R., Dickens, R., Willems, J., Termansen, M., Velthof, G., Alkemade, R., van Oorschot, 627 M., Webb, J., 2011. Chapter 22: Costs and benefits of nitrogen in the environment, in: Sutton, 628 M.A., Howard, C.M., Erisman, J.W., Billen, G., Bleeker, A., Grennfelt, P., van Grinsven, H., 629 Grizzetti, B. (Eds.), The European Nitrogen Assessment. Cambridge University Press. 630 Burton, J.I., Mladenoff, D.J., Clayton, M.K., Forrester, J.A., 2011. The roles of environmental filtering 631 and colonization in the fine-scale spatial patterning of ground-layer plant communities in north 632 temperate deciduous forests. Journal of Ecology 99, 764–776. https://doi.org/10.1111/j.1365- 633 2745.2011.01807.x 634 Chapin, F.S., 1980. The Mineral Nutrition of Wild Plants. Annu. Rev. Ecol. Syst. 11, 233–260. 635 https://doi.org/10.1146/annurev.es.11.110180.001313 636 Clark, C.M., Hobbie, S.E., Venterea, R., Tilman, D., 2009. Long-lasting effects on nitrogen cycling 12 637 years after treatments cease despite minimal long-term nitrogen retention. Global Change 638 Biology 15, 1755–1766. https://doi.org/10.1111/j.1365-2486.2008.01811.x 639 Clark, C.M., Morefield, P.E., Gilliam, F.S., Pardo, L.H., 2013. Estimated losses of plant biodiversity in the 640 United States from historical N deposition (1985–2010). Ecology 94, 1441–1448. 641 https://doi.org/10.1890/12-2016.1 642 Clark, C.M., Tilman, D., 2010. Recovery of plant diversity following N cessation: effects of recruitment, 643 litter, and elevated N cycling. Ecology 91, 3620–3630. https://doi.org/10.1890/09-1268.1 644 Cools, N., De Vos, B., 2011. Availability and evaluation of European forest soil monitoring data in the 645 study on the effects of air pollution on forests. iForest - Biogeosciences and Forestry 4, 205. 646 https://doi.org/10.3832/ifor0588-004 647 De Marco, A., Proietti, C., Cionni, I., Fischer, R., Screpanti, A., Vitale, M., 2014. Future impacts of 648 nitrogen deposition and climate change scenarios on forest crown defoliation. Environmental 649 Pollution 194, 171–180. https://doi.org/10.1016/j.envpol.2014.07.027 650 De Vries, W., Bolhuis, P., van den Burg, A., Bobbink, R., 2017a. Ongoing acidification of forests soil: 651 causes and impacts on the forest ecosystem., Vakblad voor Bos Natuur en Landschap. 652 De Vries, W., Dobbertin, M.H., Solberg, S., Dobben, H.F. van, Schaub, M., 2014. Impacts of acid 653 deposition, ozone exposure and weather conditions on forest ecosystems in Europe: an 654 overview. Plant Soil 380, 1–45. https://doi.org/10.1007/s11104-014-2056-2 655 De Vries, W., Hettelingh, J.-P., Posch, M. (Eds.), 2015. Critical Loads and Dynamic Risk Assessments: 656 Nitrogen, Acidity and Metals in Terrestrial and Aquatic Ecosystems, Environmental Pollution. 657 Springer Netherlands. 658 De Vries, W., Posch, M., Simpson, D., Reinds, G.J., 2017b. Modelling long-term impacts of changes in 659 climate, nitrogen deposition and ozone exposure on carbon sequestration of European forest 660 ecosystems. Science of The Total Environment 605–606, 1097–1116. 661 https://doi.org/10.1016/j.scitotenv.2017.06.132 662 De Vries, W., Reinds, J.G., Gundersen, P., Sterba, H., 2006. The impact of nitrogen deposition on carbon 663 sequestration in European forests and forest soils. Global Change Biology 12, 1151–1173. 664 https://doi.org/10.1111/j.1365-2486.2006.01151.x 665 De Vries, W., Schulte-Uebbing, L., 2018. Impacts of nitrogen deposition on forest ecosystem services 666 and biodiversity, in: Schröter, M., Bonn, A., Klotz, S., Seppelt, R., Baessler, C. (Eds.), Atlas of 667 Ecosystem Services: Drivers, Risks, and Societal Responses. 668 De Vries, W., Solberg, S., Dobbertin, M., Sterba, H., Laubhann, D., van Oijen, M., Evans, C., Gundersen, 669 P., Kros, J., Wamelink, G.W.W., Reinds, G.J., Sutton, M.A., 2009. The impact of nitrogen 670 deposition on carbon sequestration by European forests and heathlands. Forest Ecology and 671 Management, The relative importance of nitrogen deposition and climate change on the 672 sequestration of carbon by forests in Europe. 258, 1814–1823. 673 https://doi.org/10.1016/j.foreco.2009.02.034 674 De Vries, W., van der Salm, C., Reinds, G.J., Erisman, J.W., 2007. Element fluxes through European 675 forest ecosystems and their relationships with stand and site characteristics. Environmental 676 Pollution 148, 501–513. https://doi.org/10.1016/j.envpol.2006.12.001 677 De Vries, W., Vel, E., Reinds, G.J., Deelstra, H., Klap, J.M., Leeters, E.E.J.M., Hendriks, C.M.A., 678 Kerkvoorden, M., Landmann, G., Herkendell, J., Haussmann, T., Erisman, J.W., 2003. Intensive 679 monitoring of forest ecosystems in Europe: 1. Objectives, set-up and evaluation strategy. 680 Forest Ecology and Management 174, 77–95. https://doi.org/10.1016/S0378-1127(02)00029- 681 4 682 De Wit, H.A., Eldhuset, T.D., Mulder, J., 2010. Dissolved Al reduces Mg uptake in Norway spruce forest: 683 Results from a long-term field manipulation experiment in Norway. Forest Ecology and 684 Management 259, 2072–2082. https://doi.org/10.1016/j.foreco.2010.02.018 685 De Witte, L.C., Rosenstock, N.P., van der Linde, S., Braun, S., 2017. Nitrogen deposition changes 686 ectomycorrhizal communities in Swiss beech forests. Science of The Total Environment 605, 687 1083–1096. https://doi.org/10.1016/j.scitotenv.2017.06.142 688 Dirnböck, T., Foldal, C., Djukic, I., Kobler, J., Haas, E., Kiese, R., Kitzler, B., 2017. Historic nitrogen 689 deposition determines future climate change effects on nitrogen retention in temperate 690 forests. Climatic Change 144, 221–235. https://doi.org/10.1007/s10584-017-2024-y 691 Dirnböck, T., Grandin, U., Bernhardt-Römermann, M., Beudert, B., Canullo, R., Forsius, M., Grabner, 692 M.-T., Holmberg, M., Kleemola, S., Lundin, L., Mirtl, M., Neumann, M., Pompei, E., Salemaa, 693 M., Starlinger, F., Staszewski, T., Uziębło, A.K., 2014. Forest floor vegetation response to 694 nitrogen deposition in Europe. Glob Change Biol 20, 429–440. 695 https://doi.org/10.1111/gcb.12440 696 Dise, N.B., Rothwell, J.J., Gauci, V., van der Salm, C., De Vries, W., 2009. Predicting dissolved inorganic 697 nitrogen leaching in European forests using two independent databases. Science of The Total 698 Environment 407, 1798–1808. https://doi.org/10.1016/j.scitotenv.2008.11.003 699 Dise, N.B., Wright, R.F., 1995. Nitrogen leaching from European forests in relation to nitrogen 700 deposition. Forest Ecology and Management, Nitrex 71, 153–161. 701 https://doi.org/10.1016/0378-1127(94)06092-W 702 Dobbertin, M., 2005. Tree growth as indicator of tree vitality and of tree reaction to environmental 703 stress: a review. Eur J Forest Res 124, 319–333. https://doi.org/10.1007/s10342-005-0085-3 704 Driscoll, C.T., Fallon‐Lambert, K., Chen, L., 2006. Acidic Deposition: Sources and Effects, in: 705 Encyclopedia of Hydrological Sciences. American Cancer Society. 706 https://doi.org/10.1002/0470848944.hsa095 707 Driscoll, C.T., Lawrence, G.B., Bulger, A.J., Butler, T.J., Cronan, C.S., Eagar, C., Lambert, K.F., Likens, G.E., 708 Stoddard, J.L., Weathers, K.C., 2001. Acidic Deposition in the Northeastern United States: 709 Sources and Inputs, Ecosystem Effects, and Management Strategies. BioScience 51, 180–198. 710 https://doi.org/10.1641/0006-3568(2001)051[0180:ADITNU]2.0.CO;2 711 Duan, L., Yu, Q., Zhang, Q., Wang, Z., Pan, Y., Larssen, T., Tang, J., Mulder, J., 2016. Acid deposition in 712 Asia: Emissions, deposition, and ecosystem effects. Atmospheric Environment, Acid Rain and 713 its Environmental Effects: Recent Scientific Advances 146, 55–69. 714 https://doi.org/10.1016/j.atmosenv.2016.07.018 715 Dullinger, S., Dendoncker, N., Gattringer, A., Leitner, M., Mang, T., Moser, D., Mücher, C.A., Plutzar, C., 716 Rounsevell, M., Willner, W., Zimmermann, N.E., Hülber, K., 2015. Modelling the effect of 717 habitat fragmentation on climate-driven migration of European forest understorey plants. 718 Diversity and Distributions 21, 1375–1387. https://doi.org/10.1111/ddi.12370 719 Duquesnay, A., Dupouey, J.L., Clement, A., Ulrich, E., Le Tacon, F., 2000. Spatial and temporal variability 720 of foliar mineral concentration in beech (Fagus sylvatica) stands in northeastern France. Tree 721 Physiol. 20, 13–22. 722 Dziedek, C., Fichtner, A., Calvo, L., Marcos, E., Jansen, K., Kunz, M., Walmsley, D., Von Oheimb, G., 723 Härdtle, W., 2017. Phenotypic Plasticity Explains Response Patterns of European Beech (Fagus 724 sylvatica L.) Saplings to Nitrogen Fertilization and Drought Events. Forests 8, 91. 725 https://doi.org/10.3390/f8030091 726 Edmondson, J., Terribile, E., Carroll, J.A., Price, E.A.C., Caporn, S.J.M., 2013. The legacy of nitrogen 727 pollution in heather moorlands: Ecosystem response to simulated decline in nitrogen 728 deposition over seven years. Science of The Total Environment 444, 138–144. 729 https://doi.org/10.1016/j.scitotenv.2012.11.074 730 Eichhorn, J., Roskams, P., Potočić, N., Timmermann, V., Ferretti, M., Mues, V., Szepesi, A., Durrant, D., 731 Seletković, I., Schröck, H.W., Nevalainen, S., Bussotti, F., Garcia, P., Wulff, S., 2016. Part IV: 732 Visual Assessment of Crown Condition and Damaging Agents, in: UNECE ICP Forests 733 Programme Coordinating Centre (Ed.), Manual on Methods and Criteria for Harmonized 734 Sampling, Assessment, Monitoring and Analysis of the Effects of Air Pollution on Forests. 735 Thünen Institute of Forest Ecosystems, Eberswalde, Germany. 736 Eickenscheidt, N., Wellbrock, N., Dühnelt, P., Hilbrig, L., 2016. Kronenzustand – Steuergrößen und 737 Raum-Zeit-Entwicklung von 1989-2015, in: Wellbrock, N., Bolte, A., Flessa, H. (Eds.), Dynamik 738 Und Räumliche Muster Forstlicher Standorte in Deutschland : Ergebnisse Der 739 Bodenzustandserhebung Im Wald 2006 Bis 2008, Thünen Report. Johann Heinrich von Thünen- 740 Institut, Braunschweig, pp. 75–134. 741 Emmett, B.A., Boxman, D., Bredemeier, M., Gundersen, P., Kjønaas, O.J., Moldan, F., Schleppi, P., 742 Tietema, A., Wright, R.F., 1998. Predicting the Effects of Atmospheric Nitrogen Deposition in 743 Conifer Stands: Evidence from the NITREX Ecosystem-Scale Experiments. Ecosystems 1, 352– 744 360. https://doi.org/10.1007/s100219900029 745 Engardt, M., Simpson, D., Schwikowski, M., Granat, L., 2017. Deposition of sulphur and nitrogen in 746 Europe 1900–2050. Model calculations and comparison to historical observations. Tellus B: 747 Chemical and Physical Meteorology 69, 1328945. 748 https://doi.org/10.1080/16000889.2017.1328945 749 Erisman, J.W., Grennfelt, P., Sutton, M., 2003. The European perspective on nitrogen emission and 750 deposition. Environment International, Future Directions in Air Quality Research : 751 Ecological,Atmospheric,Regulatory/Policy/Economic, and Educational Issues 29, 311–325. 752 https://doi.org/10.1016/S0160-4120(02)00162-9 753 Erisman, J.W., Sutton, M.A., Galloway, J., Klimont, Z., Winiwarter, W., 2008. How a century of ammonia 754 synthesis changed the world. Nature Geoscience 1, 636–639. 755 https://doi.org/10.1038/ngeo325 756 Erisman, J.-W., van Grinsven, H., Grizzetti, B., Bouraoui, F., Powlson, D., Sutton, M. A., Bleeker, A., Reis, 757 S., 2011. Chapter 2: The European nitrogen problem in a global perspective, in: Sutton, Mark 758 A., Howard, C.M., Erisman, J.W., Billen, G., Bleeker, A., Grennfelt, P., van Grinsven, H., Grizzetti, 759 B. (Eds.), The European Nitrogen Assessment. Cambridge University Press. 760 Essl, F., Dullinger, S., Rabitsch, W., Hulme, P.E., Pyšek, P., Wilson, J.R.U., Richardson, D.M., 2015. 761 Historical legacies accumulate to shape future biodiversity in an era of rapid global change. 762 Diversity and Distributions 21, 534–547. https://doi.org/10.1111/ddi.12312 763 Etzold, S., Waldner, P., Thimonier, A., Schmitt, M., Dobbertin, M., 2014. Tree growth in Swiss forests 764 between 1995 and 2010 in relation to climate and stand conditions: Recent disturbances 765 matter. Forest Ecology and Management, Monitoring European forests: detecting and 766 understanding changes 311, 41–55. https://doi.org/10.1016/j.foreco.2013.05.040 767 Evans, C.D., Cullen, J.M., Alewell, C., Kopácek, J., Marchetto, A., Moldan, F., Prechtel, A., Rogora, M., 768 Veselý, J., Wright, R., 2001. Recovery from acidification in European surface waters. Hydrology 769 and Earth System Sciences Discussions 5, 283–298. https://doi.org/10.5194/hess-5-283-2001 770 Fangmeier, A., Hadwiger-Fangmeier, A., Van der Eerden, L., Jäger, H.-J., 1994. Effects of atmospheric 771 ammonia on vegetation - A review. Environmental Pollution 86, 43–82. 772 https://doi.org/10.1016/0269-7491(94)90008-6 773 Farrer, E.C., Suding, K.N., 2016. Teasing apart plant community responses to N enrichment: the roles 774 of resource limitation, competition and soil microbes. Ecology Letters 19, 1287–1296. 775 https://doi.org/10.1111/ele.12665 776 Ferretti, M., Calderisi, M., Marchetto, A., Waldner, P., Thimonier, A., Jonard, M., Cools, N., Rautio, P., 777 Clarke, N., Hansen, K., Merilä, P., Potočić, N., 2015. Variables related to nitrogen deposition 778 improve defoliation models for European forests. Annals of Forest Science 72, 897–906. 779 https://doi.org/10.1007/s13595-014-0445-6 780 Fleck, S., Ahrends, B., Sutmöller, J., Albert, M., Evers, J., Meesenburg, H., 2017. Is Biomass 781 Accumulation in Forests an Option to Prevent Climate Change Induced Increases in Nitrate 782 Concentrations in the North German Lowland? Forests 8, 219. 783 https://doi.org/10.3390/f8060219 784 Fleischer, K., Rebel, K.T., van der Molen, M.K., Erisman, J.W., Wassen, M.J., van Loon, E.E., Montagnani, 785 L., Gough, C.M., Herbst, M., Janssens, I.A., Gianelle, D., Dolman, A.J., 2013. The contribution of 786 nitrogen deposition to the photosynthetic capacity of forests. Global Biogeochem. Cycles 27, 787 187–199. https://doi.org/10.1002/gbc.20026 788 Flückiger, W., Braun, S., 1998. Nitrogen deposition in Swiss forests and its possible relevance for leaf 789 nutrient status, parasite attacks and soil acidification. Environmental Pollution, Nitrogen, the 790 Confer-N-s First International Nitrogen Conference 1998 102, 69–76. 791 https://doi.org/10.1016/S0269-7491(98)80017-1 792 Flückiger, W., Braun, S., Mainiero, R., Schütz, K., Thomas, V., 2011. Auswirkung erhöhter 793 Stickstoffbelastung auf die Stabilität des Waldes. Synthesebericht im Auftrag des BAFU. Institut 794 für Angewandte Pflanzenbiologie (IAP), Schönenbuch, Switzerland. 795 Fölster, J., Bringmark, L., Lundin, L., 2003. Temporal and Spatial Variations in Soil Water Chemistry at 796 Three Acid Forest Sites. Water, Air, & Soil Pollution 146, 171–195. 797 https://doi.org/10.1023/A:1023991910275 798 Fowler, D., Pyle, J.A., Raven, J.A., Sutton, M.A., 2013. The global nitrogen cycle in the twenty-first 799 century: introduction. Phil. Trans. R. Soc. B 368, 20130165. 800 https://doi.org/10.1098/rstb.2013.0165 801 Fowler, D., Steadman, C.E., Stevenson, D., Coyle, M., Rees, R.M., Skiba, U.M., Sutton, M.A., Cape, J.N., 802 Dore, A.J., Vieno, M., Simpson, D., Zaehle, S., Stocker, B.D., Rinaldi, M., Facchini, M.C., Flechard, 803 C.R., Nemitz, E., Twigg, M., Erisman, J.W., Butterbach-Bahl, K., Galloway, J.N., 2015. Effects of 804 global change during the 21st century on the nitrogen cycle. Atmos. Chem. Phys. 15, 13849– 805 13893. https://doi.org/10.5194/acp-15-13849-2015 806 Galloway, J.N., Aber, J.D., Erisman, J.W., Seitzinger, S.P., Howarth, R.W., Cowling, E.B., Cosby, B.J., 807 2003. The Nitrogen Cascade. BioScience 53, 341–356. https://doi.org/10.1641/0006- 808 3568(2003)053[0341:TNC]2.0.CO;2 809 Galloway, J.N., Dentener, F.J., Capone, D.G., Boyer, E.W., Howarth, R.W., Seitzinger, S.P., Asner, G.P., 810 Cleveland, C.C., Green, P.A., Holland, E.A., Karl, D.M., Michaels, A.F., Porter, J.H., Townsend, 811 A.R., Vöosmarty, C.J., 2004. Nitrogen Cycles: Past, Present, and Future. Biogeochemistry 70, 812 153–226. https://doi.org/10.1007/s10533-004-0370-0 813 Gilliam, F., Burns, D., Driscoll, C., Frey, S., Lovett, G., Watmough, S., 2018. Responses of forest 814 ecosystems of eastern North America to decreased nitrogen deposition. Environmental 815 Pollution. 816 Gilliam, F.S., 2006. Response of the herbaceous layer of forest ecosystems to excess nitrogen 817 deposition. Journal of Ecology 94, 1176–1191. https://doi.org/10.1111/j.1365- 818 2745.2006.01155.x 819 Giordani, P., Calatayud, V., Stofer, S., Seidling, W., Granke, O., Fischer, R., 2014. Detecting the nitrogen 820 critical loads on European forests by means of epiphytic lichens. A signal-to-noise evaluation. 821 Forest Ecology and Management, Monitoring European forests: detecting and understanding 822 changes 311, 29–40. https://doi.org/10.1016/j.foreco.2013.05.048 823 Godbold, D.L., Kettner, C., 1991. Use of Root Elongation Studies to Determine Aluminium and Lead 824 Toxicity in Picea abies Seedlings. Journal of Plant Physiology 138, 231–235. 825 https://doi.org/10.1016/S0176-1617(11)80276-2 826 Gundersen, P., Callesen, I., De Vries, W., 1998a. Nitrate leaching in forest ecosystems is related to 827 forest floor CN ratios. Environmental Pollution 102, 403–407. https://doi.org/10.1016/S0269- 828 7491(98)80060-2 829 Gundersen, P., Emmett, B.A., Kjønaas, O.J., Koopmans, C.J., Tietema, A., 1998b. Impact of nitrogen 830 deposition on nitrogen cycling in forests: a synthesis of NITREX data. Forest Ecology and 831 Management 101, 37–55. https://doi.org/10.1016/S0378-1127(97)00124-2 832 Gundersen, P., Schmidt, I.K., Raulund-Rasmussen, K., 2006. Leaching of nitrate from temperate forests 833 effects of air pollution and forest management. Environ. Rev. 14, 1–57. 834 https://doi.org/10.1139/a05-015 835 Hauck, M., Bruyn, U. de, Leuschner, C., 2013. Dramatic diversity losses in epiphytic lichens in temperate 836 broad-leaved forests during the last 150 years. Biological Conservation 157, 136–145. 837 https://doi.org/10.1016/j.biocon.2012.06.015 838 Hautier, Y., Niklaus, P.A., Hector, A., 2009. Competition for Light Causes Plant Biodiversity Loss After 839 Eutrophication. Science 324, 636–638. https://doi.org/10.1126/science.1169640 840 Haynes, R.J., 1982. Effects of liming on phosphate availability in acid soils. Plant Soil 68, 289–308. 841 https://doi.org/10.1007/BF02197935 842 Heinrichs, S., Schmidt, W., 2016. Biotic homogenization of herb layer composition between two 843 contrasting beech forest communities on limestone over 50 years. Applied Vegetation Science 844 20, 271–281. https://doi.org/10.1111/avsc.12255 845 Hendriks, C.M.A., Olsthoorn, A.F.M., Klap, J.M., Goedhart, P.W., Oude Voshaar, J.H., Bleeker, A., De 846 Vries, F., van der Salm, C., Voogd, J.C.H., De Vries, W., Wijdeven, S.M.J., 2000. Relationships 847 between crown condition and its determining factors in the Netherlands for the period 1984 848 to 1994 (No. 161), Alterra-rapport. Wageningen, The Netherlands. 849 Hippeli, P., Branse, C., 1992. Veränderungen der Nährelementkonzentrationen in den Nadeln 850 mittelalter Kiefernbestände auf pleistozänen Sandstandorten Brandenburgs in den Jahren 851 1964 bis 1988. Forstw Cbl 111, 44–60. https://doi.org/10.1007/BF02741658 852 Högberg, M.N., Blaško, R., Bach, L.H., Hasselquist, N.J., Egnell, G., Näsholm, T., Högberg, P., 2014. The 853 return of an experimentally N-saturated boreal forest to an N-limited state: observations on 854 the soil microbial community structure, biotic N retention capacity and gross N mineralisation. 855 Plant Soil 381, 45–60. https://doi.org/10.1007/s11104-014-2091-z 856 Högberg, P., Fan, H., Quist, M., Binkley, D., Tamm, C.O., 2006. Tree growth and soil acidification in 857 response to 30 years of experimental nitrogen loading on boreal forest. Global Change Biology 858 12, 489–499. https://doi.org/10.1111/j.1365-2486.2006.01102.x 859 Iost, S., Rautio, P., Lindroos, A.-J., 2012. Spatio-temporal Trends in Soil Solution Bc/Al and N in Relation 860 to Critical Limits in European Forest Soils. Water Air Soil Pollut 223, 1467–1479. 861 https://doi.org/10.1007/s11270-011-0958-7 862 Jaenike, J., 1991. Mass extinction of European fungi. Trends in Ecology & Evolution 6, 174–175. 863 https://doi.org/10.1016/0169-5347(91)90207-E 864 Johnson, J., Pannatier, E.G., Carnicelli, S., Cecchini, G., Clarke, N., Cools, N., Hansen, K., Meesenburg, 865 H., Nieminen, T.M., Pihl‐Karlsson, G., Titeux, H., Vanguelova, E., Verstraeten, A., Vesterdal, L., 866 Waldner, P., Jonard, M., 2018. The response of soil solution chemistry in European forests to 867 decreasing acid deposition. Global Change Biology. https://doi.org/10.1111/gcb.14156 868 Johnson, J.A., Aherne, J., Cummins, T., 2013. Contrasting responses of two Sitka spruce forest plots in 869 Ireland to reductions in sulphur emissions: results of 20 years of monitoring. Biogeochemistry 870 116, 15–37. https://doi.org/10.1007/s10533-013-9889-2 871 Jonard, M., Fürst, A., Verstraeten, A., Thimonier, A., Timmermann, V., Potočić, N., Waldner, P., 872 Benham, S., Hansen, K., Merilä, P., Ponette, Q., de la Cruz, A.C., Roskams, P., Nicolas, M., 873 Croisé, L., Ingerslev, M., Matteucci, G., Decinti, B., Bascietto, M., Rautio, P., 2015. Tree mineral 874 nutrition is deteriorating in Europe. Glob Change Biol 21, 418–430. 875 https://doi.org/10.1111/gcb.12657 876 Jonard, M., Legout, A., Nicolas, M., Dambrine, E., Nys, C., Ulrich, E., van der Perre, R., Ponette, Q., 2012. 877 Deterioration of Norway spruce vitality despite a sharp decline in acid deposition: a long-term 878 integrated perspective. Glob Change Biol 18, 711–725. https://doi.org/10.1111/j.1365- 879 2486.2011.02550.x 880 Juknys, R., Augustaitis, A., Venclovienė, J., Kliučius, A., Vitas, A., Bartkevičius, E., Jurkonis, N., 2014. 881 Dynamic response of tree growth to changing environmental pollution. Eur J Forest Res 133, 882 713–724. https://doi.org/10.1007/s10342-013-0712-3 883 Kahle, H.-P., 2008. Causes and Consequences of Forest Growth Trends in Europe: Results of the 884 Recognition Project. European Forest Institute Rese, Leiden. 885 Keith, S.A., Newton, A.C., Morecroft, M.D., Bealey, C.E., Bullock, J.M., 2009. Taxonomic 886 homogenization of woodland plant communities over 70 years. Proceedings of the Royal 887 Society of London B: Biological Sciences rspb20090938. 888 https://doi.org/10.1098/rspb.2009.0938 889 Kint, V., Aertsen, W., Campioli, M., Vansteenkiste, D., Delcloo, A., Muys, B., 2012. Radial growth change 890 of temperate tree species in response to altered regional climate and air quality in the period 891 1901–2008. Climatic Change 115, 343–363. https://doi.org/10.1007/s10584-012-0465-x 892 Klap, J.M., Voshaar, J.H.O., De Vries, W., Erisman, J.W., 2000. Effects of Environmental Stress on Forest 893 Crown Condition in Europe. Part IV: Statistical Analysis of Relationships. Water, Air, & Soil 894 Pollution 119, 387–420. https://doi.org/10.1023/A:1005157208701 895 Kolář, T., Čermák, P., Oulehle, F., Trnka, M., Štěpánek, P., Cudlín, P., Hruška, J., Büntgen, U., Rybníček, 896 M., 2015. Pollution control enhanced spruce growth in the “Black Triangle” near the Czech- 897 Polish border. Sci. Total Environ. 538, 703–711. 898 https://doi.org/10.1016/j.scitotenv.2015.08.105 899 Laubhann, D., Sterba, H., Reinds, G.J., De Vries, W., 2009. The impact of atmospheric deposition and 900 climate on forest growth in European monitoring plots: An individual tree growth model. 901 Forest Ecology and Management, The relative importance of nitrogen deposition and climate 902 change on the sequestration of carbon by forests in Europe. 258, 1751–1761. 903 https://doi.org/10.1016/j.foreco.2008.09.050 904 Löfgren, S., Aastrup, M., Bringmark, L., Hultberg, H., Lewin-Pihlblad, L., Lundin, L., Karlsson, G.P., 905 Thunholm, B., 2011. Recovery of soil water, groundwater, and streamwater from acidification 906 at the Swedish integrated monitoring catchments. Ambio 40, 836–856. 907 https://doi.org/10.1007/s13280-011-0207-8 908 Löfgren, S., Zetterberg, T., 2011. Decreased DOC concentrations in soil water in forested areas in 909 southern Sweden during 1987–2008. Science of The Total Environment 409, 1916–1926. 910 https://doi.org/10.1016/j.scitotenv.2011.02.017 911 Lovett, G.M., Goodale, C.L., 2011. A New Conceptual Model of Nitrogen Saturation Based on 912 Experimental Nitrogen Addition to an Oak Forest. Ecosystems 14, 615–631. 913 https://doi.org/10.1007/s10021-011-9432-z 914 Mathias, J.M., Thomas, R.B., 2018. Disentangling the effects of acidic air pollution, atmospheric CO2, 915 and climate change on recent growth of red spruce trees in the Central Appalachian 916 Mountains. Global Change Biology 0. https://doi.org/10.1111/gcb.14273 917 Mayer, W., Pfefferkorn-Dellali, V., Türk, R., Dullinger, S., Mirtl, M., Dirnböck, T., 2013. Significant 918 decrease in epiphytic lichen diversity in a remote area in the European Alps, Austria. Basic and 919 Applied Ecology 14, 396–403. https://doi.org/10.1016/j.baae.2013.05.006 920 Meesenburg, H., Ahrends, B., Fleck, S., Wagner, M., Fortmann, H., Scheler, B., Klinck, U., Dammann, I., 921 Eichhorn, J., Mindrup, M., Meiwes, K.J., 2016. Long-term changes of ecosystem services at 922 Solling, Germany: Recovery from acidification, but increasing nitrogen saturation? Ecological 923 Indicators 65, 103–112. https://doi.org/10.1016/j.ecolind.2015.12.013 924 Mellert, K.H., Ewald, J., 2014. Nutrient limitation and site-related growth potential of Norway spruce 925 (Picea abies [L.] Karst) in the Bavarian Alps. Eur J Forest Res 133, 433–451. 926 https://doi.org/10.1007/s10342-013-0775-1 927 Mellert, K.H., Gensior, A., Göttlein, A., Kölling, C., Rücker, G., 2008. Variation in Soil Nitrate 928 Concentrations in Two N-Saturated Norway Spruce Forests (Picea abies (L.) Karst.) in Southern 929 Bavaria. Water Air Soil Pollut 187, 203–217. https://doi.org/10.1007/s11270-007-9509-7 930 Mellert, K.H., Kolb, E., Wellbrock, N., Göttlein, A., 2017. Ernährungszustand der Wälder in Deutschland. 931 AFZ, der Wald : allgemeine Forstzeitschrift für Waldwirtschaft und Umweltvorsorge 72. 932 Mellert, K.H., Prietzel, J., Straussberger, R., Rehfuess, K.E., 2004. Long-term nutritional trends of conifer 933 stands in Europe: results from the RECOGNITION project. Eur J Forest Res 123, 305–319. 934 https://doi.org/10.1007/s10342-004-0044-4 935 Moffat, A.J., Kvaalen, H., Solberg, S., Clarke, N., 2002. Temporal trends in throughfall and soil water 936 chemistry at three Norwegian forests, 1986–1997. Forest Ecology and Management 168, 15– 937 28. https://doi.org/10.1016/S0378-1127(01)00727-7 938 Neirynck, J., Maddelein, D., Keersmaeker, L. de, Lust, N., Muys, B., 1998. Biomass and nutrient cycling 939 of a highly productive Corsican pine stand on former heathland in northern Belgium. Ann. For. 940 Sci. 55, 389–405. https://doi.org/10.1051/forest:19980401 941 Nellemann, C., Thomsen, M.G., 2001. Long-Term Changes in Forest Growth: Potential Effects of 942 Nitrogen Deposition and Acidification. Water, Air, & Soil Pollution 128, 197–205. 943 https://doi.org/10.1023/A:1010318800180 944 Nijssen, M.E., WallisDeVries, M.F., Siepel, H., 2017. Pathways for the effects of increased nitrogen 945 deposition on fauna. Biological Conservation, Nitrogen Deposition Impacts and Biodiversity in 946 Terrestrial Ecosystems: Mechanisms and Perspectives 212, 423–431. 947 https://doi.org/10.1016/j.biocon.2017.02.022 948 Nilsson, J., Grennfelt, P., 1988. Critical loads for sulphur and nitrogen; Report from a Workshop held at 949 Skokloster Sweden 19–24 March 1988. Nordic Council of Ministers, Copenhagen, Denmark. 950 Nordin, A., Strengbom, J., Witzell, J., Näsholm, T., Ericson, L., 2005. Nitrogen Deposition and the 951 Biodiversity of Boreal Forests: Implications for the Nitrogen Critical Load. AMBIO: A Journal of 952 the Human Environment 34, 20–24. https://doi.org/10.1579/0044-7447-34.1.20 953 Ochoa-Hueso, R., Mejías-Sanz, V., Pérez-Corona, M.E., Manrique, E., 2013. Nitrogen deposition effects 954 on tissue chemistry and phosphatase activity in Cladonia foliacea (Huds.) Willd., a common 955 terricolous lichen of semi-arid Mediterranean shrublands. Journal of Arid Environments 88, 956 78–81. https://doi.org/10.1016/j.jaridenv.2012.07.007 957 Ostonen, I., Püttsepp, Ü., Biel, C., Alberton, O., Bakker, M.R., Lõhmus, K., Majdi, H., Metcalfe, D., 958 Olsthoorn, A.F.M., Pronk, A., Vanguelova, E., Weih, M., Brunner, I., 2007. Specific root length 959 as an indicator of environmental change. Plant Biosystems - An International Journal Dealing 960 with all Aspects of Plant Biology 141, 426–442. https://doi.org/10.1080/11263500701626069 961 Oulehle, F., Evans, C.D., Hofmeister, J., Krejci, R., Tahovska, K., Persson, T., Cudlin, P., Hruska, J., 2011. 962 Major changes in forest carbon and nitrogen cycling caused by declining sulphur deposition. 963 Global Change Biology 17, 3115–3129. https://doi.org/10.1111/j.1365-2486.2011.02468.x 964 Pannatier, E.G., Thimonier, A., Schmitt, M., Walthert, L., Waldner, P., 2011. A decade of monitoring at 965 Swiss Long-Term Forest Ecosystem Research (LWF) sites: can we observe trends in 966 atmospheric acid deposition and in soil solution acidity? Environ Monit Assess 174, 3–30. 967 https://doi.org/10.1007/s10661-010-1754-3 968 Pannatier, E.G., Thimonier, A., Schmitt, M., Walthert, L., Waldner, P., 2010. A decade of monitoring at 969 Swiss Long-Term Forest Ecosystem Research (LWF) sites: can we observe trends in 970 atmospheric acid deposition and in soil solution acidity? Environ Monit Assess 174, 3–30. 971 https://doi.org/10.1007/s10661-010-1754-3 972 Pardo, L.H., Fenn, M.E., Goodale, C.L., Geiser, L.H., Driscoll, C.T., Allen, E.B., Baron, J.S., Bobbink, R., 973 Bowman, W.D., Clark, C.M., Emmett, B., Gilliam, F.S., Greaver, T.L., Hall, S.J., Lilleskov, E.A., Liu, 974 L., Lynch, J.A., Nadelhoffer, K.J., Perakis, S.S., Robin-Abbott, M.J., Stoddard, J.L., Weathers, K.C., 975 Dennis, R.L., 2011. Effects of nitrogen deposition and empirical nitrogen critical loads for 976 ecoregions of the United States. Ecological Applications 21, 3049–3082. 977 https://doi.org/10.1890/10-2341.1 978 Peñuelas, J., Poulter, B., Sardans, J., Ciais, P., van der Velde, M., Bopp, L., Boucher, O., Godderis, Y., 979 Hinsinger, P., Llusia, J., Nardin, E., Vicca, S., Obersteiner, M., Janssens, I.A., 2013. Human- 980 induced nitrogen–phosphorus imbalances alter natural and managed ecosystems across the 981 globe. Nature Communications 4, 2934. https://doi.org/10.1038/ncomms3934 982 Perring, M.P., Bernhardt‐Römermann, M., Baeten, L., Midolo, G., Blondeel, H., Depauw, L., Landuyt, 983 D., Maes, S.L., Lombaerde, E.D., Carón, M.M., Vellend, M., Brunet, J., Chudomelová, M., 984 Decocq, G., Diekmann, M., Dirnböck, T., Dörfler, I., Durak, T., Frenne, P.D., Gilliam, F.S., Hédl, 985 R., Heinken, T., Hommel, P., Jaroszewicz, B., Kirby, K.J., Kopecký, M., Lenoir, J., Li, D., Máliš, F., 986 Mitchell, F.J.G., Naaf, T., Newman, M., Petřík, P., Reczyńska, K., Schmidt, W., Standovár, T., 987 Świerkosz, K., Calster, H.V., Vild, O., Wagner, E.R., Wulf, M., Verheyen, K., 2017. Global 988 environmental change effects on plant community composition trajectories depend upon 989 management legacies. Global Change Biology 24, 1722–1740. 990 https://doi.org/10.1111/gcb.14030 991 Pihl Karlsson, G., Akselsson, C., Hellsten, S., Karlsson, P.E., 2011. Reduced European emissions of S and 992 N – Effects on air concentrations, deposition and soil water chemistry in Swedish forests. 993 Environmental Pollution 159, 3571–3582. https://doi.org/10.1016/j.envpol.2011.08.007 994 Pöyry, J., Carvalheiro, L.G., Heikkinen, R.K., Kühn, I., Kuussaari, M., Schweiger, O., Valtonen, A., 995 Bodegom, P.M. van, Franzén, M., 2016. The effects of soil eutrophication propagate to higher 996 trophic levels. Global Ecology and Biogeography 26, 18–30. 997 https://doi.org/10.1111/geb.12521 998 Prietzel, J., Kolb, E., Rehfuess, K.E., 1997. Langzeituntersuchung ehemals streugenutzter 999 kiefernökosysteme in der Oberpfalz: Veränderungen von bodenchemischen eigenschaften 1000 und der nährelementversorgung der bestände. Forstw Cbl 116, 269–290. 1001 https://doi.org/10.1007/BF02766904 1002 Riek, W., Talkner, U., Dammann, I., Kohler, M., Meiwes, K.J., Göttlein, A., 2016. Kapitel 8 - 1003 Waldernährung, in: Wellbrock, N., Bolte, A., Flessa, H. (Eds.), Dynamik Und Räumliche Muster 1004 Forstlicher Standorte in Deutschland - Ergebnisse Der Bodenzustandserhebung Im Wald 2006 1005 Bis 2008. Johann Heinrich von Thünen-Institut, Braunschweig, pp. 245–291. 1006 Rizzetto, S., Belyazid, S., Gégout, J.-C., Nicolas, M., Alard, D., Corcket, E., Gaudio, N., Sverdrup, H., 1007 Probst, A., 2016. Modelling the impact of climate change and atmospheric N deposition on 1008 French forests biodiversity. Environmental Pollution 213, 1016–1027. 1009 https://doi.org/10.1016/j.envpol.2015.12.048 1010 Roth, T., Kohli, L., Rihm, B., Amrhein, V., Achermann, B., 2015. Nitrogen deposition and multi- 1011 dimensional plant diversity at the landscape scale. R Soc Open Sci 2. 1012 https://doi.org/10.1098/rsos.150017 1013 Rothe, A., Huber, C., Kreutzer, K., Weis, W., 2002. Deposition and soil leaching in stands of Norway 1014 spruce and European Beech: Results from the Höglwald research in comparison with other 1015 European case studies. Plant and Soil 240, 33–45. https://doi.org/10.1023/A:1015846906956 1016 Rowe, E.C., Jones, L., Dise, N.B., Evans, C.D., Mills, G., Hall, J., Stevens, C.J., Mitchell, R.J., Field, C., 1017 Caporn, S.J.M., Helliwell, R.C., Britton, A.J., Sutton, M.A., Payne, R.J., Vieno, M., Dore, A.J., 1018 Emmett, B.A., 2017. Metrics for evaluating the ecological benefits of decreased nitrogen 1019 deposition. Biological Conservation, Nitrogen Deposition Impacts and Biodiversity in 1020 Terrestrial Ecosystems: Mechanisms and Perspectives 212, 454–463. 1021 https://doi.org/10.1016/j.biocon.2016.11.022 1022 Sanders, T.G.M., Potočić, N., Timmermann, V., Schmitz, A., 2017a. Tree crown condition and damage 1023 causes, in: Seidling, W., Michel, A. (Eds.), Forest Condition in Europe - 2017 Technical Report 1024 of ICP Forests. Report under the UNECE Convention on Long-Range Transboundary Air 1025 Pollution (CLRTAP), BFW Dokumentation 24/2017. BFW Austrian Research Centre for Forests, 1026 Vienna. 1027 Sanders, T.G.M., Schmitz, A., Edinger, J., 2017b. Trends in foliar nitrogen and phosphorus 1028 concentrations and ratios since 2000, in: Seidling, W., Michel, A. (Eds.), Forest Condition in 1029 Europe - 2017 Technical Report of ICP Forests. Report under the UNECE Convention on Long- 1030 Range Transboundary Air Pollution (CLRTAP), BFW Dokumentation 24/2017. BFW Austrian 1031 Research Centre for Forests, Vienna. 1032 Sardans, J., Alonso, R., Carnicer, J., Fernández-Martínez, M., Vivanco, M.G., Peñuelas, J., 2016a. Factors 1033 influencing the foliar elemental composition and stoichiometry in forest trees in Spain. 1034 Perspectives in Plant Ecology, Evolution and Systematics 18, 52–69. 1035 https://doi.org/10.1016/j.ppees.2016.01.001 1036 Sardans, J., Alonso, R., Janssens, I.A., Carnicer, J., Vereseglou, S., Rillig, M.C., Fernández-Martínez, M., 1037 Sanders, T.G.M., Peñuelas, J., 2016b. Foliar and soil concentrations and stoichiometry of 1038 nitrogen and phosphorous across European Pinus sylvestris forests: relationships with climate, 1039 N deposition and tree growth. Funct Ecol 30, 676–689. https://doi.org/10.1111/1365- 1040 2435.12541 1041 Sardans, J., Janssens, I.A., Alonso, R., Veresoglou, S.D., Rillig, M.C., Sanders, T.G., Carnicer, J., Filella, I., 1042 Farré-Armengol, G., Peñuelas, J., 2015. Foliar elemental composition of European forest tree 1043 species associated with evolutionary traits and present environmental and competitive 1044 conditions. Global Ecology and Biogeography 24, 240–255. 1045 https://doi.org/10.1111/geb.12253 1046 Sardans, J., Peñuelas, J., 2012. The role of plants in the effects of global change on nutrient availability 1047 and stoichiometry in the plant-soil system. Plant Physiology pp.112.208785. 1048 https://doi.org/10.1104/pp.112.208785 1049 Sauter, U., 1991. Zeitliche Variationen des Ernährungszustands nordbayerischer Kiefernbestände. 1050 Forstw Cbl 110, 13–33. https://doi.org/10.1007/BF02741237 1051 Sawicka, K., Monteith, D.T., Vanguelova, E.I., Wade, A.J., Clark, J.M., 2016. Fine-scale temporal 1052 characterization of trends in soil water dissolved organic carbon and potential drivers. 1053 Ecological Indicators, Assessing ecosystem resilience through Long Term Ecosystem Research: 1054 observations from the first twenty years of the UK Environmental Change Network 68, 36–51. 1055 https://doi.org/10.1016/j.ecolind.2015.12.028 1056 Schindler, D.W., Lee, P.G., 2010. Comprehensive conservation planning to protect biodiversity and 1057 ecosystem services in Canadian boreal regions under a warming climate and increasing 1058 exploitation. Biological Conservation, Conservation planning within emerging global climate 1059 and economic realities 143, 1571–1586. https://doi.org/10.1016/j.biocon.2010.04.003 1060 Schmitz, A., Sanders, T.G.M., Michel, A.K., Verstraeten, A., Hansen, K., Waldner, P., Prescher, A.-K., 1061 Žlindra, D., 2018. ICP Forests Brief #2 - Status and trends of inorganic nitrogen deposition to 1062 forests in Europe. Thünen Institute of Forest Ecosystems, Eberswalde, Germany. 1063 Schulte-Uebbing, L., De Vries, W., 2017. Global-scale impacts of nitrogen deposition on tree carbon 1064 sequestration in tropical, temperate, and boreal forests: A meta-analysis. Glob Change Biol 24, 1065 e416–e431. https://doi.org/10.1111/gcb.13862 1066 Seidling, W., Fischer, R., Granke, O., 2008. Relationships between forest floor vegetation on ICP forests 1067 monitoring plots in Europe and basic variables in soil and nitrogen deposition. International 1068 Journal of Environmental Studies 65, 309–320. 1069 https://doi.org/DOI:10.1080/00207230701862538 1070 Shi, S., Yu, Z., Zhao, Q., 2014. Responses of plant diversity and species composition to the cessation of 1071 fertilization in a sandy grassland. Journal of Forestry Research 25, 337–342. 1072 https://doi.org/10.1007/s11676-014-0462-1 1073 Simpson, D., Andersson, C., Christensen, J.H., Engardt, M., Geels, C., Nyiri, A., Posch, M., Soares, J., 1074 Sofiev, M., Wind, P., Langner, J., 2014. Impacts of climate and emission changes on nitrogen 1075 deposition in Europe: a multi-model study. Atmos. Chem. Phys. 14, 6995–7017. 1076 https://doi.org/10.5194/acp-14-6995-2014 1077 Slootweg, J., Posch, M., Hettelingh, J.P., 2015. Modelling and Mapping the Impacts of Atmospheric 1078 Deposition of Nitrogen and Sulphur: CCE Status Report 2015. Coordination Centre for Effects, 1079 National Institute for Public Health and the Environment, Bilthoven, The Netherlands. 1080 Solberg, S., Dobbertin, M., Reinds, G.J., Lange, H., Andreassen, K., Fernandez, P.G., Hildingsson, A., De 1081 Vries, W., 2009. Analyses of the impact of changes in atmospheric deposition and climate on 1082 forest growth in European monitoring plots: A stand growth approach. Forest Ecology and 1083 Management, The relative importance of nitrogen deposition and climate change on the 1084 sequestration of carbon by forests in Europe. 258, 1735–1750. 1085 https://doi.org/10.1016/j.foreco.2008.09.057 1086 Solberg, S., Tørseth, K., 1997. Crown condition of Norway spruce in relation to sulphur and nitrogen 1087 deposition and soil properties in southeast Norway. Environmental Pollution 96, 19–27. 1088 https://doi.org/10.1016/S0269-7491(97)00010-9 1089 Soons, M.B., Hefting, M.M., Dorland, E., Lamers, L.P.M., Versteeg, C., Bobbink, R., 2017. Nitrogen 1090 effects on plant species richness in herbaceous communities are more widespread and 1091 stronger than those of phosphorus. Biological Conservation 212, 390–397. 1092 https://doi.org/10.1016/j.biocon.2016.12.006 1093 Staszewski, T., Kubiesa, P., Łukasik, W., 2012. Response of spruce stands in national parks of southern 1094 Poland to air pollution in 1998–2005. Eur J Forest Res 131, 1163–1173. 1095 https://doi.org/10.1007/s10342-011-0587-0 1096 Stevens, C.J., 2016. How long do ecosystems take to recover from atmospheric nitrogen deposition? 1097 Biological Conservation 200, 160–167. https://doi.org/10.1016/j.biocon.2016.06.005 1098 Stevens, C.J., Mountford, J.O., Gowing, D.J.G., Bardgett, R.D., 2012. Differences in yield, Ellenberg N 1099 value, tissue chemistry and soil chemistry 15 years after the cessation of nitrogen addition. 1100 Plant Soil 357, 309–319. https://doi.org/10.1007/s11104-012-1160-4 1101 Storkey, J., Macdonald, A.J., Poulton, P.R., Scott, T., Köhler, I.H., Schnyder, H., Goulding, K.W.T., 1102 Crawley, M.J., 2015. Grassland biodiversity bounces back from long-term nitrogen addition. 1103 Nature 528, 401–404. https://doi.org/10.1038/nature16444 1104 Strengbom, J., Nordin, A., Näsholm, T., Ericson, L., 2001. Slow recovery of boreal forest ecosystem 1105 following decreased nitrogen input. Functional Ecology 15, 451–457. 1106 https://doi.org/10.1046/j.0269-8463.2001.00538.x 1107 Suding, K.N., Collins, S.L., Gough, L., Clark, C., Cleland, E.E., Gross, K.L., Milchunas, D.G., Pennings, S., 1108 2005. Functional- and abundance-based mechanisms explain diversity loss due to N 1109 fertilization. PNAS 102, 4387–4392. https://doi.org/10.1073/pnas.0408648102 1110 Sujetovienė, G., Stakėnas, V., 2007. Changes in Understorey Vegetation of Scots Pine Stands under the 1111 Decreased Impact of Acidifying and Eutrophying Pollutants. Baltic Forestry 13, 190–196. 1112 Sutton, M.A., Simpson, D., Levy, P.E., Smith, R.I., Reis, S., Van Oijen, M., De Vries, W., 2008. 1113 Uncertainties in the relationship between atmospheric nitrogen deposition and forest carbon 1114 sequestration. Global Change Biology 14, 2057–2063. https://doi.org/10.1111/j.1365- 1115 2486.2008.01636.x 1116 Suz, L.M., Barsoum, N., Benham, S., Dietrich, H.-P., Fetzer, K.D., Fischer, R., García, P., Gehrman, J., 1117 Kristöfel, F., Manninger, M., Neagu, S., Nicolas, M., Oldenburger, J., Raspe, S., Sánchez, G., 1118 Schröck, H.W., Schubert, A., Verheyen, K., Verstraeten, A., Bidartondo, M.I., 2014. 1119 Environmental drivers of ectomycorrhizal communities in Europe’s temperate oak forests. Mol 1120 Ecol 23, 5628–5644. https://doi.org/10.1111/mec.12947 1121 Talkner, U., Meiwes, K.J., Potočić, N., Seletković, I., Cools, N., Vos, B.D., Rautio, P., 2015. Phosphorus 1122 nutrition of beech (Fagus sylvatica L.) is decreasing in Europe. Annals of Forest Science 72, 919– 1123 928. https://doi.org/10.1007/s13595-015-0459-8 1124 Thelin, G., Rosengren-Brinck, U., Nihlgård, B., Barkman, A., 1998. Trends in needle and soil chemistry 1125 of Norway spruce and Scots pine stands in South Sweden 1985–1994. Environmental Pollution 1126 99, 149–158. https://doi.org/10.1016/S0269-7491(97)00192-9 1127 Tian, D., Wang, H., Sun, J., Niu, S., 2016. Global evidence on nitrogen saturation of terrestrial ecosystem 1128 net primary productivity. Environ. Res. Lett. 11, 024012. https://doi.org/10.1088/1748- 1129 9326/11/2/024012 1130 Tilman, D., Isbell, F., 2015. Biodiversity: Recovery as nitrogen declines. Nature 528, 336–337. 1131 https://doi.org/10.1038/nature16320 1132 Ukonmaanaho, L., Starr, M., Lindroos, A.-J., Nieminen, T.M., 2014. Long-term changes in acidity and 1133 DOC in throughfall and soil water in Finnish forests. Environ Monit Assess 186, 7733–7752. 1134 https://doi.org/10.1007/s10661-014-3963-7 1135 van der Heijden, G., Legout, A., Nicolas, M., Ulrich, E., Johnson, D.W., Dambrine, E., 2011. Long-term 1136 sustainability of forest ecosystems on sandstone in the Vosges Mountains (France) facing 1137 atmospheric deposition and silvicultural change. Forest Ecology and Management 261, 730– 1138 740. https://doi.org/10.1016/j.foreco.2010.12.003 1139 van der Linde, S., Suz, L.M., Orme, C.D.L., Cox, F., Andreae, H., Asi, E., Atkinson, B., Benham, S., Carroll, 1140 C., Cools, N., Vos, B.D., Dietrich, H.-P., Eichhorn, J., Gehrmann, J., Grebenc, T., Gweon, H.S., 1141 Hansen, K., Jacob, F., Kristöfel, F., Lech, P., Manninger, M., Martin, J., Meesenburg, H., Merilä, 1142 P., Nicolas, M., Pavlenda, P., Rautio, P., Schaub, M., Schröck, H.-W., Seidling, W., Šrámek, V., 1143 Thimonier, A., Thomsen, I.M., Titeux, H., Vanguelova, E., Verstraeten, A., Vesterdal, L., 1144 Waldner, P., Wijk, S., Zhang, Y., Žlindra, D., Bidartondo, M.I., 2018. Environment and host as 1145 large-scale controls of ectomycorrhizal fungi. Nature. https://doi.org/10.1038/s41586-018- 1146 0189-9 1147 van Dobben, H.F., De Vries, W., 2017. The contribution of nitrogen deposition to the eutrophication 1148 signal in understorey plant communities of European forests. Ecol Evol 7, 214–227. 1149 https://doi.org/10.1002/ece3.2485 1150 Van Grinsven, H.J.M., Holland, M., Jacobsen, B.H., Klimont, Z., Sutton, M. a., Jaap Willems, W., 2013. 1151 Costs and Benefits of Nitrogen for Europe and Implications for Mitigation. Environ. Sci. 1152 Technol. 47, 3571–3579. https://doi.org/10.1021/es303804g 1153 Vanguelova, E.I., Benham, S., Pitman, R., Moffat, A.J., Broadmeadow, M., Nisbet, T., Durrant, D., 1154 Barsoum, N., Wilkinson, M., Bochereau, F., Hutchings, T., Broadmeadow, S., Crow, P., Taylor, 1155 P., Durrant Houston, T., 2010. Chemical fluxes in time through forest ecosystems in the UK – 1156 Soil response to pollution recovery. Environmental Pollution 158, 1857–1869. 1157 https://doi.org/10.1016/j.envpol.2009.10.044 1158 Veresoglou, S.D., Peñuelas, J., Fischer, R., Rautio, P., Sardans, J., Merilä, P., Tabakovic-Tosic, M., Rillig, 1159 M.C., 2014. Exploring continental-scale stand health – N : P ratio relationships for European 1160 forests. New Phytol 202, 422–430. https://doi.org/10.1111/nph.12665 1161 Verheyen, K., Baeten, L., De Frenne, P., Bernhardt-Römermann, M., Brunet, J., Cornelis, J., Decocq, G., 1162 Dierschke, H., Eriksson, O., Hédl, R., Heinken, T., Hermy, M., Hommel, P., Kirby, K., Naaf, T., 1163 Peterken, G., Petřík, P., Pfadenhauer, J., Van Calster, H., Walther, G.-R., Wulf, M., Verstraeten, 1164 G., 2012. Driving factors behind the eutrophication signal in understorey plant communities of 1165 deciduous temperate forests. Journal of Ecology 100, 352–365. 1166 https://doi.org/10.1111/j.1365-2745.2011.01928.x 1167 Verstraeten, A., Neirynck, J., Cools, N., Roskams, P., Louette, G., De Neve, S., Sleutel, S., 2017. Multiple 1168 nitrogen saturation indicators yield contradicting conclusions on improving nitrogen status of 1169 temperate forests. Ecological Indicators 82, 451–462. 1170 https://doi.org/10.1016/j.ecolind.2017.07.034 1171 Verstraeten, A., Neirynck, J., Genouw, G., Cools, N., Roskams, P., Hens, M., 2012. Impact of declining 1172 atmospheric deposition on forest soil solution chemistry in Flanders, Belgium. Atmospheric 1173 Environment 62, 50–63. https://doi.org/10.1016/j.atmosenv.2012.08.017 1174 Verstraeten, A., Verschelde, P., De Vos, B., Neirynck, J., Cools, N., Roskams, P., Hens, M., Louette, G., 1175 Sleutel, S., De Neve, S., 2016. Increasing trends of dissolved organic nitrogen (DON) in 1176 temperate forests under recovery from acidification in Flanders, Belgium. Science of The Total 1177 Environment 553, 107–119. https://doi.org/10.1016/j.scitotenv.2016.02.060 1178 Vitale, M., Proietti, C., Cionni, I., Fischer, R., Marco, A.D., 2014. Random Forests Analysis: a Useful Tool 1179 for Defining the Relative Importance of Environmental Conditions on Crown Defoliation. Water 1180 Air Soil Pollut 225, 1992. https://doi.org/10.1007/s11270-014-1992-z 1181 Vitousek, P.M., Aber, J.D., Howarth, R.W., Likens, G.E., Matson, P.A., Schindler, D.W., Schlesinger, W.H., 1182 Tilman, D.G., 1997. Human alteration of the global nitrogen cycle: sources and consequences. 1183 Ecological Applications 7, 737–750. https://doi.org/10.1890/1051- 1184 0761(1997)007[0737:HAOTGN]2.0.CO;2 1185 Vitousek, P.M., Howarth, R.W., 1991. Nitrogen limitation on land and in the sea: How can it occur? 1186 Biogeochemistry 13, 87–115. https://doi.org/10.1007/BF00002772 1187 Waldner, P., Thimonier, A., Pannatier, E.G., Etzold, S., Schmitt, M., Marchetto, A., Rautio, P., Derome, 1188 K., Nieminen, T.M., Nevalainen, S., Lindroos, A.-J., Merilä, P., Kindermann, G., Neumann, M., 1189 Cools, N., Vos, B. de, Roskams, P., Verstraeten, A., Hansen, K., Karlsson, G.P., Dietrich, H.-P., 1190 Raspe, S., Fischer, R., Lorenz, M., Iost, S., Granke, O., Sanders, T.G.M., Michel, A., Nagel, H.-D., 1191 Scheuschner, T., Simončič, P., Wilpert, K. von, Meesenburg, H., Fleck, S., Benham, S., 1192 Vanguelova, E., Clarke, N., Ingerslev, M., Vesterdal, L., Gundersen, P., Stupak, I., Jonard, M., 1193 Potočić, N., Minaya, M., 2015. Exceedance of critical loads and of critical limits impacts tree 1194 nutrition across Europe. Annals of Forest Science 72, 929–939. 1195 https://doi.org/10.1007/s13595-015-0489-2 1196 Wellbrock, N., Bolte, A., Flessa, H. (Eds.), 2016. Dynamik und räumliche Muster forstlicher Standorte 1197 in Deutschland - Ergebnisse der Bodenzustandserhebung im Wald 2006 bis 2008, Thünen 1198 Report. Johann Heinrich von Thünen-Institut, Braunschweig. 1199 Wright, R.F., Lotse, E., Semb, A., 1993. RAIN Project: Results after 8 Years of Experimentally Reduced 1200 Acid Deposition to a Whole Catchment. Can. J. Fish. Aquat. Sci. 50, 258–268. 1201 https://doi.org/10.1139/f93-030 1202 Wright, R.F., Rasmussen, L., 1998. Introduction to the NITREX and EXMAN projects. Forest Ecology and 1203 Management, The Whole Ecosystem Experiments of the NITREX and EXMAN Projects 101, 1– 1204 7. https://doi.org/10.1016/S0378-1127(97)00120-5 1205 Yu, L., Belyazid, S., Akselsson, C., van der Heijden, G., Zanchi, G., 2016. Storm disturbances in a Swedish 1206 forest—A case study comparing monitoring and modelling. Ecological Modelling 320, 102–113. 1207 https://doi.org/10.1016/j.ecolmodel.2015.09.025 1208 1209 1210 1211 1212 1213 1214 Figure 1: Hypothetical relationship between the stage of nitrogen saturation and the effects on 1215 terrestrial ecosystems in terms of soil processes, vegetation changes and growth. This figure is an 1216 update of the figure by Aber et al. (1998) (after De Vries and Schulte-Uebbing (2018)). It illustrates the 1217 trade-off between the initial positive impact of nitrogen enrichment on tree growth and related carbon 1218 sequestration on the one hand and the negative impact on ecosystem services (e.g. water quality 1219 regulation by nitrogen retention) and on biodiversity on the other hand. 1220 1221 1222 1223 Figure 2: Relative change of throughfall deposition of inorganic nitrogen at the intensive monitoring 1224 sites of the UNECE ICP Forests programme network between 2000 and 2015 (redrawn after Schmitz et 1225 al., 2018). Large dots indicate statistically significant trends; trends represented by small dots are not 1226 statistically significant. 1227 1228 1229 Figure 3: Average deposition of oxidized, reduced and total N between 1900 and 2050 to the EU28, 1230 Norway and Switzerland according to EMEP model results (after Engardt et al. (2017), data kindly 1231 provided by Magnuz Engardt and David Simpson). Vertical dashed lines indicate the years 1990 and 1232 2018. Future reductions are expected to be small and inorganic N deposition is likely converging to a 1233 level approximately twice as high compared to 1900. 1234 1235 1236 Figure 4: Examples for the effects of N deposition on forest understory vegetation. (a) Relationship 1237 between lichen diversity (proportion of macrolichen species among all lichen species) and N 1238 throughfall deposition based on 83 forest plots across Europe. Reprinted from Giordani et al. (2014) 1239 with permission from Elsevier. (b) Relationship between the occurrence of nitrogen indicating species 1240 and N throughfall deposition based on a detrended correspondence analysis (DCA) of the floristic 1241 composition of the herb layer at 488 forest plots in the nemoral zone of Europe. Scores on the fourth 1242 axis of the DCA are positively correlated with nitrogen deposition. Redrawn from Seidling et al. (2008) 1243 by permission of the publisher (Taylor & Francis Ltd, http://www.tandfonline.com).