Strong resilience of soil respiration components to drought-induced die-off resulting in forest secondary succession

How forests cope with drought-induced perturbations and how the dependence of soil respiration on environmental and biological drivers is affected in a warming and drying context are becoming key questions. The aims of this study were to determine whether drought-induced die-off and forest succession were reflected in soil respiration and its components and to determine the influence of climate on the soil respiration components. We used the mesh exclusion method to study seasonal variations in soil respiration (RS) and its components: heterotrophic (RH) and autotrophic (RA) [further split into fine root (RR) and mycorrhizal respiration (RM)] in a mixed Mediterranean forest where Scots pine (Pinus sylvestris L.) is undergoing a drought-induced die-off and is being replaced by holm oak (Quercus ilex L.). Drought-induced pine die-off was not reflected in RS nor in its components, which denotes a high functional resilience of the plant and soil system to pine die-off. However, the succession from Scots pine to holm oak resulted in a reduction of RH and thus in an important decrease of total respiration (RS was 36 % lower in holm oaks than in non-defoliated pines). Furthermore, RS and all its components were strongly regulated by soil water content-and-temperature interaction. Since Scots pine die-off and Quercus species colonization seems to be widely occurring at the driest limit of the Scots pine distribution, the functional resilience of the soil system over die-off and the decrease of RS from Scots pine to holm oak could have direct consequences for the C balance of these ecosystems.


Introduction
Therefore we estimate that these trees have been dead for 3-11 years. The degree of pine 173 defoliation was visually estimated as the percentage of green needles relative to a non-

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Defoliated pines had less than 50% of green leaves. DBH for each type of tree was 37±7 176 cm in NDP, 59±9 cm in DFP, 58±7 cm in DP and 14±8 cm in HO (mean ± sd). The

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Hegyi competition index was significantly higher for HOs with respect to pines, 178 whereas no significant differences were found between NDP, DFP and DP (Curiel       Mixed-effects models, including tree identity as a random factor, were then used to 289 analyse the differences in annual soil respiration and its components between types of we considered marginally significant differences among means when 0.05 < p < 0.1.

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Additionally, Friedman test and its post hoc analysis were applied to the daily-averaged 294 data from the 25 campaigns, to test for possible differences in soil respiration and its 295 components between types of tree. The Friedman test is a non-parametric repeated 296 measure ANOVA. Its procedure involves ranking soil respiration from the different tree 297 types and then considering the values of ranks by campaigns.

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A u t h o r ' s a c c e p t e d m a n u s c r i p t mixed-effects models with tree type (for testing annual differences) or with the The relative contribution of RH to RS did not show differences among tree types, either 333 at annual (p=0.968) or at seasonal scales (p=0.325) (GLM models with tree identity as a 334 random factor). Nonetheless, the relative contribution of the different soil respiration  Type of tree had no effect on RA, nor on its fractions (RR and RM).

Discussion
Mean annual RS at the study site was 2.6 µmols m -2 s -1 , similar to values reported for a   We did not find higher heterotrophic respiration (RH) under DP than under NDP, which 420 is consistent with the fact that tree mortality did not apparently result in higher soil  The mechanism underlying the observed resilience of RS following Scots pine die-off is 448 likely more related to belowground colonization by HO than to the remaining,   The interactions between season-SWC-temperature and temperature-type of tree were 502 able to explain 51% of the RS variability. The positive interaction between SWC and 503 temperature indicated that the higher the SWC, the higher the temperature effect, and 504 had similar effects on RH, RR and RM fluxes. However, only during part of the spring-505 time were there simultaneous high SWC and temperature values (see figure 2). We did 506 not find support for our hypothesized higher sensitivity of RS and RA to SWC under 507 Scots pine (H2). On the contrary, we observe a higher sensitivity of RH to SWC under 508 HO (Table 2, RH model; Figure 6), which could be due to a better adaptation to the 509 strong seasonal changes in water availability of the microbial community associated    A u t h o r ' s a c c e p t e d m a n u s c r i p t A u t h o r ' s a c c e p t e d m a n u s c r i p t