Biology, Chemistry | UNDERSTAND The secret life of forests New research is revealing the previously unknown beneficial effects of tree canopies – and the secret life within them. By Rossella Guerrieri When you walk through a forest, what is it about the trees that most impresses you? For me, it’s the green canopy of leaves, with branches reaching out towards the sky. I remember when I saw a giant sequoia tree for the first time, at the Calaveras Big Trees State Park in California, USA. What really impressed me was the high evergreen canopies of these trees – which can be up to 3500 years old – receiving water from roots almost 100 metres below. In my career as a tree ecophysiologist, I am fascinated by tree canopies, which are important not only for single trees, but also for forests – and for all of us. Tree canopies make a huge contribution to mitigating global warming and climate change by taking in carbon dioxide through their leaves, which they use in photosynthesis to produce sugars. Thanks to this activity, forest canopies worldwide remove around 30% of the carbon dioxide emitted by fossil fuel combustion. At the same time, trees lose water through their leaves, which they take from the soil and return to the atmosphere through transpiration. This is a familiar story, but what is less well known, and is only beginning to be understood, is that trees also have an effect on other gases in the air, beyond carbon dioxide, oxygen and water vapour – in particular, on nitrogen and its compounds. Nitrogen cycling Tree canopies are continuously exposed to changes in the composition of the atmosphere, including those due to pollutants. Some of these pollutants are reactive nitrogen compounds, including ammonia (NH3) and its compounds, and various oxides of nitrogen known as NOx (NO, NO2 and NO3). Over the past century, there has been a rapid increase in atmospheric nitrogen compounds due mainly to global increases in industrial activity, traffic emissions, intensive 20 I Issue 46 : Spring 2019 I Science in School I www.scienceinschool.org UNDERSTAND | Biology, Chemistry Traffic emissions are a Katie Jennings/Rossella Guerrieri bibiphoto/Shutterstock.com The author (Rossella Guerrieri) amid giant sequoia trees at major source of air pollution the Calaveras Big Trees State from nitrogen compounds Park in California, USA known as NOx. livestock farming and fertiliser use in that the extra nitrogen compounds agriculture, and burning biomass for reach the soil directly and that there energy (Galloway et al., 2004). is no interaction with tree canopies, As with increases in atmospheric carbon but more recent research shows this dioxide, forests have a stabilising effect assumption may not be true. here, because they store some of the extra nitrogen compounds from the atmosphere. One of the key questions Tree canopies and pollution scientists have tried to answer since At several forests across Europe and the the 1980s is whether it is the trees USA, atmospheric nitrogen compounds themselves that have this role or only have been monitored over several the soil beneath. Earlier studies assumed decades. Studies using this data have Environmental chemistry Nitrogen cycle Microbiology Ages 16–19 This article is about tree canopies and their importance as agents of atmospheric change. The author describes the aims of her own research: to understand the diversity and function of the microbes living in and on the leaves of Scots pine and beech forests. The article could be used as a stimulus for teaching topics such as cell metabolism, biological reactions, the nitrogen cycle and, more generally, ecosystems. It could also be used as a basis for comprehension questions such as: · Which compounds are involved in the exchange between trees and the atmosphere? · What is the impact of sulfur dioxide on plants? · How do trees reduce pollution from nitrogen compounds? REVIEW The article could also be useful to show the application of techniques used for bacterial recognition, in particular DNA analyses. More generally, it highlights the sheer excitement of scientific research, and the enthusiasm of the researcher for this new area of study. Monica Menesini, science teacher, Italy www.scienceinschool.org I Science in School I Issue 46 : Spring 2019 I 21 Biology, Chemistry | UNDERSTAND Yuriy Vahlenko/Shutterstock.com Oak tree (Quercus), a species that can take up ammonia from the atmosphere Forest and Kim Starr/Flickr, CC BY 2.0 BlueRidgeKitties/Flickr, CC BY-NC-SA 2.0 Right: Black elder (Sambucus nigra), which can take up airborne NOx compounds Far right: Leaf surface showing stomata – pores guarded by elongated cells (shown green) that let gases pass in and out of the leaf shown that tree canopies have an airborne nitrogen compounds can Trees thus act as filters and contribute active role in removing air pollution actually benefit trees, as they increase to improved air quality, even in cities. by retaining some of the nitrogen the growth rate in some forests where In general, how effective a tree is at compounds and altering others that the availability of nitrogen can be a removing pollutants depends on the reach the soil. But how do they do limiting factor (Magnani et al., 2007). structure of the tree canopies (their this? It seems that, like carbon dioxide, Of course, trees can also be damaged by shape, leaf and branch density) and these nitrogen compounds are able air pollution, particularly by ozone and of their leaves (e.g. surface roughness to penetrate the leaf surface through sulfur compounds such as sulfur dioxide and waxiness). Tree species that are the stomata, and be taken up and – which has been known since the 1980s. particularly effective at taking up NOx absorbed by the plant. Other studies These gases damage the leaf cuticle compounds through their leaves have revealed that tree canopies can and stomata, causing an imbalance in include birch (Betula), poplar (Populus) transform ammonia and its compounds, the plant’s gas exchange with the air. and black elder (Sambucus nigra) converting them to nitrate, which is However, thanks to regulation, emissions species, while oak (Quercus), beech washed off into the soil to be taken that cause air pollution have decreased (Fagus sylvatica) and hornbeam up by the roots as a nutrient. These since the 1980s, especially with regard (Carpinus betulus) are good at taking processes explain another finding: to sulfur compounds. up ammonia. 22 I Issue 46 : Spring 2019 I Science in School I www.scienceinschool.org UNDERSTAND | Biology, Chemistry Microbial protectors pine and beech forests across Europe, Julia Vorholt/ETH Zurich from Sweden and Finland in the far Current research is revealing another north to Mediterranean Italy and factor in how tree canopies have these Spain in the south. Six months after I remarkable effects on the atmosphere. started my fellowship, I had a lab full This is the ‘leaf microbiome’ – the microbial life forms found inside or on the surfaces of leaves, rather like the gut microbiome within our own bodies. It’s now well known that our health and even our happiness may be significantly “discover It was amazing to that just a few affected by our gut microbiome. Remarkably, there seems to be a rather grams of leaves hide similar effect for trees, with the leaf microbiome influencing how trees an incredibly diverse respond to environmental changes. But while DNA sequencing analyses bacterial community. ” have enabled scientists to compile a global atlas of bacteria living in the soil, we have only started to learn which classes of microbes are living of soil, leaves and bottles containing in tree canopies. There are many water collected from underneath tree questions still to answer: for example, canopies. It was amazing to discover are these microbial species similar that just a few grams of leaves hide an across different forests and tree species? incredibly diverse bacterial community And which microbes are responsible (Guerrieri et al., 2017), and through the for taking up atmospheric nitrogen microscope I could see how beautifully within tree canopies, and which for the microbes line up along the leaf transforming it? nerves. In 2015, I received a research grant With my colleagues, we began to work Bacterial colonies grown by imprinting a leaf to try to answer these questions. My out which leaf microbes are doing what onto a culture medium, showing how the aim was to investigate the secret life – for example, we found that there were bacteria line up along leaf structures of the tree canopies of mature Scots more archaea than bacteria involved Takashi Images/Shutterstock.com The canopy of a primeval beech forests at Hainich National Park, Germany www.scienceinschool.org I Science in School I Issue 46 : Spring 2019 I 23 Biology, Chemistry | UNDERSTAND Rossella Guerrieri Rossella Guerrieri A single beech leaf (left), and bacteria seen on the leaf surface using fluorescence microscopy (right) in converting atmospheric ammonia Acknowledgements Experience what the Earth would look like from into nitrate. Other studies have shown space over a year of seasonal changes in The author would like to thank Newton this NASA movie. See: https://svs.gsfc.nasa. that some bacteria remove pollutants International Fellowship Alumni follow- gov/4596 from the atmosphere by using them in on funding (2013–2017) from the Royal Watch a video about nitrogen in the environment their metabolism (Wei et al., 2017). But and why we should care about it. See: www. Society, EU funding from the MSCA we have a lot more to do in this new youtube.com/watch?v=gED5f_g20_Y individual fellowship (NITRIPHYLL), research area, to understand the effects Get to know your nitrogen footprint. See: and Joan Cáliz and Mateu Menéndez- of particular microbes within the leaf www.n-print.org Serra (Centre for Advanced Studies of biome. Read about an ongoing project to monitor the Blanes, Spain) for their support with effects of air pollution on forest health. See observations at the microscope. the European ICP Forests website: http:// Leaf biomes and icp-forests.net forest health References Find out about trends in air pollution within Carrell AA, Frank AC (2015) Bacterial endophyte Europe. See the website of the European Like humans, it seems that trees communities in the foliage of coast redwood Environment Agency: themselves benefit from their microbial and giant sequoia. Frontiers in Microbiology 6: www.eea.europa.eu/highlights/air-pollution- lodgers. Recently, scientists have found 1008. doi: 10.3389/fmicb.2015.01008 agriculture-and-transport and that bacterial diversity in tree canopies Galloway JN et al. (2004) Nitrogen cycles: past, www.eea.europa.eu/data-and-maps/ helps improve resistance to pests and present, and future. Biogeochemistry 70: indicators/main-anthropogenic-air- contributes to better forest health 153–226. doi: 10.1007/s10533-004- pollutant-emissions/assessment-4 0370-0 generally. And a group of scientists in _________________________________ Guerrieri R, Peñuelas J, Mencuccini M (2017) the USA have found bacteria in sequoia Nitrification in tree canopies. In Forest leaves that protect against pathogenic conditions: ICP Forests 2017 Executive Report Rossella Guerrieri is a plant physiologist fungi (Carrell & Frank, 2015) – so pp 5-6. Eberswalde, Germany: Thuenen and forest ecologist. Her main research maybe this is the secret that has allowed Institute of Forest Ecosystems interest is understanding how forest these beautiful giant trees to resist Magnani F et al. (2007) The human footprint in the carbon cycle of temperate and boreal functioning varies in relation to major disease and survive for centuries, or forests. Nature 447: 849-851. doi: 10.1038/ environmental and anthropogenic even millennia. nature05847 changes. After receiving her PhD from In any case, after more than 20 years the University of Basilicata, Italy, she Wei X et al. (2017) Phylloremediation of air of observations in European forests, we pollutants: exploiting the potential of plant did research at the universities of New can say for sure that tree canopies – and leaves and leaf-associated microbes. Hampshire, USA, and Edinburgh, the invisible life forms they hide – are Frontiers in Plant Science 8: 1318. doi: UK, and then at CREAF (Centre for an important piece of the complex and 10.3389/fpls.2017.01318 Ecological Research and Forestry incomplete puzzle of how nitrogen is Applications) in Barcelona, Spain. cycled between land, air and living Resources She has been involved in outreach species. Find out more about the author’s research from activities for forests and climate change, her own website. See: www.rossellaguerrieri. including at the Edinburgh Science com Festival and the Barcelona Centre of Contemporary Culture. 24 I Issue 46 : Spring 2019 I Science in School I www.scienceinschool.org