Rossella Guerrieri from the University of Bologne, presents her latest study ‘Land-use legacies influence tree water-use efficiency and nitrogen dynamics in recently established European forests’, discusses the importance of long-term effects of global change and provides some advices for anyone following a research career.

About the paper

Spontaneous forest regrowth is naturally occurring in several areas of the world promoted by on-going global changes, which include land-use and climate change, increasing atmospheric CO₂ and nitrogen deposition. These ‘new’ forests may look very similar to long-established forests in terms of their structure (tree diameter and tree height) – at least when existing forests have been managed, as it was the case for our investigated beech forests. Only by looking at historical orthophotos, my colleagues (and co-authors in the paper) were able to find out that many new forest patches have developed over the recent 50 years or so in some mountainous areas of Catalonia (Northeaster Spain), where pasture used to be the dominant human activity. We were intrigued by the similar structure of the two forest types and wondered whether this could ‘hide’ differences in terms of tree ecophysiological strategies, but also nutrient dynamics. In a previous study my colleagues showed that beech trees in recent forests are growing more than trees in long-established forests. The questions then were: what does drive this difference? Is there an ‘ecological memory’ effect from the previous land-use, in terms of nitrogen availability, which would then positively affect tree intrinsic water-use efficiency (the ratio between CO₂ assimilation and stomatal conductance) and productivity? Can we find a stronger coupling between tree traits associated with physiology and productivity and nitrogen availability in recent compared to long-established forests? In order to answer to these questions, we compared beech forests recently established (post 1950) in former pastureland with long-established beech forests (pre 1950) in a mountainous region of Northeaster Spain for a number of morphological, physiological and ecological parameters measured at leaf, tree and soil levels, which included specific leaf area, foliar nitrogen concentration and nitrogen isotope composition, intrinsic water-use efficiency (estimated from leaf stable carbon isotope composition), nitrogen availability (either total nitrogen or ammonium and nitrate concentrations) and soil stable nitrogen isotope composition. To complete the puzzle, we also made use of wood density, basal area increment and richness of ectomycorrhizal fungi data that were collected in two companion studies at the investigated sites. We showed that recent forests had a higher intrinsic water-use efficiency compared to long-established ones, which was more related to a water conservative strategy (i.e., thicker leaves, higher wood density) rather than to higher nitrogen availability. Yet, as expected, the coupling between nitrogen availability, intrinsic water-use efficiency and tree productivity was stronger in the case of recent vs. long established forests. We also found important difference between the two forest types relating to nitrogen dynamics, with recent forests showing a ‘leaky nitrogen’ cycle, that is less nitrogen retention by tree and associated ectomycorrhizal fungi and increase in nitrogen loss pathways. This study was part of the big project, SPONFOREST – you can read about it and all the interesting studies carried out here: https://www6.inrae.fr/sponforest/Project). The contribution I provided to the project (with this study) was to use my plant physiologist and forest ecologist ‘eyes’ to elucidate how two apparently similar forests, but with a different history, differ in key processes underpinning carbon and nitrogen cycling. Quite a big challenge, but the puzzle is not yet completed, as new and exciting questions arose from our study.

About the research

Spontaneous forest regrowth is natural restoration process, where trees re-established after being removed for other land-use purposes (in general agriculture or livestock). This natural restoration has contributed to increase forest cover globally (by 9% over the recent decades in Europe¹ for instance) and it contrasts with on-going forest loss due to deforestation (mainly in tropical ecosystems), intensive wildfires and forest dieback associated with more frequent and prolonged droughts and heatwaves. These ‘new’ forests play a pivotal ecological role at regional and global scale. First, they represent a nature-based solution to mitigate climate change, which cannot be quantified without elucidating also the physiological mechanisms underlying it (such as those included in our study). Second, they contribute to reducing landscape fragmentation. Third, they significantly contribute to regulating hydrological cycling and improving soil functioning. 
 More studies are needed to elucidate their potential role for nature conservation and climate change mitigation – if you think about it, they are establishing in a time of rapid and abrupt changes in climatic conditions, so the solution (in terms of adaptation and plasticity to global changes) may be in the seeds they will produce and the new trees they will generate. Planting new trees is important, but understanding the complexity of the existing forests – including this new generation of forests that are spontaneously coming back on previous land-use – should come first, as they are here, now, already contributing to the land carbon sink, together with several important ecosystem services.

Besides, in our study we combined different leaf traits and soil related parameters, to explore the interaction between plant physiology and nitrogen dynamics. This, however, gave us also a picture of the present. In order to better understand how these new forests have responded to on-going global changes, the next step would be to take a dendro-isotope approach (that is the measure of stable carbon, oxygen and nitrogen isotopes in tree rings), to understand how intrinsic water-use efficiency has changed since trees establishment and what physiological mechanisms (photosynthesis or transpiration) has driven those changes. Moreover, the indication of the ‘leaky’ nitrogen cycle we found in the case of recent forests needs to be further investigated, particularly in relation to the increase in nitrogen deposition, which could contribute to significantly increasing nitrogen input and to shifting even further the balance between nitrogen retention in the ecosystem and nitrogen loss.

About The Author

I am currently an assistant professor at the University of Bologna. My research activities focus on understanding the effects of global change drivers (particularly increasing atmospheric CO₂ and nitrogen deposition) on long-term in water-use efficiency and ecosystem nitrogen dynamics. In the latter case, I am particularly interested in elucidating the role of tree canopies (and microbial life hidden within them) in processing atmospheric nitrogen deposition.  Experimental sites include nitrogen manipulation experiments and observations along nitrogen deposition gradients across Europe. Check out my webpage for more details: http://www.rossellaguerrieri.com/

I have two articles I am particularly proud of and it is hard to choose between them. They are equally important because they mark my career path, both in terms of contribution to research in my field and fantastic collaborations I have developed (or consolidated).

1. Guerrieri R, Lecha L, Mattana S, Calíz J, Casamayor E, A Barceló, Michalski G, Peñuelas J, Avila A, Mencuccini M (2020). Partitioning between atmospheric deposition and canopy microbial nitrification into throughfall nitrate fluxes in a Mediterranean forest. Journal of Ecology 108 (2): 626-640.
2. Guerrieri R, Belmecheri S, Ollinger S, Asbjornsen H, Jennings K, Xiao J, Stocker BD, Martin M, Hollinger D, Bracho-Garrillo R, Clark K, Dore S, Kolb T, Munger JW, Novick K, Richardson AD (2019). Disentangling the role of photosynthesis and stomatal conductance on rising forest water-use efficiency. Proceedings of the National Academy of Sciences of the United States of America (PNAS) 116 (34): 16909-16914.

For those following an academic career, I have some pieces of advice:

• Academic and research life can be very challenging sometimes. We are too often driven by the pressure to publish to not perish, rather than by the enthusiasm, passion, and curiosity. It is not just the number of papers, but how much they contribute to advancing our understanding on forests functioning that matters the most.
• Master one research field, but don’t be afraid to get ‘contaminated’ by other fields and explore new ideas and approaches that can take you far.
• Surround yourself by mentors and collaborative colleagues who inspire and challenge you, who celebrate your success and are there with a piece of advice when failure comes – and it comes, I tell you.  
• Be determinate, inclusive, collaborative and supportive to people you will meet along the path, but – most importantly – stay human!

¹FOREST EUROPE, 2020: State of Europe’s Forests 2020 – freely available here: https://foresteurope.org/state-europes-forests-2020/

Read the article in full here.