Forest responses to nitrogen deposition and climate extremes as assessed by combining stable isotopes in tree rings and ecosystem fluxes

Forests are central to climate-change mitigation but are increasingly threatened by global change components, such as more frequent extreme weather and climate events (particularly drought and heatwaves) and increasing nitrogen deposition, resulting in great uncertainties for the future of the essential ecosystem services they provide. Drought and heatwaves impair physiological mechanisms underpinning tree growth and forest productivity, and they may trigger tree mortality, thus constraining the forest carbon sink. On the one hand, nitrogen deposition stimulates tree growth in nitrogen-limited forests, but when exceeding the empirical nitrogen critical load could cause forest dieback, through soil acidification and nutrient imbalances, but also by making trees more vulnerable to drought. Many questions remain: How do global change components interact and affect forest functioning? Which tree ecophysiological mechanisms are involved?  Are those mechanisms synchronized at tree and ecosystem scales (in terms of temporal trends and intra-annual seasonal changes)? Does nitrogen deposition affect tree and forest responses to climate extremes under a CO₂ richer world? The NEXTRES project aims at addressing these questions by applying a multi-scale approach combining tree-based measurements (including long-term growth and stable carbon, oxygen and nitrogen isotopes together with intra-annual scale carbon isotope analyses) to ecosystem responses (Gross Primary Production and Evapotranspiration). We studied eleven forest sites along climatic and nitrogen deposition gradients (3–42 kg N ha⁻¹ yr⁻¹) across Europe, within the ICOS and ICP Forests networks, focusing on four widespread tree species (Fagus sylvatica, Quercus spp., Picea abies, Pinus sylvestris). Across sites, basal area increment generally declined during recent climate extremes (e.g. the 2018 drought), with a stronger response in the case of broadleaf vs. conifer species, followed by recovery in subsequent years at most of the sites. Preliminary isotope results for Fagus sylvatica at two sites show contrasting responses: intrinsic water-use efficiency (iWUE) increased during the 2018 drought at Sorø (Denmark), coinciding with reduced growth, whereas a severe late frost at Collelongo (Italy) reduced growth without a clear iWUE response, suggesting different plant strategies in terms of leaf gas exchanges and carbon allocation. Preliminary intra-annual δ¹³C analyses from Picea abies trees in Davos (Switzerland) reveal higher and more variable δ¹³C values during the extreme year in 2018, with elevated values in latewood compared to earlywood, highlighting strong seasonal modulation of drought responses. The coupling between tree-level and ecosystem responses will be assessed at the multidecadal and intra-seasonal scale, as well as the contribution of nitrogen deposition in modulating forest vulnerability and resilience to climate extremes.

Acknowledgments. Project funded by the European Union - NextGenerationEU under the National Recovery and Resilience Plan (PNRR) - Mission 4 Education and research - Component 2 From research to business - Investment 1.1 Notice Prin 2022 - DD N. 104 del 2/2/2022, title “Effects of nitrogen deposition and climate extremes on European forests: combining stable isotopes in tree rings and ecosystem fluxes (NEXTRES)”, proposal code 202299J927 - CUP J53D23002640006. We thank all collaborators at the forest sites for assistance in the field.