Combining stable isotopes in tree rings and ecosystem fluxes to assess the effects of nitrogen deposition and climate extremes on European forests

Forest ecosystems are particularly threatened by global change components, i.e., more frequent extreme weather and climate events (particularly drought and heatwaves) and increasing (N) deposition, resulting in great uncertainties for the future of the essential ecological, economic and social benefits that humanity relies on from forests. Drought and heatwaves impair physiological mechanisms underpinning tree growth and forest productivity, and they may trigger tree mortality, thus constraining the forest carbon sink (Adams and ì 2017; Gazol and Camarero 2022; Hartmann et al. 2022; Hubau 2020). On the one hand, N deposition stimulates tree growth in nitrogen-limited forests, under steady increase in atmospheric CO 2 (Etzold 2020; Fernández-Martínez 2017 ; Flechard 2020; Wang et al. 2017). On the other hand, increasing atmospheric N input above the empirical nitrogen critical load (above which harmful effects can occurr in the ecosystem) (Braun et al. 2022) (*) could reduce the positive effect on tree growth and could cause forest dieback, through soil acidification and nutrient imbalances, but also by making trees more vulnerable to climate extremes (Dalton et al. 2024; Etzold 2020; Ferretti et al. 2015; Flechard 2020; Gharun et al. 2021; Thomas et al. 2009). Many questions remain: How do these global change components interact and affect forest carbon, water and N cycling? Which tree ecophysiological mechanisms are involved? Are those mechanisms synchronized (in terms of magnitude and temporal trends) at tree and ecosystem scales? Does N deposition affect tree and forest responses to climate extremes? The NEXTRES project aims at answering these questions by applying a multi-scale approach (from tree to ecosystem responses) to eleven forests along a climate and total N deposition gradient (from 3 to 42 kg ha -1 yr -1 ) across Europe, selected within established monitoring networks, namely ICOS and ICP Forests (Fig. 1). We will present preliminary results combining existing ecosystem CO 2 and water vapor fluxes with dendroecological information on growth as well as stable carbon isotope ratios to explore multidecadal changes in water-use efficiency, and to elucidate underpinning tree physiological mechanisms. Moreover, we will target years characterized by climate extremes to follow the intra-annual carbon isotope fingerprint within individual in tree rings to evaluate possible divergences: among tree species in their recovery strategies, and between tree and ecosystem responses. among tree species in their recovery strategies, and between tree and ecosystem responses. Finally, we will elucidate whether an excess of atmospheric N input can affect tree and forest responses to climate extremes.