Interactive Effect of Nitrogen Deposition and Drought on Forest Carbon Sequestration

European forests are experiencing increasingly frequent and severe droughts, coinciding spatially with a long-term decline in nitrogen (N) deposition. However, the impact of this decline on forest resistance and resilience to drought remains unclear. High N deposition has been associated with lower root-to-leaf area ratios and increased canopy conductance, which may reduce the hydraulic safety margin of trees and increase their risk of hydraulic failure under drought conditions. Conversely, low N deposition decreases stomatal conductance and carbon assimilation, potentially limiting tree regrowth following drought. We hypothesize that decreasing N deposition in Europe enhances the resistance of forests to drought but decreases their resilience. To test this, we analyze how daily gross primary productivity (GPP) responds to drought events using site-level flux data from ICOS and FLUXNET and the satellite-derived GOSIF GPP product. We first quantify GPP resistance and resilience to drought events from 1996 to 2022. We then integrate drought characteristics (timing, duration, and intensity), climate factors (precipitation, temperature, vapor pressure deficit, and soil moisture), and site-specific variables (soil type, tree species, and stand age) and disentangle the interactive effects of drought and N deposition on forest resistance and resilience using a gradient-boosting machine learning approach. Our study aims to improve our understanding of forest dynamics at the carbon-nitrogen-water nexus and provide insights into sustainable, nature-based climate solutions.