Functional traits modulate nitrogen addition and climate impacts on growth–mortality trade‐offs in a temperate forest experiment

Abstract The interspecific trade‐off between growth and mortality among tree species is a key dimension of plant life‐history strategies. Atmospheric nitrogen (N) deposition and climate change are transforming forest population dynamics, yet their interactive effects on sapling growth–mortality trade‐offs remain poorly understood. In this study, we used a 5‐year N addition experiment involving 39,366 saplings from 12 temperate tree species in north‐eastern China. We tested how N addition and climate together changed demographic rates and their trade‐off. We implemented a factorial N addition experiment (0, 50 and 100 kg N ha −1 year −1 ) and assessed how sapling performance was influenced by climate variability, functional traits, soil properties, topography and neighbourhood competition. Low‐N addition increased relative growth rates by 12.5% (plot‐community level), 6.7% (species level) and 13.3% (individual level) without a concomitant increase in mortality, whereas high N addition elevated mortality by up to 23.7% without significantly enhancing growth, indicating a shift in species' positions within the growth–mortality trade‐off space. Notably, we detected non‐additive interactions between N addition and climate: increased precipitation suppressed N‐induced growth gains but amplified mortality, while warming further constrained growth responses under N addition. A realised interspecific growth–mortality trade‐off was observed under all N treatments, but its strength varied among N addition levels, weakening under low‐N addition and partially re‐intensifying under high‐N addition. Functional traits emerged as the primary proximate driver of the growth–mortality trade‐off, mediating part of the indirect effects of N addition and climate on this demographic relationship. Synthesis . Collectively, our findings reveal that N enrichment reshapes sapling life‐history strategies by altering trait‐mediated resource allocation and physiological sensitivity to climatic stress, thereby reconfiguring the growth–mortality trade‐off under global change.