Functional traits shape tree growth response to winter freeze-thaw cycle and neighborhood crowding in humid temperate forests

Climate change is intensifying the occurrence of hydric events, such as summer water availability and winter freeze-thaw cycles, which are increasingly significant in temperate regions. These events pose substantial threats to tree hydraulic functions and thereby limiting tree growth. In this study, we investigated the compound effects of climate-induced stressors and neighborhood crowding on tree growth, and tested how species functional traits mediate these responses. We combined annual growth data over 8 years from 593 individuals representing 20 tree species in Northeast China to evaluate the relative influence of water availability, freeze-thaw cycles, and their interaction with local crowding on tree growth, and how species functional traits mediate these responses. Our findings indicate that tree growth declined with increasing freeze-thaw cycles frequency, whereas summer water availability had no detectable effect. Tree growth was limited by neighborhood crowding, which appeared to operate largely independently of freeze-thaw cycles. In addition, species with higher xylem hydraulic efficiency, lower wood density and lower specific leaf area grew faster and were more sensitive to freeze-thaw cycles, while species with higher xylem hydraulic efficiency were less sensitive to neighborhood crowding. Our results demonstrate distinct and independent roles of freeze-thaw cycles and neighborhood crowding in shaping temperate tree growth, suggesting that considering the freeze-thaw cycles may improve predictions of temperate forest dynamics facing altered climate changes. Furthermore, species traits can capture how temperate trees cope with different stressors, highlighting the importance of integrating functional traits for a more comprehensive understanding of tree responses to environmental stressors.