Global tree slenderness under climate change

Abstract Forecasting tree responses under climate change is a central challenge of plant ecological strategies. Previous studies have typically used one‐dimensional metrics (diameter or height) to evaluate tree growth strategies; this may not be an integrative measure of the trade‐offs in resource allocation. Tree slenderness offers a more comprehensive alternative, as it reflects multidimensional adaptive shifts in resource allocation strategies that drive tree growth under environmental stress. It is hypothesized that tree slenderness results from the interplay of multiple ecological mechanisms, including biomechanical constraints, hydraulic limitations and light competition. However, little is known about the global patterns and drivers of tree slenderness. Here, we investigated how environmental stressors, vegetation characteristics, functional traits and human disturbance shape the tree slenderness coefficient (TSC) using allometry data from 277,384 trees across biomes worldwide. We used random forest models and SHapley Additive exPlanations to estimate critical thresholds in TSC responses to several primary factors across global and biome‐specific scales. Further, we projected changes in TSC globally and across biomes under a range of future climate scenarios (SSP1‐26, SSP2‐45, SSP3‐70 and SSP5‐85) and time periods (2021–2040, 2041–2060 and 2061–2080). We found a pronounced phylogenetic signal, supporting niche conservatism in TSC among these evolutionarily distinct lineages. Our results showed that the response of TSC to environmental factors, vegetation characteristics, functional traits and human modifications showed non‐linear patterns with ecological thresholds. Solar radiation, normalized difference vegetation index (NDVI), aridity index and specific leaf area emerged as the primary factors influencing the variation in TSC globally. Higher solar radiation and more arid conditions resulted in a decrease in TSC. In contrast, higher NDVI and specific leaf area contributed to an increase in TSC. Furthermore, warming is expected to result in smaller TSC for trees globally in the future, but the magnitude of change varies significantly among biomes. Synthesis . Our results provide new insights into the non‐linear associations between tree growth strategies and a range of environmental drivers. Linking global patterns in plant strategies with the processes that influence tree growth from the perspective of tree slenderness can enhance ecologists' understanding of tree performance under climate change.