Climate-Driven Forest Reassembly Follows Divergent Functional Pathways in Cold- and Warm-Adapted Communities

AimClimate change leads to the widespread rearrangement of forests predominantly towards warm-adapted lower-elevation species. However, understanding the context-specific community assembly mechanisms that underlie these changes requires a mechanistic understanding of plant responses. Here, we use functional traits and diversity indices to identify distinct responses of warm- and cold-adapted forest communities. MethodsOur analysis used seven traits, including leaf morphology, size, nutrient content, and foliar isotopes across 3,625 sites over 49 years. We derived functional strategies from community-weighted mean trait values, assessed assembly mechanisms using five multi-trait functional diversity indices, and estimated structural change and turnover rates using abundance-based dissimilarity metrics. ResultsForest composition shifted significantly towards warm-adapted, lowland species with increasing temperature (+0.12 {degrees}C/decade) and decreasing precipitation (0.87%/decade). Communities concurrently increased in functional and species richness, but became functionally more similar, indicating niche packing. Disturbance recruitment shifted warm-adapted forests towards acquisitive traits, and increases in anisohydric strategies, reflecting possible effects of CO2 fertilisation under drought stress. Cold-adapted forests, in contrast, shifted to conservative strategies and were subject to stronger abiotic filtering. Moreover, changes in nitrogen isotope ratios point towards the importance of ectomycorrhizal associations to persist under climate change. Ecotonal mixed forests showed both responses and experienced the highest turnover. Main conclusionsNew Zealand forests follow biome-specific trajectories, characterised by distinct assembly mechanisms and context-dependent ecological responses. While uniform trends obscure these responses, they can be disentangled using a functional diversity framework that incorporates isotopic traits, offering a practical means to detect and interpret community reassembly under climate change.