Taxonomic, functional, and phylogenetic structure of Mt. Washington’s woody plant communities (New Hampshire, USA)

Abstract Background and Aims Environmental gradients structure plant communities at fine to coarse scales. Along elevational gradients, abiotic conditions influence community assembly by filtering species based on their functional strategies, which themselves may be phylogenetically structured. However, it remains challenging to identify which abiotic factors and functional traits are most relevant for gradient-wide assembly, particularly when potential drivers covary. Methods We intensively sampled the composition and leaf traits of woody plant communities across a steep elevational gradient on Mt. Washington (New Hampshire, USA). Topo-edaphic and climatic conditions were measured to mechanistically link environmental with compositional variation. Gradient-wide trait variation was assessed using nonmetric multidimensional scaling while elevational patterns in taxonomic, functional, and phylogenetic alpha and beta diversity were quantified. Environmental drivers of trait-mediated community assembly were then identified with Bayesian generalized mixed effects regressions models. Key Results Three plant community zones were identified with hierarchical clustering: broad-leaved deciduous forest, needle-leaved evergreen forest, and alpine tundra. Plants in alpine communities often had higher leaf nitrogen and δ13C, whereas leaf area and δ15N peaked in broad-leaved deciduous forests. Alpha diversity metrics tended to increase with soil nitrogen (typically at lower elevations) and on exposed topography (typically at higher elevations), while taxonomic and phylogenetic beta diversity peaked in alpine communities. Soil clay content emerged as a driver of community composition across all communities, filtering woody plants based on water-use efficiency (measured with leaf δ13C). Gradient-wide compositional turnover was not phylogenetically structured, and models with topo-edaphic variables outperformed those with elevation alone. Conclusions Trait-based assembly processes likely predominate across Mt. Washington’s woody plant communities, with local topo-edaphic factors filtering species based on their functional strategies. In particular, functional water availability may structure plant communities based on individual- and species- water use efficiency, while diversity patterns may mirror gradient-wide variation in fine-scale environmental heterogeneity.