Lineage-specific phylogenetic structure of boreal habitats suggests different assembly processes across phylogenetic and spatial scales

Understanding how plant communities are assembled is critical to explaining the functioning of ecosystems and the maintenance of biodiversity. The phylogenetic distance among species in a community (community phylogenetic structure) has been used to infer deterministic and stochastic assembly processes, albeit with criticisms. However, the effect of the phylogenetic scale (old versus young lineages) and spatial scale on measures of community phylogenetic structure are rarely tested especially in the boreal biome, yet essential to unravel different assembly processes that might operate across species in a community. We examined lineage-specific phylogenetic structure of six plant communities defined at the habitat scale, and the phylogenetic structure of communities defined at a plot level scale (1m2). We obtained vascular plant species lists for six habitats (arctic-alpine barren, bog, fen, kalmia barren, limestone barren, and serpentine barren) on the island of Newfoundland, Canada, and 73 permanent vegetation plots within three of the above habitats to estimate their mean pairwise phylogenetic distance standardized for sample size (SES-MPD). Contrary to the expectation under the stress-dominance hypothesis of phylogenetic clustering in challenging boreal environments, the majority of clades had random phylogenetic structure. However, we observed a pattern consistent across most habitats of a shift from phylogenetic clustering at the deepest nodes of the angiosperms to no phylogenetic structure at shallower nodes, suggesting that different assembly processes may be operating at different phylogenetic scales within a habitat. As expected, all vegetation plot communities at a fine spatial scale showed strong phylogenetic overdispersion likely explained by biotic interactions. Our lineage-specific approach to estimate SES-MPD proved useful to outline testable hypotheses on the evolution and role functional traits play in community structure and assembly of boreal habitats