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

The phylogenetic distance among species in a community (community
phylogenetic structure) has been used to infer deterministic and
stochastic assembly processes, albeit with criticisms. The effect of
phylogenetic scale (old versus young lineages) and spatial scale on
measures of community phylogenetic structure are rarely tested
simultaneously especially in the boreal biome, yet are essential to
unravel different assembly processes that might operate in a community. We
examined lineage-specific phylogenetic structure for six vascular plant
communities defined at the habitat scale (arctic-alpine barren, bog, fen,
kalmia barren, limestone barren, and serpentine barren) on the island of
Newfoundland, Canada, and the phylogenetic structure of plant communities
defined at a plot scale (72 plots x 1m2). Contrary to the expectation
under the stress-dominance hypothesis of phylogenetic clustering in
challenging boreal environments, the majority of clades across the six
boreal habitats had random phylogenetic structure. However, we observed a
shift from phylogenetic clustering at the deepest nodes of the angiosperms
to no phylogenetic structure at shallower nodes (<110 mya),
suggesting changes in assembly processes with phylogenetic scale within a
habitat, and the potential role for deterministic processes at deep nodes.
The random phylogenetic structure of 1m2 plots and our modeling effort to
test the effect of an environmental stress gradient on community
composition suggest that a complex set of stochastic and deterministic
factors is responsible for species assembly at this fine spatial scale,
not just abiotic filtering in hostile environments like the serpentine as
predicted by the stress-dominance hypothesis. The interpretation of
phylogenetic structure metrics did not change when considering species
abundances or when polytomies were resolved. Taken together, inference of
assembly processes must be lineage-, habitat- and spatial scale-specific,
supplemented with knowledge on trait role and evolution for which we
outline future research hypotheses.