Plant community phylogenetic patterns suggest long-term niche conservatism and incumbency by early-diverging angiosperm lineages along a major diversity gradient in temperate deciduous forests

Background/Question/Methods

Ecological theories typically predict that species richness in communities should represent the outcome of contemporary environmental conditions and local ecological interactions operating over limited spatial and temporal scales.  However, evidence is increasing that local diversity patterns may also be influenced by longer-term, larger-scale biogeographical and evolutionary processes.  In this study we investigated plant communities along a prominent species richness gradient associated with soil pH and calcium content in Temperate Deciduous Forests to ask whether phylogenetic perspectives might provide insight to the origins of this diversity gradient.  We sampled forest vegetation and soils in fifty 0.1 ha plots located across New York and New England on sites ranging from acidic, nutrient-poor soils to circumneutral, calcium-rich soils.  For each plot we calculated the angiosperm plant community’s mean phylogenetic root distance by averaging the nodal root distance from each plant species’ family to the root of the angiosperm supertree.  An abundance-weighted measure of mean root distance was also calculated for each plot.  Null models were constructed to test for significant departures of the observed plot mean root distance scores from null distributions derived from random sampling of species from the regional species pool. 

Results/Conclusions

Species richness and estimates of plot mean phylogenetic root distance were significantly correlated with soil calcium content, with plots on fertile, high calcium soils containing both greater numbers of species and a disproportionally greater representation of species from early-diverging angiosperm lineages, such as taxa in the Aristolochiaceae, Berberidaceae, Lauraceae, Magnoliaceae, and Ranunculaceae.  The observed trends significantly diverged from null model expectations, where no correlations between species richness and phylogenetic root distance were predicted.  At the plot level, 9 of the 50 plots (18%) diverged significantly from null model expectations, with all 9 of these plots exhibiting root distance scores departing toward lower (“more basal”) scores than expected.  Significant correlations were also found between soil calcium and observed phylogenetic root distance scores that were weighted by species’ relative abundances, indicating that early-diverging angiosperm lineages also tended to be more dominant than expected by chance in the species-rich communities on high calcium soils.  These patterns suggest that fertile, calcium-rich soils in mesic Temperate Deciduous Forests may have been an important early habitat niche for angiosperm colonization of temperate climates and that the dominance of early-diverging lineages in these habitats today might reflect ancient niche preemption and long-term niche conservatism and incumbency.