Spatial and Temporal Heterogeneity Drive Vascular Plant Diversity and Endemism in the South-Central United States

Abstract— Spatial phylogenetic methods enable the integration of phylogenetic data with species occurrence records, allowing evolutionary history to be incorporated into biodiversity assessments and evaluations of biogeographic hypotheses. This study presents the first regional-scale, phylogeny-based investigation of vascular plant diversity in the South-Central US, a region that has been previously underrepresented in plant occurrence datasets and analyses. Using recently digitized herbarium data, we characterized spatial patterns of species richness, phylogenetic diversity, relative phylogenetic diversity, phylogenetic endemism, and relative phylogenetic endemism across the climatic gradients of this region. We used generalized linear modeling and spatial autoregressive modeling to identify primary abiotic drivers of these metrics. We performed a categorical analysis of neo- and paleo-endemism to identify hotspots of evolutionarily unique and geographically rare lineages. We found that spatial variability in topography and temporal variability in temperature and precipitation had much stronger relationships with diversity and endemism than the prominent precipitation and temperature gradients that characterize the region. Specifically, we found that the relatively older plant assemblages and centers of paleoendemism found in the seasonally less variable and geologically older Eastern Temperate Forest provide evidence of past refugia. Unexpectedly, this pattern was also observed in the Edwards Plateau of central Texas, which experiences greater seasonal climatic variability. The geologically younger and topographically less complex High Plains harbor widespread concentrations of short branches, signaling that recent diversification has played an important role in shaping the grassland flora. Lower precipitation and greater seasonality are associated with phylogenetic clustering in the Chihuahuan Desert, suggesting that aridity may function as an environmental filter.