Relationships between climate and phylogenetic community structure of fossil pollen assemblages are not constant during the last deglaciation

Abstract

Disentangling the influence of environmental drivers on community assembly is important to understand how multiple processes influence biodiversity patterns and can inform understanding of ecological responses to climate change. Phylogenetic Community Structure (PCS) is increasingly used in community assembly studies to incorporate evolutionary perspectives and as a proxy for trait (dis)similarity within communities. Studies often assume a stationary relationship between PCS and climate, though few if any studies have tested this assumption over long time periods with concurrent community data. We estimated Nearest Taxon Index (NTI) and Net Relatedness index (NRI), two PCS metrics, of fossil pollen assemblages of angiosperms in eastern North America data over the last 21 ka BP at 1ka intervals. We analyzed spatiotemporal relationships between PCS and seven climate variables, evaluated the potential impact of deglaciation on PCS, and tested for the stability of climate-PCS relationships. The broad scale patterns of PCS, with overdispersion increasing towards the southern and eastern parts of the study area, remained largely stable across time. Most importantly, we found that significant relationships between climate variables and PCS (slope) were unstable as climate changed during the last deglaciation and new ice-free regions were colonized. We also found weak, but significant, relationships between both PCS metrics (i.e., NTI and NRI) and climate and time-since-deglaciation, which were stable even though the baselines (intercepts) changed through time. Overall, our results suggest that (1) PCS of fossil Angiosperm assemblages during the last 21ka BP have had predictable spatial patterns, but (2) the instability in the relationships between PCS and climate brings into question their usefulness in predictive modeling of community assembly.