Understanding Spatiotemporal Variation in Richness and Rate of Within-Site Turnover for Vegetation Communities in Western Eurasia over the Last 4000 Years

Vegetation communities are intricate networks of co-occurring species. Logistical challenges in collecting primary data means research often utilises short-term data from restricted geographical areas. In this study, we examine spatiotemporal change in richness and turnover of vascular plants and bryophytes over the last 4000 years at 23 sites in western Eurasia using high-resolution palaeoecological data. We find support for the Latitudinal Diversity Gradient and Altitudinal Diversity Gradient in both the overall vegetation community (arboreal and non-arboreal species) and the shrub and herb sub-community (non-arboreal species only), as well as a significant temporal increase in the gradient of both relationships. There was a temporal increase in (alpha) richness; the rate of turnover was high but temporally consistent for the overall vegetation community and high but decreasing over time for the shrub and herb sub-community. The rate of change in turnover was affected by latitude (steeper negative relationship at higher latitudes) and altitude (steeper negative relationship at lower altitudes). The Diversity-Stability Hypothesis was supported: vegetation communities changed from "lower richness, higher turnover" historically to "higher richness, lower turnover" more recently. Causal mechanisms for these complex interlinked biogeographical patterns remain ambiguous, but likely include climate change, non-native introductions, increasing homogenisation of generalist taxa, landscape simplification, and anthropogenic disturbance. Further research into drivers of the spatiotemporal patterns revealed here is a research priority, which is especially important in the context of biodiversity decline and climate change.