Ecological novelty in space and time: Pattern, process and the drivers of late-glacial no-analog plant associations
The Pleistocene-Holocene transition in North America (17,000 BP to 8,000 BP) was a time of widespread environmental change, including the arrival of humans, the extinction of 35 genera of megafauna, and individualistic shifts in species’ ranges and abundances in response to climate change and melting ice sheets. The widespread formation of novel plant associations during this interval is presumed to be the result of novel environmental conditions, but the relative importance of biotic and abiotic drivers as causal mechanisms has remained unclear. In this study, we present a synthesis is of 7 well-dated, high-resolution pollen records from new and re-cored classic sites in the Great Lakes region of the Midwest, USA. By reconstructing the spatiotemporal dynamics of these late-glacial no-analog communities relative to the timing of local megafaunal collapse and climate change, we aim to 1) elucidate the causes and timescales of the emergence of ecological novelty, and 2) understand the drivers of between-site variability within the broader no-analog landscape during the last deglaciation.
Dissimilarity analysis indicates that the late-glacial no-analog plant associations were more narrowly constrained in this region (14.5 ka BP to 11.8 ka BP) than reported in previous sub-continental-scale syntheses (17 ka BP to 11 ka BP). Nonmetric multidimensional scaling reconstructs the trajectory of the sites in ecological space through time. Results reveal 1) abrupt changes in vegetation and increased variability within and between sites during the no-analog interval, 2) a distinct northwest-southeast time-transgressive pattern, as well as 3) interregional phenomena within the no-analog interval. Between-site differences in the behavior of key pollen types (e.g., Fraxinus nigra-type, Ostrya-type, and Picea) suggest that local site factors may have influenced the individualistic response of taxa to extrinsic abrupt forcing, including megafaunal extinction and climate change, resulting in particular novel plant associations that were unique to the region. Our results show that the last deglaciation was a natural experiment in undertanding forest resilience to multiple, interacting drivers.