SEED DISPERSAL FOLLOWING THE LAST GLACIAL MAXIMUM PART II: ANIMAL-DISPERSED TREES MORE LABILE BEFORE AND AFTER THE YOUNGER DRYAS

Climate change has been implicated in all five of the great Phanerozoic mass extinction events, is thought to be responsible for a significant fraction of background extinctions, and is widely regarded as a dire threat to biodiversity in the coming centuries. Yet, despite differing extinction selectivity regimes of morphological and ecological species traits, we know relatively little about how species with different traits reacted to episodes of past climate change theorized to be causal to extinction events.

In this study, I examine the movement of plant populations in North America and Europe during the recovery interval following the Last Glacial Maximum (LGM) as a function of their seed dispersal mechanism, using pollen data from the Neotoma database. Previous work indicates that tree distributions change little during the Younger Dryas (YD) but expand in the 1 ky periods before and after, with animal-mediated seed dispersal increasing the rate, but not the timing, of expansion, and little change happening during the YD itself. This study expands the previous study to include the entire interval from the LGM to the end of the recovery. Additionally, recent work on seed dispersal in the Miocene of North America implies that extinction selectivity regimes on seed dispersal mechanism are different depending on the intensity of climate change experienced. The LGM, including the Younger Dryas (YD) provides an excellent model for studying the effects of climate change intensity on species traits.

A sample of taxa suggests that expansion of ranges from LGM distributions began around 18 Ka, proceeded until the YD, paused during the YD but did not contract, and then resumed when the YD was over, reaching approximately modern extent around 8 Ka. There was a spurt of expansion between 14 Ka and 13 Ka. Tree species possessing animal-mediated seed dispersal move more rapidly than species without animal dispersal, typically reaching locations roughly 0.2-0.4 ky ahead of non-animal-dispersed species, but populations persist during the YD, despite the YD featuring LGM-like climatic conditions. This raises several implications: (1) that a population exists does not mean that it is sustainable; (2) even rapid climate change creates a kilo-year window during which tree species can disperse or adapt; (3) the YD, due to its brevity, is not a good analog for understanding effects of anthropogenic climate change on tree populations.