The capacity of species' ranges to keep pace with climate change remains an open question, particularly for long-lived, sessile organisms like trees. The degree to which species’ ranges maintained equilibrium with past climate change is a natural test case for how species may respond in the future. Quaternary paleoecological data has presented two conflicting models: many studies find evidence for climatic equilibrium at millennial timescales or shorter, as well as a strong vegetation response to localized abrupt climate changes. Such apparently rapid migration rates revealed by pollen data gave way to the so-called "Reid's paradox" of Quaternary migrations -- considered a paradox because such rates are orders of magnitude faster than than are predicted by contemporary dispersal kernels. At the same time, there is growing evidence of climatic disequilibrium and dispersal lags in response to postglacial warming, and contemporary biodiversity patterns often reveal a fingerprint of Quaternary climate legacies even thousands of years later. Resolving these apparent contradictions will be critical if the paleoecological record is to inform our understanding of species' range shifts in the coming centuries.
Results/Conclusions
To test trees’ capacity for maintaining climatic equilibrium, we calculated the occupied proportion of potential ranges, or “range filling,” for 24 pollen-derived tree taxa at thousand-year intervals over the 21,000 years since the Last Glacial Maximum. We found that range filling did decline following periods of abrupt climate change, but recovered by the next time step. Overall range filling trends thus do not show evidence of accumulated dispersal lags since deglaciation. However, mean range filling values have been steadily declining over the last three thousand years, and present-day values are the lowest in the entire 21,000-year study interval, despite relative temperature stability over the Holocene. We explore the mechanisms behind this trend, including the potential role of enhanced competition, changes in precipitation, and humans. The decline in modern range filling indicates that that trees may be at greater risk due to climate change than may be inferred from previous pollen-derived migration estimates. Our results suggest that managed relocation may be necessary to help trees maintain climatic equilibrium into the future.