Background/Question/Methods The forecasts of species extinctions from range displacement following rapid climate change are not supported by the Quaternary record, which shows few extinction events attributable solely to the rapid climatic changes of the late Quaternary. This “Quaternary conundrum”, described in a recent review by Botkin et al., suggests that paleorecords still have much to teach us with respect to the large-scale reorganization of ecosystems during rapid climate change. Here, we use paleoenvironmental data that describe past variations in vegetation, fire, and climate, along with climate-model simulations that suggest the mechanisms that generated those variations, to address how biogeographic shifts occur across heterogeneous landscapes. Specifically, we examine the late-Holocene assembly of a major vegetation formation in the interior Pacific Northwest: the mesic “wet belt” forests extending from northern Idaho to east-central British Columbia. More than 250 taxa are disjunct between the interior and coastal wet belts, separated by >130 km of dry forest or shrubland. Following deglaciation, the coastal forests were rapidly recolonized from the south and from northern refugia on unglaciated headlands, while the interior wet belt was recolonized from the coast and, for an unknown number of taxa, from a refugium in northern Idaho. We investigated the late-Holocene origin of the wet belt with a focus on the major tree species that today define its spatial limits.

Results/Conclusions Seven new sediment records in combination with existing data support the late-Holocene establishment of mesic forests, including a progressive north-to-south development of the mesic forests. Prior to this time forests were dominated by pines and Douglas-fir. Paleoclimate evidence from the same sediment records indicates that the onset of near-modern mesic conditions was nearly synchronous among sites, suggesting that additional factors limited range expansion in the south. Fire reconstructions indicate, however, that a decrease in fire occurrence was consistent with the onset of mesic forest types. Climatic variability may have favored fire during intervals with average climates that would have otherwise supported development of mesic forests. Regional climate model output for 6000 years BP shows steeper-than-present atmospheric lapse rates and warmer-than-present air temperature at the southern end of the wet belt. These conditions would likely favor fire which could have excluded late-successional species. Together, our data and model results suggest that, at least for common tree species, populations may spread rapidly at low abundances and later expand from outlier populations when the climate and disturbance regimes become more favorable.