Glacial populations and postglacial migration of Douglas-fir based on fossil pollen and macrofossil evidence

To understand how temperate forests might respond to future episodes of global warming, it is important to study the effects of large-scale climate change brought about by rapid postglacial warming. Compilations of fossil evidence have provided the best evidence of past plant range shifts, especially in eastern North America and Europe, and provide a context for interpreting new molecular datasets from modern forests. In western North America, however, such reviews have lagged even for common, widespread taxa. Here, we synthesize fossil evidence for Douglas-fir (Pseudotsuga menziesii) from nearly 550 fossil pollen, sedimentary macrofossil, and packrat midden macrofossil sites to develop hypotheses about the species' late Quaternary history that can be tested with molecular phylogeographic studies. For both the coastal and interior varieties, we identified alternative hypotheses on the number of glacial populations and postglacial migration patterns that can be characterized as single-population versus multiple-population hypotheses. Coastal Douglas-fir may have been subdivided into two populations at the Last Glacial Maximum (LGM) and colonized British Columbia from populations in Washington and Oregon. Interior Douglas-fir could have been subdivided along major topographic barriers into at least three LGM populations and colonized British Columbia and Alberta from populations in northwest Wyoming and/or northeast Utah. For both varieties, we calculated migration rates lower than previous studies, which could have been as high as 100-220 m/yr if Douglas-fir reached its modern distribution 9000 cal yr BP, or as low as 50 m/yr if it reached its modern range at present. The elevational range of populations in California and the southern Rockies shifted upslope by 700-1000 m. If there were multiple LGM populations, these elevational shifts suggest that those populations did not contribute to the colonization of Canada. Our findings emphasize the possibility of low-density northern LGM populations and that populations within species react individualistically in response to large-scale climate change. (C) 2010 Elsevier Ltd. All rights reserved.