My research focuses on the late-Quaternary migration history of spruce, a taxon that dominates the modern boreal forests. Two alternative hypotheses have been proposed to explain the postglacial expansion of spruce in Alaska following the end of the last glaciation. The first suggests spruce expansion to be part of the rapid postglacial migration from refuge populations in southeastern North America. The second hypothesis involves the existence of refugia in unglaciated areas of Beringia and possibly Canada during the last glacial maxima, which expanded in size and geographic extent during the Holocene.

Inherent limitations of conventional techniques (pollen and macrofossil analysis) have made it impossible to resolve this controversy. I am using a new approach to test these hypotheses through molecular analysis of modern and fossil plant tissues. We are currently analyzing spruce needles using a suite of non-coding genetic markers from the chloroplast, mitochondrial, and nuclear genomes. Preliminary results based on RAPDs (random amplified polymorphic DNAs) of modern black spruce tissue from Alaska and western Canada show genetic differentiation when compared to the midwestern United States.

These results are suggestive of Beringian refugia but inconclusive due to the potential influence of natural selection on RAPD markers. We are currently sequencing variable regions of chloroplast and nuclear DNA, which are removed from the direct affects of intense natural selection. Initial data show clear interspecific differences (black vs. white spruce at several loci as well as potentially informative intraspecific variation.

Understanding the Quaternary history of spruce is important in the context of biotic response to climatic change. The existence of spruce refugia in Beringia would imply that some spruce genotypes are resilient to extreme climatic variations. Alternatively, if spruce populations spread in one continuous northwestward wave, the remarkably fast migration rates documented by pollen records would suggest that spruce is capable of responding rapidly to climatic change. By combining molecular biology and paleoecology, this project has the potential to resolve these controversial issues and offer unique insights into ecosystem response to climatic forcing.