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The postglacial history of three Picea species in New England, USA.
Lindbladh, Matts1,2, Jacobson, George2, Scauffler, Molly2, 1 2
ABSTRACT- The postglacial occurrence of the genus Picea in eastern North America has been well documented. However, given the difficulty of separating the three Picea species -- Picea glauca, P. mariana and P. rubens (white, black, and red spruce) -- in the pollen record, little is known about their unique histories following deglaciation. Here we report the first application of classification and regression-tree analysis (CART) for distinguishing pollen grains of these spruce species. We evaluated six qualitative and seven quantitative variables of modern pollen grains collected from Maine and Maritime Canada, the only region where all three species co-occur. These variables were used to develop a classification system (classification trees) that was then successfully applied to fossil pollen from nine sites in Maine and Massachusetts. We focused on the late glacial (11,000 to 8000 BP) and the late Holocene (1500 BP to present) - the two key periods since deglaciation when Picea has been abundant in the region. The result shows a shift from a Picea forest of P. glauca and P. mariana in the late glacial to a forest of P. rubens and P. mariana in the late Holocene. The low number of P. rubens grains (<5%) identified from late-glacial samples suggests that few if any trees of this species were present near the studied sites. Where P. rubens existed prior to the late Holocene is not known, but the central Appalachian region appears likely. Previous palynological investigations of regional late Holocene vegetation reveal that, following a decline ca. 8000 BP, Picea did not again become abundant (>4% of total pollen) in most areas of Maine until 1000 to 500 BP. Thus, the results of this study indicate that the recent population expansion was likely the first time since deglaciation that P. rubens was abundant in the region.
KEY WORDS: CART, Classification tree, Forest history, Pollen analysis