Post-glacial changes in spatial patterns of vegetation across southern New England

We analysed lake-sediment pollen records from eight sites in southern New England to address: (1) regional variation in ecological responses to post-glacial climatic changes, (2) landscape-scale vegetational heterogeneity at different times in the past, and (3) environmental and ecological controls on spatial patterns of vegetation. The eight study sites are located in southern New England in the states of Massachusetts and Connecticut. The sites span a climatic and vegetational gradient from the lowland areas of eastern Massachusetts and Connecticut to the uplands of north-central and western Massachusetts. Tsuga canadensis and Fagus grandifolia are abundant in the upland area, while Quercus, Carya and Pinus species have higher abundances in the lowlands. We collected sediment cores from three lakes in eastern and north-central Massachusetts (Berry East, Blood and Little Royalston Ponds). Pollen records from those sites were compared with previously published pollen data from five other sites. Multivariate data analysis (non-metric multi-dimensional scaling) was used to compare the pollen spectra of these sites through time. Our analyses revealed a sequence of vegetational responses to climate changes occurring across southern New England during the past 14,000 calibrated radiocarbon years before present (cal yr bp). Pollen assemblages at all sites were dominated by Picea and Pinus banksiana between 14,000 and 11,500 cal yr bp; by Pinus strobus from 11,500 to 10,500 cal yr bp; and by P. strobus and Tsuga between 10,500 and 9500 cal yr bp. At 9500-8000 cal yr bp, however, vegetation composition began to differentiate between lowland and upland sites. Lowland sites had higher percentages of Quercus pollen, whereas Tsuga abundance was higher at the upland sites. This spatial heterogeneity strengthened between 8000 and 5500 cal yr bp, when Fagus became abundant in the uplands and Quercus pollen percentages increased further in the lowland records. The differentiation of upland and lowland vegetation zones remained strong during the mid-Holocene Tsuga decline (5500-3500 cal yr bp), but the pattern weakened during the late-Holocene (3500-300 cal yr bp) and European-settlement intervals. Within-group similarity declined in response to the uneven late-Holocene expansion of Castanea, while between-group similarity increased due to homogenization of the regional vegetation by forest clearance and ongoing disturbances. The regional gradient of vegetation composition