ABRUPT CHANGE OF LATE-HOLOCENE FORESTS IN NORTHEASTERN USA DRIVEN BY DROUGHT AND FIRE

Climate change has been linked to recent forest disturbance events, with forest dieback, fires, and vegetation changes predicted to increase in the coming century. However, anticipating regional-scale changes in forest composition requires an understanding of how forests have responded to past climate variability. Long-term perspectives provided by paleoecological records are particularly useful when coupled with vegetation-independent climate records. For example, between 500-600 years BP, many pollen records from the northeastern USA record a change in forest composition from mesic taxa to xeric/fire tolerant taxa. To better understand the dynamics and potential mechanisms of forest changes at this time, we reconstructed the hydroclimate, fire occurrence, and vegetation from four bogs located in Maine and New York using testate amoebae, charcoal, and pollen analysis. Our results reveal the occurrence of multiple, spatially coherent decadal-to-multidecadal drought events during the past several thousand years. Although times of widespread drought were associated with a significantly higher probability of fire events, fires rarely occurred synchronously among the sites. However, the sole exception was at 550 yr BP when all four sites experienced both drought and fire. Pollen records from the same cores directly link this widespread drought and fire event to the regionally documented forest change. Mesic species, such as Fagus grandifolia and Tsuga canadensis declined abruptly, while more xeric/fire tolerant Pinus spp. and Quercus spp. expanded and persisted at high abundance until European land-clearance. Persistence of the change may have resulted from the interaction of high-frequency drought with lower frequency temperature changes associated with the Little Ice Age. Similarly, widespread drought and fire has not occurred in the past century in the Northeast, and our results highlight the potential for abrupt regional changes in forest structure in the future.