The role of the climate niche in repeated abrupt tree declines and ecotone dynamics in the Appalachian Mountains during the Holocene

Forests in eastern North American did not achieve a stable composition during the Holocene. Both prolonged and abrupt shifts in the distributions of tree species were common. Studying the dynamics involved can help anticipate the responsiveness of forest biogeography to climate change today. Here, we evaluate changes in two >12,000-year fossil pollen stratigraphies from the Appalachian Plateaus Province, Pennsylvania. They show repeated episodes of forest turnover following the widespread collapse of hemlock ( Tsuga canadensis ) populations at ca. 5000 years before present (YBP). The changes from 5000-2500 YBP include abrupt declines in taxa such as birch ( Betula spp.) and beech ( Fagus grandifolia ), which produced unique forest phases each lasting 300-500 years. Emergent communities included the resurgence of oak ( Quercus spp.) and white pine ( Pinus strobus ), which had been important several thousand years earlier. Two oak maxima at 4800-4250 and 3800-3200 YBP mark northward shifts in the regional oak-hardwood ecotone known as the “Tension Zone” and involve centennial-scale droughts during a millennial-scale period of warming. The changes, like those elsewhere along the ecotone from Ontario to Massachusetts, differ from successional dynamics that might have been expected after the hemlock decline. Instead, the forest histories included abrupt changes, short-lived communities, and asynchronous changes across sites consistent with the interaction of a) individualistic species’ climate niches and b) multiple scales of climate variation. Comparison of the pollen records with heuristic forest history simulations demonstrates that landscape-scale forest changes, integrated across mosaics of many stands, match patterns expected from deterministic, equilibrium responses to climate change.