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), which locally coincided with severe drought and rapid warming of 1.1 °C (0.9–1.3 °C) indicated by lake-level changes and geochemical proxies. The subsequent vegetation changes from 5000-2500 YBP included abrupt declines in birch ( Betula spp.) and beech ( Fagus grandifolia ) and a series of unique forest phases each lasting 300–500 years in association with unique combinations of temperature and moisture. 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 YBP and 3800-3200 YBP mark northward shifts in the northern-temperate forest ecotone known as the “Tension Zone” during the warm-dry phase that coincided with low hemlock abundance. The changes, like those elsewhere along the ecotone from Ontario to Massachusetts, differ from successional or stochastic 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. Repeated episodes of abrupt change reveal the sensitivity of Appalachian forests to climate change during the past > 11,000 years. Two fossil pollen records from Pennsylvania show that the abrupt changes produced distinctive phases of forest composition lasting 300–500 years. An abrupt range-wide decline in hemlock ( Tsuga canadensis ) about 5000 years before present coincided locally with drought and rapid warming of ~ 1 °C. Additional abrupt declines in beech ( Fagus grandifolia ) and birch ( Betula spp.) followed the hemlock decline and were followed, in turn, by increases in taxa like oak ( Quercus spp.) and white pine ( Pinus strobus ) consistent with shifts in regional vegetation gradients and ecotones. The patterns of forest turnover differ from examples of post-disturbance succession, but conform with simulated interactions of climate change and species’ climate niches.