Range‐wide Tsuga tree decline driven by abrupt climate changes in mid‐Holocene America
Bryan N. ShumanSummary
Why do tree species' distributions occasionally collapse? Paleoecologists have puzzled over the problem since recognizing that eastern hemlock (
Tsuga canadensis
(L.) Carr.), a North American conifer tree, declined abruptly range‐wide amid the ostensibly stable climates of the Holocene. The decline
c.
5000 yr before 1950 CE (YBP) highlights risks that either ecological tipping points or unknown modes of climate variation could amplify biotic changes today.
Synchrony tests, regression, and gradient analyses applied to fossil pollen and paleoclimatic records from across hemlock's range were used to test hypotheses about the decline.
The analyses reveal a time‐transgressive decline directly linked to climatic shifts associated with Atlantic Meridional Overturning Circulation. The surprising extent of decline arose from complex regional patterns of climate change that compressed the geographic realization of hemlock's climate niche. Eastern droughts triggered an initial phase of decline after 5800 YBP even as hemlock increased in snowbelts near the Great Lakes and upslope in Appalachian highlands. Added climatic stress then eliminated these refuges during a second phase of declines
c.
4900 YBP.
Overall, the regionally patterned collapses of hemlock populations point to a strong sensitivity to interacting temperature and precipitation changes, but not a precedent for exotic insect and pathogen infestations today.