Climatic and nonclimatic controls shaping early postglacial conifer history in the Greater Yellowstone Ecosystem, USA
The last glacial-interglacial transition (20,000 to 8000 cal yr BP) was a period of rapid environmental change in western North America that featured rising summer insolation and increasing summer temperatures, receding glaciers, and new habitats for plants to colonize. While the sequence of postglacial vegetation changes is well known from paleoecological records in many regions, the relative trade-off between climate and nonclimatic factors in directing postglacial change is not clear. Our objective in this study was to reconstruct early postglacial conifer dynamics in the Greater Yellowstone Ecosystem (GYE) using new and previously published paleoecological data from 17 sites. Temporal trends in fossil pollen percentage and charcoal data were detected by fitting Generalized Additive Models (GAMs) and were compared with independent measures of climate to better understand the role of millennial-scale climate change, fire, and biotic interactions in shaping the early postglacial conifer history.
With ice retreat and increased growing season temperatures after ~16,000 cal yr BP, pollen data suggest that Engelmann spruce (13,000 cal yr BP), subalpine fir (12,500 cal yr BP), and whitebark and/or limber pine (12,500 cal yr BP) established across most elevations in the GYE. Charcoal data suggest regional fire activity was high between 12,500 and 10,000 cal yr BP, coincident with an expansion of fire-adapted and shade-intolerant whitebark and/or limber pine at the expense of Engelmann spruce and subalpine fir. As winter snowpack and available growing season moisture decreased during the middle and late Holocene, subalpine conifers became restricted to high elevations in the region (above 2600 m). At the same time, lodgepole pine steadily increased in abundance and likely outcompeted whitebark and/or limber pine at middle elevations (2000-2500 m), becoming the dominant conifer species in the GYE. Douglas-fir densities increased after 10,000 cal yr BP at middle elevations during the early-Holocene summer insolation maximum and then expanded to lower elevations (below 2000 m) after 6000 cal yr BP as summer temperatures decreased. While the long-term conifer history of the GYE highlights species’ responses to millennial-scale climate change, it also suggests that fire and interspecific competition drove important forest compositional changes. Such nonclimatic factors should be considered in projections of species’ responses to future climate change.