PARENT SESSION
Poster Session 21: Paleoecology
Wednesday, August 10, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

Spatial and temporal patterns of Late Holocene invasion of yellow birch in central Upper Michigan.

Andersen, Jennifer¹, Booth, Robert², Jackson, Stephen^*,1, ¹ University of Wyoming, Laramie, WY, USA² University of Wisconsin, Madison, WI, USA

ABSTRACT- Pollen studies in the western Great Lakes region indicate that two important mixed-forest species, hemlock (Tsuga canadensis) and beech (Fagus grandifolia), expanded their ranges during the late Holocene, and that periods of rapid expansion alternated with periods of relative stasis. The invasion history of another mixed-forest species, yellow birch (Betula alleghaniensis), is not as well understood, because discrimination among species of birch using pollen morphology is not feasible. Macrofossils (fruits and bracts) of yellow birch can be used to document arrival and expansion at individual sites, and macrofossil data can be corroborated by patterns in the birch pollen percentages. Pollen and plant macrofossils were sampled from AMS-dated sediment cores obtained from three lakes along an east-west transect in central Upper Michigan, in uplands near the Lake Superior shore. The easternmost site, Ackerman Lake, is south of Munising Bay. Lake Ann is in the Huron Mountains 135 km to the west. Partridge Lake is on the Keweenaw Peninsula, 70 km northwest of Lake Ann. Our data show a time-transgressive pattern of westward expansion of yellow birch, with first appearance 4000 yr BP at Ackerman Lake, 3500 yr BP at Lake Ann, and 3200 yr BP at Partridge Lake. This expansion occurred during an extended wet period (4000 to 2800 yr BP) documented in regional paleohydrological records. This wet period was immediately preceded by a severe, multidecadal drought (4200 to 4000 yr BP). Wildfires associated with this drought may have facilitated initial yellow birch expansion by clearing incumbent vegetation and opening up the canopy. We are expanding our paleoecological site network to the east, south, and west to corroborate these patterns and assess the influence of late Holocene climate variability and disturbance on invasion of yellow birch and other species.

Key words: paleoecology, yellow birch, climate change, invasion