Postglacial Rise and Decline of Ostrya virginiana (Mill.) K. Koch and Carpinus caroliniana Walt. In Eastern North America: Predictable Responses of Forest Species to Cyclic Changes in Seasonality of Climates

Abstract

Isopollen maps of the OstryalCarpinus pollen type, pollen accumulation rates (PAR), and a transect of fossil pollen sites across the Missouri-Arkansas Ozark border provide evidence of the postglacial rise and decline of Ostrya virginiana and Carpinus caroliniana. Rather than spreading northward from coastal plain refuge areas, these species were present in full-glacial (18,000 BP) forests throughout the region from the Ozark highlands to the Appalachian Mountains; they expanded within established forests during the late-glacial interval. Between 13,000 and 8000 BP, the OstryalCarpinus pollen type increased to over 20% of the arboreal pollen, then by 7000 BP decreased to less than 10% throughout eastern North America. PAR data from Cupola Pond, Missouri, show that population increases were logistic on mesic watersheds. Percentages of the OstryalCarpinus pollen type were highest in sites located on the mesic to xeric portion of the edaphic gradient on high interfluves in the Ozark highlands and were least in sites in the poorly drained Mississippi alluvial valley. Ostrya virginiana, which favours mesic to xeric sites today, was probably responsible for much of the pollen represented on regional isopollen maps. The late-glacial/early Holocene rise and decline of the OstryalCarpinus pollen type, as well as similar increases and decreases at the late-glacial/early interglacial transition during the preceding Yarmouthian and Sangamonian interglacials, may have been the outcome of predictable responses of forest species to cyclic changes in seasonality of climates resulting from systematic changes in solar radiation received by the earth due to changes in the timing of perihelion. In the Midwestern United States, heightened seasonality and springtime peaks in solar insolation between 13,000 and 8000 BP were manifested in plant communities unlike any that exist in eastern North America today, and of which Ostrya virginiana and Carpinus caroliniana were major constituents.