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

Isopollen maps of the Ostrya/Carpinus pollen type, pollen accumulation rates (PAR), and a transect of fossil pollen sites across the Missour i-Arkansas Ozark border provide evidence of the postglacial rise and d ecline of Ostrya virginiana and Carpinus caroliniana. Rather than spre ading northward from coastal plain refuge areas, these species were pr esent in full-glacial (18,000 BP) forests throughout the region from t he Ozark highlands to the Appalachian Mountains; they expanded within established forests during the late-glacial interval. Between 13,000 a nd 8000 BP, the Ostrya/Carpinus pollen type increased to over 20% of t he arboreal pollen, then by 7000 BP decreased to less than 10% through out eastern North America. PAR data from Cupola Pond, Missouri, show t hat population increases were logistic on mesic watersheds. Percentage s of the Ostrya/Carpinus 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 re presented on regional isopollen maps. The late-glacial/early Holocene rise and decline of the Ostrya/Calpinus pollen type, as well as simila r increases and decreases at the late-glacial/early interglacial trans ition during the preceding Yarmouthian and Sang-amonian 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 th e timing of perihelion. In the Midwestern United States, heightened se asonality 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 Carpin us caroliniana were major constituents.