XVI International Botanical Congess

Pollen and plant macrofossil studies in eastern North America indicate substantial changes in vegetation and phytogeography in response to climatic and other environmental changes. Many species underwent migrations of 103 km or more in response to climatic changes during and since the transition from glacial to interglacial conditions, while others have occupied substantial portions of their modern ranges for the past 21,000 years. At least one formerly widespread tree species underwent universal extinction during the last deglaciation. Plant communities with no modern counterparts occupied extensive areas in eastern North America between the last glacial maximum and the early Holocene. These unique communities are attributable to unique combinations of climatic and other environmental variables. Effects of lowered atmospheric CO2 concentrations pose a key challenge in understanding vegetational composition and dynamics before the Pleistocene/Holocene transition.