QUATERNARY HISTORY OF DECIDUOUS FORESTS OF EASTERN NORTH AMERICA AND EUROPE' MARGARET B. DAVIS2 ABSTRACT The temperate deciduous forest of North America is more diverse than the deciduous forest ofwestern Europe. This difference has traditionally been explained by greater survival in North Americaof deciduous species during the Quaternary. More recent investigations have shown, however, thatlate-Tertiary forests of Europe had already become dominated by conifers, with deciduous angiospermsa minor component. During the Quatemary, coniferous species and genera were lost from the Europeaflora, leaving a few species and genera of angiosperms as the dominant trees. Cold, dry, continentalclimate during the glaciations caused the extinction of conifers; deciduous trees apparently survivethese climatic conditions in pockets of favorable habitat in the eastern Mediterranean region. In easterNorth America, in contrast, temperate deciduous forests are quite similar to the forests that werpresent in the late Tertiary. During the Quaternary, relatively few extinctions occurred, althoughdeciduous angiosperms were displaced from the Appalachian mountains, surviving in small popula-tions in the lower Mississippi valley or on the southern coastal plain. Coniferous forests dominatedby spruce grew in the Great Plains, and forests dominated by pine grew on the southern part oftheAtlantic coastal plain. At the opening of the Holocene, and presumably at the beginning ofall aeprevious interglacials, tree distributions changed dramatically as temperate species rapidly extendedtheir ranges northward. Range boundaries have continued to change throughout the Holocene, asexpansions and contractions of range have occurred as the result ofclimatic change. Quaternary climatichistory caused dramatic changes in the forests of both areas, indicating that modem species distrbutions can no longer be considered relicts of Tertiary distributions. Throughout the Quaternar,species ranges have changed in response to changes in regional climate; many forest communities aof recent origin, having received their present complements of tree species within the last 5,000 yea.Forest communities in Eastern North America and in Western Europe as well have been invaddrepeatedly during the Holocene by forest species expanding from refuges far to the south. Temperate deciduous forest grows over a widearea ofeastern North America. The forest is richin numbers of species, especially the mixed me-sophytic forest communities ofthe southern Ap-palachians. These forests were traditionally com-pared with forests of the Tertiary Period (Reid,1935; Braun, 1947, 1950; Campbell, 1982), whenthe so-called Arcto-Tertiary geoflora was sup-posed to have been widespread throughout thenorthern hemisphere (Chaney, 1944). Reid (1935)and Chaney (1944) believed that severe climateduring the Quaternary Period eliminated theArcto-Tertiary geoflora entirely from many re-gions, such as western North America, while inothers, such as western Europe, extinctions elim-inated ail but a few species and genera. The mod-ern deciduous forests were thus seen as remnantsof an originally widespread, uniform vegetation. The geoflora concept has been challenged re-cently by Wolfe (1978, 1979) who argued that auniform broad-leaved forest never existed. Hisanalysis of the paleobotanical data shows thatTertiary floras were diverse, with evergreen gym-nosperms such as Sequoia dominant in sregions, and evergreen angiosperms presentwhere. He argued that a number of majorf cmatic changes occurred during the Tertiary, 'pecially during the Oligocene; these changesto local changes in abundances of varioUS cponents of the Tertiary flora. Thus changesclimate caused local adaptations rather thangrations of intact plant communities fromlatitude to another as Chaney hyo(Wolfe, 1979). Before the end of the TiPeriod, the forests of western United Statesalready dominated by conifers. A similar chad also occurred in Europe, where Plocenras contained many genera and species offers (Wolfe, 1979). Sequoia was the dontree in some regions (Traverse, 1982). Mixed coniferous-deciduous forest induring the Pliocene is a new interpretatforest history that stands in marked contthe traditional view. The traditional vie ythat deciduous forest persisted in Europthe early Quaternary Period, when incss This work was supported by the National Science Foundation. partent of log and Behavioral Biology, University of Minnesota, Minneapolis, MiesANN. MISSOURI BO. GARD. 70:550-563. 1983.
BioStor
