SPECIES-RICHNESS IN NORTH-TEMPERATE ZONE FORESTS

The long-standing premise that European temperate forests are less tax onomically diverse than their counterparts in eastern North America an d elsewhere is examined and the hypothesis advanced by Reid (1935) to account for this difference is re-evaluated. The principal difference in taxonomic diversity is at the generic level, with a higher species : genus ratio in European woody genera partially compensating for this difference so that at the species level the difference is less marked . The geography and topography of the four north-temperate forest regi ons are compared as a basis for evaluating Reid's (1935) hypothesis th at differences in these characteristics between the regions played a p rincipal role in bringing about differential extinction. The Quaternar y palaeoenvironmental and palaeovegetation history of eastern North Am erica and Europe especially is also reviewed. Reid's (1935) hypothesis can no longer be sustained in the face of the accumulated evidence of Quaternary palaeoenvironmental and palaeovegetation patterns. An alte rnative hypothesis is advanced based upon (1) the general relationship between number of taxa and area, and (2) the contrast in the areas oc cupied by temperate forest taxa during glacial stages in Europe and in eastern North America. It is hypothesized that the enhanced rate of e xtinction of genera and families in Europe during the Quaternary resul ts from the much smaller area of forest vegetation persisting during g lacial stages on that continent than on other northern hemisphere temp erate continental regions. The underlying reasons for this difference lie in the geography and climate of the northern hemisphere and in the special role of the North Atlantic basin in global ocean circulation. The higher species : genus ratio amongst European tree and tall shrub genera may result from vicariant evolution of morphologically distinc t species in different regions of southern Europe. Temperate forest ta xa have experienced extended phases of isolation in these regions duri ng the glacial stages that have dominated throughout the Quaternary. U nanswered questions remain with respect to the peculiar taxonomic dive rsity of Florida and the apparently higher rate of occurrence of apomi xy in the European flora.