SPECIES-RICHNESS IN NORTH-TEMPERATE ZONE FORESTS

The long-standing premise that European temperate forests are less taxonomically diverse than their counterparts in eastern North America and 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 regions are compared as a basis for evaluating Reid's (1935) hypothesis that differences in these characteristics between the regions played a principal role in bringing about differential extinction. The Quaternary palaeoenvironmental and palaeovegetation history of eastern North America 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 alternative hypothesis is advanced based upon (1) the general relationship between number of taxa and area, and (2) the contrast in the areas occupied by temperate forest taxa during glacial stages in Europe and in eastern North America. It is hypothesized that the enhanced rate of extinction of genera and families in Europe during the Quaternary results from the much smaller area of forest vegetation persisting during glacial stages on that continent than on other northern hemisphere temperate 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 distinct species in different regions of southern Europe. Temperate forest taxa have experienced extended phases of isolation in these regions during the glacial stages that have dominated throughout the Quaternary. Unanswered questions remain with respect to the peculiar taxonomic diversity of Florida and the apparently higher rate of occurrence of apomixy in the European flora.