Latitudinal trends in acorn size in eastern North American species of Quercus

Abstract

Latitudinal variation in acorn size was examined in 32 species of the genus Quercus geographically restricted to eastern North America. Within-species comparisons showed the existence of a prevalent trend: individuals at high latitude produce, on the average, smaller acorns than individuals of the same species at low latitude. This observed trend, clines of decreasing acorn size with increasing latitude, could represent (i) the selected or plastic response to a shorter and (or) cooler growing season constraining the size of acorns that can mature on a tree, (ii) a consequence of past size-selective dispersal that accompanied postglacial range expansion, or (iii) a combination of these effects. The within-species trend contrasted with an observed among-species trend: species with large acorns have ranged that extend into higher latitudes than species with small acorns. Current evidence strongly suggests that small acorn size is maladaptive for successful seedling establishment at higher latitudes. Our results caution against inferring selective forces operating on seed size from among-species comparisons. Key words: acorn size, Quercus L., latitudinal clines, eastern North America.