Latitudinal decrease in acorn size in bur oak (Quercus macrocarpa) is due to environmental constraints, not avian dispersal

The size of acorns produced by several species of eastern North American oaks decreases with latitude. We investigated three hypotheses for this pattern in the bur oak (Quercus macrocarpa Michx.) using samples collected over 2 years throughout the species' range. We found strong support for the hypothesis that abiotic factors, including both temperature and rainfall, constrain acorn size. There was also a smaller but important difference in acorn size related to whether sites were glaciated or not, providing support for the hypothesis that some factor associated with the history of glaciation affects acorn mass. In contrast, although blue jays (Cyanocitta cristata L.) prefer smaller acorns and are an important dispersal agent, the latitudinal patterns of acorn size and variability were not consistent with predictions of the hypothesis that they are the product of blue jay dispersal during bur oak's postglacial Holocene expansion. Assuming that there is a lower limit to the size that acorns can be and still be successful, the strong role of environmental constraints on acorn size may be important in explaining the apparently contradictory interspecific pattern that North American oaks successfully colonizing areas further north during the Holocene are those that produce relatively larger acorns.