Spatial dynamics in the absence of dispersal: acorn production by oaks in central coastal California

Abstract

We examined spatial patterns and spatial autocorrelation (synchrony) of annual acorn production in three species of oaks (genus Quercus) over a 288 km transect in central coastal California. Over small (within-site) distances of < 4 km, synchrony of acorn production between individual trees was significant but varied through time and, for coast live oaks Q. agrifolia, differed at two sites 135 km apart. On a larger geographic scale, valley Q. lobata and blue Q. douglasii oaks exhibited significant synchrony in most distance categories between trees and sites up to 135 km apart and, in the case of coast live oaks, up to the maximum extent of the transect. Spatial patterns over this geographic scale also differed among species, with valley and blue oaks, but not coast live oaks, exhibiting distinct declines in synchrony of acorn production with distance. Interspecific synchrony in acorn production was generally lower than that within species but still significant over the entire extent of the survey. Spatial synchrony between sites was to some extent related to the same environmental variables previously found to correlate with annual acorn production within a site, suggesting that the environmental factors determining acorn production locally also influence spatial patterns over larger geographic areas. These results demonstrate that mast-fruiting in oaks occurs not only on a widespread geographic scale but also across species. They also confirm that synchrony over large geographic areas and complex spatial patterns varying in time can occur in systems where dispersal does not occur and thus environmental variability (the Moran effect) alone is likely to be driving spatial dynamics.