LONG-TERM PATTERNS OF ACORN PRODUCTION FOR FIVE OAK SPECIES IN XERIC FLORIDA UPLANDS

Abstract

The number of seeds produced by a population of woody plants can vary markedly from year to year. Unfortunately, knowledge of the patterns and causes of crop-size variation is limited, and most studies have examined only single species in a single mesic environment. We examined long-term patterns of acorn crop sizes for five species of shrubby oaks in three xeric upland vegetative associations of south-central peninsular Florida for evidence of regular fruiting cycles and in relation to winter temperature and precipitation. Counts of acorns on two white oak species (Quercus chapmanii and Q. geminata) and three red oak species (Q. inopina, Q. laevis, and Q. myrtifolia) were conducted annually from 1969 to 1996 (except in 1991) on grids in southern ridge sandhill, sand pine scrub, and scrubby flatwoods associations. As the result of the dissimilar timing of reproductive events in species of red and white oaks and individual species responses to climatic variables, annual acorn production of red and white oak species in a given vegetation association were not synchronized, which dampened the variability of combined annual acorn production. Consequently, acorns were produced with reasonable abundance every year by at least one species. Oaks of nearly closed-canopy scrub produced fewer acorns than similar-sized oaks of the same species in the more open-canopied sandhill and scrubby flatwoods, suggesting light limitation. We identified regular cycles of acorn production that ranged from 2 to 2.4 yr for white oak species and from 3.6 to 5.5 yr for red oak species and found evidence that annual acorn production is affected by the interactions of precipitation, which is highly variable seasonally and annually in peninsular Florida, with endogenous reproductive patterns. Multiple regression models using precipitation variables accounted for as much as 74% of crop-size variation in Q. inopina, 65% in Q. laevis, 60% in Q. chapmanii, and 44% in Q. geminata, but only 29% for Q. myrtifolia. In contrast, acorn production showed no significant association with minimum winter temperatures. Of the various hypotheses offered to explain supra-annual variation in seed crops, our data are most compatible with the nonadaptive resource-matching hypothesis.