Acorn Production Patterns

Acorns—the fruits of oaks—are a key resource for wildlife in temperate forests throughout the Northern Hemisphere. Acorns are also economically important for extensive livestock rearing, and as a staple food have supported indigenous human populations. Consequently, differences in how individual trees and populations of oaks invest in acorn production, both in terms of the size of the acorn crop and of the size of individual acorns, are of interest both ecologically and economically. Acorn production by oaks in both California and Spain tends to be highly variable and spatially synchronous. We summarize studies conducted in the two regions that investigate the factors influencing acorn production. One hypothesis explored is that, as a consequence of management, acorn production tends to be affected by different environmental factors in the two regions; another hypothesis is that acorn production in oaks in Spanish dehesas produce larger and more predictable acorn crops than trees in less managed Spanish forests or in California woodlands. Other factors potentially influencing acorn production are summarized, including biotic factors, trade-offs with growth, trade-offs with acorn size, and pollen limitation. We conclude with a discussion of spatial synchrony and acorn production at the community level. There remain many questions concerning the mating systems of oaks, trade-offs between different oak life-history characters, and the patterns and drivers of spatial synchrony. Environmental conditions in the two regions are similar, but understanding how their subtle differences influence acorn production is likely to yield important insights about the proximate and ultimate factors affecting acorn production and masting behavior.