Patterns of Geographic Synchrony in Growth and Reproduction of Oaks within California and Beyond1

We measured patterns of spatial synchrony in growth and reproduction by oaks using direct acorn surveys, published data on acorn production, and tree-ring chronologies. The two data sets involving acorn production both indicate that acorn crops are detectably synchronous over areas of at least 500 to 1,000 km not only within individual species but among species that require the same number of years to mature acorns. Although no tree-ring data are available for California oaks, growth patterns among oaks elsewhere are statistically correlated between sites up to 2,500 km apart. These results indicate that both tree growth and acorn production patterns covary over large geographic scales and support the hypothesis that largescale weather patterns play an important role in determining these life-history parameters of California oaks. They also have important implications for the population biology of wildlife that live in California’s oak woodlands. Oaks are a dominant hardwood genus in a variety of temperate and semitropical regions throughout the world. Here in California, considerable work has been devoted to understanding the environmental factors influencing growth and reproduction by individuals and within local populations. However, virtually nothing is known concerning the factors influencing these life-history parameters on larger geographic scales. For example, our long-term work on acorn production at Hastings Reservation in central coastal California has revealed that a significant amount of variation in annual acorn crop size of blue oaks (Quercus douglasii) is correlated with weather conditions during the spring flowering period, with large crops associated with warm, dry springs and small crops associated with cold, wet springs (Koenig and others 1996). Although such results may eventually allow us to predict acorn crop size in particular localities based on local conditions, they do little to address the question of how synchronous acorn crops of valley or blue oaks are on larger geographic scales. This paper addresses this issue by asking the question: How synchronous is acorn production and other life-history parameters of oaks throughout California and beyond? Study Area and Methods We used three sets of data to address the degree of spatial autocorrelation in growth and reproduction of oaks. These are detailed below. Patterns of Acorn Production within California Data on acorn production by California oaks is currently being obtained by annual surveys of populations in 14 localities throughout California (fig. 1; table 1). Surveys at Hastings Reservation of Quercus lobata, Q. douglasii, Q. agrifolia, Q. kelloggii, and Q. chrysolepis were initiated in 1980 (Koenig and others 1994b). Surveys at two other central coastal sites of Q. lobata, Q. douglasii, and Q. agrifolia were begun in 1989. All other sites include 1 to 3 of the above species (plus one site for Q. engelmannii) and were set up in 1994. Thus, results presented here are preliminary. 1An abbreviated version of this paper was presented at the Symposium on Oak Woodlands: Ecology, Management, and Urban Interface Issues, March 19-22, 1996, San Luis Obispo, Calif. 2Research zoologist, Hastings Reservation and Museum of Vertebrate Zoology, University of California, 38601 E. Carmel Valley Road, Carmel Valley, CA 93924. 3Research director, Long-term Ecological Research Program, Cedar Creek Natural History Area, University of Minnesota, Bethel MN 55055.