The proposal will investigate the proximate factors leading to highly variable and spatially synchronized seed production in a wind-pollinated, obligate outcrossing tree species, the valley oak (Quercus lobata). This phenomenon, known as mast-fruiting or masting behavior, is believed to be driven by either environmental forcing or pollination coupling. Environmental forcing postulates that trees use weather as a cue to determine the resources they devote to reproduction in a particular year. Pollen coupling assumes that pollen is limited and that fertilization efficiency is dependent on pollen production by nearby trees. This proposal will test these hypotheses using long-term data on acorn production by Q. lobata collected on a statewide scale combined with detailed data on individual phenology, microclimate, overall pollen abundance, and the fate of female flowers.
The project will collect statewide data on acorn production, a highly variable food resource important to both game and non-game wildlife. The project focuses on a tree species of considerable conservation interest because of development, habitat conversion, apparent lack of regeneration, and susceptibility to future climate change. The work proposed here will help to emphasize the key role Q. lobata and oak woodlands in general play as a natural resource important both to wildlife and to ecosystem function. The extensive dataset on seed production by forest trees that will result from this research will be made electronically available, first in summary form and ultimately as metadata, for future researchers interesting in using the data to examine patterns and test hypotheses regarding the ecology and evolution of masting behavior.
This project is investigating the proximate factors leading to highly variable and spatially synchronized seed production in oaks, a widely distributed group of trees whose seeds (acorns) are an important resource for a wide variety of wildlife species. Particular focus has been directed toward the valley oak (Quercus lobata), a species common in California, although additional work was conducted on other oak species in California, Minnesota, and Spain. Such variable seed production, known as mast-fruiting or masting behavior, has been thought to be driven either by what is called "environmental forcing" or by "pollen coupling". Environmental forcing postulates that trees use weather as a cue to determine the resources they devote to seed production in a particular year. Pollen coupling assumes that pollen is limited and that fertilization efficiency is dependent on pollen production by nearby trees. This proposal tested these hypotheses using long-term data on acorn production by valley oaks collected throughout Cailfornia combined with detailed data on the timing of bud-burst and flowering (phenology) of individual trees, microclimate, overall pollen abundance, and the fate of female flowers. Our results, both experimental and observational, provide strong support for pollen limitation being an important factor in masting behavior. They also indicate a role for weather. We have synthesized these findings into a new hypothesis for the proximate driver of masting that combines both pollen coupling and environmental forcing. This hypothesis, which we call the phenological synchrony hypothesis, proposes that small-scale geographic variability in weather conditions results in different degrees of flowering synchrony within the population (phenological synchrony) which then results in differing amounts of pollen availability, fertilization success, and ultimately acorn production (Fig. 1). Specifically, in the case of valley oaks, warm spring weather results in relatively synchronous phenology within the population, relatively lots of pollen, high fertilization success, and a large acorn crop, whereas cool, wet spring weather results in relatively asynchronous phenology, greater pollen limitation, poor fertilization success, and a poor acorn crop. Data collected as part of this grant provide support for several of the critical steps of this hypothesis, while continuing work is designed to link these proximate drivers to the large geographic scale of masting, which in the case of valley oaks encompasses their entire geographic range throughout the state of California. Future work will continue to foster a deeper understanding of the factors driving variable seed production in forest trees and provide a basis for predicting how masting behavior is likely to be affected in the future by global climate change. The project collected statewide data on acorn production that has been used not only in our work but by a variety of studies interested in the relationship between acorn production and California wildlife including black bear, mice, band-tailed pigeons, and acorn woodpeckers. It has also been used in a comparative analysis of acorn production by oaks in California savannas compared to oaks in Spanish dehesas. Valley oaks are a species of considerable conservation concern in California due to poor regeneration, and work supported by this grant yielded data important to understanding the scale of pollen dispersal in the population. We also conducted work surveying seedlings and their relationship to cattle grazing in three central coastal California oak species that will provide a baseline for future work designed to understand the factors limiting the demographic success of California oaks.
