Spatial genetics of a rare endemic island oak, Quercus tomentella, and implications for conservation and management

Background/Question/Methods

The island oak, Quercus tomentella Engelmann, is a species endemic to five of the eight California Channel Islands and Guadalupe Island, west of Baja California. It is therefore one of the rarest of oaks and represents an important component of Channel Island biodiversity. Quercus tomentella is a member of the Section Protobalanus (golden or intermediate oaks), a small group with uncertain evolutionary origins and affinities. Despite its importance as a distinct and unique component of Channel Island biodiversity, few studies have been conducted with regard to its evolution and conservation status. We are conducting a population genetic study of island oaks to determine the levels of genetic diversity in Q. tomentella and to determine how that diversity is partitioned within and among islands. Trees have been sampled from each island where the species occurs and DNA microsatellite analysis has been used to assess genetic variability and genetic differentiation. Microsatellite loci developed in white oaks have been successfully adapted for genotyping variable loci in Q. tomentella.

Results/Conclusions

Our results suggest that Q. tomentella maintains moderate levels of genetic diversity despite having small, declining populations and low effective population sizes. However, Q. tomentella exhibits striking levels of population differentiation, both among stands on an island and among islands. Such structure is very unusual for temperate oaks, where wind-dispersed pollen typically provides reproductive connectivity over large spatial scales. Further complicating measures of genetic diversity, clonal reproduction occurs in Q. tomentella. Regeneration in some stands is primarily from acorns, and primarily from clonal growth in others, but tree growth form does not clearly distinguish this difference. Thus stem counts alone will overestimate population size and genetic surveys are needed to determine actual population sizes. Introduced ungulate herbivores have recently been eliminated from several of the islands, providing an opportunity for Q. tomentella recovery. Spatial genetic data provided by microsatellites data will help managers target stands for management and seed sources.