Comparison of conservation strategies for California Channel Island Oak (Quercus tomentella) using climate suitability predicted from genomic data

Abstract

Management strategies, such as assisted gene flow, could increase resilience to climate change in tree populations. Knowledge of evolutionary history and genetic structure of species is needed to assess the risks and benefits of different strategies. Quercus tomentella, or Island Oak, is a rare oak restricted to six Channel Islands in California, USA, and Baja California, Mexico. Previous work has shown that Island Oaks on each island are genetically differentiated, but it is unclear whether assisted gene flow could enable populations to tolerate future climates. We performed whole-genome sequencing on Island Oak individuals and Q. chrysolepis, a closely related species that hybridizes with Island Oak (127 total), to characterize genetic structure and introgression across its range and assess the relationship between genomic variation and climate. We introduce and assess three potential management strategies with different trade-offs between conserving historic genetic structure and enabling populations to survive changing climates: the status quo approach; ecosystem preservation approach, which conserves the trees and their associated biodiversity; and species preservation approach, which conserves the species. We compare the impact of these approaches on predicted maladaptation to climate using Gradient Forest. We also introduce a climate suitability index to identify optimal pairs of seed sources and planting sites for approaches involving assisted gene flow. We found one island (Santa Rosa) that could benefit from the ecosystem preservation approach and also serve as a species preservation site. Overall, we find that both the ecosystem and species preservation approaches will do better than the status quo approach. If preserving Island Oak ecosystems is the goal, assisted migration into multiple sites could produce adapted populations. If the goal is to preserve a species, the Santa Rosa population would be suitable. This case study both illustrates viable conservation strategies for Island Oak and introduces a framework for tree conservation.