COS 76-3 - Phenotypic plasticity of Quercus garryana Seedlings in Response to Climate Treatments.

Abstract: In a future of accelerating climate change, understanding the phenotypic plasticity of long-lived plants such as forest trees is critical for predicting and managing the effects of climate change on ecosystems. However, species specific knowledge of phenotypic plasticity is often lacking. This is particularly important for tree species like the Oregon white oak (Quercus garryana), the keystone species of a rare grassland ecosystem in the Pacific Northwest. To address this gap, we conducted an experiment with three climate treatments and a control. We tested for trait differences and investigated phenotypic plasticity in oak seedlings between site and treatment. Seedlings were collected from two sites within the Portland Metro region in Oregon. Open-top chambers increased the temperature by 2°C. Rain-out shelters decreased precipitation by 30%. Both structures were also used for a combination treatment. Our results showed significant differences in traits between site and treatment, with the warming treatment driving this variation (PERMANOVA, p< 0.05) and no interaction between site and treatment. In general, the warming treatment had a positive effect on plant traits related to resource acquisition and growth. The drought treatment did not affect seedling traits significantly but constrained the positive effect of warming in the combination treatment. Standardized reaction norm slopes showed a similar trend. Seedlings grown in the warming treatment had greater plasticity in measured traits and the drought treatment constrained this positive effect. Principal component analysis revealed one significant PC axis positively correlated with specific leaf area, final height, leaf number and negatively correlated with root:shoot mass ratio. Linear mixed effects modeling showed the PC axis was significantly associated with biomass, a proxy for fitness. Our findings suggest that for the keystone species Q. garryana, drought, heat, and their interaction do not limit seedling growth. We found that the magnitude of this growth response is site specific and dependent upon the combination of the two climate variables. Our results can inform seed collection strategies for conserving and restoring rare grassland ecosystems, as well as mitigating the effects of climate change on forest ecosystems through assisted gene flow, particularly those dominated by keystone species like Q. garryana.