PS 9-113 - Testing evidence for climate adaptation and adaptive differentiation in populations of bur oak (Quercus macrocarpa L) across a latitudinal gradient

Abstract: Climate change is likely to impact plant species ranges, leaving populations to adapt to the new climate, shift their ranges, or face extinction. Tree species may be able to plastically respond to changing climate conditions, as they experience climatic fluctuations within their lifespan. However, in species with broad latitudinal ranges, populations may be locally adapted, resulting in local extinction as climate shifts. We use a reciprocal transplant experiment with three populations of Quercus macrocarpa (bur oak) across a latitudinal gradient to test ecophysiological plasticity and adaptative differentiation in relation to contrasting climates. By planting seedlings from the same maternal families from three populations in gardens at three latitudes, we explicitly test the extent to which bur oak populations across the species broad latitudinal range are locally adapted and show evidence of adaptive differentiation in physiological traits. We also test the degree of seedling plasticity in response to contrasting environments. We measured growth and survival to determine fitness variation within and among populations across sites. We also measured physiological traits, including photosynthesis and stomatal conductance, as well as spectral and chemical composition of leaves at each site. We found fitness responses in the southernmost population that were consistent with climate adaptation. In the northern gardens, all three populations had similar two-year survival rates. However, in the southernmost garden, survival rates were lower for all populations, with the southernmost population showing highest relative fitness. The southernmost population in the southernmost garden also had the highest ranked photosynthetic rate, but lower photosynthetic rates than the other two populations in the more northern gardens. The two northern populations responded similarly in terms of physiological traits to each other across the three gardens. Combined, these results indicate that southern populations show adaptive differentiation from northern populations and show higher fitness under warmer conditions. However, bur oak fitness appears to be low at the southern range, indicating a possible range retraction of the species due to climate change. Our ongoing investigations reinforce the importance of genetic diversity and climatic origin in considerations of climate adaptation planning for widespread tree species.