Variation among populations of Quercus macrocarpa in a reciprocal transplant experiment across a latitudinal gradient.

Ongoing climate change will negatively impact tree populations unless they are able to acclimate to the changes in their local environment. Effective planning for climate adaptation management requires an understanding of the current state of local adaptation and physiological performance to assess whether populations are at risk of local extinction, to determine if seed movement is appropriate, and to select appropriate seed sources if intervention is needed. We established a new reciprocal transplant experiment (Adaptation to Climate and Environment [ACE]) across a latitudinal gradient in North America spanning a range of climate and soil conditions to examine the impacts of warming on three bur oak (Quercus macrocarpa) populations across much of the species' range. We established common gardens in Minnesota, Illinois, and Oklahoma in 2021 grown from seeds collected in 2018-2019 within 50 km of each of those locations from a total of 60 maternal families. In 2023, we measured physiology and 3-year growth and survival to understand local adaptation in bur oak populations. We aimed to (1) assess local adaptation in each of the populations using survival and size as fitness metrics and (2) evaluate physiological responses to different environments along the latitudinal gradient. We found evidence for local adaptation of the southern population in the southernmost garden, but that the northern populations were maladapted to high levels of warming, as evidenced by their low survival, growth, and photosynthetic rates in the warmest common garden. However, mild climate and relatively high nutrient conditions in the middle garden resulted in the highest fitness and best physiological performance for all populations. Growth and survival were correlated in the middle garden but were decoupled in the northern and southern gardens, likely due to stress associated with more extreme climates at the ends of the gradient. Our results suggest that southern seed sources may perform well in warmer conditions in the north brought on by climate change, which has important implications for managers assisting broadly ranged tree species in adapting to climate change.