Background/Question/Methods Long-standing theory (i.e., the center-periphery hypothesis) predicts that populations on a species warm range margin will be small, genetically depauperate, and live at the boundary of their climatic tolerances. Consequently, warm-margin populations will be especially vulnerable to climate change and managers have few options to reduce vulnerability. However, this theory is now in question. More recent theory suggests that warm-margin metapopulations often have the genetic diversity necessary to adapt to climate change. Under this new theory, mixing individuals from multiple warm-margin populations could help a managed population adapt to climate change by increasing genetic diversity or providing pre-adapted genotypes. We are testing whether population mixing will reduce the vulnerability of a subalpine plant (Sibbaldiopsis tridentata) to climate change near its warm range margin. In 2021, we created 90 experimental populations in three common gardens along a 5°C climate gradient, including the following population mixtures: (1) all local plants as a control, (2) a mixture of local and foreign plants from similar climates to test the effects of increasing genetic diversity, and (3) a mixture of local and foreign plants from warmer climates to test the effects of adding warm-adapted individuals.
Results/Conclusions June of 2021, when most of our planting occurred, was the hottest and driest June in the 69-year record for our study region, and therefore 17% of plants failed to survive transplanting. As expected, survival was lowest in the warmest garden and highest in the coolest garden. However, contrary to predictions, local plants had slightly higher survival than foreign plants in all gardens. The remainder of the summer was the wettest on record and very cool. Hence, the plants that survived performed well in all gardens and we saw no differences in percent cover among our experimental treatments in August or October. Our initial results fail to support the hypothesis that mixing multiple populations on a species warm range margin can help decrease climate change vulnerability. However, we suspect the benign late-summer conditions at all gardens played a strong role. We will continue to monitor the populations for multiple years and present new data from the spring of 2022. Overall, our project is contributing to new theory about the vulnerability of warm-margin populations to climate change, while also providing a rare experimental test of a commonly recommended climate change adaptation strategy that will improve biodiversity management.