Assisted Migration Experiments along a Distance/Elevation Gradient Show Limits to Supporting Home Site Communities
Art Keith1, 2, Joseph Bailey3, Thomas Whitham1, 2*
1Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011, U.S.A.
2Center for Adaptable Western Landscapes, Northern Arizona University, Flagstaff, AZ 86011, U.S.A.
3Department of Ecology and Evolutionary Biology, The University of Tennessee, Knoxville TN 37996, U.S.A.
We addressed the hypothesis that intraspecific genetic variation in the plant traits of a riparian foundation species along a distance/elevation gradient would influence the communities they support in a common garden. Answers to this hypothesis are especially important when considering the community consequences of plant assisted migration along distance/elevation and latitude gradients; in other words, if you build it will they come? We surveyed the arthropod communities occurring on the foundation riparian tree species Populus angustifolia along a distance/elevation gradient (90km distance and 530m elevation) and in a common garden where trees from along the gradient were planted 20-22 years prior to our study. Three major patterns were found: 1) In the wild, arthropod community composition changed significantly along the gradient. Trees at the lower elevation site supported up to 58% greater arthropod abundance and 26% greater species richness than more distant, high elevation trees. 2) Trees grown in a common garden sourced from the same locations along the gradient, support arthropod communities more similar to their corresponding wild trees, but the similarity declined with transfer distance and elevation. 3) Of five functional traits examined, leaf area, a trait under genetic control that decreases at higher elevations, is correlated with differences in arthropod species richness and abundance. Over this gradient of our assisted migration studies, our results argue that genetic differences in functional traits are stronger drivers of arthropod community composition than phenotypic plasticity of plant traits due to environmental factors. We also show that variation in a plant trait (leaf area) is maintained and has similar effects at the community level while controlling for environment. These results are an important demonstration of how genetically based traits vary across natural gradients and have community level effects that are maintained, in part, when they are used in assisted migration. Furthermore, optimal transfer distances for plants suffering from climate change may not be the same as optimal transfer distances for the associated communities they support.