This research will examine how climate drives patterns of genetic variation in cottonwoods and how this variation affects organisms dependent on cottonwood. Using replicated common gardens from Arizona to Alberta, genetically based variation in growth, survival, and timing of growth initiation and cessation (key adaptive traits for forest trees) among tree populations from a latitudinal gradient will be measured. With the replicated gardens spanning a broad climatic range, investigations of how temperature and precipitation interact with genetic variation to influence tree traits and to determine if trees are adapted to their local environments will be conducted. Using molecular tools, climate's effect on specific genes that influence the timing of growth initiation and cessation will be quantified. These traits can also strongly influence dependent insect herbivores. Therefore, surveys of insects feeding on the cottonwood trees, determining how genetic variation in the trees affects dependent organisms will also be conducted.
This research will have broader significance in five main ways. First, if tree populations are currently locally adapted to their environmental conditions, future climate change could render populations locally maladapted. Thus, understanding patterns of adaptation and genetic variation in trees is needed to understand the potential impact climate change will have on natural systems. This information will aid resource managers when undertaking restoration projects, common in riparian areas throughout the country. Second, understanding how genetic variation influences interactions among species is a frontier of ecology and evolutionary biology. This work will extend that frontier by identifying genes underlying traits that have consequences for biotic communities. Third, graduate student's research will promote science education by training an undergraduate assistant in field and laboratory techniques. Fourth, this study will establish a long-term international collaboration with researchers in Canada and the United States. Finally, this grant will initiate a long-term field experiment, necessary to fully understand effects of climate change on biotic communities.
Understanding how climate shapes forest tree genetic variation in growth, and its effects on dependent biotic communities. Understanding the causes and consequences of genetic variation is a fundamental goal of population genetics and evolutionary biology. Genetic variation in plants can have effects ranging from adaptation to local environments to impacts on entire communities of dependent organisms. With such wide-ranging possible impacts, it is crucial to identify what factors, historical and contemporary, affect plant genetic variation as well as how that variation may influence patterns of adaptation and distribution in both the plant and dependent species. Temperate forest trees often span large geographic ranges and associated climatic variation. Climate can influence both demographic history (when populations diverged, migration among populations, and population size) and adaptation (through divergent natural selection) in forest trees. By combining studies of plant variation in traits and DNA sequence variation, we can identify how climate has impacted both demographic history and natural selection on particular genes, and determine if these factors influence dependent organisms such as arthropods that use forest trees as habitat. We initiated a long-term replicated common garden experiment using narrowleaf cottonwood (Populus angustifolia) to examine the influence of climate on tree genetic variation. The distribution of narrowleaf cottonwood, stretching from Arizona to southern Alberta, spans a large range of climatic variation. At three sites (Arizona, Utah, and Alberta) we planted over 700 trees from range-wide samples spanning ~2500 km of latitude (Figure 1). At these sites, we measured the date trees initiated growth in the spring (bud flush) and the date trees ceased growth in the fall (bud set), two key traits that delimit the growing season and determine susceptibility to frost events. We found strong evidence that these traits are under climate-driven selection in narrowleaf cottonwood. Trees from southern locations set bud later in the fall than trees from northern locations, allowing them to grow taller. However, these southern trees were also more susceptible to frost damage than trees from northern locations, causing die-back (Figure 2). At the DNA sequence level, we found strong latitudinal patterns of variation within genes that influence these same traits, evidence of natural selection at the molecular level. Thus, we found strong signals of climate-driven selection in both tree phenotypic traits and DNA sequences in narrowleaf cottonwood. These same tree traits (height, bud flush date, and bud set date) have the potential to influence species that use the trees as habitat. Narrowleaf cottonwoods host a diverse community of dependent arthropod species. We performed surveys of the trees and found that arthropods were more abundant and more diverse on taller trees. Because tree height is related to bud flush date and in particular bud set date, climate-driven selection has indirectly influenced entire communities of arthropods through its effects on key adaptive tree traits. These studies have shown that climate is a fundamental agent of selection in narrowleaf cottonwood, and structures key adaptive traits, with impacts on dependent communities. These results have important implications in determining the best strategies for conservation and mitigation in the face of climate change, as well as choosing appropriate sources for genetic improvement programs. As climate continues to change, land managers can maximize their restoration efforts by taking climate change, genetic variation, and tree growth into account when developing strategies to preserve biodiversity and wood production. Furthermore, finding that selection on a trait has community-level consequences places community ecology more firmly in an evolutionary context, and highlights the importance of considering evolutionary perspectives
