Local adaptations driven by genetic selection over time make it possible for tree species to inhabit broader climate envelopes and to adapt to climate change. In our rapidly changing climate, however, the slow process of natural selection may not keep pace. This could create a mismatch between the near-future climate and local adaptations in today’s populations, contributing to local extinctions or range contractions in ecologically and economically important tree species. A natural solution to this problem would be for genes adapted to the near-future climate to be introduced into the local population through immigration from other populations. For this reason, understanding how the landscape and climate shapes the movement of genes through populations (a process called gene flow) will help guide management and conservation of tree species.

Andrew Shirk, a research scientist at the Climate Impacts Group, is collaborating with the U.S. Forest Service and other universities (Oregon State University, Virginia Commonwealth University, Northern Arizona University, University of Montana) to identify genetic variation that is linked to climate conditions (temperature and precipitation) in the genomes of three tree species: the southwestern white pine (Pinus strobiformus), Douglas-fir (Pseudotsuga menziesii), and fremont cottonwood (Populus fremontii). For the southwestern pine, Shirk and his collaborators will also be modeling genetic resistance to white pine blister rust in addition to climate. White pine blister rust is a fungal pathogen that has killed more than 90% of the white pines in the Central and northern Rocky Mountains. The future distribution of southwestern white pine will be determined by a complex interaction of gene flow and blister rust resistance.

According to Shirk, one of the most innovative aspects about this project is the vast spatial and conceptual scope of this work. Spatially, the study area of this project encompasses the entire U.S. mountain west and northern Mexico. In addition, this study incorporates an elevation gradient by examining low-, mid-, and high-elevation species (fremont cottonwood, Douglas-fir, and southwestern pine, respectively). This study is unique in that it will link adaptation, climate, and gene flow, on a large regional scale. The goal of this project is to map genetic adaptations to climate in the genomes these three tree species, map the spatial range of local climate adaptations, and model the ability of these adaptations to move through the population over time in a changing climate and landscape.

Shirk and his collaborators will test and quantify these gene-climate relationships using the Southwest Experimental Garden Array (SEGA), which is a series of instrumented sites along a temperature and precipitation gradient. Different genetic variants of the three tree species will be planted along the SEGA gradient and subsequently the phenotypes (e.g. survival, growth rates, etc.) will be measured and recorded. Simultaneously, landscape genetics approaches will be used to infer how this adaptive genetic variation (contained within seeds and pollen) moves through these tree populations across their range. With an understanding of both gene flow and gene-climate relationships, the researchers can then use population genetic simulations to predict how climate-adapted genes might move through the population over time in the future, given climate change scenarios.

Ultimately, this study will help inform climate adaptation planning and conservation efforts for forests in the western U.S., including spatial assessments of which genetic variants will be suited to an area’s future climate, and whether those variants are likely to arrive naturally via gene flow or would instead require assisted migration (i.e., planting seeds or seedlings).

This project is expected to be completed by 2020.

For more information on this project please contact Andrew Shirk (ashirk@uw.edu).

For more information on research at the Climate Impacts Group, visit our website.