Range-wide patterns of genetic population structure and potential geographical range shifts of Pinus contorta (ssp. latifolia, murrayana, contorta, and bolanderi)

Background/Question/Methods

As climatic niches of tree species shift in latitudinal and elevational distribution, these long-lived, sessile organisms will be forced to acclimate to novel conditions or migrate to track niche requirements. An understanding of genetic diversity and potential range shifts will be critical to assessing the resiliency of such species and essential to forest ecosystem conservation. Pinus contorta, one of the most widely distributed tree species in North America, plays a central role in the structure and function of montane forests from Baja California to the Yukon Territory. Its broad distribution includes four geographically and morphologically distinct subspecies (murrayana, latifolia, contorta, and bolanderi), with each inhabiting a discrete portion of the species’ range and growing under a unique set of environmental and climatic conditions. Consequently, the species as a whole may not concordantly respond to climatic changes, and conservation may require subspecies-level action. To quantify within- and among-population genetic structure, samples were collected range-wide at 50 sites, and genetic diversity was assessed using 16 microsatellite loci. To evaluate vulnerability to climate change and identify potential locations for a range shift, species- and subspecies-level distributional changes were modeled in MaxEnt, utilizing comprehensive occurrence records and WorldClim bioclimatic variables from future climate scenarios.

Results/Conclusions

Results to date indicate that genetic differentiation among populations is low, but analysis indicates some evidence of isolation by distance. Low population structure at neutral markers indicates a potential for high levels of gene flow between subspecies. Modeling predicts a shift in suitable habitat for Pinus contorta, with the optimal range of the species shifting higher in both latitude and elevation. Overall, however, suitable habitat is maintained and potentially expanded into central and northern Alberta. Additionally, a minimum of novel habitat is detected from this analysis, with these unsuitable conditions restricted to southern California and the southwestern United States. At the subspecies level, ssp. murrayana and ssp. latifolia are predicted to experience range loss and expansion of their current ranges both north in latitude and higher in elevation, while ssp. contorta is predicted to expand its range along the coast. Genetic analysis and modeling results from this study suggest that Pinus contorta has the potential to withstand predicted climatic changes, maintaining its extensive range of occurrence and breadth of habitat types, but that the distribution and relative abundances of morphologically distinct subspecies may shift within the species range.