Climate-related genetic variation in a threatened tree species, Pinus albicaulis

With ongoing climate change, understanding of intraspecific adaptive variation is critical for conservation and restoration of plant species. Such information is especially scarce for threatened and endangered tree species, such as Pinus albicaulis Engelm. Therefore, our principal aims were to assess adaptive variation and characterize its relationship with climate of seed origin.

We grew seedlings from 49 P. albicaulis populations representative of the interior northwestern United States in two common garden field experiments under warm-dry conditions that mimic climatic conditions predicted in the current century for areas within the species' range. Differences among populations were assessed for growth and survival. We then used regression to describe clines of apparent adaptive variation in relation to climate variation among the populations' origins.

We detected genetic divergence for growth and survival among populations of P. albicaulis. These differences corresponded to distinct climatic clines. Populations originating from locations with lower spring precipitation exhibited greater survival in response to natural drought. Populations originating from increasingly milder climates exhibited greater height growth under relatively limited stress in early years and greater fitness after 12 yr.

The results suggest that P. albicaulis exhibits adaptive variation for drought tolerance and growth in response to selection pressures associated with variation in moisture availability and temperature, respectively. Even so, clinal variation was relatively gentle. Thus, apparent differences in local adaptation to climate among populations appears to be relatively low.