Looking back to see ahead: Considering genetic divergence within tree species to anticipate responses to climate change

Background/Question/Methods

Tree species are not genetically uniform across their ranges. Geological, climatological and ecological processes partially or entirely isolate evolutionary lineages within species. These lineages may develop adaptations to different local environmental conditions, and may eventually evolve into distinct forms or species. Isolation also can reduce adaptive genetic variation within populations of a species, potentially compromising their ability to respond to climate change. Dramatic climate changes during the Pleistocene, for example, caused species ranges to contract and fragment into isolated glacial refugia before expanding and reconnecting. The genetic signals of these processes remain in several species, and are useful in guiding gene conservation strategies. Often, the current “trailing edges” of species ranges, which tend to be nearer past glacial refugia, contain considerable genetic variation and represent high levels of within-species differentiation. Such within-species evolutionary differences should be considered when predicting species responses to climate change. We do so for two widespread North American forest tree species, ponderosa pine (Pinus ponderosa) and eastern hemlock (Tsuga canadensis), applying results from range-wide molecular marker assessments and an innovative climate change modeling method. Specifically, we used the multivariate spatio-temporal clustering (MSTC) technique to predict how within-species evolutionary lineages might respond to climate change. Using the same tools, we also assessed whether populations possessing rare alleles or diminished genetic variation might experience the greatest climate change pressure. 

Results/Conclusions

In ponderosa pine, we detected and mapped 10 mitochondrial (mtDNA) haplotypes from 3,100 trees across 104 populations.  Each is an evolutionarily distinct unit that may be evolving separately and responding differently to climate change. Our environmental suitability maps, in fact, predict different climate responses for the Rocky Mountain and Pacific haplotypes of ponderosa pine. Meanwhile, our analyses of 13 microsatellite molecular markers in eastern hemlock, from 1,180 trees across 60 populations, suggest that the species consists of three or four evolutionary lineages descending from separate Pleistocene glacial refugia. These lineages face differing degrees of risk from climate change. Additionally, peripheral disjunct populations are significantly less genetically diverse than main-range populations in eastern hemlock, but some are highly genetically differentiated or contain unique alleles. Under climate change, these disjunct populations are among the farthest from projected future acceptable environmental conditions. This synthesis of phylogeography, population genetics and risk analysis should assist management and conservation planning for these widespread and ecologically important forest tree species in the face of climate change.