Looking back to see ahead: Considering genetic divergence within tree species to anticipate responses to climate change
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 may be useful in guiding gene conservation strategies. 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, one from the West (ponderosa pine [Pinus ponderosa]) and one from the East (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 within these two species might respond to climate change.
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 analysis, in fact, predicts different climate responses for the Rocky Mountain and Pacific evolutionary lineages of ponderosa pine. Specifically, the area of environmental suitability is expected to constrict much less for the Pacific (~8 percent) than the Rocky Mountain (~23 percent) haplotypes under the Hadley B1 global circulation model/scenario model for 2050. Two of the rarest haplotypes appear to be most at risk from climate change. 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, with the widespread northern lineage less at risk than the less common southern lineages. This synthesis of phylogeography, population genetics and risk assessment should assist management and conservation planning for these widespread and ecologically important forest tree species in the face of climate change.