Introduction

Climate change and associated glacial/interglacial cycles have had a profound impact on the biogeography of plant communities. Paleoecological data from pollen sediment cores and packrat middens have been used to uncover many of the past plant-climate associations, (e.g., Thompson and Anderson 2000, Thompson and others 1993, Whitlock 1993). These climatic oscillations have also affected intra-species genetic relationships that are recorded in changes in gene frequencies or adaptive traits, or both (Davis and Shaw 2001). In Western North America, most forest trees have broad, disjunct distributions that are associated with variable environmental conditions. The combination of disjunct intra-species distributions and environmental variables can contribute to barriers in gene flow and changes in adaptive or neutral gene frequencies, or both, caused by selection or genetic drift (Richardson and others 2005). Population genetics typically assess neutral genetic variation and provide estimates of genetic diversity and partitioning among a spatial hierarchy. Population genetic studies can estimate both contemporary and historic gene flow, and thereby aid in elucidating past intraspecies relationships and distributions. Such genetic data, coupled with paleoecological data, provide inferences to historical changes in plant biogeography and postglacial expansion/contraction, (e.g., Godbout and others 2005, Jackson 2006, Magri and others 2006, Petit and others 2004, Richardson and others 2002, Thompson and Anderson 2000). Knowledge of the population genetic structure and adaptation is critical for successful forest health management and conservation/restoration efforts.

Five-needled pines are keystone species in numerous ecosystems in Western North America. This paper focuses on two case studies: whitebark pine (Pinus albicaulis) and western white pine (P. monticola). Whitebark pine occupies subalpine habitats across the northern Rocky, Cascade, and Sierra Nevada Mountains and mountains of northern Nevada. Seed dispersal is dependent on a coevolved mutualism with Clark’s nutcracker (Nucifraga columbiana) that deposits seeds in caches for later consumption (Tomback 2001). Flight distances with seeds vary but have been reported as far as 22 km (Richardson and others 2002, Vander Wall and Balda 1981). Whitebark pine has been recognized as critical for wildlife (Mattson and others 2001) and watershed stability (Arno and Hoff 1989).

Western white pine occupies a wider ecological niche than whitebark pine, occurring from sea level in Washington and British Columbia to a predominately subalpine distribution from Sierra Nevada northward to the Cascades. In the northern Rocky Mountains, this species occupies wet montane to subalpine habitats (Wellner 1962). It is valued as a timber species because of its rapid growth, straight bole, and ability to regenerate as a seral species from wind-dispersed seeds. Since the early 1900s, the abundance of both five-needled pines has dropped precipitously, mainly from white pine blister rust caused by the fungus Cronartium ribicola (McDonald and Hoff 2001). Major efforts and resources have been directed toward restoration of five-needled pine ecosystems. Unfortunately, most restoration efforts have been operating on the assumption that the climate will remain stable. For successful management and restoration of five-needled pine ecosystems, future climate change must be considered and integrated into current efforts and plans for species management/conservation.

Research corroborated by diverse sciences provides strong evidence that the earth is experiencing a warming process driven by increased greenhouse gas concentrations. Records indicate that mean global temperature increased 0.6 ºC during the 20th century, and global temperature is expected to further increase 1.4 to 5.8 ºC over the next 100 years (IPCC 2001). This predicted rate of climate change is unprecedented within the available historical records. Because of the extremely fast rate of predicted climate change, determining the responses of forest tree species and populations is one of the major challenges for future threat assessment (Jump and Peñuelas 2005). Recent studies have developed plant-climate models that are highly accurate in predicting a suitable contemporary climate envelope for several Western North American species (Rehfeldt and others 2006). These models serve as means to project future climate scenarios using general circulation models (GCMs).

It is well known that historic climatic oscillations have dramatically shifted species distributions. However, these historic climate changes typically occurred at a relatively slow rate over the course of millennia. The predicted rate of climate change, due to the anthropogenic release of greenhouse gases, will greatly exceed historic rates (IPCC 2001). Studies based on several climate models have shown the predicted rate of climate warming could jeopardize plant taxa with limited seed dispersal distances or disjunct habitats (Malcolm and others 2002). In this analysis, we develop an approach to evaluate potential impacts of future climate change on genetic diversity and biogeography of whitebark pine and western white pine. Our objectives include:

1. Assess the existing genetic diversity and population structure of whitebark pine and western white pine,
2. model each species climate space, (i.e., suitable habitat) for contemporary and year 2030 climate, and
3. use the predicted biogeographical changes to prioritize genetic conservation efforts by identifying populations at risk and determining areas suitable for restoration.