Laura Koteen

The subalpine tree species whitebark pine (Pinus albicaulis) faces multiple threats to its existence. Predominant among them is the risk posed by the exotic fungus white pine blister rust, Cronartium ribicola . Throughout much of its range (Fig. 8.1), whitebark pine has suffered considerable losses; approaching 90% in some locations. Blister rust, mountain pine beetle (Dendroctonus ponderosae) and successional replacement by subalpine cohorts due to fire exclusion account for the bulk of its decline.The seeds of the white-bark pine tree are of primary importance as a food source for wildlife, most notably the threatened grizzly bear (Ursus arctos horri-bilis). In the Greater Yellowstone Ecosystem (GYE), where the grizzly's range and that of whitebark pine coincide, many biologists have concluded that the fate of the bear is linked to the survival of the tree.

According to the most recent survey conducted in 1995, blister rust infection levels in the GYE are still relatively low. Estimates suggest that 5% or less of the trees are infected, with another 7% standing dead trees due to all causes (Kendall 1996b). Biologists previously assumed that the northwestern Wyoming climate was generally unfavorable for the spread of the fungus and would thus prevent whitebark population collapse (Carlson 1978, Arno 1986). The first aim of this study was to investigate this assumption and to describe the current profile of conditions for blister rust spread that exist in the GYE, with specific reference to the region's climate. Toward this end, I analyzed available weather station data, distributed across the landscape and spanning the last 50 years, to assess the frequency of occurrence of the climatic conditions likely to produce pine infection. Secondly, this chapter discusses the prospects for future blister rust spread through a sensitivity analysis of altered climatic conditions representing predicted consequences of global climate change. Also considered here are alternative means by which whitebark pine population contraction could result from climate change, as well as the impacts of the tree's loss on grizzly bears and other wildlife.

Climate change is generally expected to hasten the recession of whitebark pine communities in the region with three separate mechanisms contributing to its decline. The first mechanism concerns the shifting of pathogen ranges into new regions or into locations that are now only marginally suitable climatically for infestation. This topic will be further elaborated and explored as an important mechanism of species loss associated with climate change. In the case of whitebark pine, climate changes that tend to increase summer moisture availability would serve to accelerate the rate of blister rust spread. Secondly, warming, associated with global climate change, may also limit the range of whitebark pine. Because white-bark pine is found among high-elevation subalpine communities, warming may lead to its competitive replacement by lodgepole pine at lower, warmer elevations (Mattson and Reinhart 1989).Through initiating upward and latitudinal migration of subalpine communities, and replacement by species better adapted to the new set of climatic conditions, the tree's range may, over time, shift up and off the region's mountain peaks (Romme and Turner 1991).Thirdly, drier conditions are predicted to accompany a warmer climate and to increase the frequency of stand-replacing wild fires. Although P. albi-caulis is adapted to the effects of small fires, vast conflagrations, such as those that burned through Yellowstone National Park in the summer of 1988, reducing whitebark pine populations by 26% (Tomback 1995, P. Farnes, pers. comm. 1999) increase the tree's overall vulnerability.

Plnus albicaulis

Plnus albicaulis

Figure 8.1. The range of whitebark pine in North America. The Greater Yellowstone Ecosystem is found at the southeasternmost portion of whitebark pine's range in Wyoming, southern Montana, and southeastern Idaho. (Diagram from Arno and Hoff 1989.)
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