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Assessing the threat of mountain pine beetle outbreaks to whitebark pine in British Columbia
Campbell, Elizabeth M.
Background/Rationale - In the United States, a large proportion of mature whitebark pine (Pinus albicaulis) were killed by mountain pine beetle during the 1940s and the 1980s when mountain pine beetle (Dendroctonous ponderosae) epidemics spread from low elevation lodgepole pine (Pinus contorta) forests upward into whitebark pine forests (Ciesla & Furniss, 1975). The effects of mountain pine beetle outbreaks on whitebark pine forests in British Columbia are poorly understood, but the potential for a major, negative impact is clear. Considering the extent of the current outbreak in low elevation lodgepole pine forests, we can expect that a massive decline of whitebark pine in British Columbia is imminent. Although mountain pine beetle outbreaks are natural disturbances in pine forests of western North America, the cumulative effects of a number of anthropogenic factors are putting whitebark pine at greater risk from beetles than ever before. Warming climates are expanding the geographic range of mountain pine beetle in British Columbia (Carroll et al., 2004), and probably creating more suitable habitat for the beetle at higher elevations (Amman, 1973). In addition, many whitebark pine forests throughout British Columbia are heavily infected by the exotic fungus white pine blister rust (Cronartium ribicola) (Campbell & Antos, 2000), which makes them more susceptible to attack by mountain pine beetle. Moreover, fire suppression activities over the last century have contributed to an increase in the extent of susceptible lodgepole pine over landscapes in BC, creating more 'sources' of beetles at low elevations (Taylor and Carroll, 2004). Considered a 'keystone' species, whitebark pine is a functionally important component of many high-elevation ecosystems in western North America and the consequences of its decline are numerous. In upper subalpine ecosystems, where harsh conditions limit the growth of other tree species, whitebark pine strongly influences patterns of snow accumulation and snowmelt, and its continued decline may alter watershed hydrology affecting slope stability and the timing, levels, and quality of stream flow. In these same harsh environments, whitebark pine also moderates microenvironments and facilitates the recruitment and growth of other plants. Whitebark pine is inexorably linked to Clark?s nutcracker (Nucifraga columbiana), which has evolved to exploit the highly nutritious seed of whitebark pine (Lanner, 1996). The pine is, in turn, entirely dependant on the nutcracker for seed dispersal and subsequent recruitment. The seed is also an important source of dietary fat for other animals such as red squirrels (Tamiasciurus hudsonicus) and grizzly bears (Ursus arctos L.), which have increased reproductive success in years of abundant seed production (Mattson & Jonkel, 1990). Any disruption of the symbiotic relationship between the bird and whitebark pine will have cascading effects, impacting many basic ecosystems processes and compromising the biological diversity of these communities (Tomback & Kendall, 2001). Expanding outbreaks of mountain pine beetle, due to warming climate conditions, or other factors, like more abundant susceptible lodgepole pine in landscapes, pose just this sort of threat. Indeed, it has been hypothesized that the current distribution of whitebark pine resulted from the inability of this species to ward off attacks by insects and pathogens that occur on pines in more climatically benign environments (Logan & Powell, 2001). The current beetle outbreak in British Columbia started in about 1992 and provincial outbreak simulations suggest beetles will reach many whitebark pine stands over the next few years (unpublished data; M. Eng, BC MoF, and E. Campbell, PFC, CFS). During a reconnaissance field tour of E. Campbell?s research plots in southern BC in 2005, we found active beetle infestations on whitebark pine and many trees that were dead or dying due to the beetle. Given that mount ...
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