Whitebark pine (Pinus albicaulis) grows in tree line habitats throughout the American West, where freezing temperatures, snow, and strong winds can occur any day. Such harsh conditions exclude most other tree species. Tight groups of wind-sculpted and gnarled whitebark pine can form in these extreme sites. These distinct krummholz formations disperse wind and aid in snowpack retention. Subalpine areas occupied by whitebark pine provide inspiring views for national park visitors.
Whitebark pine is threatened by an invasive, nonnative pathogen, white pine blister rust (Cronartium ribicola) (MacDonald and Hoff 2001), a fungus that forms lesions, or cankers, of necrotic tissue that girdle tree boles or stems (fig. 1). To complete its life cycle, the fungus must disperse from the pines to an alternative host, a shrub in the genus Ribes (currant and gooseberry) or the herbs Castilleja (Indian paintbrush) and Pedicularis (lousewort) (Geils et al. 2010).
Plump, fleshy, and nutritious whitebark pine seeds are eaten and dispersed by many animals. In particular, Clark’s nutcracker (Nucifraga columbiana) has developed a mutualistic relationship with the pine (Tomback 2001). This boisterous, robin-sized bird with flashy black-and-white wings and tail has a specialized pouch under its tongue for storing seeds, which it caches widely in the soil. Seeds that are not eaten by the nutcracker maintain populations of the pine. McKinney et al. (2009) found that nutcrackers decrease in whitebark stands as tree mortality increases.
In a study of historical data from sites in and around Crater Lake National Park, Daly et al. (2009) noted significant increases in average and winter temperatures, but no change in precipitation over the last 100 years. The implications of recent climate change for the whitebark pine ecosystem are complex and likely to operate through direct and indirect mechanisms. The most significant, observable effect of climate change is indirect: warmer temperatures in recent years have allowed mountain pine beetles (Dendroctonus ponderosae) to shift upward and persist in higher-elevation forests (Logan at al. 2010). Murray (2010) reported that mountain pine beetle is now the primary cause of whitebark pine mortality at Crater Lake, presently at a rate of 1% annually from all causes. Other indirect, pathogen-related effects could occur if climate increasingly favors blister rust. For example, the rust favors moister conditions, and increased precipitation in winter is an expected trend under climate change in the Pacific Northwest. Conversely, summers may be drier and inhibit the formation and spread of rust spores and fruiting body development. Daly et al. (2009) noted an increasing tendency for the summer dry season to extend into early fall at Crater Lake and elsewhere in the Oregon Cascades over the last few decades. Direct effects of longer, high-elevation growing seasons because of climate change could favor the pines, as many high-elevation trees are growing more rapidly today (Bunn et al. 2005). However, direct effects of longer growing seasons could increase competition among pines and other trees that can grow faster than the notoriously slow-growing whitebark pines. This could act as an indirect effect that places the pine at a competitive disadvantage, especially if whitebark pine populations cannot migrate quickly enough to competition-free environments like those in which they presently live.
Whitebark pine and its associated community of species are among the vegetation resources identified for long-term, focused monitoring by the Klamath Inventory and Monitoring Network of the National Park Service. This monitoring is designed to complement park monitoring of whitebark pines at Crater Lake National Park (Murray 2010). The network’s monitoring is also specifically designed to help identify factors related to whitebark pine infection and mortality from disease and beetles, and to track changes in all high-elevation vegetation structure, function, and composition related to climate and other factors. By tracking disease progression and beetle impacts, as well as vegetation change, we can link observed changes to direct and indirect climate-mediated mechanisms and determine the effects of climate change and other factors on whitebark pine. The network has completed its first year of monitoring, sampling 20 plots at Crater Lake National Park. We briefly highlight our initial findings here.