Abstract
In the northern Rocky Mountains of the U.S. and Canada, whitebark pine (Pinus albicaulis Engelm.) is a functionally important species in treeline communities. The introduced fungal pathogen Cronartium ribicola, which causes white pine blister rust, has led to extensive whitebark pine mortality nearly rangewide. We examined four treeline communities within the Greater Yellowstone Ecosystem (GYE) to assess structure and composition, whitebark pine prevalence and functional role, differences in growing season mesoclimate among study areas, and blister rust infection incidence. We found that (1) whitebark pine frequently serves as the majority overall, solitary, and leeward tree island conifer; (2) the prevalence of different tree species in the windward position in tree islands, and thus their potential as tree island initiators, may be predicted from their relative abundance as solitary trees; and (3) white pine blister rust infection incidence ranged from 0.6% to 18.0% across study areas. White pine blister rust poses a threat to treeline development and structure and the provision of ecosystem services in the GYE. Increasing blister rust resistance in nearby subalpine whitebark pine communities through seedling planting or direct seeding projects should eventually result in higher levels of blister rust resistance in whitebark pine in treeline communities.
Highlights
Conifer communities in temperate zone alpine-treeline ecotones—the transition between open subalpine forest and alpine tundra, hereafter, “treeline” [1]—provide important ecosystem services such as snow retention, soil stabilization [2,3], and wildlife habitat (e.g., [4])
We found that the mean white pine blister rust infection incidence was highest for the two study areas west of the Continental Divide: 18.0% at Paintbrush Divide and 14.4%
We found that whitebark pine is locally abundant, regionally common, and frequently serves as the majority conifer in the Greater Yellowstone Ecosystem (GYE) treeline communities we sampled
Summary
Conifer communities in temperate zone alpine-treeline ecotones—the transition between open subalpine forest and alpine tundra, hereafter, “treeline” [1]—provide important ecosystem services such as snow retention, soil stabilization [2,3], and wildlife habitat (e.g., [4]). Community structure and composition at treeline have been assessed in many locations, and differences exist within and across regions, mainly in response to topographic and climatic variation, and in response to subalpine species distribution, seed sources, site suitability, and snow deposition (e.g., [11,12,13,14]). (“nurse”) object shapes plant community spatial structure in stressful environments [17,18,19]. Solitary trees shapes plant community spatial structure in stressful environments [17,18,19]. Solitary trees frequently establish establish leeward leeward of of protective protective objects, objects, such such as as rocks frequently rocks or or plants, plants, or or in in micro-topographic micro-topographic depressions (small hollows), which facilitate seed and seedling survival (Figure depressions (small hollows), which facilitate seed and seedling survival (Figure 1)
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