Whitebark pine at treeline in the Greater Yellowstone Ecosystem: Prevalence, facilitation, and biophysical characteristics of leeward microsites

Wagner, Aaron

Facilitative interactions are particularly important in the climatically stressful alpine treeline ecotone (ATE), the transition zone from closed canopy forest to alpine tundra. Within some ATEs on the harsh Eastern slope in the Rocky Mountains, whitebark pine (Pinus albicaulis)—a foundation and keystone species—plays a central role in tree island development by facilitating the survival and growth of leeward conifers more frequently than other species. However, the structure, composition, and dynamics of ATE formation are not well known for some geographic regions of western North America, including the Greater Yellowstone Ecosystem (GYE). Although facilitative interactions are considered important to treeline community development, comparisons of biophysical attributes among the leeward microsites of nurse objects and trees are also lacking. We conducted an exploratory study of four treeline communities in the GYE to investigate (1) the prevalence of whitebark pine, (2) the proportional occurrence of whitebark pine as a tree island initiator, (3) whether solitary whitebark pine abundance predicts its prevalence as a tree island initiator, (4) whether whitebark pine better ameliorates local biophysical conditions relative to other plants or objects, (5) the relationship between common nurse objects and solitary/tree island initiator establishment, and (6) the presence and severity of Cronartium ribicola, an invasive pathogen that attacks and kills whitebark pine, in these communities. We found that whitebark pine is the most prevalent solitary conifer and tree island component, and initiates tree island formation in direct proportion to its abundance as a solitary tree. We also found that whitebark pine leeward microsites did not consistently experience the most moderate microclimate compared to spruce, fir, rock and unprotected leeward microsites; differences in microclimatic conditions by microsite type depended substantially on the general climatic conditions observed at each study area. Among our study areas, C. ribicola infection rates ranged from less than 1% to 18%. Our findings suggest that whitebark pine is an important treeline species in the Greater Yellowstone Ecosystem. Losses of whitebark pine from infection by C. ribicola will lead to proportional changes in the composition and structure of ATE communities in the GYE, including decline in tree island formation on the landscape.