Invasive pathogen alters facilitation interactions at treeline: Potential implications for response to climate change
In some Rocky Mountain treeline ecotones, whitebark pine (Pinus albicaulis) serves as tree island initiator more frequently than Engelmann spruce (Picea engelmannii) or subalpine fir (Abies lasiocarpa). Whitebark pine mortality from white pine blister rust (pathogen = Cronartium ribicola) reduces opportunities for facilitation interactions. Will declines in whitebark pine alter treeline response to climate warming? We have surveyed various treeline communities from the Greater Yellowstone Area to whitebark pine’s northern limit. From 2010 to 2013 at Line Creek, MT, and Divide Mountain, MT, we examined facilitation mechanisms by simulating whitebark pine death from blister rust, measuring tree shoot lengths, and evaluating performance of common treeline leeward microsites with respect to seed germination and seedling survival and 11 biophysical variables. In both study areas, plus Parker Ridge, AB, we sampled 80 randomized 15 m x 15 m vegetation plots to characterize community composition and structure with respect to local climate, hydrology, digital elevation models, and incidence of blister rust. Using predictive modeling, we examined possible climate and topographic correlates of whitebark as tree island initiator both within and across study areas, and used agent-based modeling (ABM) to examine response to climate change in the presence of blister rust.
At Line Creek and Divide Mountain, blister rust incidence was 19.2% and 23.6%, respectively. Among solitary conifers, whitebark pine was most abundant and had the greatest vigor and longest shoot lengths. Death of windward whitebark pine reduced shoot length and vigor in leeward trees, demonstrating facilitation. In planting experiments, new seedlings had higher survival in whitebark microsites. Microsites leeward of whitebark pine and spruce, relative to rocks or exposed sites, had lower PAR, sky exposure, and wind speeds, and more moderate soil and air temperatures. Thus, whitebark pine prevalence as a tree island initiator results from its hardiness, relative abundance, and the protective qualities of its leeward microsites. Modeling indicates that growing season temperature and slope curvature may predict the incidence of whitebark pine facilitation at local scales. In certain treeline ecotones, whitebark pine is abundant but facilitation is less prevalent, likely because of more moderate climate. ABM combining climate warming and blister rust indicated that the latter slowed the overall advance of trees while slightly favoring spruce and fir in relative abundance. Our studies suggest that whitebark pine tolerates harsh treeline climate better than associated conifers, and losses to blister rust will affect treeline response to climate warming.