Drivers of forest change in the Greater Yellowstone Ecosystem

Questions: Global climate change is predicted to cause widespread shifts in the distribution and composition of forests, particularly in mountain environments where climate exerts strong controls on tree community arrangement. The upslope movement of vegetation has been observed in association with warming temperatures and is especially evident in ecotones-the transition zones between vegetation types. We explored the role of drought and tree mortality on recent changes in high-elevation forests. Location: Greater Yellowstone Ecosystem, USA. Methods: We established 19 forest demography plots along an elevational gradient spanning dominant high-elevation vegetation types. Results: Tree establishment dates indicated the upslope movement of Pinus aibicaulis (whitebark pine) treeline and ecotone shift from meadow to forest starting in the 1950s. An expansion of the growing season likely contributed to the upward expansion of the treeline. Comparisons between overstory and understory tree composition suggested ongoing succession in the absence of fire at lower elevations, namely the replacement of Pinus contorta (lodgepole pine) by Abies lasiocarpa (subalpine fir). P. contorta seedlings were distributed at higher elevations than overstory trees of the same species, suggesting some potential for upslope movement with warming conditions; P. albicaulis seedlings, conversely, were distributed throughout all elevations of the transect. Significant tree mortality occurred in Pinus spp. and disproportionately affected P. albicaulis, as a result of a regional Dendroctonus ponderosae (mountain pine beetle) outbreak (2008-2012). Mortality events were strongly associated with drier than average conditions 2-3 years prior to tree death. Conclusion: Rising sensitivity to arid conditions in the mid-20th century amid already dense, aging forests appears to have increased susceptibility to beetle-induced mortality during the most recent drought. Tree species in the study area responded individually to global change stressors, which acted on these forests in complex ways and led to both ecotone shifts and stability. This work highlights the interplay between succession, forest disturbances and climate-related growth responses in driving forest compositional change in subalpine and treeline environments.