| Summary: | Mountains have been warming faster than lower elevation ecosystems, and because of tight coupling between organisms and a compressed growing season, the impacts of change may be more pronounced in high elevation systems. Further, in the climatically extreme alpine environment, biotic interactions between neighboring species may be important to alpine species persistence or colonization by lower elevation species. For species whose upper distributional range is within or near the alpine-treeline ecotone, climate change will likely relieve cold temperature limitations to higher elevation establishment. Taken together, climate change is likely to impact alpine plant phenology, species interactions, and may cause species range shifts. However because many alpine plants are long-lived, they may persist in the midst of change resulting in disequilibrium with climate. In the first chapter, I examine the effects of experimental warming and watering on alpine plant phenology and evaluate the mechanisms driving change. I ask does warming act directly through temperature or indirectly through snowmelt or drier soils to influence community flowering? I found that earlier snowmelt, not warmer temperature, drives advances in alpine plant community flowering. Because of strong synchrony of alpine phenology to a short growing season, community level flowering duration was conserved. Early flowering species with strong coupling to snowmelt timing responded most strongly along with forbs and graminoids, while longer lived cushion plants and succulents were more resistant to change and did not take advantage of a prolonged growing season. My second chapter examines the role of species interactions between native alpine vegetation and subalpine conifers, which have the potential to migrate into the alpine ecosystem. Consistent with the stress gradient hypothesis, which would predict greater benefits from neighbors at higher elevations, a shade and moisture tolerant conifer requires neighbors to establish in the alpine, while a sun and drought tolerant conifer Is equally likely to establish aside neighbors or in vegetation gaps. Contrary to the stress gradient hypothesis however, a native alpine herb benefits from the presence of neighbors even at the low elevation end of an environmental stress gradient. In the final chapter, I use a decade long observational dataset from four mountain summits, at four elevations, as part of the Global Observation Research Initiative in Alpine Environments. Over a longer, 40-year time period, maximum and minimum temperatures have risen, while snowmelt date advanced at a nearby weather station. On the summits, community-wide vegetation cover decreased while richness increased over the decade of observations. Long-lived alpine plants were generally slow to respond, but there is some evidence for colonization of the lowest elevation, the most rugged, and the highest elevation summit. Long-lived alpine species may be able to resist change resulting in disequilibrium with climate but continued rising temperature and decreased snow duration will likely have an impact on future composition, performance and persistence of plant species in alpine tundra communities.
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