Changes in alpine vegetation over 21 years: Are patterns across a heterogeneous landscape consistent with predictions?

One significant unanswered question about biotic responses to climate change is how plant communities within topographically complex landscapes will respond to climate change. Alpine plant communities are strongly influenced by topographic microclimates which can either buffer or compound the effects of more regional climatic changes. Here, we analyzed species changes over 20_ years in a complex alpine landscape with pronounced gradients in microtopography and consequently large variation in temperatures, snow depths, and nitrogen availability across small (10 m) scales. Using data from long-term monitoring plots from six community types, we asked how species composition and functional diversity changed over time in these different areas of the landscape, and whether fine-scale heterogeneity allowed species to move in response to temporal changes in the environment. We found sitewide patterns of increasing species and functional diversity. However, the majority of variability in composition over time was non-directional, both within and between community types. Within community types, Carex-dominated snow banks and wet meadow communities were the most variable in composition over time, while Sibbaldia-dominated snow banks, fellfield, dry meadow and moist meadow exhibited moderate change. Over forty percent of the plots also transitioned between community types during the census intervals, but these also were largely transient, with a shift occurring in one time interval and then shifting back in the next interval. Thus, even with evidence of directional change over time in climate, N deposition, and release from grazing, vegetation is tracking finer-scale variability both in time and space. Environmental heterogeneity may allow vegetation to track this finer-scale variability and enhance resilience to underlying directional changes in alpine and other topographically-complex environments.