Shrub expansion and alpine plant community change: 40-year record from Niwot Ridge, Colorado

Background: Alpine and arctic tundra are particularly sensitive ecosystems to the impacts of global climate change. Yet, warming studies versus observational studies in these ecosystems yield different and sometimes conflicting results. Many short-term changes may be phenotypic responses to warming rather than shifts at the community level. Aims: We posit that long-term monitoring of permanently marked plots provides data for making predictions of the impact of climate change on alpine tundra communities. Methods: We surveyed plant community composition and cover at four sampling time points over a forty-year span (1971-2011). Results: We observed an expansion of shrub cover, both within the shrub tundra and encroaching into moist and dry meadow communities. This shrub cover expansion coincides with increased litter and decreased species richness at the plot level. Overall, despite some shifts in functional group cover and species richness, plant community composition remained mostly intact across forty years. Conclusions: Our results corroborate with other published studies demonstrating the rapid expansion and impacts of woody shrubs in tundra ecosystems. This study demonstrates a surprising resistance of alpine tundra plant communities under current global climate change over a 40-year period. Additional studies are needed to disentangle the many potential drivers that explain plant community shifts and community stability.