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Global change affects alpine ecosystems by altering plant distributions and community composition. However, determining the global magnitude of alpine vegetation change is challenged by a scarcity of observations at fixed plots spanning decadal-time scales, and the bioclimatic and landscape heterogeneity of these environments. Similar to other alpine ecosystems, tundra on Niwot Ridge is structured by the combined interactions of topography, wind, and snow. Sparsely vegetated fellfield and dry meadows are typical of exposed locations experiencing cold winter temperatures, strong winds and low snow cover. Topographically sheltered areas have deeper snow, higher soil moisture, and more diverse moist meadow and shrub tundra communities. We investigated changes in this alpine tundra over 40-years by estimating cover and abundance of vascular plant species in 30 permanent plots using the same sampling methods in 1971 in 1991, 2001, and 2011. Based on a classification of plant communities in 1971, we explored how these communities changed among sampling dates using Non-metric Multidimensional Scaling (NMS), and tied these changes to differences in diversity, species cover and abundance, and plant growth forms.
Results/Conclusions
The NMS included data from all sampling years and resulted in a 2-dimensional solution where communities ordered along the same gradients identified in 1971. Along axis one communities ordered along a soil-moisture gradient; snow-depth defined communities along the second axis. Only during the longest intervals (30 and 40 years) were there significant changes in some plant communities. Between 1971 and 2011, shrub tundra and wet meadow vegetation changed significantly in ordination space. Diversity generally decreased between 1971 and subsequent sampling intervals, but not among all communities. Diversity declined consistently only for shrub tundra, while other communities had lower diversity at some dates, but showed no trend across the 40 years. These changes are largely explained by increases in the deciduous shrub Salix planifolia and losses of forbs and graminoids between 1971 and 2011. While the expansion of shrub tundra is the most striking finding of the 40-year resampling, other evidence suggests that wet meadows are shifting towards vegetation states more typical of wetter site conditions and fellfields toward states more typical of drier conditions. While other tundra studies report increases in shrub vegetation with climate warming, a number of location-specific drivers, such as anthropogenic N-deposition and recovery from domestic grazing may be responsible on Niwot Ridge. Although we found compelling evidence of change in the vegetation structure, the precise mechanisms underpinning these changes remain unclear.
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