Snow avalanches and patterns of plant species composition: Examples from the San Juan Mountains, Colorado

Background/Question/Methods

Near Silverton, Colorado, a series of snow storms in January of 2005 resulted in many avalanche paths running full track.  Many avalanches cut fresh trimlines, widening their tracks by uprooting, stripping, and breaking mature trees.  Powerful avalanches deposited massive piles of snow, rocks, and woody debris in their runout zones.  We evaluated landscape ecology approaches to characterize snow avalanche paths based on patterns of plant species composition and evidence of past disturbance.  We sampled patterns of plant species diversity and vegetation damage on the elevation gradient of an example avalanche path (including the starting zone, track, and runout zone), using 168m² circular forest monitoring plots (7.3m radius plots with three 1m² subplots).  We also examined cross-section discs and cores of representative downed trees to detect dendro-ecological signals of past avalanche disturbance. 

Results/Conclusions

We recorded a total of 221 plant species from an initial set of 7 plots on the example avalanche path, 72% of which were found on only one plot.  Although our results are preliminary and subject to final taxonomic identification, they indicate that snow slide activity contributes to the high local plant species diversity and provides opportunities for both native and non-native seedling establishment.  Evidence of avalanche disturbance included the geographic distribution of stripped, broken, and uprooted trees, and direct impact scars on trees from the moving snow and associated wind blast.  We also found signals of past disturbance events recorded in the growth layers of woody plants, including the development of reaction wood in response to tilting, variation in the relative width of annual growth rings, the presence of traumatic resin ducts, and the ages of trees growing in the avalanche path.  Documented observations of avalanche incidents provide the most reliable record of avalanche events for an area, but historic records are often incomplete or strongly influenced by patterns of human activity.  These examples support the conclusions of many other scientists who have found that signals recorded in the annual growth layers of woody plants, landscape patterns of forest cover, and distributions of plant species can be used to quantify and map the frequency and magnitude of snow slide events in avalanche terrain.