Plant species composition reveals temporal and spatial dynamics of snow slides in the San Juan Mountains, Colorado

The magnitude and frequency of snow avalanche events are difficult to study due to the uncertainty of forecasting when and where they will occur, and the dangers of traveling in snow-covered avalanche terrain.  Approaches from landscape ecology offer practical methods to characterize avalanche paths based on patterns of plant species composition and evidence of past disturbance.  Linear swaths of open vegetation within forested mountain slopes and other landscape patterns of plant diversity can be used to quantify and map the frequency and magnitude of past snow slide events.  A series of snow storms in January of 2005 in the San Juan Mountains of southwestern Colorado resulted in many avalanche paths running full track at 30 and 100 year return frequency magnitude.  Near Silverton, Colorado, powerful avalanches cut fresh trimlines in the forested margins of existing paths, widening their tracks by uprooting, stripping, and breaking mature trees.  Massive piles of snow, rocks, and woody debris were entrained and deposited in the runout zones.  Cross-section discs and cores of representative downed trees were used to evaluate dendro-ecological approaches to detect past snow avalanche events.  Signals of avalanche disturbance included the relative width of annual growth rings, traumatic resin ducts, reaction wood in response to tilting, and impact scars from the moving snow, debris, and powder blast.  Plot-based measurements of plant species along the elevation gradient of an avalanche path near Silverton indicate high species diversity, low overlap in plant species composition and structure among neighboring sites, and new opportunities for seedling establishment.