Snowmobile effects on height and live stem abundance of wetland shrubs in south-central Alaska

Abstract

Snowmobiling is an important recreational activity at higher latitudes and in alpine areas where snow occurs. The Kenai National Wildlife Refuge in south-central Alaska allows snowmobiling access to >500,000 ha between December–April but not until snow depths are ≥50 cm to reduce snowmobile impacts to vegetation. We used a Before-After-Control-Impact experimental design to determine snowmobile effects on height and abundance of live stems of dwarf birch (Betula nana), shrubby cinquefoil (Dasiphora fruticosa) and sweet gale (Myrica gale) at snow depths ≥50 cm and <50 cm. We traversed 50x2-m transects with 10 snowmobile passes, taking snow depth measurements before and after treatments in January 2012 (≥50 cm) and April 2012 (<50 cm). Vegetation was measured in September 2011 and August 2012. Data were analyzed with general linear mixed effects models. Snowmobile traffic compacted snow by >40%. Dwarf birch was the most intolerant of snowmobiles with ≥7% decline in mean height (p ≤ 0.05) and ≥28% decline in mean live stem abundance (p ≤ 0.02). Shrubby cinquefoil was the most resilient as plant height was unaffected and mean live stem abundance declined by 20% only when snow was <50 cm (p = 0.01). Mean height of sweet gale was also unaffected at both snow depths but mean live stem abundance declined by ≥ 15% regardless of snow depth (p ≤ 0.04). When snowmobiles did affect height or live stems, the impacts were greater in all three species at snow depths <50 cm. In contrast, both vegetative metrics for all three species remained the same or increased in the control group. Snow depths ≥50 cm reduce snowmobile impacts to wetland shrubs but did not entirely eliminate them. Thinning snowpack in a rapidly warming climate suggests more damage to vegetation is likely where snowmobiling occurs.Management implications: Land managers may specify a minimum snow depth to protect vegetation from the physical damage of snowmobiles and their indirect effects through degradation of the subnivean environment. Snow depths ≥50 cm observed in our study reduced the negative effects of snowmobile traffic on three wetland shrub species but did not eliminate them. Snowmobiling as a winter recreational activity is likely to decrease in the foreseeable future in a warming climate. However, declining snowmobile opportunity does not necessarily translate to declining impacts in fact, the opposite may be true at least in the near term. Our data suggest snowmobiling over poor snowpack likely means more vegetation damage. We recommend partial openings when snow depth in the regulated area varies above and below 50 cm over large areas. The development of a maintained trail system, rather than open access, is an alternative way to reduce landscape-scale impacts.