Niwot Ridge Takes its Snowfence to Arctic Tundra for a Cross-site Comparison
In high-elevation or high-altitude environments, enhanced snowpack is commonly predicted as a consequence of global change. Studies have demonstrated that enhanced snowpack can greatly alter decomposition and trace-gas dynamics, provided this snowpack modifies the subnivean thermal regime by extending the time interval that H2O remains in liquid form (e.g. Brooks et al. Biogeochemistry 32:93-113). Three different projects funded by LTER and NSF-DPP are being used to measure the relative importance of enhanced snowpack in different plant community types found in alpine and arctic tundra.
A 60-meter long by 2.8 meter high snowfence was installed at Niwot Ridge in summer of 1993 as part of a long-term experiment to study effects of enhanced snow deposition on species and ecosystem processes of moist and dry communities in alpine tundra. In 1994, two similar fences were constructed on moist and dry tundra sites at Toolik Lake, Alaska (Arctic LTER).
All sites have been instrumented with temperature and moisture monitoring equipment. Vegetation plots along the snow enhancement gradient created by these fences were established within the drift zones and were paired with control plots outside the snow manipulation area. Measurements were taken of community composition, microclimate, permafrost and snow temperature, decomposition, soil characteristics, plant phenology, growth and reproduction. These sites are being used as experimental sites by REU students and graduate students, as well as being monitored for long-term changes.
In the Colorado alpine, the enhanced snowpack reduces growing season length by almost 50 percent in the deepest part of the drift, and the snow can raise the average soil surface temperature of a previously wind-blown dry alpine area by about 12° C. These changes have substantially altered the vegetation and dramatically increased surface litter decomposition rates. The arctic results suggest more modest changes in both growing season and soil temperature changes, and preliminary results suggest that surface litter decomposition is at most weakly influenced, if at all, by these differences.
Enhanced snowpack tends to stress the dominant species of the dry sites. As species replacements are very slow (and perhaps slower in the alpine than in the arctic due to extreme thermal stresses during the short growing season), this stress translates to a period where plant productivity is diminished while decomposition is enhanced. This response, similar in many respects to that observed in a clearcut, produces a pulse of mineralized nutrients that do not appear to be retained by the system. We will continue to study these sites to see if, as in the forest response, an accretion phase of nutrients is associated with recovery.
The two ecosystem types as well as the two different communities being studied within these ecosystem types exhibit unique responses that are related to the way that enhanced snowpack expresses itself in terms of specific microclimatic modifications. These changes then produce a transitional period where the biotic responses, including both a functional response of organisms present at the time of disturbance as well as the successional effects induced by species replacements, further alter the microclimate regime of the snowfence areas.
In addition to enhanced snowpack, these sites have also been used in warming experiments. Small greenhouse plots have been placed within and adjacent to snowfence areas to observe the concurrent effects of warming on plant response. This latter experiment is part of the International Tundra Experiment (ITEX), which is replicated in over 20 circumpolar countries.