Canopy-snow Interaction in a sub-Arctic Shrub Tundra With Shrubs Buried by Snow

The accumulation and ablation of the winter snowpack is strongly affected by the structure and distribution of vegetation. Near the tree-line where the boreal forest transitions to sub-Arctic tundra, wind scours snow from the open tundra and deposits it near the edges of shrub tundra and forests. The Trail Valley Creek (TVC) watershed is located just north of the forest-tundra transition at 68°45'N, 133°30'W in the Northwest Territories and is characterized by gently rolling hills with some deeply incised river valleys. Tundra vegetation dominates much of the upland areas, with shrub tundra and sparse black spruce found on hill slopes and in the valley bottoms. Typically, open tundra and areas with short shrubs are snow-covered for most of the winter and exhibit a high winter albedo (~0.8) characteristic of the snowpack, whereas dense forests are very efficient at trapping solar radiation and can maintain a low albedo (~0.2-0.3) for much of the winter. The winter albedo of sparse forests and unburied shrubs falls between these values. Tall shrubs are interesting because the effect of the shrubs on the snowpack and energy exchange is dependent on whether the shrubs remain standing during the winter, or are bent over and buried by the snowpack, subsequently springing up during the spring melt. In this study the influence of the canopy on snowmelt and energy exchanges is examined at a tall tundra shrub (TTS) site in the TVC basin for the 2003 spring melt. The shrubs were bent over and buried during the winter and sprang up over a short period during the spring melt. Measurements from a nearby tundra upland (TUP) site serve as a basis of comparison and the interpretation is aided using simulations performed with the Canadian Land Surface Scheme (CLASS). Snow accumulation was higher at TTS than at TUP, but lower than the over-winter snowfall, suggesting that transport out of TTS by wind and/or sublimation was significant. The albedo was similar at the two sites (~0.8) at the start of the melt, but decreased quickly by almost 50 percent at TTS as the shrubs sprang up. The smaller albedo at TTS resulted in an additional 6.8 MJ m-2 d-1 of absorbed solar radiation (6.3 MJ m-2 d-1 modelled) during the primary melt period, relative to TUP. According to CLASS simulations, additional shading from the canopy at TTS resulted in a smaller net solar radiation over the snowpack of about 1 MJ m-2 d-1 relative to TUP. However, modelled values of net longwave radiation at the snowpack show an increase at TTS relative to TUP following the shrubs springing up, with daily average values being 1.9 MJ m-2 d-1 larger. The result is an increase in modelled net radiation over the snowpack at TTS relative to TUP of 0.9 MJ m-2 d-1. Both snow surveys and CLASS simulations show that following the shrubs springing up, the melt rate was initially faster at TTS relative to TUP but this lasted only a few days. The measurements and simulations show that much of the additional absorbed solar radiation at TTS was expended through larger turbulent heat fluxes (particularly sensible heat) during this time. Most of the larger initial SWE at TTS was depleted shortly following the shrubs springing up, after which the melt proceeded at a similar rate at both sites which subsequently became snow-free at about the same time.