The effects of tree islands and timberline on soil carbon, soil nitrogen, and soil phosphorus in high montane systems.

Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) tree islands modify the characteristics of surface soils in alpine tundra. Soil C content of the approximate A horizon (top 15 cm) of soil was measured during the summer of 1994 on windward, leeward, upslope and downslope sides, and interiors of tree islands on Niwot Ridge, Colorado, USA. A subset of samples from these sites were also used for CHN analysis and were measured for total phosphorus using persulfate digestions and colorimetric measurements. Results indicate significant (p<.0001) reductions of percent of dry mass represented by C in soil and significant (p<.04) declines in absolute C storage among soils on the windward sides of tree islands as compared to the upslope and downslope controls, and a tendency for reduced C on the leeward sides as well. Surface organic matter (O horizon) accumulations averaging 9.6 +/- 1.02 kg/m^2 are found in the interior of tree islands, but this material, in addition to roots, is not stabilized in the A horizons of soil. The movement of tree islands can therefore be regarded as disturbances to soil building processes in alpine tundra. Timberline forest and adjacent tundra patches of similar aspect and slope were also sampled for comparisons of soil C content. Results indicated similar C storage beneath trees and tundra at this lower elevation. The wind-induced movement of tree islands across the tundra creates enhanced snowpack within the trees and on their leeward sides. Shading and moisture conditions of the soil are altered, leading to C deposition and decomposition dynamics which differ from that of unimpacted tundra surface soils. However, at timberline, adjacent tundra lacks the ability to exhibit the enhanced C storage of alpine tundra at higher elevations. Snowpack within trees and adjacent tundra at timberline may be relatively constant such that biophysical factors affecting soil characteristics are relatively unchanged by plant life-form