Thaw Response of Tussock-Shrub Tundra to Experimental All-Terrain Vehicle Disturbances in South-central Alaska
The vehicle-induced subsurface thaw response in a tussock tundra area was experimentally measured in relation to increasing traffic (10, 50 and 150 passes) applied by different types of lightweight (100-450 kg) all-terrain vehicles (ATVs) compared with a heavier (1200 kg) tracked Weasel (M-29) at four different times during the thaw season: (1) early June, (2) early September. (3) at weekly intervals for 10 weeks from mid-June to early September, and (4) in late July of two successive years. Two years later, in August 1987, three frost-table profiles were constructed for each of 144 test lanes 30 m long by probing at 10 cm intervals along three horizontal reference lines. The test site in south-central Alaska is underlain by "warm" permafrost with a 35 cm thick organic horizon over an ice-rich mineral soil. Early in the thaw season when thaw depths are 10-20 cm, traffic by ATVs can produce as much or more subsurface thaw than a heavier Weasel. Later, in September, the Weasel produced more than the ATVs. Traffic intensity (number of passes) also had a greater effect on thaw response in the spring than in the fall. The thaw response produced by traffic driven at weekly intervals throughout the summer was greater than that produced by traffic confined to early June or September. The downward progression of thaw from May to September results in changing soil moisture levels, bearing strengths and compressibility of the organic and mineral soil horizons.
Key words: all-terrain vehicles, tundra disturbance, permafrost thaw, experimental traffic, Alaska, Wrangell-St. Elias National Park, southcentral Alaska