406 G. O. Batzli et al. 



transport nutrients from both basins and rims to troughs. 



Polygons form in drained lake basins, which have relatively uniform 

 topography and sediments and, therefore, an even distribution of soil 

 nutrients before polygon formation begins. The nutrient-transport hy- 

 pothesis provides an explanation for the development of the current pat- 

 terns of biological activity as polygonal ground develops. The sequence 

 of events can be hypothesized as follows. 



Troughs develop over ice wedges, and the wedges continue to ex- 

 pand to produce the rims that surround the basins (Figure 1-10). Drain- 

 age is impeded in the central basins and the deepest parts of the troughs. 

 Although the basins of low-centered polygons hold water during early 

 summer, they are higher and relatively drier than the troughs by midsum- 

 mer. Snow cover is deeper in the troughs during winter. Lemmings place 

 their winter nests in troughs where deeper snow improves the microhabi- 

 tat. Foraging lemmings move out from the troughs under the snow, but 

 most feces and urine are deposited in the troughs near the nests. Nutri- 

 ents accumulate in the troughs as a result of lemming activities. Nutrients 

 are depleted in the basins where lemmings remove forage but deposit few 

 wastes. Summer (June through September) burrows and nests are located 

 on the relatively dry rims. The rims attain an intermediate nutrient status 

 because they are the site of summer nests and near the winter nests. Ac- 

 cumulation of nutrients is associated with higher primary production and 

 higher concentration of nutrients in the trough vegetation (Barel and 

 Barsdate 1978, Tieszen, pers. comm.). Improved forage reinforces the 

 preference of lemmings for troughs. Nutrient depletion has the opposite 

 effect in the basins. Increased activity of decomposers and soil in- 

 vertebrates occurs in response to higher quality of litter and concentra- 

 tion of soil nutrients in troughs (see Chapter 1 1 for details). Accumula- 

 tion of organic matter in the soil is slowed, and rates of nutrient cycling 

 increase. Again, the opposite trends occur in the basins. 



Although these events have been presented sequentially to empha- 

 size causal relationships, all occur simultaneously. The result is a slow 

 transition from relatively homogeneous distribution of soil properties 

 and biological activity in drained lake basins to the marked spatial heter- 

 ogeneity seen in polygonal terrain. According to the nutrient-transport 

 hypothesis, spatial differences in biological activity are largely a result of 

 different availability of nutrients. The pattern of nutrient availability is 

 imposed by activities of lemmings. 



Unfortunately, we do not have sufficient data to test the nutrient- 

 transport hypothesis. The trends in soil properties and biological activity 

 associated with polygonal terrain have been used to construct the hy- 

 pothesis and cannot be used to test it. Ultimately, the causal links will 

 need to be tested by field observations and experiments. In the absence of 



