480 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



less rectilinear ditches. Mounded polygons are called "high center" 

 polygons, and hollowed out ones "low center." The basins of the latter 

 type usually hold a few inches to several feet of standing water (pi. 4) . 

 The ditches of both types also usually contam at least a little standing 

 water. In very general terms, soil polygons can be said to arise 

 through the differential effects of perennial freezing and thawing 

 in the surface layers. Convection movements churn and sort the par- 

 ticles into a more or less regular pattern. Some kinds of polygons are 

 related to the development of discontinuous ice lenses in the subsoil. 

 Whatever their precise geomorphic origin and significance, they 

 are extremely significant to plant growth in that they create considera- 

 ble microrelief in many areas otherwise flat and thereby create a 

 greater diversity of habitats. Consequently, xerophytic and hydro- 

 phytic species can exist side by side. 



Let me digress for a moment to emphasize the extreme importance 

 of microrelief in general to the development of vegetation in the Arctic. 

 Vast areas of tundra are essentially flat (pi. 3) and poorly drained, and 

 the slightest difference in elevation, even a few inches, can change the 

 moisture-holding capacity and drainage characteristics of the soil 

 toward a more xeric condition. Often the texture of the soil is coarser 

 on the mounds too. Consequently, on the drier tussocks and hum- 

 mocks one finds a sliglitly more xerophytic vegetation. If the differ- 

 ences in elevation are pronounced, the differences in vegetation may 

 also be quite pronounced. Phenological traits are frequently affected. 

 The wetter the soil the longer it takes in early summer for the soil 

 temperature to warm up sufficiently for growth, and I have seen again 

 and again the same species in full bloom in the depressions but with 

 nearly mature fruits on the adjacent mounds, with scarcely more than 

 a 6-inch difference in relief. This can only be due, I think, to a rela- 

 tively slight differential in soil moisture and/or temperature. 



Frost action leads to still other phenomena of significance to plants. 

 On medium-to-steep slopes broad bands or lobes of surface material, 

 rubble or turf, tend to slump during the early thawing period when 

 the soil is saturated. This is one of several phenomena referred to as 

 solifluGtion (pi. 10, fig. 1) . The hard upper surface of the permafrost 

 zone functions like a shear plane, and large masses of surficial ma- 

 terial, consolidated and unconsolidated, simply creep slowly down- 

 slope under their own weight. After a period of years, sizable rup- 

 tures may be created, where primary ecological succession can begin 

 again. Whole turfs and herbmats may be displaced downslope, but 

 the movement is usually slow enough that little if any significant 

 damage is suffered, apart from the actual displacement itself. 



On the higher slopes and plains of the Tundra, Arctic and Alpine, 

 annual freezing and thawing may eventually have almost violent 

 consequences for the soil, heaving it upward and rupturing the sur- 



