202 P.J. Webber et al. 



upon inorganic nutrients being absorbed or deposited upon the moss sur- 

 face or diffusing to the surface, since most mosses cannot absorb 

 minerals through a root system from the soil profile. 



Interrelations Between Environmental Factors 

 and Plant Characteristics 



The links between the major environmental factors and the patterns 

 of plant characteristics can be diagrammed to clarify and assess the cur- 

 rent state of understanding (Figure 6-8). Although the variation in en- 

 vironmental factors is continuous, a comparison of the interactions on 

 exposed microtopographic units, such as tops of high-centered polygons, 

 with those on more protected units, such as polygon troughs or sloping 

 creek banks, provides some basis for assessing the direction of influence. 

 Within the macroclimatic environment the patterns of vegetation are re- 

 lated to topographic gradients and associated environmental factors. 



For example, the major environmental variables are all influenced 

 by wind. Sites with relatively high wind have thin snow cover, low soil 

 water and lower temperatures during the growing season (Table 6-1; 

 Oberbauer and Miller 1979). High wind leads to increased removal of lit- 

 ter, which is indicated in the higher litter turnover rate of the unprotected 

 Luzula heath relative to the Salix heath, and to reduced nutrients because 

 of the wind-blown nutrient loss (Figure 6-4, Table 6-1). The low soil 

 water also increases soil aeration, which can lead to increased acidity and 

 phosphorus immobilization. 



These environmental factors influence several plant processes. High 

 wind and thin snow cover lead to increased loss of plant parts due to 

 abrasion (Savile 1972), which favors low growth forms and species in 

 which the buds are protected. The lower temperatures in the active sea- 

 son should lead to reduced growth (Warren Wilson 1966a, Larcher et al. 

 1973), although the relation between temperature and growth is poorly 

 quantified for the Arctic (Miller et al. 1979). The reduced growth may 

 also lead to low growth forms. The low growth forms lead to increased 

 plant temperature, since ground surface temperatures are higher than air 

 temperatures. The higher plant temperatures will cause higher vapor 

 pressure differences from leaf to air, thus potentially increasing water 

 loss and water stress unless compensated by high leaf resistance. High 

 leaf resistances are associated with low photosynthesis rates, which lead 

 to greater leaf longevity that provides time to recover the leaf construc- 

 tion costs (Johnson and Tieszen 1976, Miller and Stoner 1979). The need 

 for greater leaf longevity is reinforced by the low nutrients (Beadle 1962, 

 Monk 1966, Small 1972, Stoner et al. 1978b). 



Thin snow cover results in earlier exposure of the surface in spring. 



