Biophysical Processes and Primary Production 75 



port of heat and water vapor. Soil surface temperatures affect the air 

 temperatures immediately above the surface and the deeper soil tem- 

 peratures. Thus there is an interacting system comprising 1) the vertical 

 profiles of leaf area index, leaf inchnation, leaf width, leaf absorptance, 

 leaf conductance to water loss, and stem area index; 2) the absorptance 

 of the soil and the properties of the soil affecting heat conduction; 3) the 

 profiles of the processes of energy exchange; and 4) the profiles of plant 

 temperature. 



During the course of the growing season, the foliage area index of 

 the canopy of the Carex-Oncophorus meadow develops from zero at the 

 beginning to between 0.8 and 1.2 by the last half of July (Caldwell et al. 

 1974). (The foliage area index was calculated as the total of the leaf and 

 stem area indices, m^ plant surface per m^ ground, one side of the leaf 

 considered.) In years without intensive lemming grazing the standing 

 dead material with a foliage area index of 0.3 to 0.5 was present 

 throughout the season but was most conspicuous early in the season. Lit- 

 ter with an area index of 0.3 to 0.5 was concentrated within 2 cm above 

 the moss surface. Standing dead material and litter included dead mate- 

 rial in various stages of decay from several previous years. From mid- 

 July to the end of the growing season leaf material produced in the cur- 

 rent year senesced and was added to the crop of standing material. New 

 leaves grow from the stem base, located in the moss layer, and must grow 

 through the shade cast by the dead material. From above, the appearance 

 and albedo of the canopy were dominated by the light brown dead mate- 

 rial until early June when the darker green live material began to pre- 

 dominate in the upper levels. However, the conversion from light to dark 

 color was not complete because the current year's growth senesces and 

 turns light brown before the new leaves dominate the canopy. 



The graminoids (Dupontiafisheri, Carex aquatilis and Eriophorum 

 angustifolium) developed the highest foliage area index, about 0.2-0.4 

 (Figure 3-3). However, foliage area indices and mean inclinations vary 

 systematically across microtopographic units and were highest near the 

 wet end of the gradient (Caldwell et al. 1974, Dennis et al. 1978). In 

 standing water, foliage area indices of pure stands of Dupontia and Arc- 

 tophila fulva were as high as 5.2 and 8.5, respectively. At the other ex- 

 treme, the centers of low-centered polygons, occupied predominantly by 

 Carex, had foHage area indices of 0.1 to 0.2. The seasonal progression of 

 aboveground biomass increased with the foliage area and was 80 to 1(X) g 

 m"^ by late July in the moist meadow studied. 



The foliage areas were not uniformly distributed vertically in the 

 canopy. Foliage was concentrated in the lowest 5 cm of the canopy (Fig- 

 ure 3-4) and in late July was stratified by species and growth form, with 

 the grasses and sedges growing above the dicotyledonous plants. Most of 

 the leaves and stems were steeply inclined, with angles 60° to 90° from 



