10 IS 20 29 so 

 JUNE 70 I 



10 IS 20 2S 30 4 

 JULY TO I 



9 14 It M n 

 AueuST TO I 



Figure 17. Seasonal nitrogen contents, site 2 control plots, Barrow, Alaska. 



The maximum leaf area supported by the community, LAI = 1.2, was contained within a zone 

 20 cm high and was greater in the 0-5 cm increment than any other. However, a greater amount of 

 leaf material of D. fischeri and C. aquatilis was in the 5-10 cm increment than was the case in 

 E. angustifolium. The standing dead (attached growth from previous years) also accounted for a 

 LAI of 1.2 at this time. This material is very significant in the canopy as a non-functional absorber 

 of radiation, and the consequent effect of the shading of photosynthetic leaves must be known. 

 These dead leaves were distributed mainly in the bottom of the canopy where their influence on 

 the vascular plants is probably reduced. Leaf angles varied greatly from predominantly horizontal 

 in some of the forbs to quite erect in the three major producers. The functional significance d" 

 this will be tested in the primary production model and should be validated next summer. 



Leaf area indices varied widely from one community to another as did the total primary pro- 

 duction values. The highest indices were obtained in very wet habitats and were found in pure 

 stands of Duponlia lischeri and Arctophila ftilva where the leaf area indices (5.1 and 7.5 respec- 

 tively) were associated with equally high values for total production (734 and 541 g/m'). Pre- 

 sumably the erect nature and marked stratification of the leaves in these species are related to 

 the high potential leaf area and production values. Figure 18 contains plots of seasonal LAI values 

 for the three principal species and the total foliage. 



Requirements for stand photosynthesis and production predictions require a knowledge of air 

 temperature in the canopy and its relationship to radiation intensity and wind. In conjunction with 

 the micromet program, aspirated thermocouples were placed in control, mulch, clip and clear, and 

 the greenhouse plots at heights of 0, 2.5, 5, 10 and 15 cm. Air temperatures in the control canopy 

 showed some profile development, with highest temperatures near ground level. Temperature in 

 instrumented leaves was usually tightly coupled to air temperature, although occasionally, under 

 clear conditions, leaf temperatures were considerably above air temperature. Much more significant 

 are the differences in both leaf and air temperatures associated with seemingly minor changes in 

 micraelief. 



Efficient utilization of the 24 hours of radiation near the summer solstice demands that leaf 

 production and expansion be attained as soon in the growing season as possible, since the light 

 available around midnight decreases rapidly after about 15 July. Accompanving this limited radia- 



3.3 



