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P. C. Miller et al. 



15 



10 



1200 2400 



Hour 



5 2.5 



1200 

 Hour 



10 



2400 1.0 



W 



FIGURE 3-10. a) Isopleths of solar irradiance (300 to 3000 nm) absorbed 

 by leaves (J m'^ s'^) at different heights in the canopy through the day on 

 about 15 July 1971. b) Isopleths of leaf temperatures through the day for 

 15 July, c) Vertical profiles of live (m) and dead (o) foliage area indices 

 (Af). 



of dead material was present. Interception by evergreen shrubs was more 

 constant through the growing season than was interception by grasses 

 and sedges because shrub leaf and stem areas were more constant. Thus 

 photosynthesis was possible earlier in the season in evergreen shrubs. On 

 21 June, the interception efficiency of a canopy with leaves incHned 65 ° 

 and a foHage area index of 1.0 was about 0.6. With similarly inclined 

 leaves and foliage area index of 2.0, interception was about 0.96. 



The fraction of incoming direct beam radiation intercepted by the 

 canopy increased as solar altitude decreased; thus interception was high 

 with relatively low foliage area indices (Figure 3-9). On 21 June at solar 

 midnight with the sun 5° above the horizon, interception was almost 

 complete with foliage area index of 0.5. At solar noon with the sun about 

 40° above the horizon, interception was only 0.3 with the same foHage 

 area index. At this time complete interception required a foliage area in- 

 dex of about 4.0. The Dupontia and Arctophila stands, after developing 

 foliage area indices of 5 and 8 respectively, should intercept all incoming 

 solar radiation. 



Leaf absorptances were lower in regions with higher solar irradiance 

 than in regions with lower solar irradiance, and were higher in the alpine 

 than in the Arctic (Billings and Morris 1951, Mooney and Billings 1961, 

 Mooney and Johnson 1965). In the simulation models for vegetation of 



