Photosynthesis 133 



400 



o 



c 



a 



2 4 6 8 



Foliage Area Index 



10 



FIGURE 4-18. A simulation il- 

 lustrating the effect of increasing 

 live (upper graph) and dead (lower 

 graph) foliage area index on photo- 

 synthetic rates per unit ground area 

 (a) and per unit leaf area (b). Other 

 input is for the standard day. In the 

 upper graph, ground area rates are 

 the sum of all vascular plants. In the 

 lower graph ground rates are for 

 Dupontia only with a foliage area 

 index of 0. 125 in the presence of a 

 total live foliage area index = 0. 744 

 plus a dead foliage area index given. 

 (After Miller et al. 1976.) 



rial (Tieszen 1972b). As an intercepting and emitting component in the 

 canopy, the effect of the accumulation of standing dead material on 

 photosynthesis and soil thaw could be quite significant. But because of 

 the decreased absorptivity of the dead material its effects on photosyn- 

 thesis are less than that of live material (Figure 4-18). In the simulations a 

 dead fohage area index of 5.0, 2 to 3 times that usually measured in the 

 field, results in a 64% reduction of photosynthesis. Accumulation of 

 dead material has a depressing effect on photosynthesis, which is mini- 

 mized as long as winter snows and some decomposition reposition this 

 potentially intercepting material at the bottom of the canopy. At the bot- 

 tom of the canopy, however, the effects of dead material will be greater 

 in Dupontia than most other species, since Dupontia has more leaf area 

 near the base of the canopy. 



Canopy relationships also have profound effects on mosses. A vas- 

 cular plant canopy under moist conditions reduces the carbon dioxide 

 uptake by mosses from the levels simulated in the open. The canopy 

 reduces radiation and temperature at the moss surface, which would tend 

 to decrease photosynthesis, especially during periods of low radiation. 

 The canopy also reduces turbulence and results in a lowered vapor densi- 

 ty gradient between the moss and the air. Therefore the reduction in car- 



