132 L. L. Tieszen et al. 



The graminoid leaf, especially in the single-shooted growth form, is 

 closely coupled to air temperature. The temperature correspondence is 

 mainly a result of the low boundary layer resistances associated with nar- 

 row leaves and the generally turbulent wind conditions of the Biome 

 research area. As leaf width increases, leaf temperatures should increase 

 and more closely approach the temperature optimum for photosynthesis. 

 However, at the same time, the water vapor density gradient between the 

 air and the leaf, and the transpiration rate, increase, resulting in a poten- 

 tially large water deficit and in a decrease in leaf water potential. Altering 

 leaf width from 4 to 15 mm under standard conditions has no effect on 

 carbon dioxide uptake. The radiation load is low; therefore the leaf tem- 

 perature remains close to the air temperature. 



The rate of photosynthesis for any given leaf will also vary as a func- 

 tion of its position in the canopy, since irradiance and leaf temperatures 

 are markedly influenced by the canopy. Leaves at the top of the canopy 

 protrude above the standing dead material and are occasionally light- 

 saturated although they are usually at low temperatures. The trends are 

 reversed for the leaves positioned at the bottom of the canopy. Because 

 of the effect of canopy density on thermal and radiance properties, the 

 foliage area index will determine the range of photosynthesis rates by all 

 leaves. Increasing live foliage area will reduce available light, since the 

 absorptivity of visible wavelengths by live leaves is high. Mean photosyn- 

 thetic rates should decrease, although stand photosynthesis should in- 

 crease up to a maximal foliage area index. Beyond this point self-shading 

 should result in a decrease in carbon dioxide uptake. Mean leaf photo- 

 synthetic rates decrease as the live foliage area index exceeds 0.74 (Figure 

 4-18). With a live foliage index of 1.0, a common upper value, mean leaf 

 rates are decreased to 14% of the open canopy, a decrease which results 

 principally from the absorption of radiation by the live leaves, resulting 

 in high light extinction in the canopy. Thus, with a foliage area index of 

 1.0 or higher, relatively few leaves in the 10 to 15 cm stratum are 

 saturated and then for only 1 hour around solar noon. Although mean 

 photosynthetic rates on a foliage area basis continue to decrease, com- 

 munity uptake increases up to a foliage area index of from 3 to 6. With a 

 foliage area index of 6 as much as 34 g CO2 m"^ day"' is assimilated. In 

 terms of carbon dioxide exchange, a foliage area index as high as 8, 

 which has been measured (Dennis et al. 1978), can be supported by the 

 graminoid vegetation. With such a high foliage area index the lower 

 leaves are in a negative carbon balance. Canopy architecture becomes in- 

 creasingly important in affecting photosynthesis at high foliage areas. In 

 the standard canopy with a dead area index of 1.24, photosynthesis in- 

 creases at all times of the day as leaf inclination increases. 



One of the characteristic features of graminoid canopies, especially 

 in the absence of lemmings, is the accumulation of standing dead mate- 



