Primary Producers 235 



it certainly would help the pond plants and reduce the amount of 

 carbohydrate translocated from the roots and rhizomes. 



Light 



The standing dead material absorbs a great deal of solar radiation in 

 the canopy of the tundra. For this reason, the absence of standing dead in 

 the ponds produces a better light climate for each leaf. The light climate is 

 improved by each leaf being very erect so that it is close to perpendicular 

 to the sun's rays. Dennis et al. (1978) report that 79% of the graminoid 

 leaves at Barrow were inclined at 30° or greater and 77% of the Carex 

 aquatilis leaves were inclined 60° or more. When the sun angle is low, and 

 the average at Barrow is 25° on 21 June and 19° on 21 August, the 

 combination of leaf angle and low sun angle results in almost complete 

 absorption of all sunlight within the canopy (Caldwell et al. 1974). Bunnell 

 et al. (1975) calculated that leaf area indices of 0.5 and 1 .0 intercept 62 and 

 94% of the incoming solar radiation on 21 June; on 21 August they 

 intercept 68 and 98%. 



Early in the growing season, the rate of photosynthesis is clearly 

 limited by the small amount of biomass. Accordingly, the plants are 

 adapted to a rapid production of leaves immediately after the snowmelt by 

 mobilizing stored carbohydrates from the roots and rhizomes and 

 translocating them to the leaves. In terrestrial plants, this rapid early 

 growth reaches 0.2 to 0.25 g (g dry wt) ' day ~ ' for the first week then falls 

 to 0.03 g (g dry wt) ' day^ after 10 days (Bunnell et al. 1975). The 

 growth rate in the ponds was 0.07 g (g dry wt) ' day " ' at mid-season 1971 

 which compares very well with rates close to 0.06 g (g dry wt) ' day^ 

 measured in a Carex rostrata marsh in Minnesota (Bernard 1974). 



Photosynthesis is a direct function of light in the Barrow tundra 

 plants (Tieszen 1978a). The net photosynthesis of C aquatilis was 

 maximum at 15°C with severe inhibition above this temperature (Figure 5- 

 33). Light saturation occurred at low light levels (about 0.1 ly min~^) at 

 0°C and at lower temperatures but occurred at 0.24 ly min " ' at 15°C. 

 These values are photosynthetically useful radiation (50% of incoming 

 solar radiation). This saturation level is high relative to the usual 

 insolation at Barrow because of the low solar angle and the high 

 percentage of cloud cover during the summer. For example on 14 July 

 (Figure 5-34a) the light level was saturating only around noontime. Other 

 experiments show that the photosynthesis rates at optimum conditions 

 were 18.5 mg CO2 dm "^ hr ' for C. aquatilis and 19.6 for A. fulva. 

 Tieszen's studies also show that there is positive net photosynthesis {Ps 

 greater than R) above an insolation of 0.01 cal cm '^ min '. Thus there is 

 positive CO2 uptake throughout most of the growing season. Only when 

 the sun begins to set in August does the CO 2 uptake become negative at 



