236 



V. Alexander et al. 



20 



- 16 - 



12 - 



o 8 

 o 



E 4 



? 



0.08 



0.16 



ly mm 



0.24 



I 



0.32 



0.40 



FIGURE 5-33. Net photosynthesis of Carex 

 aquatilis at various temperatures and radiation 

 levels. The ly min'^ are for the 400 and 700 nm 

 range (Tieszen, unpublished data.) 



night (Figure 5-34b). Final evidence for the limiting effect of light on 

 vascular plant photosynthesis at Barrow is the direct relationship between 

 daily solar radiation and primary production per day reported by Tieszen 

 (1978a) for three species of sedges and grasses (including Carex aquatilis). 



Temperature 



The vascular plants at Barrow are well adapted to low temperatures. 

 Photosynthesis, as described above, has a 15°C optimum and takes place 

 even below - 4°C (Tieszen 1978b). Another adaptation is significant 

 respiration and translocation of carbohydrates at close to 0°C (Allessio 

 and Tieszen 1973) and significant root elongation near 0°C (Shaver and 

 Billings 1975). Other evidence for this adaptation to low temperature is the 

 lack of relationship between daily temperature and daily photosynthesis in 

 the tundra at Barrow (Webber 1978). 



In summary, the vascular plants in the ponds are well adapted to local 

 conditions of a short growing season and cold temperatures. Once free 

 from' snow and ice, their rate of growth per gram of plant is higher than 

 temperate plants for a brief period and as high for most of the summer. 

 While total plant photosynthesis would be greater at higher temperatures, 

 the net photosynthesis may actually be lower due to the rapid increase in 

 respiration. Indeed, the photosynthetic efficiency of 0.46% of the 

 photosynthetically active radiation is respectable (Bunnell et al. 1975) and 

 may be twice as high for the pond plants. Warming the tundra will have 

 little effect on the net photosynthesis as generally the plants are operating 

 below their light saturation on the part of the response curve where 

 photosynthesis is insensitive to temperature (Figure 5-33). 



