136 



L. L. Tieszen et al. 



240 



FIGURE 4-21. The simu- 

 lated effect of season length 

 on annual net photosynthesis 

 for Dupontia fisheri in the 

 canopy of the Carex-Onco- 

 phorus meadow. The fre- 

 quencies of yearly thaw sea- 

 son lengths of periods with 

 above 0°C mean daily tem- 

 peratures for the years 1922 

 to 1973 are given also. Other 

 input is for the standard 

 season. (After Miller et al. 

 1976.) 



Season Length, days 



tion of carbohydrates for leaf production which, in the coastal tundra at 

 Barrow, is controlled to a large extent by the availabihty of soil nutrients, 

 principally nitrogen and phosphorus. The spatial pattern of above- 

 ground primary production reflects these topographic and nutrient rela- 

 tionships; and the magnitude of production depends upon climatic fac- 

 tors. Simulating the seasonal courses of carbon dioxide uptake for 

 Dupontia fisheri can be used to assess the response of the graminoids to 

 some of these variables. 



Season length was simulated by initiating the season earlier or later 

 than the 15 June date used for 1971. Environmental factors and rate of 

 canopy development were assumed to be the same as in the 1971 simula- 

 tion. Under these conditions, there is a marked effect of season length on 

 total carbon uptake (Figure 4-21), primarily because plants can more 

 fully utilize the high levels of radiant energy at the time of the solstice. 

 During the simulated long season, Dupontia attained a foliage area index 

 of 0.22 on 4 August, nearly 30% greater than the standard season. Con- 

 versely the foliage area was reduced by 30'yo during the short season. Un- 

 fortunately, long-term field observations covering this range of season 

 lengths are not available for comparison with productivity estimates. 

 Production and carbon dioxide uptake are increased in the short season 



