Photosynthesis 137 



if the canopy delays senescence and continues to develop until a standard 

 canopy is attained. The simulations and an analysis of the seasonal 

 course of CO2 uptake (Figures 3-1, 3-2) suggest that production is very 

 sensitive to the rate of allocation to leaves. We have estimated (Tieszen 

 1978b) that a 25% increase in leaf production results in a 45% increase in 

 CO2 uptake. Thus, it appear that both the date of initiation and the pat- 

 tern of canopy development are among the most important factors con- 

 trolling CO2 uptake and primary production. 



Variations in the amount of radiant energy received during a season 

 of constant length affect seasonal uptake and production. An increase in 

 incoming solar radiation equivalent to one standard deviation results in a 

 13% increase in uptake. 



The overall sensitivity of the carbon dioxide uptake system to tem- 

 perature variations is much less significant. Decreasing temperatures by 

 one standard deviation results in a mean temperature of -l-3.0°C, or 

 2.3 °C lower than the long-term mean, but reduces carbon dioxide uptake 

 by only 1 .4%. Early in the season, low temperatures inhibit carbon diox- 

 ide uptake on a leaf basis. However, the small foliage area at this time 

 makes its effect on the total season budget nearly negligible. Increasing 

 temperatures by one standard deviation increases carbon dioxide uptake 

 by only 0.4% (Tieszen 1978b). 



In order to determine the effect of several factors interacting simul- 

 taneously, two seasons were simulated, one warmer and brighter than the 

 mean and one colder and with less irradiance than the mean. The warmer 

 season with higher irradiance resulted in an 1 1 % increase in carbon diox- 

 ide uptake whereas the colder season with less irradiance reduced carbon 

 dioxide uptake by 17%. If these patterns are associated with changes in 

 rates of allocation, the effect is great. When the warmer, brighter season 

 was combined with a 20% increase in live foliage area index, net photo- 

 synthesis increased 53%. When the colder, darker season was combined 

 with a similar decrease in the foliage area, net photosynthesis decreased 

 by 41%. The carbon dioxide uptake system, however, appears relatively 

 insensitive to temperature, is quite dependent on radiant energy, and is 

 very dependent on allocation to photosynthetic tissue. The sensitivity of 

 allocation to various environmental factors must now be understood 

 because photosynthesis, per se, functions quite well at the prevailing am- 

 bient temperatures. 



SUMMARY 



The photosynthetic rates of plants of the coastal tundra at Barrow 

 show consistent patterns among growth forms that are comparable to 

 similar plant types in more temperate zones. Maximal rates of carbon 



