Photosynthesis 125 



0600 1200 1800 



0600 1200 1800 

 Solor Time 



0600 1200 1800 



FIGURE 4-12. Isopleths of simulated net photosynthesis (mg CO2 dm~^ 

 hr^J through the day at different levels in the canopy, using the stand 

 photosynthesis model with different temperature optima. (After Miller et 

 al. 1976.) 



are still positive at 0°C. The significance of the breadth and shape of the 

 response curve is illustrated by simulations in which the curve was en- 

 hanced or depressed at O^C. When the response at low temperatures was 

 increased so rates at 0°C were increased lOO^o, the daily increase was on- 

 ly 7%. When the rates were decreased by 50% at 0°C, daily photosynthe- 

 sis was reduced only 3%. If, however, the curve was depressed so carbon 

 dioxide uptake at 0°C was zero, daily uptake was reduced 27%. Thus a 

 photosynthetic capability at 0°C is very important, and it appears that 

 the Dupontia response curve is well-adapted to prevailing temperatures. 

 Carbon dioxide uptake is much more temperature-sensitive in moss- 

 es than in graminoids. In simulations where the temperature optimum 

 for photosynthesis is varied and the shape of the response surface held 

 constant, a temperature optimum of 5°C yields the highest uptake rates 

 for the most frequently observed temperatures 1-3 °C (Figure 4-10). Up- 

 take is suppressed only slightly at a temperature optimum of 10 °C under 

 these conditions. However, temperature optima of 15 °C and higher 

 result in large depressions in carbon uptake at ambient temperatures be- 

 low 10 °C. The high temperature optima typical of the mosses result in 

 significant losses of carbon. For example, the observed temperature opti- 

 ma of 11 to 19 °C in Dicranum result in a seasonal carbon uptake 25% 

 lower than that possible with lower optimum temperatures (Figure 4-13) 

 (Oechel et al. 1975). Other moss species at the Biome research area show 

 similar patterns (Oechel 1976, Oechel and Sveinbjbrnsson 1978). The 

 temperature optimum in Dicranum elongatum does not acclimate season- 



