126 



L. L. Tieszen et al. 



FIGURE 4-13. The simulated effect of different tem- 

 perature optima on the seasonal course of net photo- 

 synthesis in Dicranum elongatum. The environmental 

 input data are from 1973. (After Oechel et al. 1975.) 



ally in a manner that would maximize carbon uptake, despite the fact 

 that the low temperature optima necessary to maximize carbon uptake in 

 Dicranum have been observed in Dicranum fuscescens in the Subarctic 

 (Hicklenton and Oechel 1976). The controls on low temperature acclima- 

 tion are not understood. The carbon uptake benefits of such acclimation 

 or genetic adaptation are known, but the costs are not. 



Because of the temperature sensitivity in mosses, they should be af- 

 fected more by climatic temperature changes than are the vascular 

 plants. Compared to the standard climate, photosynthesis in the simu- 

 lated cold climate described above decreased from 22% in Dicranum to 

 72% in Calliergon. Under the hot climate photosynthesis increased 

 slightly in Dicranum, relative to the standard year. Photosynthesis de- 

 creased in Calliergon, because of water stress induced by the higher tem- 

 peratures. Although mosses are more temperature-limited than are vas- 

 cular plants, increased temperatures may reduce the success of certain 

 species of moss by creating an unfavorable moisture balance (Stoner et 

 al. 1978b). 



Maximum Rates and Competency 



Simulations show that Dupontia is very sensitive to the light- 

 saturated rate of photosynthesis (Figure 4-14). Doubling the rate of light- 

 saturated photosynthesis results in a 58% increase in daily uptake of car- 

 bon, whereas a reduction of the saturated rate to 25% results in a reduc- 



