112 L. L. Tieszen et al. 



since arctic mosses tend to reach light saturation at lower radiant fluxes 

 than vascular plants— 98 J m'^ s"' (400 to 700 nm) (Figure 4-3). Mosses 

 are generally light-saturated for most of the midday periods. The tend- 

 ency for light intensities above saturation to reduce the rate of photosyn- 

 thesis in Pogonatum alpinum may be a result of photo-inhibition or 

 photo-oxidation of the photosynthetic apparatus (Oechel and Collins 

 1976). This pattern is in contrast to graminoids, which increase photo- 

 synthesis to radiation levels approaching full sunlight, and represents a 

 major response difference between Pogonatum alpinum, especially 

 populations from low light environments, and the graminoids. The dif- 

 ferent light saturation requirements result in different daily responses be- 

 tween mosses and vascular plants. During the season, mosses show only 

 a slight daily dependence of photosynthesis on total daily irradiance 

 (Oechel and Sveinbjornsson 1978), in marked contrast to vascular plants 

 (Tieszen 1975). 



Temperature 



An effective photosynthetic system at low ambient temperatures is 

 essential for maintaining a positive carbon balance. Although mean am- 

 bient air temperatures during the growing season are less than 4°C and 

 graminoid leaf temperatures are closely coupled to air temperatures, de- 

 tailed studies at Barrow (Tieszen 1973, 1978b) and in other areas (Mayo 

 et al. 1977) have shown a temperature optimum for leaf photosynthesis 

 between 10 and 15 °C and significant carbon dioxide uptake at 0°C. 

 Photosynthesis in Dupontia is generally active until the leaf freezes, 

 which may not occur until -4 to -7°C. However, the destruction of en- 

 larging cells was observed at a temperature of -4°C, which usually repre- 

 sents the lower limit of photosynthesis. Very low temperatures are infre- 

 quent in July and do not appear to affect carbon dioxide uptake as sig- 

 nificantly as other growth processes. 



The underlying physiological and biochemical bases for the temper- 

 ature response curve are not clear. All resistances in the graminoids, in- 

 cluding leaf resistance, remained low down to 5 °C (Figure 4-4). Leaf re- 

 sistance did not increase at 0°C. At higher temperatures there was a 

 slight increase in leaf resistance but the mesophyll components of resist- 

 ance became significantly more important, indicating an internal diffu- 

 sion or carboxylation limitation to photosynthesis. A substantial increase 

 in light respiration may account for the net photosynthesis decrease at 

 temperatures greater than 15 °C. 



Mosses also have relatively high photosynthetic rates at low temper- 

 atures (Figure 4-5). Temperature responses are similar to those observed 

 in vascular plants with temperature optima between 10 and 19°C (Oechel 



