Photosynthesis 123 



canopy is added to the moss surface water film. Water flows between the 

 green moss surface and the non-green moss and peat layers below. Sur- 

 face water can be evaporated directly or absorbed into the moss tissue, to 

 be lost by transpiration later. 



The seasonal progression of microchmate and production was simu- 

 lated using a standard set of climatic conditions and deviations from the 

 standard. The standard input cUmate is based on climatic and microcli- 

 matic data collected at the Biome research area during summer of 1973. 

 Climate data were adjusted using long-term records to produce two other 

 sets of conditions which were each 3 standard deviations above or below 

 the standard temperature conditions. The two contrived chmates are 

 hereafter referred to as hot and cold. The microclimatic data from 1973 

 have been used in other studies (Ng and Miller 1975, Stoner and Miller 

 1975, Miller et al. 1976, Ng and Miller 1977, Stoner et al. 1978b) and 

 were used here as the standard case to aid in interpreting the results. 



Simulation Results 



Temperature Relationships 



Analyses suggest that the vascular plant photosynthetic system has 

 adapted to function at near maximal capacities under existing tempera- 

 tures of the coastal tundra at Barrow while maintaining a leaf tempera- 

 ture optimum above mean ambient temperatures (Figure 4-11). Large 

 amounts of leaf area occur in positions of the canopy where tempera- 

 tures are ameliorated at times of the day when irradiances are high 

 enough for carbon dioxide uptake to respond to temperature. Further- 

 more, since within-season temperature changes are small, the results sug- 

 gested that seasonal acclimation responses, i.e. compensatory shifts in 

 the response curve, are not necessary to maintain high daily photosynthe- 

 sis rates. Strong accUmation responses have not been seen in the Barrow 

 tundra plants (Oechel and Sveinbjornsson 1978, Tieszen 1978b). 



As the temperature optimum for photosynthesis increases, the stra- 

 tum which supports the highest photosynthetic rates on a leaf area basis 

 shifts to lower levels in the canopy (Figure 4-12). However, the total daily 

 photosynthesis rate of each stratum remains similar, because strata with 

 high irradiance and potentially high photosynthetic rates have less leaf 

 area than do strata at the base of the canopy. The model simulations sug- 

 gest that dwarf shrubs and cushion plants should be characterized by 

 higher optimum temperatures for photosynthesis than the graminoids, 

 which are more closely coupled to ambient air temperatures. 



In part, plants are capable of effective photosynthesis because the 

 response curves are broad enough that with an optimum of 15°C rates 



