Photosynthesis 135 



FIGURE 4-20. Simulated 

 responses of a Dupontia 

 fisheri tiller to grazing. Leaf 

 photosynthetic rates were 

 estimated for 1971 environ- 

 mental data and 1972 leaf 

 growth data. Grazing was 

 simulated by the complete 

 removal of photosynthetic 

 tissue. The numbers repre- 

 sent the percent change in 

 seasonal CO2 uptake result- 

 ing from four grazing 

 events. 



is altered dramatically. Leaves near the moss surface are exposed to more 

 intense radiation and have lower temperatures, since convectional losses 

 are greater (Figure 4-20). 



Early season grazing by lemmings has little effect on photosynthesis 

 in Dupontia because photosynthetically competent tissues are not avail- 

 able to be harvested and because mean photosynthetic rates of new tis- 

 sues increase in a more open canopy. Midseason grazing, however, is 

 very detrimental to seasonal carbon dioxide uptake. In plants grazed in 

 midseason, carbon dioxide uptake was 42% less than in ungrazed plants. 

 This results mainly because photosynthesis is limited by the available leaf 

 area, and grazing at midseason removes photosynthetic tissue at a time 

 when its contribution is greatest. In addition to the major effect on eco- 

 system carbon balance, grazing reduces storage reserves (Tieszen and Ar- 

 cher 1979), which will affect plant performance for one or more growing 

 seasons. We have not assessed the occasional severe grazing pressure 

 which can result in rhizome and stem base destruction. This may have 

 dramatic effects on population structure. 



Seasonal Course of Carbon Dioxide Uptake 



Primary production varies both spatially and from season to season. 

 Since the rate of net photosynthesis is not strongly depressed because of 

 the low temperatures, other factors must account for major portions of 

 the variation. The primary factors limiting primary production or can- 

 opy development appear to be: 1) the length of the growing season (see 

 Figures 3-1, 3-2), which is dictated by the duration of snow cover and 

 related to topography and seasonal radiation patterns, and 2) the alloca- 



