Photosynthesis 109 



Oechel 1974). As the season progresses, new tissue is produced with high 

 photosynthetic capacities. The moss growth pattern has the potential for 

 significant late season photosynthesis since no end-of-season senescence 

 is observed. However, mortality of older age classes is high, and the 

 amount of older tissue decreases markedly at ages greater than 1 year 

 (Collins and Oechel 1974). 



Nutrients 



Nutrients can limit photosynthesis at the leaf and plant level if the 

 allocation for photosynthetic structures exceeds the support capabilities 

 of available nutrients. Under field conditions Dupontia appears to con- 

 trol allocation to produce a complement of photosynthetic structures 

 operating at near optimal capacities. The main response to chronic and 

 intense fertilization (Schultz 1964) was an increase in productivity due to 

 the stimulation of greater plant density (Dennis et al. 1978) and a two 

 times greater leaf area index. Although fertilizer stimulated a slight in- 

 crease in leaf width in Dupontia, there was no significant difference in 

 carboxylation activity and presumably no difference in leaf photosyn- 

 thesis. The short-term responses at site 2 were similar (Dennis et al. 1978) 

 and resulted in statistically significant, but small, increases in plant phos- 

 phorus and potassium, but not nitrogen (Chapin et al. 1975). 



In an attempt to document the spatial variability of photosynthesis 

 and to determine the extent to which large changes in production were 

 associated with changes in photosynthetic rates, a study was made along 

 a productivity and growth form gradient (Tieszen, unpubl.). Although 

 aboveground production ranged from 21 g m"^ in the basins of low- 

 centered polygons to 215 g m"^ in a disturbed vehicle track, there were no 

 significant (p = 0.95) correlations among photosynthesis and soil or leaf 

 potassium, nitrogen and phosphorus (Tieszen 1978b) (Table 4-3). This 



TABLE 4-3 Range of Nutrient Concentrations in 

 Leaves (%) in which Photosynthesis 

 was Independent of Leaf Nutrient 

 Concentration (P = 0.95) 



Dupontia Carex Eriophorum 



fisheri aquatilis angustifolium 



Nitrogen 1.83-3.28 2.74-3.28 1.50-3.21 



Phosphorus 0.07-0.24 0.07-0.40 0.15-0.31 



Potassium 0.64-1.59 0.55-1.55 0.45-1.14 



