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FIGURE 4-2. Seasonal progression of the carboxylation activity for 

 entire tillers (summation of leaf activity times leaf area) in 1971 of: 

 Dupontia fisheri (D.f.), Carex aquatilis (C.a), Eriophorum angusti- 

 folium (E.a.), and the total foliage area index (A,) of the community. 

 The foliage area indices of the species are given in Figure 3-3. Ab- 

 solute rates not directly comparable among species. (After Tieszen 

 1978b.) 



On both a daily and seasonal basis, leaf photosynthetic rates are 

 highly correlated with carboxylation activity (Tieszen 1978b). The inte- 

 gration of carboxylase activity among all leaves suggests that on a tiller 

 basis the greatest potential for photosynthesis occurs well before the time 

 of maximum standing crop or leaf area index (Figure 4-2). Although the 

 greatest conversion efficiency on a green leaf area basis should occur on 

 20 July, the increased canopy or tiller density later in the season results in 

 a greater efficiency on a land basis. Mosses begin each season with a high 

 proportion of chlorophyllous tissue which may equal 50% of the maxi- 

 mum for the season. This tissue is photosynthetically competent under 

 the snow, with potential in situ photosynthesis rates of about 25% of the 

 normal seasonal maximum (Tieszen 1974, Oechel and Sveinbjbrnsson 

 1978). In contrast to vascular plants, the photosynthetic moss tissue does 

 not decrease in activity during the growing season (Oechel 1976), but net 

 photosynthesis is reduced as the tissue ages. Early in the season photo- 

 synthesis is carried out by tissue from the previous 1 or 2 years. This pat- 

 tern permits early season photosynthesis but at rates for 2- and 1 -year- 

 old tissue of only 40% to 75% of new tissue, respectively (ColHns and 



