Primary Producers 233 



only 0.05% P in the leaves and Ulrich and Gersper (1978) pointed out that 

 this low a concentration can affect photosynthesis. Calcium (Table 5-8) is 

 low in the new leaves but increases in older tissue. This could either be a 

 result of its low mobility or a result of increase in secondary cell wall 

 content. Chapin el al. (1975) show that calcium, magnesium, and iron all 

 increase in concentration through the growing season (e.g., Ca in Figure 5- 

 32). 



Nitrogen is highest in the youngest leaves (Table 5-8) but this 

 element, along with phosphorus and potassium, is actively retranslocated 

 back to the roots and rhizomes late in the growing season. The nitrogen 

 standing stock of Carex on a tundra site (Figure 5-32) actually peaked in 

 mid-July and then was reduced to about 50% of the maximum by late 

 August. The start of this decrease may have preceded the biomass peak. 

 Exactly the same thing occurred in Carex and Arctophila from the ponds 

 (Table 5-9). Chapin et al. (1975) reported that the ratios of end of season 

 (24 August) standing stock to maximum standing stock were 0.47 for N, 

 0.45 for P, 0.36 for K, 1 .0 for Ca, and 0.62 for biomass. Thus 53% of the N 

 was retranslocated and even greater amounts (64%) of the K. Calcium was 

 not retranslocated but 38% of the biomass was lost from the aboveground 

 plant parts. A separate measurement (McCown 1978) showed that Barrow 

 graminoids contain 30 to 40% non-structural carbohydrate at the time of 

 maximum biomass so that it appears that these carbohydrates are also 

 efficiently extracted from the senescing shoots. 



700 



'E 600 



cr 



E 



500 



c 

 a> 



o 



'z 400 



o 



(D 



300 



200 



2 100 

 en 



E 



56 

 48 

 40 



E 



I 32 

 o 



24 



o 



c 



16 



^ 8 



n — r- 



1 1 r- 



-| 1 1 1 1 r 



32 



28 



'E 

 24 o. 



20 ° 

 o 



-16 



o. 



H'2 5 



c 

 ■6 

 c 

 o 



FIGURE 5-32. Biomass and standing stocks of nitrogen and calcium in 

 shoots of Carex aquatilis on tundra near Barrow. (After Chapin et al. 

 1975.) 



