Control of Tundra Plant Allocation Patterns and Growth 171 



TABLE 5-6 Estimated Nutrient and Standing Crop Removed from 

 Shoots by Retranslocation Below Ground 



Note: Assumes that all nutrient disappearance from time of maximum standing stock of 

 nutrients until 24 August results from retranslocation. 

 •Calculated from Tieszen (1972b). 

 Source: Chapin et al. (1975). 



phorus are probably leached slowly (Tukey 1970, Morton 1977) so that 

 the decrease in aboveground concentration of nitrogen and phosphorus 

 after 25 July must be due primarily to downward translocation. 



Because of downward translocation, shoots begin acting as a nutri- 

 ent source rather than a nutrient sink even before peak standing crop is 

 achieved (Figure 5-13). Graminoids retranslocate more than 40% of their 

 maximum aboveground standing crop of nutrients below ground before 

 the end of August (Table 5-6), a quantity comparable to that observed in 

 other communities (Goodman and Perkins 1959, Morton 1977, Chapin et 

 al. 1980). Autoradiography indicates that phosphorus, which is absorbed 

 in late season (September), is stored in the rhizome/stem base rather than 

 being translocated to other plant parts (Chapin and Bloom 1976). Clear- 

 ly, nutrient storage plays an essential role in the strategy of tundra 

 graminoids. 



Effect on Shoot Growth and Photosynthesis 



The pronounced increase in aboveground plant production when 

 nutrients are added to the tundra system indicates that growth is strongly 

 limited by nutrients under natural conditions (Russell et al. 1940, Schultz 

 1964, Bliss 1966, Haag 1974, Chapin et al. 1975). Nitrogen and phos- 

 phorus have been specifically identified by field fertilization studies as 

 two of the most important Hmiting nutrients in the tundra at Barrow 

 (McKendrick et al. 1978). Yet the precise nature of the effect of these two 

 nutrients upon production remains unclear. Phosphorus and nitrogen 

 concentrations in young leaves of Dupontia collected in the field were 



