152 F. S. Chapin III et al. 



1976). In these tissues carbohydrates play an active metabolic role, pro- 

 viding the energy necessary for rapid growth in a short growing season 

 and protecting important tissues from freezing. It is mainly in the mature 

 tillers that carbohydrate storage as polysaccharide becomes important. 

 Hence, high TNC levels in plants at Barrow may reflect quite different 

 processes, depending upon tissue age (Shaver and Billings 1976). 



There is a gradual accumulation of carbohydrate reserves in rhi- 

 zomes and shoots as the growing season progresses (Figure 5-5). 

 Overwintering green leaf sections enclosed within the sheath bases may 

 be important reservoirs for respiratory energy and precursor molecules 

 utilized during the rapid production of photosynthetic tissue in spring 

 (McCown 1978) and are an important energy source for grazers during 

 the winter and spring months. In early spring sugar levels are high, 

 presumably the result of hydrolysis of the polysaccharides built up the 

 previous autumn (Shaver and Billings 1976, McCown 1978). After the 

 period of rapid leaf production in June, sugar levels drop and do not rise 

 again until autumn. The seasonal pattern of lipids is unclear. In 1970 ear- 

 ly season levels of leaf hpids were unusually high (17%), but rapidly 

 dropped to a constant level of 697o (McCown 1978). In other years Hpid 

 levels remained low throughout the growing season (Figure 5-5). High 

 lipid levels in early spring could reflect energy storage (Bliss 1962b, 

 Hadley and Bliss 1964) or could reflect small cell size and the abundant 

 membrane lipid associated with meristematic tissue and high metabolic 

 activity (Kedrowski and Chapin 1978). Early season lipids appear to have 

 a lower melting point than those observed later and hence would func- 

 tion effectively in membranes at lower temperatures (McCown 1978). 



The TNC concentration of Dupontia exhibits greater seasonal 

 stabiHty than that of temperate plants (McKendrick et al. 1975, McCown 

 1978), a feature of arctic plants also noted by Warren Wilson (1966a). 

 The seasonal constancy of TNC pool size reflects a seasonal stability of 

 allocation patterns as demonstrated by '"C translocation studies in 

 Dupontia. From snowmelt until time of maximum aboveground biomass 

 (which was the duration of the study) new photosynthate is largely re- 

 tained in the shoot where it is synthesized, presumably to support the 

 continuing production of new leaves through the season (Figure 5-6) 

 (Allessio and Tieszen 1975b). Except in VI tillers, which are in their first 

 year of aboveground growth and which produce the majority of new 

 roots and rhizomes, there is relatively little translocation of photosyn- 

 thate to belowground structures during this first half of the season. 

 Moreover, production of new leaves by a tiller is largely self-supported 

 and is relatively independent of carbohydrate reserves stored below 

 ground. Even reproductive tillers are largely self-sufficient and do not 

 withdraw large quantities of photosynthate from their own or neighbor- 

 ing rhizomes (Allessio and Tieszen 1975a). These observations indicate a 



