Control of Tundra Plant Allocation Patterns and Growth 175 



nitrogen, despite the importance of length of photosynthetic season in 

 hmiting total carbon gain (Chapter 4, Miller et al. 1976). At Barrow, the 

 probability of a killing frost increases after early August and may select 

 for downward nutrient translocation at this time to minimize the proba- 

 bility of large nutrient losses. Moreover, root and rhizome growth is 

 quite active in late July, and the belowground demand for nitrogen may 

 be met in part by nitrogen translocated from leaves (Chapin et al. 1980). 



REPRODUCTIVE ALLOCATION 

 AND POPULATION STRUCTURE 



Tiller Interaction 



Growth and allocation patterns have been discussed in terms of a 

 single tiller unit. However, the tiller system is the genetic unit in Dupon- 

 tia, and it responds to selection and interacts to maximize short-term car- 

 bon and nutrient gain and long-term reproductive success for the entire 

 tiller system. 



'"C and '^P translocation studies (Allessio and Tieszen 1975b, 

 Chapin and Bloom 1976) indicate that although mature tillers act largely 

 as independent physiological units, there is still considerable transloca- 

 tion from old V2 and V3 tillers to young VO and VI tillers that have little 

 or no photosynthetic tissue. Experiments in which individual tillers are 

 severed from the tiller system corroborate these conclusions (Mattheis et 

 al. 1976). Only tillers in their first season of leaf production were highly 

 dependent upon other members of the tiller system for normal seasonal 

 growth. Mature tillers initiated and supported more daughter tillers when 

 their reserves were not tapped by the tiller system (Mattheis et al. 1976). 

 Tiller interdependence may be particularly important during regrowth 

 following grazing. Similar intertiller relationships have been observed in 

 temperate grasses (Evans et al. 1964, Marshall and Sagar 1968). 



Neighboring tillers compete for available resources. Nutrients are 

 probably more critical than carbohydrates in governing dry weight in- 

 creases of tillers. Even young tillers have high available carbohydrate 

 concentrations. The number and the length of new VO rhizomes are 

 strongly correlated with rhizome nutrient concentration (Table 5-3). 

 Clearly, the nutritional status of the plant governs the balance between 

 growth of an individual shoot and vegetative propagation and hence has 

 profound effects upon population structure. 



