176 F. S. Chapin III et al. 



Population Dynamics 



New shoot production and the interaction between members of a 

 single tiller system are important primarily because of implications for 

 population structure. Each tiller may draw upon the resources of older 

 tillers to colonize, grow and reproduce, particularly during its first two 

 years, thus minimizing carbon costs of growth and respiration and the 

 risks of mortality during the early stages of growth. Unlike Carex big- 

 elowii (Callaghan 1976) Dupontia tillers show minimal mortality until 

 after flowering occurs in V3 or V4 tillers. Consequently, VO, VI, V2 and 

 V3 tillers have similar frequencies, whereas V4 tillers are relatively un- 

 common (Figure 5-17). Tiller interdependence averages out the growth 

 and reproductive performance over an environment that is variable in 

 both time and space. The vegetative reproduction characteristics of Du- 

 pontia and other graminoids are not an ahernative to sexual reproduc- 

 tion but rather a strategy of expanding the number of loci at which 

 flowering may eventually occur (Lawrence et al. 1978). Vegetative repro- 

 duction is important under situations where 1) a given tiller has a low 

 probability of survival, e.g. due to grazing, 2) the probability of success- 

 ful seed set in any given year is low, or 3) the probability of seedling 

 establishment from the seed crop produced in a given year is low. All of 

 these conditions characterize the coastal tundra at Barrow and may be 

 important in selecting for extensive vegetative reproduction. The selected 

 strategy, of allocating most resources to competition and to growth 

 rather than to extensive short-term reproductive output, typifies tundra 



Sexual 



V4 



Vegetative 



V4 



V3 



V2 



V3' 



V2' 



VI' 



VO 



FIGURE 5-17. Relative frequency and den- 

 sity of Dupontia fisheri tillers in various 

 age classes and sexual conditions in moist 

 meadow tundra in August 1973. (Lewis and 

 Tieszen, unpubl.) 



