Control of Tundra Plant Allocation Patterns and Growth 183 



the following season, which is perhaps why the entire root system of 

 Eriophorum is replaced annually. Annual replacement avoids the cost of 

 maintenance respiration during the second year when the roots may be 

 less functional (Shaver and Billings 1975). Carex produces long-lived, 

 thick primary roots that exploit intermediate soil horizons and thin se- 

 condary roots that are quite abundant in surface horizons. Carex invests 

 proportionately more tissue in roots than do the other two species and is 

 most successful in nutrient-poor situations. 



Interspecific differences in growth and allocation, tiller interdepend- 

 ence, and age class distribution lead to distinct population structures in 

 the three graminoid species. Dupontia from the nutrient-rich habitat 

 shows considerable tiller interdependence, low tiller mortality, and a uni- 

 form age class structure (Shaver and Billings 1975, Allessio and Tieszen 

 1975a, Lawrence et al. 1978). Tiller interdependence may be important in 

 allowing Dupontia to survive acute and chronic grazing. 



In contrast, the longer-lived Eriophorum tillers lose rhizome con- 

 nections and become physiologically independent within two or three 

 years (Shaver 1976). Heavily grazed Eriophorum tillers cannot rely upon 

 the reserves of an interconnected tiller system and tend to occur less fre- 

 quently in heavily grazed polygon troughs. Lack of tiller interdependence 

 may be important in explaining Eriophorum 's apparent success in sexual 

 reproduction, since its reserves may accumulate to support the inflores- 

 cence rather than being continuously siphoned away into the rest of the 

 tiller system. Allocation to sexual reproduction and the windblown seed 

 dispersal pattern in Eriophorum are in part responsible for its abundance 

 in disturbed habitats. An annual rooting pattern is adaptive in disturbed 

 sites, where roots are subject to breaking by frost heaving. Little is cur- 

 rently known about mortality of Eriophorum tillers, but the greater tiller 

 independence in Eriophorum than in Dupontia may well lead to greater 

 variability in recruitment and death in the former and hence more varia- 

 tion in age class structure. 



Carex has a dual tillering pattern. Some tillers (clumping tillers) 

 have very short rhizome internodes so that new shoots are produced ad- 

 jacent to the parent tiller (Shaver and Billings 1975). Other (spreading) 

 tillers have elongated rhizome internodes so that daughter tillers occupy 

 space quite far from the parent tiller. The spreading pattern is most com- 

 mon in phosphorus-deficient sites like polygon basins. The low phos- 

 phorus-to-nitrogen ratio may stimulate rhizome elongation, minimizing 

 competition between parent and daughter tillers. Such an effect has been 

 proposed in temperate grasses (Evans et al. 1964, Langer 1966). 



These consistent differences in allocation patterns among the vari- 

 ous graminoids are consistent and appear to explain their distribution 

 patterns. Differences in life forms are more pronounced and are dis- 

 cussed in Chapter 6. 



