148 F. S. Chapin III et al. 



Roots of tundra plants, of necessity, grow at temperatures below 

 5 °C and can resume active elongation even after being temporarily fro- 

 zen (Billings et al. 1977). Graminoids at Barrow have lower optimum 

 temperatures for root initiation, elongation, and hence production than 

 do comparable temperate species, when grown in the growth chamber 

 (Chapin 1974a). In part due to low temperature sensitivity of root 

 growth, root production in the field is not correlated with root tempera- 

 ture but appears to be controlled largely by photoperiod (Shaver and 

 Billings 1977). For example, elongation of primary roots in Dupontia oc- 

 curs during the first half of the growing season when root temperatures 

 are lowest. Secondary root production predominates in August (Shaver 

 and Billings 1975, 1977). In Eriophorum angustifolium most of the root 

 tips are close to the retreating permafrost table, so the zone of most rapid 

 elongation occurs in the coldest soil. 



The primary roots of all graminoids at Barrow have large diameters 

 and contain aerenchyma. These roots transport sufficient oxygen to pro- 

 duce an aerobic rhizosphere (Barsdate and Prentki, unpubl.) in a soil 

 that is frequently oxygen-deficient. Because tundra plants are effective in 

 transporting oxygen to roots, it is unlikely that metabolism of primary 

 roots is ever limited by an inadequate oxygen supply. The root diameter 

 probably results from a balance between selection for large diameter for 

 adequate oxygen transport and selection for large surface-to-volume 

 ratio for effective nutrient absorption (Chapin 1978). Both Dupontia and 

 Carex produce secondary roots with small diameter, which are found pri- 

 marily in the surface aerobic soil horizons and are important in nutrient 

 absorption because of their large surface-to-volume ratio. These thin 

 roots probably meet most of their oxygen requirement with soil oxygen 

 rather than by diffusion through the primary roots. The small diameter 

 rooting strategy allows continued exploitation of the surface soil horizon 

 with a minimal carbon investment (Chapin 1978). 



Graminoids at Barrow produce very few root hairs in the field, as is 

 typical of aquatic and emergent plants (Sculthorpe 1967), Root hairs pre- 

 sumably require an external oxygen supply for proliferation and main- 

 tenance and for this reason are of minimal importance in Barrow tundra. 



ALLOCATION OF CARBON COMPOUNDS 



Seasonal Patterns 



Growth and production depend in part upon availability of photo- 

 synthate in the form of total nonstructural carbohydrate (TNC), a pool 

 that includes sugars and storage polymers. The TNC concentrations in 

 leaves, stem bases and rhizomes of Dupontia are quite high, ranging 



