Control of Tundra Plant Allocation Patterns and Growth 147 



Root Growth 



Primary root primordia begin accumulating new photosynthate in 

 the spring while the soil is still frozen (Dadykin 1954, Allessio and Ties- 

 zen 1975a, 1978), but growth and elongation do not begin until after soil 

 thaw. In Dupontia elongation of primary roots is complete by late July, 

 whereas other major graminoids of the tundra continue primary root 

 elongation throughout the growing season (Shaver and Billings 1975). 

 Dupontia initiates two to four roots per rhizome node. Autoradiography 

 shows that these roots remain functional throughout the life of the tiller, 

 frequently remaining alive even after the shoot dies (Allessio and Tieszen 

 1975a). A few shorter, slender roots are initiated in later seasons from 

 the nodes of older leafing phytomers and elongate upward between the 

 dead sheaths; they may be important in retrieving leached nutrients from 

 stem flow. Primary roots of Dupontia do not elongate after their first 

 season (Shaver and Billings 1975). Toward the end of the first season 

 lateral roots are initiated with a larger surface-to-volume ratio (Shaver 

 and Billings 1975). As with primary roots, these roots actively accumu- 

 late new '"C photosynthate much earlier in the season than they com- 

 mence visible elongation (Allessio and Tieszen 1975a). 



Root biomass in the tundra at Barrow shows more variation both 

 among and within microtopographic units than between sample dates. 

 Therefore the seasonality of root production is difficult to determine by 

 the harvest method (Dennis and Johnson 1970, Dennis 1977, Dennis et 

 al. 1978). '"C translocation studies (Allessio and Tieszen 1975a) and di- 

 rect observations of root elongation (Shaver and Billings 1975) suggest 

 that much of the root production for the wet meadow tundra occurs in 

 July and August, after the early flush of leaf production. It appears that 

 early in the season new photosynthate is allocated primarily to new shoot 

 growth, and that the root growth that does occur at this time proceeds 

 largely at the expense of rhizome carbohydrate reserves acquired in pre- 

 vious seasons. In the community as a whole, approximately 25% of the 

 root biomass (i.e. 100 g m~^) may turn over each year (Shaver and Bill- 

 ings 1975). This percentage is considerably lower than that found in 

 many temperate ecosystems such as the eastern deciduous forest (Harris 

 et al. 1977), but is approximately the same as that reported for grassland 

 communities (Dahlman and Kucera 1965). The seasonality of root loss 

 through senescence is not known, but loss is assumed to occur largely 

 during winter. Because of the large ratio of belowground to aboveground 

 biomass, roots and rhizomes constitute a major carbon and nutrient in- 

 put to the saprovore food chain and are probably a relatively more im- 

 portant carbon-nutrient source than in temperate ecosystems (Chapter 

 12). 



