466 F. S. Chapin III et al. 



tundra at Barrow are similar to rates observed at much higher tempera- 

 tures in the shortgrass prairie (Chapter 5). However, at Barrow the grow- 

 ing season is short, and 90% of the aboveground vascular biomass senes- 

 ces each fall. Consequently, the tundra exhibits a small standing crop of 

 photosynthetic tissue. The net quantity of carbon fixed by photosynthe- 

 sis at Barrow (174 g m"' season"'; Figure 12-1) is about half the quantity 

 fixed by a shortgrass prairie. This corresponds to a Barrow growing sea- 

 son which is half as long as that in the prairie (Coleman et al. 1976). It 

 thus appears that the annual carbon input into the coastal tundra ecosys- 

 tem at Barrow is limited not so much by light and temperature effects 

 upon photosynthesis as by the shortness of the growing season, which in 

 turn limits the standing crop of photosynthetic tissue (Miller et al. 1976). 

 Carbon flux through the tundra at Barrow is slow. Less than 1% of the 

 ecosystem carbon pool turns over annually. This contrasts sharply with 

 tropical systems, where 40% of the organic carbon in a rain forest is 

 fixed and respired each year (calculated from Odum 1970). Radiocarbon 

 dating of surface and buried organic matter from soils in the Barrow re- 

 gion yields ages of as much as 10,000 years (Brown 1965). These suggest 

 that there are large pools of soil organic matter with very slow turnover 

 rates and other pools that turn over much more rapidly than the ecosys- 

 tem average, as demonstrated in temperate and tropical soils (Jenkinson 

 and Rayner 1977, Jenkinson and Ayanaba 1977). 



The partitioning of photosynthetic carbon and its subsequent loss in 

 respiration emphasize the belowground nature of the system. Of the 214 

 g m"^ of total carbon fixed annually (total net daytime photosynthesis), 

 only 19% is lost as aboveground dark respiration, in part because of the 

 long periods of daylight during summer (Figure 12-1). Another 22% is 

 converted to aboveground biomass, and the remaining 59% is translo- 

 cated below ground. Of the carbon translocated below ground, approxi- 

 mately half is converted to new tissue and half is lost in respiration. This 

 contrasts with the shortgrass prairie (Coleman et al. 1976), where 34% of 

 the total carbon fixed is respired above ground, 9% is converted to 

 aboveground production, and of the 57% translocated below ground, 

 85% is converted to new biomass. Apparently tundra plants produce 

 shoots efficiently because of the long period of daylight, but much of the 

 belowground carbon is used in maintenance respiration for the large 

 standing crop of roots and rhizomes. The large proportion of litter re- 

 leased below ground, where decomposition rates are low, may be one 

 factor leading to organic accumulation in tundra. However, in grass- 

 lands, where a substantial proportion of the litter is also shed below 

 ground, soil conditions are more favorable for decomposition, and there 

 is less accumulation of soil organic matter. 



The relative importance of plants and soil organisms as sources of 

 soil CO2 is unclear. The proportion of soil respiration accounted for by 



