482 F. S. Chapin III et al. 



biological activity and by causing a low ambient temperature during that 

 season. Because they have evolved numerous characteristics that enhance 

 activity at low temperature and lengthen the season of activity, tundra 

 organisms are generally relatively insensitive to the direct effects of low 

 temperature, while still strongly influenced by the indirect temperature 

 effects upon other environmental factors, such as length of growing 

 season, permafrost formation and soil aeration. 



Low temperature and precipitation and a short growing season limit 

 nutrient input to the tundra ecosystem from precipitation, weathering 

 and nitrogen fixation. Therefore, the system depends almost entirely 

 upon recycling of organically bound nutrients. Decomposition, the main 

 bottleneck in nutrient recycling, is ultimately limited by low temperature. 

 The nature of this limitation is largely indirect: the presence of perma- 

 frost, resulting from negative mean annual temperature, restricts drain- 

 age and soil aeration and thereby decomposition. In lower soil horizons 

 aerobic decomposition is severely restricted, so nutrients accumulate in 

 organic matter and pH decHnes. Soil microorganisms, and therefore 

 decomposition, are more severely restricted in their activity by tundra 

 conditions than are other trophic groups, not because they are any less 

 well adapted, but because they must bear the full brunt of interaction 

 between low temperature and anaerobiosis. Moreover, tundra decom- 

 posers have a season of activity only slightly longer than that of primary 

 producers, whereas in temperate regions the season of decomposition 

 may greatly exceed that of most primary producers. Nutrients slowly ac- 

 cumulate in soil organic matter, where they are unavailable to plants un- 

 til decomposed. As a resuh, primary productivity, and therefore energy 

 flow, are strongly limited. 



Animals may increase nutrient release to plants by stimulating or by 

 short-circuiting the slow decomposition process. Soil invertebrates con- 

 sume microorganisms, and lemmings consume vegetation. Both release 

 much of the nutrient content from their food into the soil in soluble 

 form, thus bypassing the decomposition process. Lemmings also fell 

 standing litter and live plant biomass, thereby improving the quality of 

 the substrate and the temperature regime for decomposition. 



The gradual accumulation of nutrients in soil organic matter through 

 succession reduces rates of primary production and nutrient cycling. The 

 continued functioning of the tundra ecosystem may depend upon per- 

 turbations in the steady-state nutrient cycles. Perturbations that assist 

 long-term cycling of nutrients include grazing, thaw lake cycles, and 

 frost action. Thus the present functioning of the tundra system must be 

 viewed in the context of processes that operate on time scales of hun- 

 dreds and thousands of years. 



