affecting photosynthesis. Photosynthesis on a daily basis is efficient 

 due to the long photoperiod and generally low temperatures . During the 

 "night," however, and particularly in the lower levels of the canopy, 

 carbon dioxide uptake is limited by the availability of light. At these 

 lower levels senescence is initiated by July 15. Carboxylation in all 

 species is mainly with ribulose, 1-5, diphosphate as substrate. Grasses 

 all show a high carbon dioxide compensation concentration, a pronounced 

 oxygen inhibition, and a low light requirement for saturation. Within 

 species, photosynthesis varies as a function of leaf position, age, and 

 season. Maximal rates or capacities of CO2 uptake are low in the 

 grasses due apparently to a high diffusion resistance or possibly a slow 

 rate of translocation. 



On all terrestrial sites, but particularly on the wetter meso-sites, 

 production is directly influenced by the slow decay of undipped standing 

 dead material. It is hypothesized that the availability of nutrients to 

 plants for growth is modulated by the lemming population through accumula- 

 tion and storage of certain elements (N, K, P) in, and their ultimate re- 

 lease from, feces and other dead organic matter. The lemmings, by clip- 

 ping the slowly decomposing standing dead hasten nutrient release as the 

 newly fallen litter decomposes faster than the standing dead. 



The long-term rate of total decomposition in the Barrow area is ap- 

 proximately equal to production, although initial rates of organic 

 matter accumulation on recently drained lake beds are rapid. Based on 

 radiocarbon evidence, depth of thaw measurements, and the absence of 

 widespread organic terrain, organic accumulation and decomposition under 

 uniform climatic conditions are assumed to reach a steady state within a 

 relatively short period (presumably less than 100 years) . Although the 

 rate of organic decomposition is slow, wetter habitats show higher rates 

 of nutrient loss through decomposition compared to better-drained, drier 

 habitats . 



Tundra decomposers are limited by cold temperatures, shortness of 

 season, low inorganic nutrient concentrations , and in the wetter soils by 

 anaerobic conditions. Three microbial zones are easily recognized within 

 the active layer: surface organic, mineral and buried organic layers. 

 The bulk of the microbial biomass and activity is located and takes place 

 within the surface layer of grass roots, mosses, litter, and peat. Early 

 in the summer bacteria are active in decomposing dead plant material, 

 plant exodate, and sloughed off root material. Less actively decomposing 

 peat is found at the lower portion of the surface layer. During mid 

 summer, there is an increase in the quantity of yeast and filamentous 

 fungi and slight decrease in the bacterial flora. The microflora is 

 largely psychrophilic , although optimum growth temperature of the majority 

 of the microbes is around 20°C . The most common bacteria observed are 

 gram negative rods which belong to the genera; Pseudomonas , Achromobacter , 

 and Flavobacterium. Yeast are present in unusually high numbers and blue- 

 green and green algae are present in significant numbers in the soil. 



There is little organic matter in the mineral layer; therefore, the 

 microflora has a low biomass and activity. There are few algae, yeasts 



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