Control of Tundra Plant Allocation Patterns and Growth 165 



all suggest that the large pools of nonstructural carbohydrate contained 

 in belowground structures of Dupontia are not redistributed to any great 

 extent through the growing season and mainly provide substrate for 

 maintenance respiration and, except in the reproductive tillers, are re- 

 plenished during August and September in readiness for metabolism the 

 following season. Simulations support '"C translocation studies (Allessio 

 and Tieszen 1975b) showing that inflorescence development is self- 

 sustaining and not dependent on reserve utilization, even though the rhi- 

 zome rapidly loses weight early in the season. Later development of the 

 inflorescence may well depend on photosynthesis produced by its own 

 chlorophyllous tissue. 



NUTRIENT ABSORPTION 



Seasonal Patterns 



The high levels and relatively stable pool size of nonstructural car- 

 bohydrate strongly suggest that nutrient availability, absorption rates 

 and/or translocation rates are among the important factors limiting 

 plant growth at Barrow. The nitrogen and phosphorus status of the soils 

 is quite low (Chapter 7). More than 99% of the nitrogen and phosphorus 

 present in the rooting zone is organically bound and available to plants 

 only after being released by decomposition, a process that occurs slowly 

 at low temperatures and low oxygen levels. Nitrogen is frequently cited 

 as a key limiting nutrient at the Biome research area and in other tundra 

 systems (Russell et al. 1940, Warren Wilson 1957, BHss 1966, McKend- 

 rick et al. 1978). Exchangeable ammonium, the principal ionic form of 

 nitrogen, remains seasonally constant in the rooting zone but increases 

 through the growing season in lower soil horizons (Flint and Gersper 

 1974), suggesting that nutrient absorption by plants has a significant im- 

 pact upon soil nutrient dynamics. In contrast to nitrogen, available soil 

 phosphorus and potassium decrease in concentration through the grow- 

 ing season (Bar^l and Barsdate 1978), suggesting that phosphorus and 

 potassium will be absorbed by plants earlier in the season than will 

 nitrogen. 



An important asynchrony is apparent in environmental favorability 

 for aboveground and belowground plant processes. Light and air temp- 

 erature regimes are most favorable for photosynthesis immediately fol- 

 lowing snowmelt, whereas soil temperatures and thaw depth increase un- 

 til late July or early August. Seasonal changes in depth of thaw, soil 

 temperature, and the quantity and activity of root biomass influence the 

 seasonal pattern of nutrient absorption. However, simulations suggest 

 that these parameters are less important than seasonal changes in soluble 



