Control of Tundra Plant Allocation Patterns and Growth 167 



soil phosphorus in determining the seasonal pattern of phosphorus up- 

 take (Chapin and Bloom 1976). The seasonal pattern of phosphorus re- 

 lease from decomposers is probably the single most important factor 

 governing phosphorus availability and therefore phosphate uptake by 

 vascular plants (Chapin et al. 1978). Absorption of phosphate continues 

 actively until late September when the soil begins freezing from the sur- 

 face downward (Figure 5-10). More than 40*^0 of the total phosphorus 

 absorbed by a given root biomass is acquired after 25 July, the date when 

 shoots begin a net downward translocation of phosphorus for below- 

 ground winter storage. Clearly, aboveground phenological patterns are 

 an inaccurate gauge for determining periods of plant activity. Because 

 total root biomass increases through the growing season, end-of-season 

 nutrient absorption is probably even more important than the above dis- 

 cussion would indicate. 



Physiological Basis 



Tundra graminoids differ from their temperate counterparts in hav- 

 ing higher phosphate absorption rates under standard measurement con- 

 ditions (Figure 5-11). Furthermore, tundra plants maintain substantial 

 rates of phosphate absorption at temperatures that would inhibit active 

 uptake by most temperate plants (Sutton 1969, Chapin 1974a, Carey and 

 Berry 1978). For example, Dupontia grown in the field still maintains 

 35% of its 20 °C phosphate absorption rate at 1 °C (Chapin and Bloom 

 1976), which suggests that tundra plants actively absorb phosphate from 

 cold soils and do not depend upon daily or seasonal warming of the soil 

 to fulfill their phosphate requirements. Phosphate absorption by tundra 

 plants is relatively insensitive to temperature changes and has an opti- 

 mum temperature of at least 40 °C. It would appear that the phosphate 

 absorption process in graminoids has adapted to low temperature by a 

 decrease in temperature sensitivity below optimum temperature, by an 

 increased affinity of roots for phosphate at low temperatures (Chapin 

 1977), and by an increase in uptake rate at all temperatures, but not by 

 any change in temperature optimum. Similar conclusions were reached 

 for the photosynthetic process (Chapter 4). The ability to acclimate in 

 compensation for temperature changes is not well developed in plants at 

 Barrow, as might be anticipated in a thermally stable environment such 

 as the tundra soil (Chapin 1974b). The overall effects of temperature 

 upon rate of phosphate absorption by Dupontia are such that the rate at 

 the bottom of the soil profile (0.2 °C) is approximately 75% of the rate at 

 the top of the profile (5.0 °C) (calculated from Chapin and Bloom 1976). 



A plant's capacity to absorb nutrients depends upon its nutrient 

 status and allocation pattern. Plants with low concentration of an essen- 



