Primary Producers 249 



lack of standing dead material may be an increased rate of decomposition 

 of the leaf parts that are in contact with the water or mechanical damage 

 by ice. 



The canopy of leaves is very effective in absorbing nearly all of the 

 solar radiation. Each leaf is erect and is angled so that it is close to 

 perpendicular to the sun's rays (it is 85% from the horizontal for C. 

 aquatilis). The sun angle is low, only 21° to 25° during the summer. 



Early in the growing season, the rate of photosynthesis is clearly 

 limited by the small amount of green plant material. Accordingly, the 

 plants are adapted to a rapid production of leaves immediately after the 

 snowmelt by moving stored carbohydrates and nutrients from roots and 

 rhizomes into leaves. This early growth rate can be as high as 0.25 g (g of 

 plant)"' day ' but falls to 0.07 g (g of plant)' day' in mid-season. 

 Plants in the pond may also get a jump on the terrestrial plants because the 

 ice preserves some green tissue over the winter. In the spring, Carex in the 

 ponds has 5 to 6 cm of green tissue on each leaf, whereas terrestrial plants 

 will have only 2 cm. 



The maximum photosynthesis rate of Carex occurs at 15°C but 

 positive net photosynthesis occurs as low as 12°C. The Ps vs. light curve is 

 saturated at a relatively high level so that on a typical day the Ps is 

 saturated only briefly, around noon. There is continuous positive net 

 photosynthesis from mid-June until early August when the sun begins to 

 set. Thus, photosynthesis of the plants is directly related to the solar 

 radiation throughout the summer. 



The Carex and Arctophila are similar to other tundra plants in that 

 they can function at low temperatures. Even at 0°C, roots grow, net 

 photosynthesis occurs, and translocation of carbohydrates takes place. 

 The low temperatures also reduce respiration so that the photosynthesis 

 efficiency of the terrestrial tundra plants was 0.46% and that of the aquatic 

 plants was even higher. These values are about the same as for temperate 

 plants. 



Experiments with terrestrial plants of the Barrow tundra suggest that 

 phosphorus concentrations in the soils limit the plant production. Yet, the 

 concentrations of phosphorus are lowest in the pond sediments where the 

 productivity of the plants is the highest. It appears that the phosphorus in 

 the ponds is more available than that in the soils and, indeed, the binding 

 process is very different in these two systems. Thus, plant production in the 

 ponds is higher than that in the terrestrial system in part because of greater 

 availability of phosphorus. Improved light conditions for the pond plants 

 will also aid in higher productivity rates. 



When compared with temperate plants, phosphate absorption of 

 arctic plants is faster at low temperatures. This is, in part, tied to the high 

 amounts of roots in arctic plants and to root elongation at low 

 temperatures. At Barrow, plants from the ponds had a high Vmax and a 

 high K for phosphate absorption when compared to terrestrial plants. 

 These indicate that the phosphorus is more available in the ponds. 



