MODEL OF STAND PHOTOSYNTHESIS FOR WET MEADOW TUNDRA AT BARROW, ALASKA

A model of radiation, air temperature and vapor density in the vegetation canopy, plant water relations and photosynthesis, developed in the primary production research program of the U.S. Tundra Biome, IBP (International Biological Program), is described and results for the years 1970-1973 presented. The model calculates daily courses of direct and diffuse solar radiation; IR radiation; wind; air temperature and humidity; leaf temperatures; convectional and transpirational exchange by leaves, stems and dead material; leaf H2O content, leaf water potential; leaf resistance to H2O loss; internal resistance to CO2 diffusion; and net photosynthesis. Climate varied from year to year: 1970 and 1971 were similar, 1972 was relatively warm and dry and 1973 was relatively cold and wet. Plant parameters were obtained for Carex aquatilis, Dupontia fischeri, Eriophorum angustifolium and Salix pulchra. Leaf area indices varied by species and by year. Leaf areas of all species were lowest in 1973. Of the incoming solar radiation .apprxeq. 20% is reflected back and 32%-53% absorbed by the canopy, increasing with leaf area index. In the canopy 5-10 times more heat is lost by convection than by evaporation. Total seasonal vascular plant CO2 uptake ranged from 400-627 g CO2 .cntdot. mG-2 (subscript G refers to ground). In 1971 cuvette estimates for total seasonal vascular plant CO2 uptake were 602 while the model predicted 627. Individual species incorporated about 4 g CO2 .cntdot. mG-2 .cntdot. day-1 at most. The daily CO2 incorporation was larger than the downward CO2 flux from the atmosphere, implying that soil respiration is a source of CO2 for the vascular plants. Photosynthesis increased with solar radiation, air and ground temperatures and air vapor density and decreased with increasing IR radiation for the sky and root resistance to water uptake. Factors increasing transpiration without directly affecting photosynthesis tend to decrease photosynthesis because of the effect on water stress. Net photosynthesis is higher at the top of the canopy, despite more favorable temperatures within the canopy, because of light limitation. Total accumulated CO2, after subtracting growth costs of leaves, become positive late in the season and is highest at the lower levels because of the longer duration of leaf area at these levels. Photosynthesis seems adjusted to maximize C gain under the most frequent conditions but not under all conditions or extreme conditions.