anisnis which control photosynthetic rates in an attempt to identify the ways in which tundra 

 conditions may limit physiological processes. The present model was expanded and modified from 

 an earlier physiological model, so as to take advantage of the existing arctic tundra data base. 

 Because of this, the model is ready for first order validation of predictions of tundra photosynthesis 

 using 1970 and earlier data. 



The model first calculates vertical profiles of temperatures of sunlit and shaded leaves by 

 setting the leaf temperature equal to air temperature and repeatedly calculating the leaf tempera- 

 ture with corrected infrared radiation profiles until the leaf temperatures do not change more than 

 0.1°C. After these leaf temperatures are calculated the energy budgets for sunlit and shaded 

 leaves at each level are calculated. The increment of water deficit is calculated from the trans- 

 piration rate and a rate of water uptake. At present, water uptake is assumed to be constant. Then 

 gross photosynthesis is calculated from absorbed solar radiation and leaf resistance. The lower 

 rate of gross photosynthesis is selected and reduced proportionally to the temperature departures 

 from the reference temperatures if absorbed solar radiation is not limiting photosynthesis. Res- 

 piration is calculated in relation to leaf temperatures and net photosynthesis is calculated. The 

 effects of standing dead vegetation on the radiation and wind regimes in the canopy were not in- 

 cluded; these are being added to the model. Since no data on air temperatures were available, 

 prior to 1970 air temperatures within the canopy were set equal to the air temperatures above it. 



The stand photosynthesis model calculates the net production of leaves. This net produc- 

 tion is assumed to be allocated to roots for storage or growth and to leaves for new leaf area. 

 The allocation process defined by the model is constrained by optimum leaf/root ratio and by the 

 energy economy of leaf production. Allocation to seed storage is not yet included in the model. 

 Leaf area expands at a given level depending upon net photosynthesis and an arbitrary maximum 

 leaf area for each level. Surplus in excess of the requirements for already established canopy 

 levels results in a growth of a new level at the top of the canopy. Species position and light 

 utilization characteristics are to be considered in expansion of the model. 



For the purpose of validation, the year 1965 was selected as the most complete set of data 

 for the Barrow area. Numerous transformations of the published and unpublished data were nec- 

 essary for production of a consistent set of units. 



Several conclusions may be drawn fromthe analysis of the 1965 data (recently supplemented 

 by 1970 data): 



1. Even with a low canopy and small leaf area indices, light extinction in the canopy is high 

 because of low solar altitudes. 



2. Production is limited primarily by light and temperature. Warming the area should increase 

 production. 



3. Infrared radiation and ground surface temperatures do not affect transpiration or net pro- 

 duction greatly, because their influence on leaf temperature is slight due to high convectional ex- 

 change. Ground temperatures will affect root respiration and the accumulation of root mass in 

 the growing season. 



4. Evaporation rates calculated with a small leaf area index are similar to those measured 

 in 1965 by Dennis. 



5. E^imary production increases with leaf area index up to a leaf index of about 0.5 then 

 decreases, using photosynthesis-light curves representative of average tundra species measured 

 in 1969 and a leaf slope of 15°. Evaporation continues to increase with leaf area index. 



6. Leaf temperatuies are consistently about 0.5°C below air temperature because of the low 

 intensity of solar radiation, the relatively large negative infrared radiation balance, and the high 

 convectional exchange potential of the leaves. 



36 



