138 L. L. Tieszen et al. 



dioxide uptake are greatest in leaves of short duration, for example 

 grasses and forbs, and are lowest in the evergreen dwarf shrubs, mosses 

 and lichens. The dominant graminoids attain rates around 17 to 21 mg 

 CO2 dm"^ hr"' and the mosses 1 to 5 mg CO2 gdw' hr^'. 



Leaves rapidly develop photosynthetic competency following snow- 

 melt, and maximal rates are highly correlated with carboxylation activ- 

 ity. The continuous irradiance and a relatively open canopy result in high 

 daily rates of carbon dioxide uptake. Net rates for whole plants are high- 

 est in early July, 200 to 400 mg CO2 dm"^ hr"', and decrease progressively 

 until early September as senescence progresses, self-shading increases, 

 and the irradiance decreases following the summer solstice. Efficiencies 

 of energy conversion are above 1 % for the graminoids and are as high as 

 2.3% for Salix pulchra. The net seasonal incorporation is 602 g m"^ a 

 field value corroborated by the aerodynamic assessment of carbon diox- 

 ide exchange and the simulation model. Approximately two-thirds of the 

 seasonal incorporation occurs after the canopy has developed and has re- 

 plenished belowground reserves. 



Plants are well adapted to prevailing tundra environments. The vas- 

 cular plants and mosses have similar, low light compensation require- 

 ments (5.6 to 15.8 J m'^ s"', 400 to 700 nm), but differ with respect to 

 light saturation. Grasses saturate around 279 J m"^ s'' and mosses satu- 

 rate around 98 J m"^ s"'. On a daily basis vascular leaves are rarely light- 

 saturated, but mosses may be inhibited by high irradiances, especially in 

 open canopies. 



Temperature optima for leaf carbon dioxide uptake are commonly 

 around 15 °C or well above mean ambient temperatures. The high uptake 

 efficiencies on a daily and seasonal basis suggest that this optimum 

 allows plants to function effectively under climatic conditions of the 

 coastal tundra at Barrow. Simulations confirm that a temperature opti- 

 mum of 15 °C allows vascular plants to take up carbon dioxide efficiently 

 across the range of temperatures experienced. This occurs in part because 

 the leaf area is concentrated at the base of the canopy where leaf temper- 

 atures are higher and because the leaves often function on the light- 

 dependent portion of the light response curve. Seasonal temperature ac- 

 climation is not apparent. Mosses, however, show a greater sensitivity to 

 temperature changes and a greater vulnerability to water loss. Thus, they 

 show more frequent water stress than the vascular plants, which are 

 rarely water-stressed even though leaf resistances are low. This results 

 from a low evaporative demand and a high soil water potential. 



Annual carbon dioxide uptake and net primary production are 

 mainly limited by the availability of photosynthetic leaf area. In a typical 

 season, photosynthesis on a land area basis is strongly limited because 

 the canopy is not well developed until late July when solar irradiance is 

 already decreasing. Season length, or more precisely, the date of snow- 



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