110 L. L. Tieszen et al. 



independence of photosynthesis and potassium, nitrogen and phosphor- 

 us over a large range of field concentrations provides strong evidence for 

 precise control of allocation in response to available nutrients. Plants of 

 the coastal tundra at Barrow do not appear to produce additional leaf 

 area unless they can operate at near maximal capacity. Thus, under field 

 conditions, plants seem to avoid nutrient limitations of photosynthesis 

 by Hmiting the amount of photosynthetic tissue within the support capa- 

 bilities of the available nutrients (see also Chapter 5). Ulrich and Gersper 

 (1978), however, show that these plants are always on the borderline of 

 being nutrient-limited; and the addition of phosphorus and nitrogen 

 clearly stimulates production (Chapin et al. 1975, Dennis et al. 1978). 



EXTRINSIC FACTORS AND THE RATE 

 OF PHOTOSYNTHESIS 



Light 



Arctic tundras have often been described as light-limited ecological 

 systems. Daily totals of irradiance can be high at Barrow, but instantane- 

 ous irradiances are generally low because of the low sun angle and fre- 

 quent cloudiness. Light response curves for vascular plant species are 



TABLE 4-4 Irradiance (300 to 3000 nm) 



Required for Photosynthesis to 

 Equal Respiration as Determined 

 from Field Measurements at 

 Temperatures near 0°C 



Irradiance 

 Species (J m'^ s"') 



Vascular plants 



Dupon tia fisher i 1 6 . 7 ± 2 . 1 



Carex aqua t His 9. 1 ± 0.7 



Eriophorum angustifolium 9.1 ±0.7 



Salix pulchra 14.0 ±2.1 



Vascular plant mean 12.6 ±3.5 



Mosses 



Pogonatum alpinum 9.1 ±2.1 



Calliergon sarmentosum 1 0. 5 ± 3 . 5 



Dicranum angustum 5.6 ± 2.8 



Dicranum elongatum 10.5 ±4.2 



Polytrichum commune 1 1 .2 ± 1 .4 



Moss mean 9.1 ±2.1 



