1.2S 



fO.so -- 



o.es -- 



0.00 



Z3 



I I I I I I I I I I I I I 1 1 1 I I 1 1 I I I I I 1 1 I I ill I I I ) I I I 1 1 I I I I I I I I 1 1 1 1 I I I I I I I I ill 1 1 ) I I I 1 1 ' I '' I '''' I I ''' I ''■' I 



IS 10 IS ao ss ad s lo is so zs 30 h 9 m is zt zi 



JUNE TO JUL! 70 nUQJST 70 



Figure 18. Seasonal LAI values, site 2 control plots, Barrow, Alaska. 



tion supply is the effect of leaf shading in the canopy, a factor which is substantially increased 

 by the standing dead material accumulated in the absence of a lemming high. Again in cooperation 

 with the micromet prc^am, photocell systems were positioned in the canopy to record seasonal 

 changes in light extinction through the canopy. Light intensity in the canopy was reduced sub- 

 stantially below 15 cm, and at ground level was generally less than 10% of incoming visible ra- 

 diation. Light intensity differed markedly between noon and midnight and was reduced by the end 

 of the season to about half the amount available at the beginning of the growing season. As is 

 indicated by the photosynthesis studies, leaves are frequently light-limited. 



The approach of the subproject in modeling primary production is based on a prediction of 

 stand production from a knowledge of species responses under various environmental conditions. 

 Thus, detailed photosynthetic characterization of many of the main species was necessitated. In 

 addition, aspects of the project attempted to determine unique adaptations or responses in these 

 plants. Therefore, mechanistic aspects of photosynthesis were investigated. 



The photosynthesis studies included a survey of carboxylation systems in all native vascular 

 plants. From the standpoint of photosynthetic efficiency in CO^ uptake, one might expect some 

 phosphoenol-pyruvate carboxylation plants. The fact that this system is usually associated with 

 tropical plants and taxonomic groups, however, would suggest its absence from this region. In 

 all cases carboxylation activity was higher with ribulose, 1-5, diphosphate than with phosphoenol- 

 pyruvic acid as substrate, although often there was considerable PEP activity. Associated with 

 the high RUDP activity were high COj compensation concentrations (range = 35-80 ppm). Associated 

 with these biochemical and physiological comparisons were more detailed studies of the grasses 

 at Barrow; and, in addition to the above features. aU showed a marked stimulation of photosynthesis 

 in the absence of oxygen. This stimulation was not accompanied by an increase in transpiration 

 or a decrease in stomatal diffusion resistance. All grasses possessed a CO2 dependency at and 

 below ambient concentrations, thus indicating that ambient COj concentrations were limiting photo- 

 synthesis at high intensities. 



The photosynthesis characterizations showed that all responses were similar to those generally 

 associated with grasses possessing high RUDP activity. Thus maximal (light saturated) rates 

 were low and in these grasses usuaUy averaged less than 10 mg COj/dmVhr. These low values, 

 however, are considerably below those characteristic of similar grasses from temperate latitudes. 



34 



