Two grasses {Arctopnila {ulva and Elywus arenarius) had distinctly higher rates and also pos- 

 sessed very high growth rates. The light intensity curves for photosynthesis indicate that satur- 

 ation is approached between 2500 and 5000 ftc but in most cases is not attained at 1000 ftc. 

 Light intensities required for compensation are somewhat high due probably to the high respira- 

 tion rates which characterized all grasses. 



Temperature optima for net photosynthesis were usually around 15°C. The COj uptake de- 

 creased at lower temperatures but at O'C there was stiU substantial photosynthesis. At the 

 higher temperatures net photosynthesis decreased rapidly. This was associated with a marked 

 stimulation of the generally high respiration rates by temperature. The temperature optima for 

 the isolated carboxylation enzymes were higher than those for leaf photosynthesis, indicating 

 tliat the decrease in CO^ uptake at high temperatures must be due to other factors. It should be 

 noted, however, that there were species differences in the temperature stability of RUDP car- 

 boxylase. 



Major components of the production studies were the detailed growth rate and phenology of 

 photosynthesis determinations on five species. Growth and photosynthesis were initiated about 

 15 June with the production of the first leaves. This leaf in Dupontia, for example, was the most 

 efficient leaf in CO^ uptake until around 1 July at which time senescence and a decrease in COj 

 uptake ensued. During this time, however, leaves number 2 and 3 emerged and began replacing 

 leaf number 1 as the most important photosynthetic structure. Interestingly, growth as measured 

 by increase in plant height or leaf number attained its optimum between 15 July and 1 August 

 which agrees with the period of maximum production in the entire community. It must be stressed 

 that the last leaves to emerge and develop did so after "maximal production" had occurred. Thus, 

 since they are still actively incorpaating COj, it must be at this time that large proportions of 

 photosynthate are being allocated to reproductive and/or storage tissue. In stiU other species, 

 e.g. Ranunculus nivalis and Saxi/raga cernua, there were distinct differences between the acti- 

 vities of the basal leaves and those attached to the flowering stems. Basal leaves were usually 

 considerably more important in COj uptake. In all cases, however, the actual COj uptake rates 

 were dependent upon light intensity. Thus, time of day, position in the canopy, and the canopy 

 structure were very important factors affecting COj uptake. 



Although the emphasis in the above discussion is on the effect of available light intensity, 

 it must be mentioned that other factors also control production on this and other sites. Some of 

 these comparisons are made in the report on the activities at Prudhoe Bay. At Barrow, obser- 

 vations on the community gradient site (4) reveal additional differences. This site included 6 

 study plots which encompass a moisture and topographic gradient from low, wet meadow and 

 polygon through to raised, dry polygon top. Peak-of-season leaf area indices varied from 1.50 

 (wet meadow) to 0.56 (eroded polygon top), and were associated with similar changes in above- 

 ground production of from 71.6 to 16.6 g/m^ Similarly, the extent of vertical development of the 

 canopy could be related to this moisture gradient. At the alpine site on Niwot Ridge, Colorado, 

 where radiation is considerably more intense, the maximal production of above-ground material in 

 Kobresia meadow averaged 120 g/m^ a value only slightly higher than those obtained from site 

 2 at Barrow. Thus, factors other tlian light intensity and temperature are extremely important in 

 determining the magnitude of the production at different sites. These factors will need to be 

 taken into account as we attempt to generalize from the Barrow site to other tundra areas. 



Primary productiou model 



Phil Miller* San Diego State College 



Lan-y Tieszen* Augustana College 



Each of the previously discussed studies was designed to provide input into a model to pre- 

 dict commiuiity photosynthesis in tundra regions. The model simulates the physiological mech- 



* Principal authors 



35 



