118 



L. L. Tieszen et al. 



9 

 ■a 



8 



E 



in 

 a> 



x: 



c 



o 



Z 



I I I I I I I I I 



J I I L 



J L 



12 16 20 24 4 8 

 Alaska Standard Time 



12 



16 



20 24 



FIGURE 4-8. Diurnal patterns of CO2 flux for alternate 10-day 

 periods through the 1973 growing season simulated for Pogo- 

 natum alpinum (P. a.), Calliergon sarmentosum (C.s.), Dicran- 

 um elongatum (D.e.), anc^Dicranum angustum (D.a.). The envi- 

 ronmental input was the 10-day average for the hour simulated. 

 Periods began on 24 June (A), 14 July (B), 3 Aug (C), 23 Aug 

 (D). and 12 Sept (E). (After Miller et al. 1978a.) 



elation of the linear equation relating daily totals of carbon dioxide up- 

 take to radiation suggests that for the three graminoids slightly less than 

 4.2 MJ m"^ day"' is required to compensate for daily aboveground respir- 

 atory carbon dioxide losses. The value is somewhat greater than the com- 

 pensation points actually measured during night runs, and may suggest a 

 higher respiration rate during daytime than at night. 



Late in the season the combination of shorter photoperiods, reduced 

 irradiances, a developing senescence, and self shading results in a de- 

 crease in the daily incorporation of carbon dioxide. Thus, by 25 August, 

 daily photosynthetic totals for some graminoids are well below 50 mg 

 CO2 dm-^ day-'. 



A multiple linear regression analysis (Tieszen 1975) suggests that in 

 all species there is a highly significant change in photosynthesis which is 

 independent of the seasonal changes in radiation for the entire plant, 

 which could be caused by an increase in the proportion of supporting or 

 other non-chlorophyllous tissues, developing senescence, or other phen- 

 omena. The overall seasonal trend of photosynthesis is one of decreasing 



