72 P. C. Miller et al. 



and the aerodynamic method (Dennis 1968, Tieszen 1972a, b, 1975, 

 1978b, Coyne and Kelley 1975, Miller et al. 1976, Dennis et al. 1978, 

 Oechel and Sveinbjornsson 1978). 



Estimates of carbon dioxide incorporation with the cuvette and the 

 simulation model showed a constant increase in carbon dioxide uptake 

 from the beginning of the season to mid-July (Figure 3-1). The above- 

 ground harvests for the first 30 days of the season showed a constant rate 

 of carbon incorporation that was approximately equal to the initial rates 

 estimated by the cuvette and by the model (Figure 3-1). During the re- 

 mainder of the season the rate of carbon dioxide uptake by vascular 

 plant tops declined. The comparison indicated that during the first 30 

 days, when net photosynthesis by the canopy was increasing, the rate of 

 photosynthate allocation to aboveground biomass productivity was con- 

 stant and the allocation to belowground parts was increasing. In spite of 

 this allocation to belowground parts, the weight of the belowground 

 parts decreased because of respiratory costs associated with maintenance 

 (Chapter 5). During the second half of the season, when green tissues 

 were gradually senescing, both net photosynthesis by the canopy and 

 aboveground production rates were decreasing at about equal rates, and 

 allocation to belowground parts continued. During this period below- 

 ground weights were increasing (Chapter 5). Net photosynthesis in the 

 latter part of the season remained greater than the rate required to main- 

 tain aboveground biomass at peak season levels. The reduction in 

 aboveground biomass must have been triggered by intrinsic controls. By 

 4 August, the standing crop of live aboveground biomass had begun to 

 decline from the peak season level. Aboveground production was becom- 

 ing negative, indicating mobihzation of aboveground organic and/or in- 

 organic nutrients and translocation of these nutrients to belowground 

 parts. Mosses show a more or less constant rate of seasonal CO2 incor- 

 poration (Oechel and Sveinbjornsson 1978). High early season carbon 

 uptake makes mosses active at a time when vascular plants are not highly 

 productive (Figure 3-1, Miller et al. 1978a). 



The seasonal course of atmospheric carbon dioxide flux, estimated 

 by the aerodynamic method, showed a fairly constant rate of carbon di- 

 oxide removal from the atmosphere during the first 20 days of the sea- 

 son, indicating that most of the carbon dioxide incorporated in net pho- 

 tosynthesis was counterbalanced by plant root and soil respiration. From 

 about 10 July to 25 July, atmospheric carbon dioxide flux increased, sug- 

 gesting that respiratory sources of carbon dioxide were insufficient to 

 maintain the observed increase in net photosynthesis. During August, 

 when net photosynthesis declined, ecosystem respiration sources gradu- 

 ally assumed greater importance and carbon dioxide flux from the at- 

 mosphere declined. 



Net photosynthesis, and consequently primary production, was lim- 



