164 F. S. Chapin III et al. 



of the live compartments. Most of the photosynthate produced in the 

 blade is retained within this compartment. Both the stem base and rhi- 

 zome export carbohydrate in early July but become net importers as the 

 season progresses. The calculated rate of net photosynthesis increases 

 gradually through the season even though the blade biomass decreases 

 from early August onward. Net photosynthesis rates calculated from the 

 flux analyses are consistently lower than measured rates until the end of 

 August but then persist at unexpectedly elevated levels until freeze-up. 

 No convincing explanation can now be offered for these discrepancies. 

 The simulated patterns of growth and allocation in flowering tillers 

 (Figure 5-9) contrast strikingly with those in vegetative tillers. In flower- 

 ing tillers the growth rate of the rhizome is strongly negative at the begin- 

 ning of the season, and the rhizome continues to lose weight throughout 

 the season, although it never assumes the role of a major exporter of 

 photosynthate. The stem base grows rapidly and blades develop their ex- 

 porting capacity early in the season. The phase of positive leaf growth is 

 truncated compared with vegetative tillers and never attains the same 

 maximum rate. The second, smaller peaic in leaf growth may be associ- 

 ated with the production of the flag leaf. The inflorescence exhibits the 

 highest growth rate, particularly through midseason, and represents the 

 largest sink for photosynthate. In contrast to vegetative tillers, there is no 

 late-season recovery of stem base and rhizome because the photosyn- 

 thetic rate decreases and inflorescence maintenance respiration increases 

 throughout the season. 



Implications of Biomass Flux Analysis 



Simulations suggest that although the rhizome rapidly decreases in 

 weight early in the season, there is relatively little upward translocation 

 of reserves at this time. Maintenance respiration accounts for the major 

 change in rhizome weight. In general, these simulations suggest that the 

 rapid early-season development of leaves is self-sustaining and that re- 

 mobilization of reserve carbohydrates is of minor importance. This con- 

 clusion is supported by evidence from radiocarbon and carbohydrate 

 analyses and contrasts strongly with earlier conclusions concerning arctic 

 and alpine forbs. Russell (1940), Mooney and Billings (1960) and Fonda 

 and BUss (1966) concluded on the basis of correlations between growth 

 and changes in carbohydrate levels that rhizome reserves were utilized to 

 support rapid spring shoot growth. Further research is necessary to 

 determine whether the difference in conclusions results from differences 

 in methods and interpretation or from differences between graminoid 

 and forb growth patterns and phenology. 



Simulations, seasonal measurements of TNC, and autoradiography 



