230 V. Alexander etal. 

 Productivity 



The primary production was measured in two ways. One was by 

 measuring the accumulated aboveground standing crop at the end of the 

 growing season. The other was by measuring the uptake of carbon-14 by 

 plant parts incubated under water in clear or dark glass jars for 4 to 6 

 hours. After incubation, the dried plant material was combusted, the 

 labeled carbon dioxide trapped in a strong base (NCS from Amersham- 

 Searle), and the '*C counted by liquid scintillation. 



Based on the maximum aboveground biomass attained each year, two 

 different projects estimated that the production of Carex in ponds was 290 

 to 370 g dry wt m ^ (Table 5-7) and 89 to 219 g dry wt m^^ (Tieszen, 

 Mandsager, and Vetter unpublished data). Terrestrial production at 

 Barrow is 80 to 130 g dry wt m ^ (Miller and Tieszen 1972) so these values 

 are reasonable. All these values, however, refer only to aboveground 

 biomass, and production data for belowground plant material cannot be 

 obtained by a single harvesting procedure. Shaver and Billings (1975) 

 suggest from longevity and standing stock measurements that 100 g dry wt 

 m~^ is a likely value for root production in tundra plants. This value 

 agrees with the data of Dennis and Johnson (1970). 



The production of root biomass in the ponds is probably also equal to 

 the aboveground production; a likely value is 140 to 180 g C m " yr^ 

 (388 to 500 g dry wt m~^). The total root biomass could, therefore, be 

 replaced every 3 years. 



Productivity estimates based on '"C uptake in the pond did not agree 

 with the biomass measurements and so have only relative value (Figure 5- 

 31). A rough integration of the curve of the photosynthesis per day gives 

 9.4 g C m ^ or about 20 g dry wt which is at least an order of magnitude 

 too low. It is likely that the water contained too little carbon dioxide to 

 permit maximum photosynthesis. For example, the highest photosynthesis 

 rate in Figure 5-31 is equal to 0.27 mg C g^^ hr '. Yet the rates of carbon 

 dioxide '^C uptake measured by Tieszen and Johnson (1975) peaked at 5 

 to 9 mg C g " ^ hr ~ ' for Carex in the air. When the leaves are incubated in 

 500 ml of pond water, there is only 0.1 to 2 mg C available for 

 photosynthesis (Chapter 4) so obviously carbon soon became limiting. 



Productivity of the plants on the terrestrial site has also been 

 measured by continuous measurement of gas exchange using an IRGA 

 (infrared gas analyzer). For the whole community, Tieszen (1978a) 

 estimated a net incorporation of carbon dioxide carbon of 207 g C m "^ 

 yr '. This agreed well with the 210 g estimated by the aerodynamic studies 

 of Coyne and Kelley (1975). Tieszen (1978b) suggested that the carbon 

 budget might be as follows: net production of shoots, 46 g C; respiration of 

 shoots, 39 g C; net production of belowground rhizomes and roots, 61 g C; 

 respiration belowground, 61 g C m " yr '. 



It is difficult to know how closely the parallel can be drawn between 

 production in the terrestrial and aquatic systems. Based on Tieszen's 



