CARBON FLOW AND STORAGE IN A FOREST ECOSYSTEM 357 



When contributions of all vegetation components were added up, total 

 daytime C0 2 uptake for the forest was 4.43 kg m year J and dark respiration 

 was 0.43 kg m year , resulting in an annual net CO2 uptake equivalent to 

 4.00 kg m year . Converting C0 2 fluxes to carbon resulted in net carbon 

 uptake of 1.09 kg C m year . Dark respiration resulted in the release of 0.12 

 kg C m year . A lower bound on gross carbon fixation was approximated by 

 assuming that daytime respiration was equivalent to nighttime rates and adding 

 leaf respiration to daytime C0 2 uptake. The procedure neglected photorespira- 

 tion, a phenomenon that has yet to be evaluated in most forest species. Thus the 

 first estimate of total annual carbon influx (net annual uptake plus two times 

 dark respiration) by woody vegetation is 1.3 3 kg C m 2 year 1 based on 

 initial gas-exchange data. 



Respiration of Woody Tissues 



Carbon efflux due to respiration of lateral roots was estimated from 

 manometric determinations of various-sized yellow poplar roots. Respiration 

 rates (C0 2 efflux) at 15°C were 0.13 mg g" 1 (dry weight) hr" 1 for roots <0.5 cm 

 in diameter, and 0.08 mg g" 1 hr" 1 for larger roots. Multiplying these C0 2 

 respiration rates by the total standing crop of two lateral root-size classes yielded 

 an hourly C0 2 release of 0.07 g m 2 hr l (<0.5-cm diameter roots) and 0. 14 g 

 m hr l (>0.5-cm diameter roots). On an annual basis, respiration by small 

 roots would result in release of 0.59 kg C0 2 m 2 year '. Larger roots would 

 respire 1.23 kg C0 2 m 2 year 1 resulting in 1.82 g C0 2 m 2 year ' total efflux. 

 Conversion of these values to carbon flux yields a total annual carbon loss by the 

 root component of this system equal to 0.496 kg C m 2 year l . 



Shoot respiration data for Fagus sylvatica (Mbller et al., 1954) were used for 

 estimation of carbon efflux by this component of the system. Branch tissue was 

 assumed to respire 0.11 g C0 2 g" 1 year" 1 and bole tissues 0.03 g g" 1 year" 1 . With 

 these provisional values and standing crop of bole and branch weight (Table 2), 

 total respiration by these components was estimated to be equal to 593 g C0 2 

 m year (branch = 322 g/m 2 , bole = 271 g/m 2 ) or a carbon equivalent of 

 162 g C m year total annual carbon loss due to stem respiration, excluding 

 stumps. 



Annual net daytime carbon influx by vegetation was estimated at 1.21 kg C 

 m year 1 . Leaf respiration resulted in a loss of 0.12 kg C m" 2 year * , yielding a 

 net carbon uptake equal to 1.02 kg C m year . Respiration by shoot and root 

 tissues consumed 0.66 kg C m year l of fixed carbon, resulting in a 

 gas-exchange estimate of net carbon accumulation of 0.43 kg C m 2 year * by 

 woody vegetation. 



Heterotrophic Respiration 



Carbon dioxide evolution rates from the forest floor were measured for 

 24-hr intervals biweekly with an infrared gas analyzer, using a technique that 



