CARBON FLOW AND STORAGE IN A FOREST ECOSYSTEM 363 



of up to 360 gC m" 2 year 1 root death from harvest data indicate that the 

 calculated input of root carbon to detritus is not unreasonable. 



CONCLUSIONS 



There are two major reservoirs for carbon in the biosphere exclusive of 

 incipient fossil fuels: the oceans and the terrestrial landscapes of which forests 

 comprise one- third the area and two- thirds of the total terrestrial carbon pool 

 (Olson, 1970; Keeling, 1970). In varying degrees the assimilation of organic 

 matter by ecosystem components of the biosphere compensates for some of the 

 excess C0 2 (increasing at 0.2% or 0.7 ppM/year) released by the combustion of 

 fossil fuels. In addition, the clearing of land continues to release much carbon 

 from tree standing crop and humus pools, which suggests that this ecosystem 

 impact has had a large historic effect on the world's carbon budget. Early 

 International Biological Program synthesis of research (Olson, 1970) indicates 

 that terrestrial plants absorb substantially more carbon by photosynthesis than 

 do aquatic plants. Keeling (1970) has recently used Brookhaven estimates that 

 the biosphere would respond to increased atmospheric C0 2 by an approximate 5 

 to 8% increase in primary production per 10% increase in atmospheric C0 2 . All 

 these factors underscore the importance in understanding the ecological factors 

 affecting the carbon cycle in local and regional ecosystems. 



Residence times of carbon in components of forest ecosystems vary 

 considerably, depending on input and output fluxes as well as pool sizes. Some 

 components such as foilage have a turnover time of 1 year or less, whereas others 

 such as small roots may have a surprisingly rapid turnover rate (350 gC m 

 year _1 /1670 gC m" 2 = 0.21 years" 1 or a residence time of approximately 

 5 years). Rapidlv decomposable components of fresh litterfall may have 

 residence times of only 0.5 years, whereas the turnover rate (0X O4 -8 year ) of 

 soil organic matter in the tulip poplar forest is equivalent to a turnover time of 

 approximately 200 years. Turnover of carbon through the mortality of woody 

 components of trees (50 gC m~ 2 year" 1 /6990 g C m" 2 = 0.0071 year" 1 ) is 

 equally slow and yields a residence time for carbon in this ecosystem component 

 of nearly 140 years. Overall mean residence time for carbon in the tulip poplar 

 forest was 15 years (1465 gC m" 2 year" 1 /21, 510 g C m" 2 = 0.068 year" 1 ), 

 calculated from the ratio of R E to total ecosystem carbon pool. How these 

 individual and collective turnover rates for carbon are affected by seasonal 

 phenomena and how they vary among ecosystems of different type and age are 

 questions that yet need to be answered before we can accurately describe in 

 detail the dynamics of carbon in terrestrial ecosystems. 



ACKNOWLEDGMENTS 



The research reported in this paper [contribution No. 54 from Eastern 

 Deciduous Forest Biome (U. S.-IBP)] was supported in part by the U. S. 



