APPENDIX 375 



view and adds that terrestrial detritus pools are probably shrinking as well 

 through decreases of input and increases in decay rate. Thus, either the estimates 

 of the flux of carbon through the biota, or of the rate of solution into the ocean 

 are in error. This raises a variety of questions, some of which are outlined below. 

 Most of the questions are related to terrestrial ecosystems since these systems 

 account for about two-thirds of the world's net primary productivity and more 

 than 99% of the world's biomass (Whittaker and Likens, this volume). 



Photo synthetic Responses to C0 2 Enrichment 



There is abundant evidence that enhanced C0 2 concentrations stimulate 

 higher photosynthetic rates under conditions of nonlimiting light, water, and 

 nutrients. The important questions here are: (1) How much stimulation of net 

 annual ecosystem production can be expected, and (2) does increased 

 photosynthesis automatically lead to higher biomass, and thus larger carbon pool 

 sizes? 



First, it is not at all clear how much increased primary production can be 

 expected over the earth's surface with CO2 enrichment. Many vegetation types 

 are limited by other factors for large portions of the time. For example, a 10% 

 increase in C0 2 might lead to a 10% increase in net photosynthesis under ideal 

 conditions, but factors other than C0 2 might be limiting for 75% of the growing 

 season. Thus annual yields of net production might increase by only 25 X 0.10, 

 or 2.5%. 



A second point concerns ecosystems dominated by nonwoody plants or 

 small woody plants with characteristically high rates of biomass turnover and 

 prominent grazing pathways of energy flow. Perhaps the best examples of such 

 situations are agricultural lands: as more biomass is produced, more is consumed, 

 and no net gain in carbon storage is possible. 



Third, we have the case of ecosystems dominated by woody plants of 

 indeterminant growth form, such as forests. Actually there are limits to sizes of 

 all plants, although it is not always clear whether they are set by genetically 

 controlled senescence or by site factors. Assuming forest productivity is 

 enhanced by increased C0 2 levels, and assuming this extra increment of 

 production is largely stored in biomass or in larger standing states of detritus, 

 again there must be some limit to the biomass and detritus increases. If not set 

 by genetic limits, the increases will be set by some other environmental factor. If 

 some new maximum biomass level is attained, then there will be a new limit to 

 carbon storage. If true, then the land plant biomass could, at best, act as a net 

 carbon sink for only a few decades. This effect would occur mainly in forests, 

 and, to the extent that forests are harvested, the effect would be limited. This 

 result is demonstrated in a simulation of forest growth by Botkin in this volume. 



Broecker, Li, and Peng 1 6 concluded that the biota is probably not an 

 important sink for excess carbon. Their conclusion, however, was based on much 

 lower estimates of living and dead organics (detritus) than those used in this 



