CARBON FERTILIZATION EFFECTS IN A SIMULATED ECOSYSTEM 343 



DISCUSSION BY ATTENDEES 



Richardson: I thought this was an extremely interesting paper, but your 

 model bypasses the difficult question of how great an increase of atmospheric 

 C0 2 would be necessary to increase tree growth rates by 10, 20, 50, or 100%. Is 

 there any rough estimate of this that you could give us, on the basis of presently 

 available data? 



Botkin: No, there are no data to indicate how much C0 2 produces how 

 much growth increase for a community. I bypassed that by saying, "Suppose 

 C0 2 did produce a growth increment, what would happen then? Suppose it 

 produced a 50% increase, then what?" There are studies, however, that show 

 kinds of growth. For example, there is a study that shows that quite a large 

 increase in C0 2 doubled the litter production of birch seedlings. 



Allen: Botkin's model predicts mixed stand response, assuming equal effects 

 of C0 2 fertilization on all species. However, two points need to be investigated 

 closely to determine first-order plant response. One need is to model realistically 

 the physics of the C0 2 diffusion pathway from the atmosphere to the C0 2 

 fixation sites in leaves. The second need is to determine stomatal opening 

 response to C0 2 concentration. I will describe briefly the predictions of a 

 microclimate and C0 2 uptake model (SPAM) developed by Stewart at 

 Dr. Lemon's project at Ithaca. The most important inputs to this model are 

 photosynthesis-response functions of leaves to light and to C0 2 concentration, 

 which are coupled to responses of stomatal diffusion resistance to light and to 

 C0 2 concentration. Many other climatic and plant parameters of somewhat 

 lesser importance go into this monoculture model. 



The C0 2 uptake has been simulated over a daylight period for corn on the 

 basis of plant and microclimatic data of Aug. 18, 1968, at Ithaca, N. Y. The 

 simulations were run at 315 ppM and at 400 ppM, a 27% increase. If no stomatal 

 closure occurred (typical of some terrestrial plants), the model predicted a 21% 

 increase in daily total C0 2 uptake. If partial stomatal closure occurred, on the 

 basis of corn data, the model predicted only a 9% increase in C0 2 uptake rates. 



Plans are underway to extend this model to a complex ecosystem and even 

 to the global scale. For an ecosystem, knowledge of the vertical and horizontal 

 distribution of types of leaf material and their light responses and stomatal 

 responses to ambient C0 2 concentration are required to predict increased C0 2 

 uptake due to increased background C0 2 concentration. 



The SPAM model uses physics and physiology to predict first-order C0 2 

 uptake. The next questions to be answered are what the consequences are of the 

 increased C0 2 uptake for local and global ecosystems and how fast the 

 additional fixed carbon will be recycled. Botkin's model can give answers to 

 these types of questions, but it does not handle adequately the physics and 

 physiology of the initial C0 2 uptake. 



Wright: Preliminary data on response of dominants in the Brookhaven 

 oak— pine forest to C0 2 enhancement give strong indication that response of 



