80 EKDAHL AND KEELING 



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DISCUSSION BY ATTENDEES 



Allen: Dr. Ekdahl, you ended your presentation with the comment that we 

 need to know more about the effect of the increase in atmospheric C0 2 

 on the biota. 1 will comment briefly on the effects of CO2 concentra- 

 tion on a short-term basis on plants. Dr. Lemon's project at Ithaca, N. Y., 

 has developed a computer model of the soil— plant— atmosphere (SPAM) 

 which can be used to predict the effects of increased C0 2 concentration on 

 plant-community photosynthesis. The most critical parameter in this model is 

 the response of leaf stomata to C0 2 concentration. Many terrestrial higher 

 plants open their stomata in response to light. Of these types of plants, some 

 partially close their stomata in response to higher than normal C0 2 concentra- 

 tion and others do not respond very much to external C0 2 concentration. When 

 the atmospheric C0 2 concentration was increased by 27% (from 315 ppM to 

 400 ppM), there was an increase of 7 to 9% in net photosynthesis using the 

 partial stomatal closure model. Simulations of plants that do not close stomata 

 in response to C0 2 gave a 21% increase. Furthermore, increases in diffuse 

 radiation which may accompany climatic changes often gave larger predicted net 

 photosynthesis rates. In general, with 30% of total solar radiation coming from a 

 diffuse hemispherical source, predicted net photosynthesis rates were higher 



