PART IX — TERRESTRIAL ECOSYSTEMS 



vapor. We understand these matters 

 well but still need much additional 

 work in this area. It is the energy 

 exchange for a leaf which drives all 

 other processes critical to the life 

 of the plant. 



The next part of the process, the 

 gas exchange of carbon dioxide and 

 oxygen release, is not well worked 

 out. The chemical kinetics of photo- 

 synthesis and respiration are rate 

 processes which depend on light, 

 temperature, and gas concentration 

 and which are driven by the avail- 

 able energy. In order to understand 

 plant adaptation and response to cli- 

 mate and environment, we must un- 

 derstand the entire process of energy 

 exchange, gas flow, photochemistry, 

 thermochemistry, and physiological 

 reaction. 



Each species of plant has a bio- 

 chemical response which is enzyme- 

 controlled. Some plants photosyn- 

 thesize well at low temperatures and 

 some at high temperatures, some at 

 low light levels and some at high 

 light levels, and so on. More knowl- 

 edge is needed immediately concern- 

 ing these enzyme-mediated processes. 

 Schemes are needed to determine the 

 basic biochemical response functions 

 of chloroplasts and mitochondria 

 within whole leaves as a function of 

 leaf temperature, light intensity, and 

 concentrations of oxygen and carbon 

 dioxide. These measurements must 

 be separated from the whole process 

 which involves gas diffusion and the 

 physical environment. 



The matter of photorespiration, 

 which occurs in most plants, must be 

 understood much better. We want to 

 know precisely how it is that net 

 photosynthesis productivity depends 

 on the climate conditions of radiation, 

 air temperature, wind speed, and hu- 

 midity for each specific kind of plant. 

 Only now are we putting together 

 a complete model that incorporates 

 in a self-consistent manner energy 

 flow, gas diffusion, leaf morphology, 

 anatomy, physiology, and biochem- 

 istry. Such a model is essential if we 



are to understand primary produc- 

 tivity, including the exchange of ox- 

 ygen, carbon dioxide, water vapor, 

 and other gases including pollutants. 

 This is not only important for our 

 understanding of ecosystems but also 

 for our management of crops for food 

 production. 



Energy Relations of Animals 



The energy budget of specific an- 

 imals has been worked out for the 

 first time only in recent years. From 

 the particular properties of a specific 

 animal we are able to predict the 

 climate within which the animals 

 must live in order to survive. Con- 

 versely, for a given set of climatic 

 conditions we can predict the met- 

 abolic rate required for survival and 



this in turn puts limits on the avail- 

 able food supply. Earlier work con- 

 cerning the response of an animal 

 to climate was highly qualitative and 

 descriptive. (See Figure IX-7) Al- 

 though useful, this is not sufficient, 

 since we are dealing with an extremely 

 complex response to a multiple set 

 of variables all of which act simul- 

 taneously. 



Our lack of good physiological 

 knowledge for any particular animal 

 is likely to be enormous. Informa- 

 tion concerning metabolic rates, res- 

 piratory moisture loss, evaporative 

 water loss, and thermal insulation of 

 animals is usually poor and inade- 

 quate. This information is essential 

 to an understanding of the energy 

 balance of animals and their specific 

 response to climate and environment. 



Figure IX-7 — ENERGY BUDGET OF A HORSE 



> INFRARED THERMAL RADIATION 

 FROM GROUND 



The diagram depicts, simply and qualitatively, the multiple energy inputs and outputs 

 that affect a horse. Although not quantified in the diagram, it is possible to describe 

 each input mathematically so that the energy balance of the animal can be com- 

 puted. The result can be used further as a part of a larger model describing the 

 energy balance in a field or pasture where grazing takes place. 



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