284 WHITTAKER AND LIKENS 



may be low. The values, first computed as dry matter, ' have been 

 multiplied by 0.45 to obtain the values for C given. These values are net primary 

 production, i.e., the amount of organic matter (or its energetic equivalent) 

 synthesized by photosynthesis of green plants that remains after the respiration 

 by these plants. The total organic matter produced by the plants (exclusive of 

 photorespiration, but before true respiration) is gross primary productivity. The 

 fraction of gross productivity respired by plants apparently ranges from 50 to 

 75% in many forests (with the higher of these values in the Tropics) downward 

 20 to 40% in many other terrestrial and aquatic communities. 4 1 Too few 

 measurements of plant respiration in the field are available to permit effective 

 estimation of gross primary production for the world. In general, however, the 

 higher respiration rates for forests and lower ones for other communities should 

 compensate for one another and give a gross primary production for the world 

 that is approximately twice the net primary production 80 (compare also 

 Ref. 25). 



Columns 5 and 6 of Table 1 give mean energy equivalents for organic carbon 

 in plants of the different ecosystem types 34 and energy fixed in net primary 

 productivity. Lieth estimates world net photosynthetic efficiency (energy of 

 net primary production/energy of sunlight at the earth's surface) at 0.13%. This 

 value is convergent with estimates based on less detailed analysis of caloric 

 values — 0.25% for net primary production relative to sunlight in the visible 

 range at the earth's surface, and 0.24% for gross primary production relative 

 to the full spectrum. 25 Efficiency of gross primary production for light in 

 the visible spectrum should be about 0.5%. The magnitude of world energy 

 fixation 1 in gross primary production, about 1.4 X 10 1 8 kcal/year, far exceeds 

 man's total use of fossil fuels and other industrial energy, which was about 

 4.7 X 10 1 6 kcal/year in 1970. 



PRODUCTION AND BIOMASS RELATIONS TO ENVIRONMENT 



The second approach to production estimation is through quantitative 

 relations to environmental variables. Efforts to deduce the amount of productiv- 

 ity by way of what ought to be possible for the photosynthetic processes are 

 likely to give exaggerated values. The works of Riley, 53 Ryther, 62 and 

 Russell-Hunter 61 are approaches of this sort that seem unrealistic in relation to 

 field measurements. A more reliable approach is that of induction, establishment 

 of the relations of actual production measurements to environmental variables. 



For terrestrial communities the principal variables are moisture availability 

 and temperature; additional variables are sunlight intensity, nutrient availability, 

 and seasonal change in climatic factors. A number of people have established 

 correlations of productivity with these variables or combinations of them. 



"7 f\ 



Walter showed that in grasslands of fairly dry climates the aboveground 

 production increased with precipitation in a nearly linear manner, at 1 g m 2 



