Primary productivity is the rate at which energy is bound or organic matter 

 created by photosynthesis, per unit of the earth's surface, per unit time. 

 In contrast, the amount of organic matter present at a given time is biomass. 

 Production of animals and a saprobes in communities is referred to as sec- 

 ondary productivity. 



The amount of organic matter created or energy bound, per unit area and 

 time, that is left after plant respiration is the net primary productivity. 

 This is the amount of total (gross) productivity available for harvest by man 

 or other animals. 



Only a part of the net primary productivity of rangeland usually is 

 measured. Often only the aerial portion of the plants are sampled, and usually 

 only at one time per year. This aerial portion of production may be less than 

 bellow ground net production, or about forty percent of total production 

 (Sims and Singh 1971). 



Measuring productivity at one time during the year poses some problems in 

 measuring community productivity. The species comprising a community may have 

 peak yearly aerial accumulations of organic matter at different times. Further, 

 the timing of these peaks varies from year to year. Coupland's (1950) forage 

 yield data for six years exhibited a nearly five fold variation and a coef- 

 fecient of variation of 0.68. Yield of short-gross communities is variable 

 even during a series of good years. (Albertson and Weaver, 1944). It is even 

 possible that there may be more than one yearly peak in yield, as in, for 

 example, years with a heavy late summer-early fall rain. 



Single measurements of peak community yields are known to be an under- 

 estimate of productivity. In experiments on four grassland areas in Montana, 

 sampling at the time of peak standing crop gave results which were, on the 

 average, sixty-six percent of the production as measured by summing multiple 

 samples at the species peaks (Kelly et al , 1974). 



Community yield measurements made under field conditions have additional 

 problems. Sampling must be scheduled long before the spring precipitation and 

 plant phoenologies are known. Exclosures can be made to prevent grazing by 

 ungulates, but insects and rodents are still free to consume plants in the 

 sampling area. Lesser amounts of yield may be lost to micioflora, and through 

 leaching and exudation. 



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