age are important space characteristics, while weight is the 

 quantitative expression of the total mass of structural materials, 

 food substances, protoplasm, and other substances that have re- 

 sulted from the metabolic processes and form the basic reservoir 

 of materials for additional growth and for the endurance of, and 

 recovery from, unfavorable conditions. It is the total weight of 

 these substances that constitutes the forage value of the herbage. 

 Height and area are expressions of the distribution of the mass 

 in space, and determine the availability of the herbage for graz- 

 ing. Most of the research on weight has been done on the above- 

 ground parts of plants, but in recent years some work has been 

 done on the weight of root systems, and of leaves separately from 

 the stems. 



Clipping of the plants within sample areas to secure green or 

 dry weights has been widely practiced for a long time. The plants 

 may be clipped with grass shears, hand sickles, lawn mowers, 

 field mowers, or plucking by hand to more closely resemble graz- 

 ing, but even this may not yield data comparable to grazing be- 

 cause of the differentiating effects of the livestock. Sheep, espe- 

 cially, are highly selective in choosing plants or parts of plants. 

 In order to protect vegetation from grazing, fenced plots and 

 movable cages, varying from a few square feet to several square 

 yards or more, are often employed. A commonly used size of cage 

 covers 9.6 sq ft because the weight in grams from such a plot, 

 multiplied by 10, equals the weight in pounds per acre. Cages may 

 be located permanently, annually, or for shorter periods, depend- 

 ing upon the frequency and duration of the clipping operation. 

 When only the total herbage yield is needed the procedure is 

 rapid, but when the yield of each species is wanted, the separa- 

 tion of the plants of each kind, before or after clipping, is tedious 

 and time-consuming, but the resulting data may be essential in 

 solving some problems. Significant differences in botanical com- 

 position may occur between grazed and mowed stands because 

 the latter may change more rapidly in species composition. To 

 avoid many difficulties the sampling areas should be relocated at 

 fairly frequent intervals. 



As discussed under the section on height, estimation methods 



Analytic CHaracteristics o£ the Community • 109 



