THE STRUCTURAL CHARACTERISTICS OF THE COMMUNITY 33 



especially in regions with a dry resting period, necessarily demand 

 several examinations per year. Between dry and wet years there will 

 often be (particularly in steppes and deserts) very great differences in 

 the cover of the species on the same area. 



Determination of Space. — The degree of dominance of the species in 

 plankton communities is given by the number of individuals of a species 

 in a unit volume (1 cc.) of water, multiplied by the volume of one indi- 

 vidual. In rooted communities the estimate of cover always gives an 

 approximate idea of the space occupied by the species. 



Del Villar (1925) has attempted a determination of the space 

 occupied by shrubby species in the Spanish meseta. He assumed the 

 volume of the shrubs to be ellipsoidal and calculated the cubic contents 

 from the vertical and horizontal diameters. For Salsola vermiculata he 

 found 208 cu. m. per hectare; for Thymus zygis, 197 cu. m. The fre- 

 quency percentage (according to Raunkiaer) gave for Salsola only 

 7.2 per cent; but for Thymus, 55 per cent. In a certain sense the 

 measurement of the cubic volume of standing timber by the forester 

 may be considered as a determination of dominance. According to 

 Hausrath, Pinus silvestris on an average soil produces in 100 years 404 

 cu. m. of wood; in 120 years 430 cu. m., whereas Picea excelsa in the 

 same time produces 739 and 806 cu. m. respectively. 



Weighing. — The method of weighing the shoots has hitherto been 

 apphed almost exclusively to pasture studies. The grass of a small 

 measured piece of ground is sheared off, the shoots of each species are 

 sorted out, and the amount of each species is given in percentage by 

 weight. Or 1 sq. ft. of turf is dug out and the percentage by weight is 

 determined for both aerial and buried parts (c/. Stebler and Schroter, 

 1892). 



Kultiassoff (1927) in Central Asia investigated the decrease in 

 weight of the root mass with increasing depth. 



Table 2. — Decrease of Root Mass with Depth in Soil in the Thebophyte 

 Steppe Near Taschkent 

 Depth, Root mass, 



centimeters grams 



OtolO 1,400.2 



10 to 20 84 



20 to 30 20.6 



30 to 40 3.0 



40 to 50 2.2 



50 to 60 3.0 



60 to 70 1.5 



In the therophyte steppes of Taschkqnt the weight of roots exceeds 

 the weight of tops fifteen times (tops per cubic meter about 100 g., 



