HISTORICAL. 13 



In the case of most fairy rings, the fungus produces a temporary stimulating 

 effect only, and the ring is indicated merely by the increased size, vigor, and 

 chlorophyll-content of the annuals and the perennial grasses. 



The stimulation of the grasses and other plants which produced the inner 

 and outer zones is probably due to the presence in the soil of nitrates and 

 ammonia salts derived from (1) the reduction of the organic matter of the soil, 

 (2) the decay of the mushrooms, and (3) the decay of the mycelium. The 

 bare zone results from the death of the vegetation as a consequence of a lack 

 of available soil moisture. Water penetrates very slowly into the sod filled 

 with mycelium when it is once dry. The increased growth in the outer zone 

 hastens the drying-out of the soil and, once dry, the latter is not wetted by 

 heavy and continued rain. The vegetation is not noticeably damaged during 

 growing seasons uniformly wet, but it quickly shows the effect of dry years or 

 periods of drought. The secondary sere initiated by the fairy rings is essen- 

 tially like that caused by any other disturbance in the short-grass association. 



Shantz and Aldous, 1917. — In the field instructions for classifying public 

 lands under the terms of the Stock Raising Homestead Act of 1916, Shantz 

 and Aldous have made the most comprehensive use of indicators for the pur- 

 pose of land classification. Ninety different types are recognized as indicator 

 communities and are described briefly, though usually without a statement of 

 the correlated conditions. Of these, 32 belong to the prairie-plains grassland 

 climax, 20 to the sagebrush climax, 16 to the desert-scrub climax, and 9 to 

 the chaparral. The types are designated by the names of dominants and 

 subdominants and represent both serai and climax communities. Density, 

 percentage of grasses and grass-like plants, and height of shrubs are also made 

 use of for minor indications, while overgrazed areas are given especial atten- 

 tion. A key to correlation conditions and crop-producing capabilities was 

 filed with the Geological Survey and is used by it in the interpretation of 

 the types. 



Weaver, 1919.— While the work of Shantz (1911), Weaver (1915), Sampson 

 (1914, 1917), and of Cannon (1911, 1913, 1917), Markle (1917), and others 

 had laid the basis for the consideration of root systems in connection with 

 indicator values, the first special and comprehensive study of the indicator 

 significance of roots was made by Weaver in 1918. This investigation derives 

 its importance not only from the thoroughness of the methods, but especially 

 also from the large number of species concerned, the wide range of the com- 

 munities, and the consistency of the instrumental results. Approximately 160 

 species were investigated, involving the examination of about 1,150 individual 

 plants. These were largely grasses and grassland herbs, but they included 

 shrubs, undershrubs, weeds, and forest herbs as well. The communities 

 represented were the prairies of Nebraska and the Palouse region of the North- 

 west, the short-grass plains and the sandhills subclimax of Colorado, the gravel- 

 slide and half-gravel-slide associes, and the forest climax of the Pike's Peak 

 region. In practically all these, readings were made cf water-content, humid- 

 ity, temperature, and light, and in critical ones of transpiration as well. In 

 showing the community relations of competing root systems use was made of 

 the quadrat-bisect (plate a; cf. Weaver, 1919, for plate 26a). Many of the 

 detailed results have been utilized in the discussion of particular indicators 

 in Chapters IV and VI. 



