PLANT SUCCESSION AND EANGE MANAGEMENT. 35 



chiefly from the first foot of soil (fig. 11). The surface soil is some- 

 what matted but seldom bound so firmly as entirely to prevent the 

 presence of other species. AMiere the second-weed stage is well es- 

 tablished little serious erosion takes place. 



CONDITIONS OF GROWTH AND PRODUCTION. 



The foxglove-sweet-sage-yarrow consociation thrives wherever con- 

 ditions are favorable to the growth of porcupine grass and yellow 

 brush, and even where the soil is not so good. As in the porcupine- 

 grass-yellow-brush consociation, however, the best development of the 

 dominant species of the second-weed stage is found where the soil is 

 fairly well decomposed, mellow, and reasonably moist. In general, 

 the waterholding power of the soil is lower where the perennial weed 

 species predominate than where plant types higher in the succes- 

 sion prevail. This difference in the water-holding capacity of the soil 

 is also associated with differences in soil fertility, and to some extent 

 at least with its physical texture, as Table 2, showing the relative 

 chemical properties of typical soil samples, would imply. The soil 

 representing the porcupine-grass-yellow-brush cover is richer than 

 the soil characteristic of the second-weed stage in all chemical constit- 

 uents here considered, and the difference in the total organic matter of 

 81 per cent in favor of the porcupine-grass-yellow-brush soil is par- 

 ticularly significant. Aside from the fact that this high percentage 

 of organic matter implies the presence of correspondingly large 

 amounts of available nitrates and other plant foods, it has a direct 

 bearing upon the water-holding capacity and the power of water 

 retention of the soil. The average difference in the available water 

 content of the soil samples was 4.6 per cent in favor of the porcupine- 

 grass-yellow-brush area. Obviously, blue foxglove, sweet sage, yar- 

 row, and the associated species will thrive on the moister and richer 

 soils ; but owing to more highly developed root systems, greater lon- 

 gevity, and other life-history characteristics of the plants of the 

 higher developmental stages, the second- weed-stage species are sooner 

 or later forced to yield their places to the more permanent species. 

 However, plants of the foxglove-sweet-sage-yarrdw consociation, 

 particularly the dominant species, are comparatively resistant to 

 drought, and maintain themselves w;ell under adverse climatic con- 

 ditions. 



