232 



(2) SoiL 



Existence 

 of Desert, 

 Grassland 



depend 

 the Soil. 



Pioneer 



becomes less as the elevation increases. Hence in high moun- 

 tains. also, the effect of moisture on the vegetation may often 

 be clearly seen. On the lower slopes with a heavy rainfall 

 we find forests, at higher altitudes grassland and finally, if the 

 line of perpetual snow does not extend to the grassland, desert. 

 As already noted also the quantity of water available for deep- 

 rooted plants at high elevations depends not only on the 

 amount of the rainfall but to a great extent also on the tempera- 

 ture of the soil. 



212. A study of the gradual 



changes in the soil and vegetation which may pften be seen 

 occurring in an area covered with naked rock, before 

 or luxuriant vegetation finally establishes itself there, demonstrates 

 ver ^ c l ear ty the dependence of the type of vegetation on 

 the soil. The rocks are first weathered and broken up, 

 slowly it may be but none the less surely, by the mechanical 

 action of heat and cold and the chemical action of the 

 atmosphere and water. On the decomposing rocks and 

 rocky particles small algae and lichens usually soon establish 

 themselves. Lichens aid in the work of decomposition by 

 keeping the rock surface moist and by the action of their 

 absorbing hyphae. The decaying remains of such plants, 

 mixing with the particles of rock, produce a little soil on 

 which mosses, grasses, and other higher plants can establish 

 themselves. In cracks and clefts in the rock also where 

 more soil accumulates than elsewhere, woody plants soon 

 appear which by the action of their roots play an important 

 part in the disintegration of the rock and in the formation 

 of new soil. Many trees and shrubs which in themselves may 

 not be valuable on account of their timber or other products 

 are for this reason of the greatest value to the Forester, they 

 being the pioneers which prepare the way for more valuable 

 species and make the existence of the latter possible. Among 

 such pioneer plants should be noted the very common and wide- 

 ly distributed Salai (Boswellia serrata) and Kulu (Sterculw 

 urens). As this preparatory work proceeds and the accu- 

 mulation of soil increases, more and more trees succeed in estab- 

 lishing themselves, and eventually woodland arises where at 

 first there was only barren rock. Thus here, as the naked rock 

 becomes converted at first into shallow and later into deep 

 soil, we can trace corresponding changes in the vegetation, the 

 desert at first passing into grassland, while the latter eventual- 

 ly gives way to woodland. It is important for the Forester to 

 realize the part played in nature by the pioneer plants noted 



