THE HALF-GRAVEL-SLIDE COMMUNITY. 



99 



The soils of the half-gravel-sUde are much more favorable for plant 

 growth. Although from one-third to one-half of the surface may still 

 be unoccupied, enough plants are present to prevent almost wholly 

 the slipping of the soil, even the thin surface layer of pebbles being 

 moved between plant clumps only during heavy showers. The first 

 8 to 10 inches of soil has a rich brown color, due to the presence of con- 

 siderable humus formed by the decayed vegetation. It has many more 

 fine particles and fewer large, coarse ones than the corresponding layer 

 on the gravel-slide. Although there is considerable variation, the rock 

 is decayed to a greater depth, due undoubtedly in part to the excre- 

 tions of plant roots and the resultant porosity of the soil, and to greater 

 water penetration following the death and decay of the roots. The 

 soil underlying the surface layer already described, while still con- 

 sisting largely of the decayed granite, is looser in texture and has 

 fewer large particles and much more sand intermixed with it, thus 

 affording a more congenial home for roots. In the gravel-slide only 

 local areas of soil about the sparsely spaced plants are filled with roots, 

 especially in the surface layer. In the half-gravel-slide, on account 

 of the greater number of plants and especially grasses, all of the soil 

 is well filled with roots to a depth of at least 18 to 20 inches or more, 

 while many roots penetrate to a depth of 3 feet. 



The amount and distribution of the precipitation is practically 

 identical with that of the gravel-slide and the actual available water- 

 content is about the same also (table 18). This results from the greater 

 water-holding capacity of the soil, due to the presence of more humus 

 coupled with more perfect rock decomposition, and occurs in spite of 

 the increased absorbing and transpiring surface offered by the plant 

 population. The evaporating power of the air as shown in table 19 is 

 considerably less than that of the gravel-slide, owing to the greater 

 transpiration and shade and the reduced wind movement resulting 

 from the denser community. Notwithstanding the fact that one- 

 third to one-half of the soil surface may still be unoccupied, root 

 competition is rather severe. This is well illustrated in the bisect 

 shown in plate B, though this was made in a rather open portion of 

 the community. It should be borne in mind that the roots seen repre- 

 sent only those actually occurring in a rectangle of soil 7 feet long and 

 4 inches wide. Undoubtedly competition is one of the large factors 

 in determining root distribution. This must be taken into account 

 with the soil conditions in reaching a logical explanation of the root 

 habits of the plants of this habitat. 



