10 VEGETATION OF A DESERT MOUNTAIN RANGE. 



find root. There are many deep soil-filled crevices through which the 

 roots of trees are able to penetrate to bodies of deep-seated soil of 

 favorable moisture content. 



The restricted areas of alluvial soil in the desert and lower mountain 

 regions are of a fine sand or sandy loam and possess considerable humus, 

 in contrast to the soils of the slopes. At the forested elevations the 

 soil is similar to that of the evergreen oak region. The soil of the lower 

 pine belt is scarcely superior in depth or humus content to that of the 

 upper oak region. Above 7,500 feet, however, the amount of humus, 

 as well as the amount of surface litter, increases with the increasing 

 density of the stand of pines. On the north-facing slopes which are 

 clothed with fir forest the soil is not much if at all deeper than in the 

 heavy stands of pine, but is notably richer in organic matter. 



The alluvial slopes which immediately surround the mountain are 

 so closely related to it in all of their physical and biological features 

 that it will be necessary in the following pages to give some considera- 

 tion to their vegetation. Throughout the arid southwest the long 

 straight profiles presented by the outwash slopes of the hills and moun- 

 tains form one of the characteristic features of the landscape. The 

 distinct character of these slopes is to be attributed to the manner in 

 which they have been laid down under conditions of torrential rainfall 

 and of violent and intermittent stream flow, and their distinctness from 

 the parabolic alluvial slopes of the humid regions has caused Tolman * 

 to designate them technically by their popular Spanish name "bajada."]' 



The bajadas constitute almost the total area of all the intermontane 

 valleys of southern Arizona. The only portions of the valleys topo- 

 graphically separable from them are the stream beds, the flood-plains, 

 and the "play as" or undrained areas into which one or more streams 

 flow and deposit their load. To the student of vegetation there are 

 marked differences between the upper and lower portions of all bajadas. 

 The differences in the physical features presented by upper and lower 

 bajadas of the same elevation have been only superficially investigated ; 

 the differences in their vegetation are very obvious, as will be described. 

 The principal environmental features which appear to differentiate the 

 high and low bajadas are the coarser character of the soil in the high 

 bajadas, the possibility of higher soil moisture in them, at least at a 

 depth of several feet, and the greater development of calcareous incrus- 

 tations, or " caliche," in the soil of the low bajadas. The layers of 

 caliche lie near the surface in some places, while in others the upper- 

 most ones have been covered by deposition; they extend downwards 

 for a few feet in some cases, or more frequently recur to a depth of 

 100 feet or more. 



The bajada of the southern face of the Santa Catalinas has been 



* Tolman, C. F. Erosion and Deposition in the Southern Arizona Bolson Region. Jour. Geol., 

 vol. 17, pp. 136-163, 1909. 

 t Pronounced bahada. 



