GENERAL CONCLUSIONS. Ill 



The principal departures of the vegetation from the ideal gradient 

 that would be found on a geometrical cone are expressed in the irregu- 

 larity of the upper or lower limits of vegetations or of individual species 

 as observed in different habitats. The chief departure is that due to 

 slope exposure, by virtue of which the vegetation of north-facing and 

 south-facing slopes at the same elevation shows striking differences. 

 A second departure is that due to the influence of streams and the 

 high moisture content of the soil of arroyos and flood-plains, by reason 

 of which the plants of all altitudes are carried below their normal 

 lowest occurrences on slopes. Another departure is due to the influence 

 of ridges, on which the plants of all elevations (and particularly those 

 of the Desert) find their highest occurrences. These departures seldom 

 result in the occurrence of distinctive plant communities, but are 

 operative rather in the carrying of the usual and widespread communi- 

 ties into elevations at which they are exceptional. The effect of slope 

 exposure is to carry the normal vegetation of a given elevation both 

 up and down the mountain, so that its lowest occurrences are on north 

 slopes and its highest on south slopes. The effect of streamways is 

 to carry either the normal or the streamside vegetation down the moun- 

 tain, so that the extreme lowest occurrences of almost all Encinal and 

 Forest plants may be sought along the streamways. The effect of 

 ridges is to carry the vegetation (or more particularly individual 

 species and small groups of species) up the mountain, so that all highest 

 occurrences of Desert and Encinal species are to be found on narrow 

 ridges — the highest occurrences of Forest plants are not reached on 

 the Santa Catalina Mountains, and they are controlled by a very 

 dissimilar group of factors. 



It is impossible to study the distribution of vegetation in a region 

 where pronounced differences may be found within short distances 

 without being impressed with the independence which each species 

 exhibits in its allocation. Plants which are associated on the Lower 

 Desert Slopes, for example, range to very different maximum altitudes, 

 and plants which are associated in the Upper Encinal are found to be 

 in part at the upper edges of their ranges, in part at the lower edges, 

 and also in part rather closely restricted to that region. It is nowhere 

 possible to pick out a group of plants which may be thought of as 

 associates without being able to find other localities in which the asso- 

 ciation has been dissolved. Certain plants may be thought of as having 

 closely identical physical requirements because of their associated 

 occurrence in the same spot. Nevertheless, the fact that the vertical 

 ranges and habitat characteristics of these species will reveal more 

 or less pronounced differences goes to show that each of them has 

 survived in a particular section of the climatic gradient. It is true in 

 the Santa Catalina Mountains, as it is true in all other places, that the 

 associated members of a plant community are not able to follow each 



