DEPARTMENT OF BOTANICAL RESEARCH. 85 



matic gradients of the mountain with the vertical differences of the 

 vegetation. The mountain is characterized by desert on its lower 

 slopes, by open evergreen-oak forest or encinal at its middle elevations, 

 by pine forest above 7,000 feet, and by fir forest on the highest summits. 

 Nearly all the species of plants are distributed so definitely with rela- 

 tion to altitude and habitat as to indicate that they are controlled in 

 their movements and establishment by the operation of physical 

 factors. The major differentiation of vegetation on the mountain is 

 controlled by the factors which are in turn due to differences of alti- 

 tude. The minor influences of slope-exposure and other topographic 

 features cause local departures from the normal altitudinal gradient 

 of vegetation, but these departures are merely such as to bring a given 

 type of vegetation to an altitude higher or lower than that in which it 

 is commonly found. Rainfall, soil-moisture, evaporation, and temper- 

 ature have been studied at a series of stations reaching from 3,000 to 

 9,000 feet, at 1,000-foot intervals. The influence of slope-exposure on 

 the conditions of soil-moisture and evaporation and the influence of 

 topography in modifying the theoretical conditions of temperature 

 have been particularly emphasized. The rainfall at the forested eleva- 

 tions is about 2h times as great as it is on the desert, and the soil- 

 moisture in the driest portion of the year is from 5 to 15 times as great, 

 according to the slope-exposure. The evaporation is 3 to 4 times as 

 great on the desert as it is on the summit of the mountain. The daily 

 and seasonal temperatures are approximately 30° F. lower on the sum- 

 mit of the mountain than on the desert, while the frostless season is 

 about half as long in the former as in the latter locality. 



The ratio of evaporation to soil-moisture in the Santa Catalinas has 

 already been commented upon (see Annual Report, 1912), and it 

 appears to be the climatic feature wliich limits the distribution of 

 the mountain plants at the edge of the desert. The upward limitation 

 of desert plants appears, on the other hand, to be due to the operation 

 of winter temperature conditions. It has been necessary to study the 

 vertical gradient of temperature with special reference to the operation 

 of cold-air drainage, which is very pronounced throughout the Ughtly 

 forested or unforested portions of the mountain. For instance, the 

 difference between the minimum temperature in the floor of a canon at 

 6,000 feet and on the summit of a ridge at the same elevation has been 

 as great as the normal difference between two stations of the same 

 topographic site located nearly 3,500 vertical feet apart. Compari- 

 sons have been made between the chmatic gradients of the Santa 

 Catahnas and gradients derived from the Weather Bureau stations 

 of southern Arizona, situated at different elevations in the valleys of 

 the adjacent region. These comparisons are particularly significant 

 with respect to the rainfall conditions, showing that the isolated moun- 

 tains have a greater rainfall at 4,000 and 5,000 feet than locaUties in 



