CLIMATE OF THE SANTA CATALINA MOUNTAINS. 59 



individuals of all Encinal and Forest species are subjected to conditions 

 of water supply which are perhaps below any conditions that have 

 previously occurred during their lives, or are surely in the case of 

 perennials the most trying conditions when considered in the light 

 of the plants having grown to greater size and heavier water-demand 

 than during the dry periods of their earlier existence. 



SOIL MOISTURE. 



It is obvious, from a consideration of the monthly distribution of 

 rainfall in the Santa Catalinas, that at all elevations there are annually 

 two periods of high soil moisture, coinciding with the humid mid- 

 summer and the humid winter, and two periods of decreasing soil mois- 

 ture content, coinciding with the arid fore-summer and arid after- 

 summer. The influence of the earliest rains of summer and winter is 

 quickly exerted in an elevation of the soil moisture, but at the close 

 of these seasons it is with relative slowness that the soil falls to low 

 percentages of moisture, particularly at the highest altitudes. The 

 minimum moisture content of the year is usually to be detected just 

 before the first heavy rain of the humid mid-summer, but the content 

 in September or October may sometimes be quite as low. 



At low elevations in the Santa Catalinas the annual march of soil 

 moisture may be expected to be analogous to that which has been 

 described by the writer for Tumamoc Hill, the site of the Desert 

 Laboratory.* Marked differences will result from a comparison of the 

 two localities, however, owing to the difference in the character of 

 the soil. The very fine clay of Tumamoc Hill is conservative in its 

 changes of moisture content, both with respect to increases and de- 

 creases of moisture, while the coarse loam found at the lower elevation 

 in the Santa Catalinas possesses a greater permeability and a lesser 

 holding power. The soils of elevations of 7,000 feet and more are 

 richer in organic matter than those of the Desert and Encinal regions 

 of the mountain, and are doubtless more like the clay of Tumamoc 

 Hill in the smoothness of their curves of change in moisture content. 



The few readings of soil moisture content that have been made were 

 directed toward a determination of the soil conditions in the most arid 

 portion of the year. It is obvious that it is these annual minima which 

 are of the greatest importance to plants, particularly to such plants 

 as are near the lowest limit of their vertical occurrence. Much less 

 interest attaches to the high moisture contents which might be found 

 in the midst of the rainy seasons. It is true that these high moistures 

 are the ones which call forth general vegetative activity and condition 

 the appearance of ephemeral plants at the lower altitudes. It is like- 

 wise possible that high and protracted soil moistures may be of some 

 importance as a limiting factor for desert species at the upper edges 



* Shreve, Forrest. Rainfall as a Determinant of Soil Moisture. The Plant World, 17 : 9-26, 1914. 



