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. Rainfallasa Determinant of Soil Moisture. The Plant World, 17:9-26, 1914. 
