CLIMATE OF THE SANTA CATALINA MOUNTAINS. 47 
their effect is soon overcome by the desiccating conditions of March 
and April. The hot and rainless weeks which precede the mid-summer 
have been designated the ‘‘arid fore-summer.” On the desert this is 
a season in which the temperature conditions are conducive to activity 
on the part of plants, while the soil moisture conditions are increasingly 
deterrent to it. Asa result of these conflicting conditions activity may 
be observed in the trees which grow near a constant water supply, as 
Populus sp. (cottonwood) and Salix sp. (willows), trees which possess 
deep-seated root systems, as Prosopis velutina (mesquite), and plants 
which contain stores of water, as all species of cacti. The activity of 
Populus and Prosopis consists in both flowering and leafing-out, as well 
as in shoot growth; in the cacti it consists in flowering and in some 
species also in growth. Among all desert plants other than those 
indicated the arid fore-summer is a period of drought-rest. 
With respect to the water relations of plants the arid fore-summer 
is the most trying season of the year, combining low soil moistures with 
atmospheric conditions that compel active transpiration. Inallrespects 
in which moisture conditions may be critical for the survival of individuals 
or thelimitation of the distribution of species it isin the arid fore-summer 
that the critical intensity of these conditions must be sought. 
The retardation of spring which accompanies increasing altitude 
results in a shortening of the arid fore-summer from a length of 15 
weeks on the desert to 11 weeks at 6,000 feet and 6 weeks at 8,000 feet 
(see fig. 2). Not only does this trying season decrease in length with 
altitude, but its physical conditions becomeameliorated, as will beshown. 
The “humid mid-summer” commences on July 8 and lasts until 
September 12, these being the average dates, for 8 years, of the first 
and last rains of 0.50 inch or more. In this season the moisture con- 
ditions of desert and mountain top are more nearly alike than at any 
other time. It is the season of greatest vegetative activity on the 
desert and in the forest also. On the desert it is the only season in 
which germinations take place among the perennials, and it is the chief 
season of growth among all perennial plants, including those that have 
been in leaf during the arid fore-summer. In the Encinal region the 
evergreen oaks renew their foliage at the advent of spring, but the great 
mass of vegetative activity awaits the humid mid-summer. In the 
Forest the pines also commence growth with the cessation of frost, 
but make their chief growth during July and August. The humid 
mid-summer is also the chief period of activity for the herbaceous 
perennials and small shrubs of the forested elevations. Heavy snow- 
fall during mid-winter or the occurrence of exceptionally late winter 
rains may bring about growth among the herbaceous perennials of the 
forest during the arid fore-summer. In fact a few species, notably 
Frasera speciosa and Dugaldia hoopesti, commence growth before the 
last frosts of spring. 
