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. As a 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. In all respects 

 in which moisture conditions may be critical for the survival of individuals 

 or the limitation of the distribution of species it is in 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 become amehorated, as will be shown. 



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 hoopesii, commence growth before the 

 last frosts of spring. 



