Ecology of Desert Plants. IV. Combined Field and Laboratory Work on Germination of Annuals in the Joshua Tree National Monument, California

Abstract

In the studies of the annuals of the Joshua Tree National Monument, Death Valley, and the Coachella Valley, made prior to 1948, it appeared that the temperatures which prevailed at the time of the infrequent rainfalls were a critical factor in determining which species would germinate. The difference in summer and winter flora was shown to be due to this factor (Went 1948, 1949, Went and Westergaard 1949). The extension of these observations in the present paper consists of the attempt to define in more detail the correlation between rainfall and temperature, and germination. The study has extended from the fall of 1948 to the fall of 1953. It was thought that temperature, in combination with rainfall, might explain other differences than those of summer and winter, for it is a conspicuous feature of the flowering of the desert annuals that a rather small portion of the number of native species will blossom in any one year. At one time, two or three species may dominate, appearing most profusely; the following year these may scarcely appear at all, quite different species predominating. Yet three, five, or even seven years later the first ones may appear again, and in the same locations. The present study is directed toward the question whether their appearance might be due to recurrence of favorable rain-temperature