Ecology of Desert Plants. I. Observations on Germination in the Joshua Tree National Monument, California

Abstract

Plant communities exist due to interaction between the individual members and between these and the environment. This introduces such a complexity, that a physiological analysis of the community as a whole seems impossible at present. But as the starting point of such an analysis the desert communities offer an unusually favorable opportunity. In the first place the vegetation cover is not closed, so that 'interactions between plants are greatly reduced, and their influence on each others' immediate surroundings is relatively small. Under such conditions direct correlations between plants (the influence of one plant on the other) are more easily found, as shown by Went ('42). This wide spacing of the plants is of special importance in germination, because that process tends to occur much more frequently in open areas than in a closed cover of existing vegetation. In still another respect, desert plants are particularly favorable experimental material. They are subjected to more extreme conditions than other plants, so that their responses can be expected to be geared to greater climatic differences. Therefore laboratory experiments are likely to show larger differences in response than when plants of more equitable climates are used. Finally, everything which happens in the desert vegetation can be rather easily attributed to distinct climatic occurrences such as a particular rain. With these considerations in mind a series of trips were made, during the years 1945-1947, to study the vegetation of the Joshua Tree National Monument, located 200 km. (130 miles) due east of Los Angeles. The altitude of the areas most commonly visited varies between 800 and 1500 m. (2500-5000 ft.). The San Bernardino and San Jacinto mountains shield the area from the climatic influence of the ocean. Along its west boundary the Little San Bernardino Mountains, reaching a height of 1700 m., separate the area from the Coachella Valley. From the west the altitude of the broad valleys of the National Monument falls gradually to 800 m. in the north and east, with intervening mountain ranges. The climate can best be described with the help of figures 1, 2 and 3. Climatic records are kept by the Cooperative Weather Station at Twentynine Palms, just north of the Monument at 650 m., and daily temperatures are recorded at Hidden Valley at 1400 m. by Mr. Randolph.' Annual rainfall at Twentynine Palms amounts to 136 mm. but at Hidden Valley and higher altitudes in general the rainfall is considerably greater, especially during July, August and September, when thunderstorms break over the mountains. Figure 1 shows the monthly distribution of rainfall, the number of cloudy days and the relative humidity in the afternoon at Twentynine Palms, with an indication of the seasonal differences in rainfall. The winter rains are fairly evenly distributed over the whole National Monument, but the summer rains are highly localized, as figure 2 shows. This figure gives the approximate location of rainbursts ac-