Effects of Rainfall and Temperature on the Distribution and Behavior of Larrea Tridentata (Creosote‐Bush) in the Mojave Desert of Nevada

The effects of rainfall and temperature on the distribution and certain behavioral characteristics of Larrea tridentata (Creosote—bush) at and near its northern limits in the northern Mojave desert of southern Nevada, were investigated at 39 sites with Larrea and 20 sites without Larrea in eight drainage basins at elevations of 915—1,770 m over a 2,600—km2 area of the Nevada Test Site. Data used were (1) rainfall records for 9 yr (1963—71) for each site; (2) maximum and minimum air temperature records for each site, November 1962—February 1972; (3) percentage cover by all shrubs species and by Larrea; (4) height and density data for Larrea; and (5) percentage of germinable seeds from 29 of the Larrea populations for three seasons (1963—65) in relation to the seasonal rainfall for each site. Total percentage cover by all shrubs species is highly correlated with mean annual rainfall, less well correlated with elevation. Percentage cover by Larreafollows two patterns of relationship with rainfall: (1) where mean rainfall is low to intermediate, the same pattern as total shrub cover in relation to rainfall, and (2) on sites of high mean rainfall, consistently low cover, a function of low density of Larrea on these sites. In general, in undisturbed communities, the taller the Larrea plants the fewer there are of them, but the relationship is not strictly linear. Using height as an index to plant volume, numbers of Larrea plants are highly correlated with total plant volume. Mean height is not strongly correlated with mean annual rainfall or temperature parameters, but is well correlated with the ratio of mean precipitation/mean temperature. Tall plants (>1 m) occur in low density and on sites with high rainfall (mean 160—183 mm). The prevailing low minimum air temperatures and their extremes in the lowlands of enclosed drainage basins are inferred to be the primary cause of the absence of Larrea in three discrete vegetation zones (Atriplex confertifolia, Lycium pallidum, and L. shockleyi) in Frenchman Falt, and over most of the basin floor of Yucca Flat, where the communities are Atriplex and Grayia—Lycium andersonii. The year—round low minima in the lowlands of these basin result from nocturnal cold air drainage phenomena and formation of a cold air layer of variable depth.Average extreme minima on these sites were mostly below O° F; the extreme minimum was —18° F in one of the Atriplex communities of Frenchman Flat. Larrea occurs over the bajadas of Frenchman Flat on sites above the lower layers of cold air. In Yucca Flat, at its northern limits, it is restricted to certain upper bajada sites and notably one site on the basin floor. Average extreme minimum air temperatures on all Larrea sites were above 1°F; the absolute minimum was —8°F. Upper altitudinal limits of Larrea apparently are not determined by minimum temperatures since minima (including the extremes) in Coleogyne vegetation, which replaces Larrea altidudinally on the slopes, are well within the range of those recorded on Larrea sites. There is no pattern of relationship between maximum temperatures and the distribution of Larrea, although the highest extreme maxima usually occur on non—Larrea sites in the lowlands of Frenchman Flat. Mean annual rainfall on the Larrea sites ranged from 118 to 183 mm. Altitudinal and latitudinal limits of Larrea coincide with a maximum mean rainfall of 183 mm. Mean altitudinal rainfall of 160—183 mm appears to be critical to the behavior of Larrea. Germination trials support the inference of a deleterious effect of high rainfall on Larrea populations through time: there were high correlation coefficients (negative or positive, depending on the year) between the rainfall of the effective rainfall season and the percentage of germinable seeds; highest mean germination percentages (20%—60%) occurrred with 80—150 mm of seasonal rain, and either lower or higher seasonal rainfall resulted in lower percentages of germinable seeds (0%—20%).