A Comparison of Seasonal Primary Production of Mojave Desert Shrubs During Wet and Dry Years

Abstract

Estimates of net primary productivity were made for five major shrub species in the northern Mojave Desert during 2 years of contrasting moisture regimes using harvest and gas exchange techniques. Production increased by 224% (harvest) to 260% (gas exchange) in 1973 over 1972. Production gains varied among species depending on whether the species was favored by the cool, moist spring of 1973. Lycium andersonii and L. pallidum, which have high photosynthetic rates and start growth in the early spring, showed the greatest increases. Larrea tridentata and Krameria parvifolia, species adapted to growing under warmer, drier conditions, exhibited the smallest changes in production. Ambrosia dumosa, which has high photosynthetic rates but responds more favorably to warmer spring temperatures than the Lycium species, had moderate production gains. Reasons for the different estimates obtained by these two methods are discussed. INTRODUCTION The determination of primary productivity in terrestrial ecosystems is a necessary but difficult step in providing information on the basic. energy or carbon input into biological systems. Productivity estimates are important for predictive models of a general nature in ecosystem analysis. In general, two methods have been used for net primary production estimates. The first method is a direct harvest technique in which whole plants or samples are clipped throughout the growing season, dried and weighed, and productivity calculated on dry weight per unit area. The second method involves measuring the total CO2 or carbon exchange per weight of photosynthetic organs, and calculating productivity as carbon or biomass fixed minus respiratory losses. There have been several measurements of net primary production and biomass in deserts made by the harvest and other methods (Burk and Dick-Peddie, 1973; Chew and Chew, 1965; Pearson, 1965; Rodin and Bazilevich, 1967). Measurements of CO2 exchange rates in desert plants have been made in the Negev Desert (Lange et al., 1969) and in the North American deserts (Bamberg et al., 1975; Caldwell, 1972; Patten and Dinger, 1969; Strain, 1969, 1970; Strain and Chase, 1966). Attempts at relating some of these gas exchange rates to production are only now being done. The US/IBP Desert Biome is currently obtaining production estimates from several sites in the western United States. 1 Present address: EG&G Environmental Consultants, 9025 E. Kenyon Avenue, Denver, Colorado 80237. 2 Present address: University of Idaho, Research and Extension Center, Route 1, Kimberly, Idaho 83341.