The relationship of precipitation seasonality to the flora and stable isotope chemistry of soils in the Vizcaíno desert, Baja California, México

Abstract

The proportion of winter to summer rainfall decreases with decreasing latitude in the Vizcaino desert of Baja California, Mexico. Vegetation characteristics, soil carbon contents, the stable carbon isotope composition of soil organic matter and pedogenic carbonate, and the stable oxygen isotope composition of carbonate were measured at four locations along this precipitation transect. The number of species (≈ 20), the ground cover (≈ 30%), and the proportion of succulents to total species (≈ 60%) were similar at all sites. The northenmost site possessed species (e.g. Ephedra californica, Yucca schidigera) peculiar to winter rainfall zones while the southern site had several species typically common only to summer rainfall zones (e.g. Cercidium praecox, Lemaiveocereus thurberi). Organic carbon contents of the soils were low and varied little with rainfall distribution. σ13C values of organic carbon were approximately 2 to 3‰ more positive at the southernmost site than at the other locations, possibly reflecting a greater proportion of biomass from either succulents or C4 summer grasses. The σ13C value of tissue from a representative CAM plant (Opuntia echinocarpa) did not vary with precipitation patterns suggesting that the σ13C values of CAM species are insensitive to the climate variations in this area. σ13C value of pedogenic carbonate varied little with location (−4·1 to −5·8‰), reflecting small differences in soil respiration rates with latitude. σ18O values of pedogenic carbonate varied significantly from north (−3·6‰) to south (−7·3‰), and are interpreted as reflecting the differences in the relative contribution of isotopically ‘heavy’ winter storms from the Pacific to the isotopically ‘light’ summer and fall storms from the southern Pacific and the Gulf of Mexico. The results of the study reinforce the observation that subtle climatic differences in arid regions are reflected in the floral composition and in the chemistry of the soils. The Holocene data obtained in this study provide a basis for interpreting the climatic significance of isotopic data from paleosols in the Vizcaino Desert region.