PLANT-SPECIES DIVERSITY AND CHEMICAL-PROPERTIES OF SOILS IN THE CENTRAL DESERT OF BAJA-CALIFORNIA, MEXICO

A gradient in plant species diversity was investigated to determine ho w soil chemical properties limit the distribution of desert plants. Sp ecies-poor communities (lacking succulents) growing on basalt-derived soils were compared with richer communities on soils that developed on basalt or quartz diorite. Species richness and density were determine d in three 100-m2 quadrats at five sites. Soil samples were collected from A (0-10 cm) and B (10-30 cm) layers near the center of each quadr at. Leaves of species growing in or near quadrats were collected as in dicator tissue. Elemental concentrations were measured in soil saturat ion extracts and leaf tissue digests. Surface soils were non-saline an d had near-neutral pH. Soil saturation extracts were high in NaCl and adequate in Ca but low in N, P, B, K, Mg, S, and Fe. Leaves of indicat or plants (Viguiera spp.) were high in K, Ca, and Na, medium in B, and low in Mg, P, and some micronutrients. Analysis of variance revealed that the Ca:Mg ratio in the soil solution differed significantly betwe en sites and was lowest at species-rich sites. Simple regression revea led a moderate positive dependence of species richness on Mg in the so il solution and a negative dependence of species richness on soil solu tion Ca:Mg. But a marked dependence was found when species diversity ( Shannon-Weiner H' Index) was regressed on Ca:Mg in the soil solution. These results indicate that the geographic ranges of desert species (a nd of succulents in particular) can be restricted by even moderate Ca in the rooting medium when Mg is critically low. Magnesium concentrati on in leaves of species studied in Baja California was among the lowes t, and Ca:Mg ratios among the highest ever reported in desert plants.