We examined trends in foliar nutrient concentrations in two physiognomically similar desertscrub plant communities on contrasting rocks, limestone (L) and granite (G), in southeastern Arizona. ANOVA revealed significant species and site effects for most of the 10 elements studied. Discriminant analysis and principal components analysis revealed separation of species' populations in nutrient space, with a site effect most pronounced on the second principal component. Positions of populations on this component were positively correlated with foliar concentrations of Cu and Fe and negatively correlated with P and Mg. Foliar Cu concentrations appeared to reflect relative availability in the soil, with higher levels found on the limestone site. Populations on limestone had higher foliar concentrations of Fe than those on granite, despite an opposing trend of Fe availability in the respective soils; this may be the consequence of special adaptations of the L populations for Fe acquisition under alkaline conditions. The L populations were near deficiency levels for foliar Mg, probably because of absorption antagonism with the abundant Ca in the L soil. This antagonism may have caused selective pressure for regulation of Ca and Mg uptake in the L populations: concentrations of these two elements were strongly correlated among L populations, and the Ca:Mg ratios in leaf tissues of the L populations were reduced relative to that in the L soil. We hypothesize that low Mg content in tissues of the L populations may exacerbate problems of P acquisition from the P-deficient L soil, resulting in low foliar P.
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