Direct demonstrations of competition between higher plants for specific soil resources are seldom possible1-3. Previous neighbour plant removal experiments in desert environments have shown increased growth and improved water status of the remaining plants4-6. However, these responses were only measured several months after the removal of neighbours, after which period there would have been ample time for major adjustments in the root distribution of surviving plants. We report here field experiments using dual-isotope labelling to measure opportunistic phosphate acquisition by shrubs within 2 weeks of partial defoliation of neighbouring grass plants. Phosphate isotope uptake from interspaces shared with defoliated grasses increased to as much as six times that of uptake from interspaces shared with unperturbed neighbours. The experiments indicate immediate competition for phosphate and the influence of root physiological activity on this competition.
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