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We investigated the spatial distribution of the three Acacia and two Tamarix tree species and related the spatial distribution of these two genera to their O-18 value, i.e. their O-18/O-16 isotopic ratio in the Negev desert, Israel. At the regional spatial scale, there was a significant difference in the overall distributions of these two genera, although there was considerable overlap. At the 1 km(2) scale, there was no deviation from homogeneity. At a finer scale, no deviation from a random distribution was found in three ephemeral rivers between the two largest trees in these two genera, Acacia raddiana and Tamarix nilotica. We found that Tamarix used deep water from aquifers while Acacia trees used an alternate source, most likely surface water. Spatial segregation of Tamarix and Acacia occurs at the large spatial scale but not at the small scale, perhaps because of the high tolerance of Tamarix species for salinity and the low tolerance of Acacia species for saline conditions. It appears that the trees are not spatially segregated on the small scale because they use different water sources. (C) 2009 Elsevier Ltd. All rights reserved.