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This study quantified honey mesquite (Prosopis glandulosa) root growth and water use efficiency following chronic soil drought or wetness on a clay loam site in north Texas. Root systems of mature trees were containerized with barriers inserted into the soil. Soil moisture within containers was manipulated with irrigation (Irrigated) or rain sheltering (Rainout). Other treatments included containerized precipitation-only (Control) and non-containerized precipitation-only (Natural) with three trees per treatment. After four years of treatment, soil cores to 2.7 m depth were obtained beneath each tree canopy. Mesquite and grass roots were extracted, separated into size classes, and weighed. Root length density (m m(-3)) was quantified for mesquite roots. Averaged over the entire sample depth, Irrigated trees doubled root length density of small (< 2 mm diam.) roots compared to Control trees (232 vs 105 m m(-3)). Below 90 cm depth, root length density of large (2 to 10 mm diameter) roots was five times greater in Rainout (36 m m(-3)) than Control trees (7 m m(-3)). Over all depths, root biomass was greatest in Rainout trees and root:shoot (biomass) ratio was three times greater in Rainout than Control or Irrigated trees. Mesquite leaf carbon isotope ratio (delta C-13) was lower (more negative) in Irrigated trees than other treatments, suggesting these trees had lower water use efficiency. Leaf delta C-13 was not different between Rainout and Control trees. Mesquite adapted to chronic wet or drought cycles through increased root growth but patterns of distribution differed as Irrigated trees emphasized growth of small roots throughout the profile and Rainout trees grew large roots into deeper soil layers.