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Recent studies suggest that savanna trees in semi-arid areas can increase understorey plant production. We hypothesized that one of the mechanisms that explains the facilitation between trees and grasses in East African savannas is hydraulic lift (HL). HL in large Acacia tortilis trees was studied during the first 3 months of the dry season during a relatively wet year (1998) and a very dry year (2000). In 1998, we found distinct diel fluctuation in soil water potential (ψ_s), with increasing values during the night and decreasing again the following day. These fluctuations in ψ_s are consistent with other observations of HL and in A. tortilis were found up to 10 m from the tree. In 2000, during a severe drought, ψ_s measurements indicated that HL was largely absent. The finding that HL occurred in wetter years and not in drier years was supported by data obtained on the δ¹⁸O values in soil, rain and groundwater. The …