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The effect of leaching fraction (LF = drainage volume/irrigation volume) and tree planting size on the carbon isotope ratio measured in leaves of live oak (Quercus virginiana), desert willow (Chilopsis linearis) and tall fescue (Festuca arundinacea; LF only) were investigated in an arid climate. Irrigations were applied to maintain LFs of + 0.25, 0.00 or -0.25 (theoretical). Live oak and desert willow were planted as 3.8, 18.9 or 56.81 nursery container plants in lysimeters where hydrologic balances were maintained weekly. Leaf carbon isotope ratios varied by species across the three LFs (live oak > desert willow > tall fescue). In live oak, leaf carbon isotope ratios increased as LF decreased but did not change with planting size. In desert willow the ratios increased with decreasing LF and with increasing planting size. In tall fescue the leaf carbon isotope ratio decreased as the LF increased. Leaf carbon isotope ratios also varied with stomatal conductance, leaf xylem water potentials and water-use efficiency (WUE). In live oak, both average midday stomatal conductance and leaf xylem water potential decreased with increasing leaf carbon isotope ratio. Average stomatal conductance for live oak was negatively correlated with average leaf WUE. Average leaf WUE in live oak and tall fescue were correlated with average leaf carbon isotope ratios. However, in tall fescue leaf WUE increased as the leaf carbon isotope ratio became more negative. In live oak the ratio became more positive as leaf WUE increased. This study has demonstrated that leaf carbon isotope ratios can be used as a way of screening plant response for low water-use landscapes, but only after careful correlations for each species have been established between growth form and growing conditions. (C) 1997 Academic Press Limited.