Highlight: Sap velocities in pinyon (Pin& edulis Engelm.) and juniper (Juniperus osteosperma (TOW.) Little) trees in southeastern and southwestern Utah were studied for 1 year using the heat-pulse technique. Measured velocities were related to nearby environmental factors and Dalton's simplified aerodynamic method for estimating evaporation from water. Sap velocity was independent of the dry weight of green biomass within both pinyon and juniper trees. From 90.5 to 99.0% and from 66.0 to 99.0%

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... f the variability in diurnal sap velocities in pinyon and juniper trees, respectively, could be accounted for by using an 8-variable multiple regression equation. Dalton's equation accounted for 4.0 to 86.0% of the variability in diurnal sap velocities for both tree species on select days. Amount of variability in sap velocities explained by multiple regression equations was reduced markedly when sap velocities and environmental data were pooled over all sampling dates.