A comparison of sap flux and water relations of leaves of various isolated trees with special reference to foundation movement in clay soil

Diurnal variation in sap flux (S) through stems of six trees, two each of Ulmus procera SALISB., Melaleuca styphelioides SM. and Prunus cerasifera EHRH. ‘Nigra’ (referred to hereafter by their generic names), were estimated from measurements of heat pulse velocities. Leaf water potential (ψ), stomatal conductance (g _s ) and transpiration from leaves (T) of all replicate trees were measured at 1300–1500h, once during the summer. On two separate occasions measurements were made of S, ψ, (g _s ) and T for one each of Ulmus and Melaleuca trees to study diurnal variations in these parameters. A 12×12 m² area around each tree was kept covered to simulate the condition of trees growing on pavements adjacent to residential properties. Sap flux for these tree species was in the order Melaleuca>Ulmus>Prunus. It is suggested that the smaller canopy and sapwood area in Prunus compared to the other two species is responsible for lower water potential and lower transpiration rate than the other species. Detailed analysis of the diurnal variation in sap flux and water relation of leaves of Melaleuca and Ulmus indicated sap flux of Melaleuca to be greater than that of Ulmus at the same transpiration rate per unit leaf area although the sapwood area of the two species was marginally different. This may have been due either to the difference in canopy conductance or in leaf area between the two species. With the assumption that sap flux closely resembles the rate of soil water extraction for both species, results indicate that Melaleuca is likely to extract soil water at a higher rate than Ulmus and hence is capable of causing greater shrinkage and soil movement than Ulmus.