Tree/pasture interactions at a range of tree densities in an agroforestry experiment. III. Water uptake in relation to soil hydraulic conductivity and rooting patterns

Abstract

Eucalyptus grandis was planted in a Nelder fan design in November 1983 into a previously established pasture dominated by Setaria sphacelata cv. Kazungula, at the CSIRO Sanford Pasture Research Station, Queensland, Australia. Nine concentric rings of 18 trees were planted at radii of 4.4-61.6 m, giving a range of tree densities which decreased from 3580 to 42 stems/ha. Tree transpiration was studied at three tree densities (2150, 304 and 82 stems/ha, representing high, medium and low densities) over a 'drought' period of approximately 1 yr (Nov. 1985-Sep. 1986) and related to rooting patterns and soil hydraulic properties. Over the range of soil water contents studied, the ratio of tree transpiration rate to equilibrium evaporation rate (T/Esub(eq)) decreased linearly with decreasing mean soil water content at each tree density. To investigate the effects of soil hydraulic conductivity and root length density on the total transpirational flux, overall soil conductances (Ksub(s)) were calculated, with soil conductance in each horizon weighted according to the length of root in that horizon. At each tree density, decreases in the ratio T/Esub(eq) were related to decreases in ln Ksub(s) measured at 1.2 m from the stem. A more rapid decrease in T/Esub(eq) with decrease in water content observed at the low tree density was attributed to a greater decrease in Ksub(s) as mean water contents decreased. The greater decrease in Ksub(s) at low tree densities was associated with a larger proportion of water extracted and a higher proportion of total root length in surface soil horizons, which showed a greater decrease in hydraulic conductivity than subsoil horizons for the same decrease in water content.