Planting density effects of water use efficiency of trees and pasture in an agroforestry experiment.

Water use of trees and pasture were studied at three tree densities in an agroforestry experiment where trees were planted at densities of approximately 2150, 304, and 82 stems/ha in a Nelder design. Tree transpiration and pasture evaporation were related to their respective biomass production to estimate water use efficiencies at each tree density over a 2-year study period throughout 1985 and 1986. Tree planting density was found to modify productivity and water use of both trees and pasture. During dry conditions biomass production per tree was lowest at high tree densities being 19 g/day compared with 69 and 45 g/day at medium and low tree densities respectively. Transpiration rates per tree were also lowest at high tree densities being 12.9 x 10-3 m3/day compared with 54.5 and 72.9 x 10-3 m3/day from medium and low densities respectively. However, water use efficiency was found to be highest in the densely planted trees where mean values for 1985 and 1986 were 4.6 and 3.9 kg/m3 respectively in 1986. Pasture evaporation was generally lowest under the densely planted trees where soil water contents in the pasture root zone were lowest. Pasture production was greatest at the intermediate tree density and this was associated with a higher water use efficiency than was found from pasture under high and low tree densities.