RESPONSE OF LONDON PLANE AND CORKSCREW WILLOW TO VARIABLE IRRIGATION
We investigated growth and water relations of London plane and corkscrew willow irrigated at 80% and 0% replacement of potential evapotranspiration (ETo). In Spring 1991, whips were planted in a randomized complete-block design in a silt loam soil that was clean-cultivated through two seasons. In 1992, tree response was measured in water relations [water potential (ψ)] at predawn and midday and dawn-to-dusk stomatal conductance (gs), trunk growth, and total leaf area. Soil-water depletion was monitored with a neutron probe. Measured ETo was 98.6 mm, and actual water applied based on final leaf area was 92% and 38% of ETo for plane trees and willows, respectively. Nonirrigated trees received 4% of ETo from rain. Soil water content at the 0.90-m depth was lower in the 0% ETo treatment. There were, however, no differences in predawn ψ through the season. Plane trees had consistently higher dawn-to-dusk gs than the willows, but there were no differences in gs or midday ψ between irrigation treatments for either species. Despite lower gs, willows had greater total leaf area and trunk growth than the plane trees, but again, there were no differences among irrigation treatments. Lack of detectable water-stress effects suggests that, in the absence of competition from other species, an expanding perimeter of root growth explored new soil and allowed nonirrigated trees to exploit soil water ahead of moisture depletion within the root zone.