Hydrological processes and herbage production in shrub invaded Poplar Box (Eucalyptus populnea) woodlands.

Abstract

The hydrological processes of relevance to herbage production in shrub invaded semi-arid popla box (Eucalvptus popuhea) woodlands are reviewed. Climatic limitations to herbage production in the poplar box lands are discussed in relation to the annual distribution of rainfall and evaporation. The limiting effects of trees and shrubs on herbage production within a woodland are then evaluated using data from a study site near Coolabah in north-western New South Wales. Hydrologic measurements made at the site include soil water storage, run-off and infiltration rates. Marked spatial variability in infiltration of water into the soil was in part related to the distribution of plants and redistribu- tion of water from ridges to adjacent flats occurred because of low rates of infiltration into bare soil areas between groups of trees and shrubs. From measurements of the depletion of soil water which accumulated during an exceptionally wet period the potential rate of evapotranspiration from the poplar box component of the woodland was estimated to be 0.5 times that from a free water surface. Bare soil evaporation accounted for approximately one half of water losses. Herbage production is shown to be poorly related to soil water storage alone, but the regression between herbage production and the product of water and nitrogen availabilities accounts for 70% of the vari~ce. It is concluded that the role of trees and shrubs in competing with herbage for nutrients, as well as water, is in need of clarification.