Impact of regrowth and remnant brigalow (Acacia harpophylla) trees on wheat production in southern Queensland

Abstract

The potential for integrating native brigalow (Acacia harpophylla F. Muell. Mimosaceae) vegetation with dryland cropping systems is being investigated in the Tara Shire, southern Queensland. Of particular interest is regrowth brigalow, a key means for landscape revegetation following broadscale land clearing last century. Integration of brigalow vegetation and cropping can result in competition for water, light and nutrients between trees and crops, and potentially, a reduction in agricultural production. Competition dynamics must be assessed to understand the trade-offs involved with brigalow-cropping integration and to guide sustainable landscape management. Extent of the brigalow-crop competition zone, i.e. the horizontal distance over which crop production is reduced, has been measured at four sites where regrowth and remnant brigalow are adjacent to wheat. Evaluated on the basis of grain yield, the competition zone ranged from 19 m for 3 year old regrowth (3YRG), to 44 m for remnant brigalow > 70 years old (> 70YRM). When expressed as a ratio of tree height, the competition zone ranged from 4 tree heights for > 70YRM to 13 tree heights for 3YRG. The tree heights results suggest that in a relative sense, regrowth brigalow produces a larger competition zone compared to remnant brigalow. Further collection of field data as well as simulation modelling will allow for investigation of the impact of land use history and seasonal conditions on brigalow-crop competition. The overall aim is to develop a simple spreadsheet model that will allow farmers to explore different brigalow-cropping scenarios and their associated economic and ecological trade-offs.