Establishment of Woody Weeds in Western New South Wales. 2. Growth and Competitive Potential.

Abstract

Growth and survival of hopbush (Dodonaea attenuata), turpentine (Eremophila sturtii) and punty bushes (Cassia eremophila and C. artemisioides) were studied in relation to the effects of grazing and shrub density, at four sites in the Bourke-Wanaaring region of western New South Wales during 1979 to 1982. Hopbush shrubs grew faster than turpentine and punty bushes. Small hopbush grew better on open areas, while mature shrubs grew better in dense stands. Turpentine showed better performance in open areas, and punty bush growth was unaffected by shrub population density. All these species showed a high survival rate, particularly in height classes greater than 25 cm, and they kept growing even during drought periods. The effects of grazing on survival were inconclusive. Hopbush and turpentine root development was rapid, attaining depths of over 1.1 m in wet soils, 14 weeks after germination. Mature plants in the field, both hopbush and turpentine, were observed to have extensive lateral root networks at either 22 cm or 30-70 cm soil depth respectively, as well as a tap root system. The characteristics of both species help explain the competitive advantage of these shrubs over herbaceous species, and their survival capability in times of drought. Competition of hopbush with the perennial grass woollybutt (Eragrostis eriopoda) and annual herbage was also examined. In spring, survival of hopbush seedlings was favoured within woollybutt tussocks, this situation being reversed in autumn. Woollybutt biomass was negatively affected by the proximity of hopbush shrubs. Annual herbage seemed not to have any effect on hopbush survival, although shrub seedlings grew better under conditions of moderate herbage cover. Microtopography affected the establishment of both grasses and shrubs, with depressions favouring the growth of grasses at the expense of shrubs during periods of adequate moisture, and the growth of shrubs during drier times. The findings suggest that the woody weed problem is unlikely to wax and wane and that turpentine may be particularly difficult to control. Maintenance of high levels of pasture cover from perennial grasses and annuals may slow down recruitment and growth in open areas with scattered parent shrubs, where control will be less costly, and may contain future spread.