Shrub removal with grazing management fails to reinstate the characteristic herbaceous understorey in a Eucalyptus populnea savanna woodland near Bourke, New South Wales, Australia

  • Published source details Harrington G.N. & John G.G. (1990) Herbaceous biomass in a Eucalyptus savanna woodland after removing trees and/or shrubs. Journal of Applied Ecology, 27, 775-787


The introduction of domestic stock to savanna woodlands may result in an increase in shrubs due to growth of unpalatable woody species at the expense of the natural grass-dominated herbaceous ground flora. An investigation initiated in 1975, examined the effect on an Australian savanna woodland of mechanically clearing woody vegetation, combined with the use of goats to control subsequent woody regrowth. The hypothesis was that this would release more soil moisture and nutrients for use by herbs, and thus go someway to restoring an open woodland with a grassy ground layer.

Study site: The study was undertaken near Bourke, New South Wales (146º30'E, 30º55'S) in open Eucalyptus populnea woodland (trees 10-15 m in height, average density 36 trees/ha Eucalyptus, canopy cover 15%). The shrub layer was strongly aggregated around the trees.

Prior to the study rainfall had been poor, and livestock grazing had left no herbaceous plants visible above ground; the subsequent response to rain indicated very few herbaceous perennial plants had survived.

Twelve 1-ha study sites were selected for their similarity in position on the catena,
slope and tree density. Three scrub-clearing treatments were imposed, each with and
without grazing, with each treatment combination replicated twice:

i) uncleared - no mechanical treatment of the vegetation;

ii) shrubs cleared - shrubs and trees with a trunk diameter of less than 30 cm at 1.5 m height were pushed over by a bulldozer and left in place;

iii) all woody plants cleared, plus ring-barking of large trees and monthly sucker removal.

Grazed plots were stocked with 0.5 goats/ha for 5 years until 1980 when the food supply ran out.

Stratified sampling was used to account for the strong influence of Eucalyptus trees on the flora. Two trees were selected within each plot and 360 permanent 1 m² quadrats were sampled on three lines radiating up, down and across the slope from the bole of each sample tree to the centre of the inter-tree area, which encompassed the different habitat zones within the savanna. Herbaceous and shrub biomass was monitored for 9 years.

An unusually wet period during early 1976 resulted in rapid shrub regeneration from both seedlings and root-suckers on the cleared plots. During this first major wet period, the herbaceous biomass on ungrazed plots cleared of shrubs or of all woody plants, as a proportion of that of the controls was 4.3 and 6.7, respectively. As the shrubs regrew, these values declined to around 1 and 2.5 respectively after 9 years. Herbaceous biomass was negatively correlated with biomass of tree and shrub leaves.

Grazing reduced the herbaceous biomass but sustained the goats until early 1980 when they were removed to prevent starvation. The two subsequent periods of herbaceous growth in subsequent wet periods (in 1981 and 1983) showed no residual effect of grazing, the biomass of the herbaceous layer being similar on uncleared and shrub-cleared treatments for both grazed and ungrazed plots. However, there was evidence of an indirect effect as grazing induced a reduction in shrub biomass on the cleared treatments where herbaceous growth was greater. This effect was never significant on the 'shrubs-cleared' treatment, but there was a significantly higher herbaceous biomass on the previously grazed 'woody plants-cleared' treatment than on the ungrazed plots. Grazing did not significantly affect shrub biomass on the uncleared because palatable shrubs had been eliminated by commercial grazing prior to the experiment.

Conclusions: The effects on the herbaceous layer of heavy grazing and complete protection for 5 years were not significantly different after 3 years. Thus this and the rapid shrub regrowth appears to indicate that dominance by shrubs and occasional flushes of ephemeral herbs is now the characteristic livestock-grazing induced state of these woodlands that has developed from the natural vegetation compositon where trees and perennial herbs were dominant.

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