Woody overstorey and herbaceous understorey biomass in Acacia harpophylla (brigalow) woodlands

Abstract

AbstractExtensive areas of Acacia harpophylla (brigalow) woodlands have been cleared for pasture production in Queensland. The woody regrowth of A. harpophylla influences pasture production and composition following initial development of these woodlands.Biomass component regressions were developed using tree basal area as the predictor variable and used to estimate component yields of regrowing A. harpophylla communities. Leaf biomass of A. harpophylla reached a maximum of 12 t ha‐1 with a leaf area index of 2.5 when regrowing plants were 2.5 m high and about 10 years old. Pasture production, pasture basal area and the proportion of sown pasture species were lower at higher tree basal area of A. harpophylla. The greatest decrease was seen between 0 and 2 m2ha‐2tree basal area. Annual grasses and broadleaf herbaceous species represented a greater proportion of pasture biomass at high tree basal area.The poor control achieved by herbicide sprays on A. harpophylla plants that are 2–2.5 m tall would be partly due to poor coverage of the herbicide spray, as leaf area index is at a maximum at this stage. A density of 1000 plants ha‐1is a threshold value above which regrowth control would be necessary at some stage to maintain acceptable pasture production and composition.The non‐linear regression relationship between pasture production and tree basal area was similar to that observed in Eucalyptus spp. However, in the equation that relates pasture yield (Y) to tree basal area (X), Y = A + B*e(‐kX)the value of k for A. harpophylla communities was generally higher than observed in Eucalyptus spp. communities with the same potential pasture production in the absence of trees. Within even‐aged stands, the use of tree leaf biomass to predict pasture production showed no advantage over the use of tree basal area as the predictor. However, there are advantages in using tree leaf biomass as a predictor when comparing communities with different size‐ or age‐class structures.