Herbaceous biomass in a Eucalyptus savanna woodland after removing trees and/or shrubs.
Herbaceous biomass was monitored for 9 years in an intact, semiarid savanna woodland near Bourke, New South Wales, with a 15% canopy cover of E. populnea trees. Shrubs had largely replaced the perennial grasses since the introduction of domestic stock 100 years previously. The effect of clearing all the woody plants and the shrubs alone was compared with the uncleared control. Grazing was maintained as a subtreatment for 5 years. During the 1st major wet period the herbaceous biomass on ungrazed plots, which were either cleared of shrubs or of all woody plants, as a proportion of that of the controls was 4.3 and 6.7, resp.; as the shrubs regrew these values decreased to unity and 2.5, resp., after 9 years. Herbaceous biomass was negatively correlated with biomass of tree and shrub leaves. Per unit wt, tree leaves were twice as effective as shrub leaves in reducing herbaceous biomass. Modelling flux of herbaceous biomass on the woody plants-cleared treatment suggested that: (a) no increase would be obtained in any month unless rainfall exceeded 10 d potential evaporation; (b) herbaceous biomass would accrue at the rate of 0.5 g/m2 for each mm of monthly rainfall over the above threshold; (c) herbaceous biomass would decline by 0.1% per month for every mm by which monthly rainfall failed to reach the monthly potential evaporation equivalent. The effects on the herbaceous layer of heavy grazing and complete protection for 5 years were not significantly different after 3 years. This and the rapid shrub regrowth indicated that dominance by shrubs and occasional flushes of ephemeral herbs was a resilient characteristic of these woodlands that had developed from a previous state where trees and perennial herbs were dominant.