Herbaceous Biomass in a Eucalyptus Savanna Woodland After Removing Trees and/or Shrubs

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Abstract
(1)Herbaceous biomass was monitored for 9 years in an intact, semi-arid savanna woodland with a 15% canopy cover of Eucalyptus 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. (2) During the first 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, respectively. As the shrubs regrew these values declined, reaching unity and 2.5 respectively after 9 years. (3) Herbaceous biomass was negatively correlated with biomass of tree and shrub leaves. Per unit weight, tree leaves were as effective as shrub leaves in reducing herbaceous biomass. (4) Modelling flux of herbaceous biomass on the woody plants-cleared treatment suggested that: (i) no increase would be obtained in any month unless rainfall exceeded 10 days potential evaporation; (ii) herbaceous biomass would accrue at the rate of 0.5 g m-2 for each mm of monthly rainfall over the above threshold, (iii) hervaceous biomass would decline by 0.1% month-1 for every mm by which monthly rainfall failed to reach the monthly potential evaporation equivalent. (5) 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 indicates that dominance by shrubs and occasional flushes of ephemeral herbs is resilient characteristic of these woodlands that has developed from a previous state trees and perennial herbs were dominant.