Long-term changes in the vegetation after the cessation of livestock grazing in Eucalyptus marginata (jarrah) woodland remnants

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Abstract

This paper documents changes in the floristic composition of Eucalyptus marginata Donn (jarrah) woodlands over 7 years of recovery from continual, intensive livestock grazing. In remnants of native woodland left after agricultural clearing, which have been subjected to livestock grazing, comparisons were made between the floristics of fenced exclosure plots and open plots that continued to be grazed. The vegetation in nearby remnants, which had not been subjected to livestock grazing, was also surveyed. An initial increase in annual exotic pasture species after grazing relief was only temporary and highly influenced by fluctuations in annual climatic patterns, particularly rainfall distribution and abundance. Subsequent years saw a decrease in exotic annuals in exclosure plots and an increase in native perennials, in a trend towards becoming more floristically similar to the ungrazed sites. Germination of overstorey species was observed in the exclosure plots, however, development of seedlings and saplings was sparse. Results indicate that for jarrah woodland in southwestern Australia, natural regeneration is possible after the removal of livestock, with the return (within 6 years) of native species richness to levels similar to those found in ungrazed vegetation. Re-establishment of cover, however, appears to take longer. The floristic dynamics are described in terms of a nonequilibrium model. Two vegetation states exist, degraded remnants with an understorey dominated by annual species, and ungrazed vegetation with an understorey dominated by perennial shrubs and herbs. The former state is maintained by continual heavy grazing by livestock. Upon relief from grazing, the vegetation undergoes a transition towards floristic similarity to ungrazed vegetation. After 6 years, vegetation change in the exclosure plots appears to be continuing and therefore it is still in transition.

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