Conservation of the Grassy White Box Woodlands: Relative Contributions of Size and Disturbance to Floristic Composition and Diversity of Remnants

Abstract

Before European settlement, grassy white box woodlands were the dominant vegetation in the east of the wheat-sheep belt of south-eastern Australia. Tree clearing, cultivation and pasture improvement have led to fragmentation of this once relatively continuous ecosystem, leaving a series of remnants which themselves have been modified by livestock grazing. Little-modified remnants are extremely rare. We examined and compared the effects of fragmentation and disturbance on the understorey flora of woodland remnants, through a survey of remnants of varying size, grazing history and tree clearing. In accordance with fragmentation theory, species richness generally increased with remnant size, and, for little-grazed remnants, smaller remnants were more vulnerable to weed invasion. Similarly, tree clearing and grazing encouraged weed invasion and reduced native species richness. Evidence for increased total species richness at intermediate grazing levels, as predicted by the intermediate disturbance hypothesis, was equivocal. Remnant quality was more severely affected by grazing than by remnant size. All little-grazed remnants had lower exotic species abundance and similar or higher native species richness than grazed remnants, despite their extremely small sizes (< 6 ha). Further, small, littlegrazed remnants maintained the general character of the pre-European woodland understorey, while grazing caused changes to the dominant species. Although generally small, the little-grazed remnants are the best representatives of the pre-European woodland understorey, and should be central to any conservation plan for the woodlands. Selected larger remnants are needed to complement these, however, to increase the total area of woodland conserved, and, because most little-grazed remnants are cleared, to represent the ecosystem in its original structural form. For the maintenance of native plant diversity and composition in little-grazed remnants, it is critical that livestock grazing continues to be excluded. For grazed remnants, maintenance of a site in its current state would allow continuation of past management, while restoration to a pre-European condition would require management directed towards weed removal, and could take advantage of the difference noted in the predominant life-cycle of native (perennial) versus exotic (annual or biennial) species.