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.

Tree decline and the future of Australian farmland biodiversity

“…First, we calibrated the demographic model for trees by applying four combinations of plausible mortality and recruitment rates to tree data from grazed sites and ungrazed remnant patches. Although many factors determine whether tree density at a given site will increase or decrease, it is widely accepted in our study region that the best examples of functioning woodlands are ungrazed remnant patches where livestock grazing is excluded (34,35). Tree recruitment, in particular, is significantly higher in ungrazed patches (22,36), and recruitment rates in ungrazed patches provide the best available estimates of natural recruitment rates.…”

Role of plant functional traits in determining vegetation composition of abandoned grazing land in north‐eastern Victoria, Australia

“…Box woodlands (Eucalyptus albens) in New South Wales were prone to weed invasion under slight disturbance through grazing, even though they had not been cleared (Prober & Thiele 1995). Fragmentation also increases the likelihood of the movement of nutrients, fauna and plant propagules into remnant vegetation (Saunders et al 1991), increasing the potential for invasion by introduced species (Hobbs & Huenneke 1992).…”

Overstorey tree density and understorey regrowth effects on plant composition, stand structure and floristic richness in grazed temperate woodlands in eastern Australia

“…In addition, pastoral land management in many grazing systems often involves removing or reducing the tree layer in remaining wooded areas to increase native grass production for livestock grazing (McIvor and McIntyre, 2002). As a consequence, eucalypt woodland remnants with near natural understorey are now rare and often restricted to patches that vary considerably in size, quality and isolation (Prober and Thiele, 1995;Yates and Hobbs, 1997;Prober et al, 2002). It is now clear that in temperate grazing landscapes in Australia, where woodlands have inadequate representation in nature reserves, regional biodiversity conservation may best be achieved if combined with current production systems (Chilcott et al, 1997;Kirkpatrick et al, 2005;Dorrough et al, 2007).…”

“…In contrast, the negative effects of grazing have been well documented. Grazing may alter the composition of under-storey species (Prober and Thiele, 1995;Clarke, 2003), prevent seedling recruitment (McIntyre and Lavorel, 1994), contribute to soil erosion and compaction (Yates and Hobbs, 1997) and enhance the invasion of exotic species (Prober and Thiele, 1995;Clarke, 2003).…”

“…In spite of these benefits, the clearing of remnant vegetation and re-clearing of woody regrowth in grazing landscapes has resulted in the loss and severe modification of large areas of woodlands in eastern Australia (McIvor and McIntyre, 2002). In particular, grassy temperate woodlands that once covered millions of hectares in eastern and southern Australia have largely been cleared for cropping or modified for sheep and cattle grazing (Prober and Thiele, 1995;Prober etal., 2002). In addition, pastoral land management in many grazing systems often involves removing or reducing the tree layer in remaining wooded areas to increase native grass production for livestock grazing (McIvor and McIntyre, 2002).…”