remnant vegetation ~ values and threats

Transcription

1 remnant vegetation ~ values and threats Jann Williams School of Environmental and Information Sciences, Charles Sturt University, Albury, NSW. I NTRODUCTION The South West Slopes of New South Wales is an agricultural area of national significance and the value of the land for cropping and grazing has led to widespread clearing of the native vegetation. This has created patches of remnant vegetation around which most or all of the original vegetation has been removed (see Figure 1). Much of this native vegetation is located on steeper, rocky or infertile soils and some on travelling stock routes and railway lines. Few examples exist, however, of the vegetation types which once occurred on more fertile and accessible areas. Any remnants in these areas can be assumed to have high conservation value, even if they are degraded. The remnant vegetation in the South West Slopes has many shapes, sizes and degrees of isolation. It represents a number of vegetation associations and occurs on a range of soil and ownership types. For example, State Forests manage remnant forests and woodlands which can cover an area of several thousand hectares. Typically however, remnants are small, including most of the conservation reserves in the region. These reserves do not adequately sample the range of biodiversity in the South West Slopes. For example, plant communities occurring on particular geological types, or grassy White Box (Eucalyptus albens) woodlands are not adequately represented in reserves. Because remnants on private land contain much of the biodiversity outside these reserves, their ongoing management is critical for biodiversity conservation. However, while remnant vegetation has inherent and precious biodiversity value, it also has qualities that are particularly relevant to the sustainability of agriculture. Benefits of remnant vegetation include the prevention and amelioration of dryland salinity, minimisation of soil loss, habitat for native wildlife (important in natural pest control), the provision of shade and shelter for crops, pasture and stock, aesthetic values, the opportunity to maintain genetic diversity and to take advantage of a cheap regeneration method, via self-regeneration. Remnant health is also increasingly being acknowledged as one of the indicators of sustainable agriculture. (G. Johnson) Figure 1. Remnant vegetation is typically found along roadsides or travelling stock routes. Page 7

2 VALUES OF REMNANT VEGETATION Prevention and amelioration of dryland salinity Deep rooted, perennial native vegetation helps slow the development of surface salinity by utilising ground water. This water may otherwise contribute to rising water tables, waterlogging and accumulation of salt on the surface. Minimising soil loss Soil erosion removes valuable top soil and nutrients which causes pollution, salinisation, siltation and algal blooms in water bodies. The only effective way to reduce soil erosion is to maintain groundcover and keep surface run-off to a minimum by ensuring the soil structure is good enough to allow good infiltration of water into the soil. Retaining remnants on farms, particularly on steeper areas and along water courses, can help minimise soil loss and erosion and therefore contribute to farm productivity. Biodiversity conservation As already noted, remnant vegetation often represents the best remaining examples of particular ecosystems. Therefore, remnants are particularly important in the conservation of biodiversity and ecological processes in the South West Slopes. Natural pest control Remnant vegetation may attract native wildlife that prey upon pasture, tree and crop insect pests, thus providing an alternative to pesticides, which are often harmful to useful species. Wildlife also assist in keeping remnant vegetation healthy, by preying on pests and by carrying predatory parasites and diseases that also reduce pest numbers. As the range and quantity of wildlife that preys on pests increases with habitat diversity, natural pest control is enhanced if there is a diverse habitat, with understorey being particularly important (see Practical Information Note Natural Pest Control). Shade and shelter Remnant native vegetation can provide protection for plants and animals from wind, cold and heat stress. It can shelter stock which reduces lamb and sheep offshears mortality and improves growth rate. It can shade stock, and reduce heat stress, leading to higher weight gains and improved fertility in sheep. Remnant vegetation can protect newborn sheep and cattle by sheltering them from wind and sun and reduces water Habitat for native wildlife Remnant vegetation provides important habitat for local native wildlife including insects, birds, mammals, lizards and frogs. Hollows, which generally do not start forming until a tree is at least 100 years old, provide essential sites for nesting and shelter for animals such as birds, mammals and bats (see Figure 2). Due to their size, mature trees also provide more food resources than younger trees. Essential habitat is provided for insects, reptiles, frogs and birds by branches, twigs and leaves that fall to the ground (litter). Litter also reduces erosion and maintains soil fertility as it breaks down. Figure 2. Hollows in old trees are nesting sites for many native birds. (G. Johnson) Page 8

3 loss from crops and pastures by providing shade. The extent of the shelter partially depends on the height of the trees, with mature trees providing more benefits than recently planted trees. Enhanced shade and shelter is also provided if the vegetation is growing in clumps or in strips (common along roadsides) and includes understorey plants such as grasses and shrubs. Economic values There are potentially many sources of direct income from remnant vegetation. These include grazing, browse and fodder; tourism, education and amenity; specialty timbers, craftwood and fuelwood; native seed; cut flowers, foliage and fruits; tannins, resins and essential oils; bush foods and honey (see Chapter 12. The economic use of remnant vegetation, for details). Aesthetic and other values Remnants have aesthetic, educational, recreational and scientific values. They contribute an additional visual dimension to human-modified landscapes because they retain a relatively natural appearance. They are also valuable indicators of what the landscape used to be like. Mature trees, which can be up to 400 or 500 years old, are particularly valued for their beauty and contribution to wildlife habitat. Trees planted today will take at least 100 years to attain a similar dimension in the landscape. Because remnants are generally dispersed across the landscape, they are relatively accessible to local communities. They are also suited for research and interpretation. Taken together, these characteristics create a significant educational tool. Maintenance of genetic diversity Remnant vegetation, whether it is a lone paddock tree or a patch of bushland, provides a source of plant and animal genetic material that can generally be enhanced by encouraging regeneration. This is particularly important as remnants are often the best remaining examples of once-intact ecosystems. Unfortunately, most remnant vegetation is either disappearing through continued clearing, or degrading more slowly due to factors such as livestock grazing, weed invasion and rural dieback. Once lost, these complex systems are extremely difficult, if not impossible, to recreate. Indicators of sustainable agriculture It has been proposed by the Commonwealth/State Standing Committee on Agriculture and Resource Management that the total area and health and degree of fragmentation of native vegetation (remnant and replaced) may be a useful indicator of sustainability in broadacre cropping areas. The contribution of native vegetation will depend on the health and degree of fragmentation (i.e. corridors or isolated patches of vegetation), the plant species involved, fauna habitat provided, soils, hydrology and topography. THREATS TO REMNANT VEGETATION Most remnants have been modified by activities such as grazing, weed infestation, forestry, fire wood collection, herbicide, pesticide and nutrient drift, and altered fire regimes. Rural dieback is also a major factor in the decline of remnants on farms (see Practical Information Note Rural Dieback). These activities threaten not only existing vegetation, but also can have a major impact on the regeneration of native species, and hence the long-term viability of remnants. Altered disturbance regimes (fire, impacts of domestic stock, water and nutrient transfers) Fragmentation of natural vegetation into small patches generally leads to changes in the types of disturbances that occurred prior to clearing. This has the potential to alter the long-term functioning of the communities that remain. Alterations to the types of disturbance in remnants include changes in the frequency, timing and intensity of fires, or changes in grazing pressures. Additional disturbance may arise from new transfers of nutrients, pesticides and water between natural Page 9

4 vegetation and adjacent agricultural land. These transfers are likely to affect the dynamics of the remnants. In particular weed species may be favoured by greater nutrient availability. Domestic stock do not only cause changes to the way native vegetation is grazed. They break down the soil structure by trampling, increase soil nutrient levels through their waste products (which appears to make trees prone to insect predation), ringbark trees (a particular problem with stringybarks) and introduce exotic seeds via faeces or body surfaces. Pest plants and animals Invasion of weed species into remnant vegetation is of particular concern. Once established, it is likely that weeds will have a detrimental effect on the vegetation by impeding regeneration of native species, through their impact on the availability of nutrients, water and light and potential alterations to the fire regime through changes in fuel characteristics (see Chapter 6. Environmental weeds). Pest animals such as foxes and cats can also have a detrimental impact on some of the fauna that inhabit remnants. species, age classes and fauna. The pressure from grazing by insects is usually severe and sustained, leading to sparse crowns and considerable epicormic growth, and tree disorder and death is common (see Practical Information Note Insects and Tree Decline). Lack of regeneration of native species All causes of tree mortality such as ageing, disease, drought, flooding, exposure to extreme weather, insect damage, lightning, hares and rabbits must be viewed in relation to the regeneration of new individuals. The increase in grazing intensity and trampling by cattle and sheep has reduced, and in many cases eliminated, the natural replacement of trees and shrubs (see Figure 3). This is because livestock have not been fenced out of bushland remnants on most farms. Grazing by pest animals such as goats can also be a problem in remnants on outcrops and in the ranges. Rural dieback Clearance of native vegetation, mainly by heavy machinery and herbicides, is a continuing threat to remnants. A more insidious loss of natural ecosystems on farms, however, results from rural dieback. Rural dieback is the premature and relatively rapid decline and death of native trees on farms, apparently as a consequence of interacting environmental stresses (see Practical Information Note Rural Dieback). Deliberate tree clearing and the intensification of land use associated with it are pivotal factors in the development of rural dieback. Remnants with dieback are usually small and lack diversity of vegetation structure, numbers of plant Figure 3. Typical isolated paddock trees not regenerating and lacking associated understorey. (F. Stelling) Page 10

5 THE FUTURE While the conservation and management of remnant vegetation is challenging, with strategic planning, sound technical advice and the enthusiasm and commitment of land managers and local communities, the long-term future of remnant vegetation in the South West Slopes is looking much brighter. REFERENCES AND FURTHER READING Bos, D. & Lockwood, M. 1996, Flora, fauna and other features of the South West Slopes biogeographic region, NSW, Johnstone Centre Report No. 59, Charles Sturt University, Albury, NSW. Davidson, R.L. 1995, Farmers as conservators of a sustainable production base, in R.A. Bradstock, T.D. Auld, D.A. Keith, R.T. Kingsford, D. Lunney and D.P. Sivertsen (eds), Conserving biodiversity: threats and solutions, Surrey Beatty and Sons, Chipping Norton. Department of Land and Water Conservation 1995, Native vegetation protection and management in NSW, Information paper, Department of Land and Water Conservation, Sydney. Hale, P. & Lamb, D. 1997, Conservation outside nature reserves, Centre for Conservation Biology, University of Queensland. Norris, E.H. & Thomas, J. 1991, Vegetation on rocky outcrops and ranges in central and south-eastern New South Wales, Cunninghamia, vol. 2, pp Morgan, G. & Terrey, J. 1992, Nature conservation in western New South Wales, National Parks Association of NSW, Sydney. Rural Industries and Research Development Corporation (RIRDC) 1997, Sustainability indicators for agriculture: introductory guide to regional/national and on-farm indicators, RIRDC Publication No. 97/72, RIRDC, Canberra. Saunders, D.A., Arnold, G.W., Burbidge, A.A., & Hopkins, A.J.M. (eds) 1987, Nature conservation: the role of remnants of native vegetation, Surrey Beatty & Sons, Chipping Norton. Stelling, F. 1994, Revegetation guide for Northeastern Victoria, Department of Conservation and Natural Resources, Victoria. Yates, C.J. & Hobbs, R.J. 1997, Temperate eucalypt woodlands: a review of their status, processes threatening their persistence and techniques for restoration, Australian Journal of Botany, vol. 45, pp Page 11