Chapter 6 Native Grasslands in the NSW Riverina
Total Page：16
File Type：pdf, Size：1020Kb
Chapter 6 Native Grasslands in the NSW Riverina Ian Lunt Johnstone Centre, Charles Sturt University Albury NSW What is a native grassland? woodlands and shrublands dominated by Old Man Saltbush (Atriplex nummularia), Native grasslands are areas dominated by Boree (Acacia pendula) or Bladder native grasses with few, if any, widely- Saltbush (Atriplex vesicaria). However, spaced shrubs and trees. Many native these species were depleted across large grasslands in Australia were treeless areas by heavy grazing in the late 1800’s before European settlement ('natural when sheep numbers were extremely grasslands'), but others ('secondary’ or high. The loss of these low trees and ‘derived grasslands') have been created shrubs has created large areas of since settlement, following clearing or secondary native grasslands. Despite the heavy grazing. This chapter focuses on loss of woody species, many native the botanical and conservation values of grasses and herbs survived this ecological native grasslands in the Riverina. Grazing calamity, and these large secondary management is discussed in more detail grasslands are now very important for in Chapter 7. sustainable stock production and biodiversity conservation. As well as grasses, grasslands contain a wide variety of small plants, including native chenopods (saltbushes), lilies, Grasslands – changing in daisies, and sedges. Grasslands usually space and time contain many more species of small herbs Whilst most grasslands are superficially (daisies, lilies, peas, etc) than larger similar (flat and open), closer inspection grasses. This diversity of small herbs reveals great variability in their adds to their pasture value for stock, as composition across the Riverina. This well as being important for biodiversity variability is due to many natural and conservation. Although native species management influences, including dominate, introduced species from other underlying soil patterns, the effects of countries (including grasses, medics and grazing and past landuse, and seasonal other herbs) are usually common. weather changes. Native grasslands occupy large areas in Soil patterns the Riverina, especially in the 'Hay Grassland composition often changes Plains' region bounded roughly by Hay, greatly with subtle soil changes. Most Deniliquin, Jerilderie and Narrandera. As native grasslands in the Riverina occur on well as the original natural grasslands, heavy clay soils, and a variety of much of this region supported open woodland communities (often very Soil changes do not only control degraded) persist on lighter sandier soils. grassland composition, they also control Subtle differences in clay soils, usually patterns of grass productivity. Many hard, related to drainage patterns, cause red-brown clay soils have very low different plant associations to grow. nutrient levels (nitrogen, phosphorus and Thus, grey self-mulching clays in low- potassium) and are very impermeable to lying areas support different grassland water. Consequently, grass biomass is plants to better-drained, red-brown clays. held in check by the harsh conditions, and A common pattern is for White-top relatively little grass grows, even in good wallaby grasses (Austrodanthonia spp.) seasons. These areas often support short and fine spear-grasses (Austrostipa spp.) wallaby grasses and spear grasses and to dominate on well-drained, red-brown many small herbs, often with much open soils and other grasses, including tall ground between the grass tussocks. Plains Grass (Austrostipa aristiglumis) Despite their low productivity, many of and Rigid Panic (Homopholis proluta), to these areas are important for biodiversity grow on grey clays in lower-lying areas. conservation, and many uncommon Many soils vary greatly over small areas, plants and animals (including the Plains- forming complex mosaics of different wanderer) prefer these habitats. grassland plants. These mosaics are most pronounced on hummocky 'crabhole' or By contrast, crumbly, self-mulching and gilgai soils. cracking grey soils in low lying areas often contain more soil nutrients, and water can rapidly enter the cracked soil when it rains. Much of the rainfall that falls on impermeable red clays runs off and enters the cracking grey soils in low- lying areas, so these areas receive more effective rainfall than nearby red soil areas. Thus these areas usually support more grass growth than the harder red soils. Grazing effects As well as soil-induced patterns, grassland composition is also strongly affected by management, especially grazing. Heavy grazing in the past has greatly degraded grassland soils and plants in many areas. Heavy grazing often causes tall, long-lived, deep-rooted perennial species to be replaced by Figure 1: A diverse native grassland shorter, short-lived, shallow-rooted dominated by White-top (Common plants, especially annuals. Over the past Wallaby-grass) on low productivity, red- 150 years, long-lived perennial grasses brown clay soil. like Curly Windmill-grass (Enteropogon acicularis) and Kangaroo Grass (Themeda australis) have declined in plants germinate in intervening years. many areas, to be replaced in heavily Thus, the brief periods that are favourable grazed areas by annuals including fine to germination create a new generation of Spear-grasses (eg. Austrostipa scabra) plants which may appear to be relatively and introduced Barley-grass (Hordeum stable for long periods thereafter. leporinum), Rye-grass (Lolium species), Wild Oats (Avena fatua) and others. Grasslands also change rapidly in dry periods (especially when heavily grazed), Notwithstanding the degradation caused and productive species like low by over-grazing, conservative stock saltbushes can rapidly disappear. Such grazing appears to be compatible with changes can occur extremely quickly, and many grassland conservation values. can be effectively permanent thereafter, Many high quality grasslands have especially if there are few seeds in the traditionally been grazed without obvious soil to enable later regeneration. As in degradation or conservation problems. many Australian environments, extreme Indeed in some cases, conservative weather conditions (both dry and wet) grazing has positive outcomes for can trigger massive changes in grassland biodiversity conservation. If grazing is vegetation, which might then remain totally excluded for long periods on relatively stable for long periods. productive soils, the dominant grasses can increase in stature and smother out Exotic grassland plants smaller species, forming poorer associations with fewer species, and Introduced species are often abundant in making the grassland less suitable for native grasslands, especially in degraded some fauna. There is more potential for and heavily grazed areas. Exotic species this negative change to occur in include undesirable plants, such as productive low-lying areas than on dry noxious weeds (eg. Bathurst Burr, unproductive soils (eg. hard-packed red Xanthium spinosum), and also many earths) where grass cover does not species that provide useful feed in increase greatly even in good years. rangeland pastures. Exotic annuals such Strategic grazing regimes can maintain as Wimmera Rye-grass and Barrel Medic sustainable production returns and (Medicago truncatula) provide feed biodiversity values in many cases. during the autumn feed gap before perennial species begin to grow rapidly. Seasonal changes Whilst some exotic species are useful The only thing that stays the same in from a pastoral perspective, conservation grasslands is continual change. Native managers usually wish to promote native grasslands are always changing in species and minimise exotic species, as response to grazing management, exotic plants compete for water, nutrients seasonal rainfall and long-term weather and space against native species. The changes. Seasonal weather changes create impact of this competition is illustrated in rapid changes in grassland composition an historical quote from northern and structure. Some of these rapid Victoria, where Audas (1950) considered changes have very lasting after-effects. that the spread of Wimmera Rye-grass in The long-lived, Curly Windmill-grass is the 1940s, "must be considered a abundant in wet years, and few new blessing, were it only to redeem the interminable acres which are now given irrigated crops promote foxes that eat over to Helipterums or Sunrays" (both native fauna. These conflicts are are paper-daisies). Unfortunately the regrettable but real. Conservation and "interminable" native paper-daisies are development benefits can be maximised now very rare on the plains of northern by obtaining comprehensive advice on Victoria, although many remain abundant biodiversity values as part of the across large areas of the Riverina. development process. The recent declaration of grassland conservation Managing grasslands for reserves such as Terrick Terrick National Park in northern Victoria and biodiversity conservation Oolambeyan National Park in NSW will A primary aim of this Native Vegetation provide further benefits for conservation Management Guide is to promote and ecotourism in the region. biodiversity conservation in the Riverina. Consequently, this chapter focuses on managing native grasslands for Monitoring grassland conservation purposes, rather than for changes production values. Grazing management Grasslands are always changing. Some issues are discussed in Chapter 7 and changes are good, others bad, and many many superb extension materials describe