Suzanne Prober is a research associate with The Johnstone Centre, Charles Sturt University (PO Box 789, Albury, NSW 2640, Australia, Email: firstname.lastname@example.org), and Kevin Thiele works as a freelance ecologist for Ecological Interactions (5165 Bonang Road, Martins Creek, VIC 3888). This article was prepared to promote a broad-based approach to restoring grassy ecosystems, providing relevant contexts for goal-setting and a functional basis for identifying and overcoming barriers to restoration.
Box 1. Grassy White Box woodlands: what were they like before European settlement?
Grassy woodlands with an overstorey formed by mosaics of White Box (Eucalyptus albens), Yellow Box (Eucalyptus melliodora) and Blakely's Red Gum (Eucalyptus blakelyi) were once common in the eastern half of the vast New South Wales western slopes regions, extending also into northern Victoria and southern Queensland. Because they occur on productive country, most of these woodlands have been cleared or modified for agriculture. Less than 0.05% remain in little-modified condition, and they are listed as threatened at State and Federal levels. Conservation of these woodlands relies on conserving and restoring the woodlands that still remain, and even reconstructing more of them. A first step to achieving this is simply knowing what they were like, and perhaps even how they ‘worked’.
Because there is so little intact woodland remaining, it's hard to know exactly what the woodlands were like before European settlement. We've built a picture of woodland understoreys (Fig. 3) based on early records and what we can still see in some of the least disturbed places across the NSW western slopes. These are typically country cemeteries, rail easements, travelling stock reserves and other areas that have escaped cultivation and regular grazing by livestock (Prober & Thiele 1995; Prober 1996).
[ Diagram of little-disturbed grassy White Box woodland understorey indicating species composition and soil attributes inferred from a range of reference sites. (Photo: Courtesy S. Prober and K. Thiele.) ]
Soils were generally soft, deep and moderate to rich in most nutrients, except that levels of available nitrogen and phosphorus were low. Nitrogen was tied up in the thick roots and other parts of the dense perennial grasses, and little became available in the topsoil in any season. Topsoils beneath trees were often higher in nutrients (total nitrogen, total carbon, available phosphorus, available potassium and cations) and pH than topsoils in open areas, probably due to factors such as nutrient concentration by extensive tree root systems (Prober et al. 2002a).
On the deeper, more fertile soils, true grassy woodland shrubs such as Acacia decora and Indigofera adesmiifolia were naturally scattered or patchy in the woodland understorey. Other shrubs such as Dodonea viscosa, Cassinia arcuata and Maireana microphylla increased on shallower or drier soils or with disturbance.
A diverse ground layer
Over much of the NSW western slopes region, the natural dominants in the grassy White Box woodland understorey were Kangaroo Grass (Themeda australis), especially in more open or frequently burnt areas, and Snow Tussock (Poa sieberiana), which was often dominant beneath trees (Prober 1996; Prober et al. 2002a). These tussock grasses played a significant role in the functioning of this ecological community, and distinguish it from other grassy woodlands on the plains to the west. They influenced levels of important soil nutrients, protected the soil surface, and provided habitat for birds, reptiles and invertebrates. A wide range of other native plants relied on the gaps between the tussocks, and their diversity was particularly high where the thick thatch of Kangaroo Grass was occasionally removed by fire, or where the dominant grasses were less competitive for other reasons (e.g. beneath trees or on less fertile soils).
Many of the woodland forbs and grasses were naturally widespread across the region, but some were more prominent in southern winter rainfall areas, and others in northern, summer rainfall areas. On heavy black clays in some northern areas, grasses like Plains Grass (Austrostipa aristiglumis), Blue Grass (Dichanthium sericeum) and Native Sorghum (Sorghum leiocladum) became more prominent (Prober 1996).
Box 2. Grassy White Box woodlands: how has the understorey changed?
With European settlement, the once abundant grassy White Box woodlands (Box 1) were transformed into a productive landscape of crops and pastures. Even in areas that weren't cleared or cultivated, livestock grazing and other landuses caused many changes in the woodland understoreys and the soils they grew in. Native diversity was reduced to varying extents under different grazing or cultivation regimes, soils were depleted or enriched depending on management and other disturbance, shrub cover was lost, and different grass species, both native and introduced, became dominant under different conditions (Prober & Thiele 1995; Prober et al. 2002b). One key change across all types of degraded remnants has been an increase in soil nitrate levels, which in turn encourages the growth of annual exotic plants (Prober et al. 2002b). Figure 4 describes some of the different ways that the woodlands have changed and identifies some of the degrading processes. Other types of changes, such as replacement by perennial pasture species or invasion by perennial exotics such as Coolatai Grass (Hyparrhenia hirta), have also occurred.
[ Five typical degraded states of grassy White Box woodland understorey, indicating some associated changes in topsoil properties. Note that these changes don't necessarily occur in a linear sequence, rather, they can result from differing management influences as indicated. (Photo: Courtesy S. Prober and K. Thiele.) ]
Box 3. Approaches for restoring Kangaroo Grass/Snow Tussock understoreys in grassy White Box woodlands
Understanding changes that have occurred in grassy White Box woodlands (Box 2) provides us with clear directions for determining ecological methods for their restoration. Common goals in restoring woodland understorey are to increase native plant diversity and cover, to decrease exotic abundance, to re-establish scattered shrubs, and to re-establish the original perennial dominant grasses Kangaroo Grass and Snow Tussock.
Modifying conditions to favour desirable native species and disadvantage exotics is a key to long-term success. We can use tools such as fire, carbon addition and seed reintroduction to restore more natural soil nutrient levels, soil seed banks, and competitive environments. These techniques can be used in conjunction with agronomic techniques such as cultivation, herbicides and direct drilling, or can be applied directly to remnants to help tip the balance to favour native species over exotics. Below, we introduce some restoration techniques that we have begun to experiment with, focusing on reducing annual exotics. Annual exotics are important competitors with native species in many degraded remnants. They are ecologically very different from most native plants, which are mostly perennial species with lower nitrogen requirements.
Soil nitrate depletion
In most degraded remnants, soil nitrate rises to high levels over the summer and autumn, encouraging lush growth of annual exotics as they germinate in autumn. Suppressing this nitrate peak is particularly effective for reducing the vigour of annual exotics, and hence enhancing establishment and competitiveness of desirable natives (Fig. 5). We can reduce soil nitrate temporarily by adding sugar or other carbon sources (e.g. sawdust) to the soil. This increases the carbon: nitrogen ratio, causing soil microbes to flourish and use up any available nitrogen. In preliminary field trials we added 0.5 kg sugar per m2 at 3-monthly intervals, but further studies are needed to determine effective minimum rates of carbon application for different types of remnants (S. Prober, K. Thiele & I. Lunt, unpub. data, 2004).
[ Addition of sugar or other carbon-rich materials to the soil temporarily reduces available soil nitrogen by encouraging growth of soil microbes. Plots on the left (a, b) were not treated with sugar and support robust annual exotics. Plots on the right were initially similar to those on the left but regular sugar applications led to (c) dramatically reduced weed growth after 1 year, and (d) successful establishment and growth of re-seeded native Kangaroo Grass within 2 years. (Photo: Courtesy S. Prober and K. Thiele.) ]
Seed bank manipulation – Exotics
Degraded remnants usually support a large seed bank of annual exotics (Lunt 1990). The seed bank of one important group of exotics, the cool-season annual grasses (e.g. Wild Oats and Bromes), can be reduced by burning in spring before established plants set seed (Fig. 6). Pulse grazing, herbicides or repeated slashing in spring may be similarly effective. Techniques involving spring biomass removal are best for sites with few broadleaf annual exotics, as many broadleaf annuals have longer-lived seed banks and can increase on bare soil (Prober et al. 2004). Scalping (removal of a complete thin layer of topsoil from a remnant) is another technique that has been tried for removing weed seed banks. This technique may also influence soil nutrient levels, but effects on soils or vegetation in grassy woodlands have not yet been well-documented.
[ Spring burns can be used to minimize seeding and discourage re-establishment of exotic annual grasses in weedy remnants. They are relatively safe to undertake because the surrounding countryside is still green but can be difficult to achieve. For small areas, a gas-powered weed burner is effective, as in this degraded remnant near Young, NSW. For larger areas, weeds can be dried-off before burning using knock-down herbicides or steam. Burns should be undertaken in patches to minimize soil erosion and impact on native fauna. (Photo: Courtesy S. Prober and K. Thiele.) ]
Manipulation of weed seed banks is best accompanied by augmentation with seed of native perennials (especially Kangaroo Grass), to ensure the exotics are replaced by natives rather than other weeds.
Seed bank manipulation – Natives
Native species that no longer persist on a site are unlikely to return quickly without assistance – their seed banks are short-lived (Lunt 1997), and their distribution throughout the landscape is too sparse to rely on natural seed dispersal. Thus, reintroduction of seed of desired species is usually a necessary part of the restoration process. Native seed is best sourced locally. If seed is limited, restoration can begin with small patches and further seed harvested as patches mature.
Perennial native sward establishment
Re-establishment of a dense sward of native perennial grasses, particularly the original dominants Kangaroo Grass and Snow Tussock, is important for restoring the natural functioning of the grassy woodland ecosystem. In particular, dense native grasses are likely to play an ongoing role in out-competing weeds, through above-ground competition and by maintaining low levels of available nitrogen and phosphorus. Preliminary data suggest, for example, that Kangaroo Grass is superior to other native or exotic grasses for reducing soil nitrate levels. Techniques for re-establishing Kangaroo Grass are increasingly well known. It has large, awned seeds that drill themselves into the soil, making cultivation or direct-drilling less critical for their establishment. Spring burns can enhance germination by releasing seed dormancy, as well as controlling annual grasses. Carbon amendment (e.g. sugar application) or herbicides can enhance establishment and growth by reducing competition with weeds (Cole & Lunt 2005; S. Prober, K. Thiele & I. Lunt, unpub. data, 2004). Establishment needs of Snow Tussock are currently poorly understood.
Perennial exotic control
No ecological methods are yet known for dealing with serious invasive perennials such as St. John's Wort (Hypericum perforatum) or Coolatai Grass (Hyparrhenia hirta), as these species are similar to native plants in their ecology. All invasive perennial exotics should be dealt with promptly with the aid of herbicides or hand removal, otherwise they may heavily infest the site and can build up long-lived seed banks. Avoiding disturbances that encourage their invasion is likely to be one important preventative measure (e.g. Coolatai Grass often begins invading after roadside scalping). Development of biological control methods for invasive exotics is another unexplored option for addressing these threats.