Semi-arid Woodlands Of Eastern Australia Research Articles

Risks to carbon dynamics in semi-arid woodlands of eastern Australia under current and future climates

Extreme disturbance events, such as wildfire and drought, have large impacts on carbon storage and sequestration of forests and woodlands globally. Here, we present a modelling approach that assesses the relative impact of disturbances on carbon storage and sequestration, and how this will alter under climate change. Our case study is semi-arid Australia where large areas of land are managed to offset over 122 million tonnes of anthropogenic carbon emissions over a 100-year period. These carbon offsets include mature vegetation that has been protected from clearing and regenerating vegetation on degraded agricultural land. We use a Bayesian Network model to combine multiple probabilistic models of the risk posed by fire, drought, grazing and recruitment failure to carbon dynamics. The model is parameterised from a review of relevant literature and additional quantitative analyses presented here. We found that the risk of vegetation becoming a net source of carbon due to a mortality event, or failing to realise maximum sequestration potential, through recruitment failure in regenerating vegetation, was primarily a function of rainfall in this semi-arid environment. However, the relative size of an emissions event varied across vegetation communities depending on plant attributes, specifically resprouting capacity. Modelled climate change effects were variable, depending on the climate change projection used. Under ‘best-case’ or ‘most-likely’ climate scenarios for 2050, similar or increased projections of mean annual precipitation, associated with a build-up of fuel, were expected to drive an increase in fire activity (a 40–160% increase), but a decrease in drought (a 20–35% decrease). Under a ‘worst-case’ climate scenario, fire activity was expected to decline (a 37% decrease), but drought conditions remain similar (a 5% decrease). These projected changes to the frequency of drought and fire increase the risk that vegetation used for carbon offsetting will fail to provide anticipated amounts of carbon abatement over their lifetime.

Response of vertebrates to fenceline contrasts in grazing intensity in semi-arid woodlands of eastern Australia

Response of vertebrates to fenceline contrasts in grazing intensity in semi-arid woodlands of eastern Australia

Effect of fragmentation, habitat loss and within-patch habitat characteristics on ant assemblages in semi-arid woodlands of eastern Australia

The reliability of ants as bioindicators of ecosystem condition is dependent on the consistency of their response to localised habitat characteristics, which may be modified by larger-scale effects of habitat fragmentation and loss. We assessed the relative contribution of habitat fragmentation, habitat loss and within-patch habitat characteristics in determining ant assemblages in semi-arid woodland in Queensland, Australia. Species and functional group abundance were recorded using pitfall traps across 20 woodland patches in landscapes that exhibited a range of fragmentation states. Of fragmentation measures, changes in patch area and patch edge contrast exerted the greatest influence on species assemblages, after accounting for differences in habitat loss. However, 35% of fragmentation effects on species were confounded by the effects of habitat characteristics and habitat loss. Within-patch habitat characteristics explained more than twice the amount of species variation attributable to fragmentation and four times the variation explained by habitat loss. The study indicates that within-patch habitat characteristics are the predominant drivers of ant composition. We suggest that caution should be exercised in interpreting the independent effects of habitat fragmentation and loss on ant assemblages without jointly considering localised habitat attributes and associated joint effects.

Foraging pits of the short-beaked echidna ( Tachyglossus aculeatus) as small-scale patches in a semi-arid Australian box woodland

Many animals create disturbances on the soil surface while constructing habitat and resting sites, or foraging for food. This soil disturbance, which is sometimes known as biopedturbation, is a major contributor to landscape patchiness in arid and semi-arid environments. In the semi-arid woodlands of eastern Australia, the Short-Beaked Echidna ( Tachyglossus aculeatus) creates a mosaic of foraging pits close to the canopies of large trees. The effects of pits on physical, chemical and biological properties of soils were compared at seven sites, each with two levels of disturbance (foraging pit vs. surface) and two canopy locations (under the canopy, in the open) associated with two tree species ( Eucalyptus intertexta, Alectryon oleifolius). Foraging pits trapped twice the mass of litter compared with adjacent non-pit surfaces, and there was more litter under the tree canopies than in the open. Pits contained more bark and leaf material, and larger pits tended to trap more litter. Soil electrical conductivity levels were lower in the pits, and although there were greater concentrations of soil nutrients under the canopy, the concentrations of total carbon, nitrogen and sulphur were lower in the pits compared with the surface. Changes in litter mass did not explain differences in soil carbon, nitrogen or phosphorus. Soil in the pit was moister and more porous, and surface temperatures below the litter in the pits about 2 °C lower than at the surface. Respiration was about 30% greater in the pits, and both the early (sorptivity) and late (steady-state infiltration) stages of infiltration were significantly greater in the pits. Soil micro-arthropods were more abundant in the pits, which supported a different complement of taxa, but a similar diversity, to non-pit surfaces. Our results indicate that echidna foraging pits act as substantial resource traps. Given their extensive distribution in semi-arid woodlands, and their marked influence on soil biogeochemistry, echidnas should be seen as important ecosystem engineers in woodland critical for the maintenance of small-scale patchiness and, therefore, the efficient functioning of arid and semi-arid ecosystems.

Integrated shrub management in semi-arid woodlands of eastern Australia: A systems-based decision support model

What is causing the increasing densities of native shrubs, or so-called ‘woody weeds’, in some semi-arid pastoral lands and how might they be most effectively managed? This question has been on the rangeland policy agenda in Australia for more than one hundred years. This paper describes a fresh examination of this question using a systems approach. A key component of the approach involved ‘mapping the problem’. Using a systems-based approach, landholders developed four system diagrams broadly describing the ecology of woody weed re-occurrence, control options, property economics and management constraints with diagrams identifying how different factors related to, or influenced, each other. Agency personnel also constructed a system diagram describing institutional and regulatory constraints, and their interactions. Later, all these system diagrams formed the basis for an adaptive management model with capabilities for developing and quantitatively evaluating alternative management strategies relating to woody weeds. This model is called the Woody Weed Planner. The Woody Weed Planner contains mathematical relationships developed through field experimentation over the last 25–50 years covering the ecology of woody weeds, control options and control economics. These relationships enable the user to generate mathematical responses as a result of changing model parameters. A key component of the model is the ability to simulate the effects of alternative management responses given different rainfall scenarios. To enable this to occur, the Planner allows the user to replay historical rainfall patterns and ask the question “what impact will these have on woody weeds, stocking rates and economic performance on my property?”

Integrated shrub management in semi-arid woodlands of eastern Australia: ground and aerial application of defoliant to shrubs regenerating after disturbance

This paper describes experiments undertaken at several sites in semi-arid woodlands of eastern Australia to determine if chemicals applied either on the ground or from the air reduce the density of shrubs regenerating after disturbance. Ground-spraying of Roundup® in the autumn was more effective than spring application in defoliating shrubs, especially 2-year-old coppice growth. Spraying of Roundup with a hand-held boom at 0.5 up to 2.5 kg glyphosate/ha identified rates to be used for boom spraying. Aerial spraying experiments were then undertaken across several sites and involved several target species. The location of sufficiently large areas where shrub regeneration was of an optimum age (i.e. about 2–3 years) proved to be extremely difficult due to prevailing drought conditions precluding the use of prescribed fire as a preliminary treatment. Nonetheless in one experiment, young (1-year-old) regrowth of firebush (Senna pleurocarpa) exhibited increased sensitivity to Roundup with significant shoot mortality recorded after it had been applied at 0.5 kg glyphosate/ha. Aerial spraying based on an ultra-low volume application of 10 L/ha further enhanced cost-effectiveness on this occasion. Economic analyses structured around 20-year partial budgeting and determination of net present value (NPV) suggested a profitable return could be expected where treatment was based on Roundup applied at this threshold rate 2 years after a prescribed fire, especially when the rehabilitation costs were spread over an entire paddock that had been only partially sprayed. Finally, operational aspects involving aerial spraying in these semi-arid woodlands are also discussed.

Detecting environmental change in eastern Australia: rangeland health in the semi-arid woodlands

We examined changes in rangeland health in the semi-arid woodlands of eastern Australia at fixed sites between 1989 and 1999. Over the 11-year period there were significant declines in the quality of the vegetation, and changes in plant species were driven largely by seasonality, and to a lesser extent, amount of rainfall. Three indices of rangelands health (composition, function and stability) developed using site-based vegetation and landscape data, indicated that the majority of sites had intermediate values of the three indices, and few sites had either very low or very high values. The indices of composition and function were strongly correlated with the subjective ratings applied to each site at each measurement period. The results of this study highlight the difficulty of detecting change over extensive areas of rangeland, and of separating management-induced effects from climatic effects in an environment which experiences wide spatial and temporal variation in rainfall.

Integrated shrub management in semi-arid woodlands of eastern Australia: effects of chemical defoliants applied after an initial disturbance

Previous fire experiments using artificial fuel have shown that annual fires, especially those applied in the autumn, can effectively control coppicing understorey shrubs in semi-arid woodlands. Such frequent fire is impossible to apply under natural conditions given the limited time available for sufficient herbage fuel to accumulate. Preliminary screening studies were therefore undertaken to test the hypothesis that chemical sprays applied at concentrations less than those normally recommended could be used to mimic high-frequency experimental fires. The effectiveness of 11 chemicals (7 arboricides and 4 dessicants) applied at a range of concentrations was assessed on one site by spot-spraying 5-year-old coppice regeneration of Eremophila mitchellii (budda or false sandalwood) and E. sturtii (turpentine). Chemical activity was assessed by regularly monitoring leaf effect, i.e. by rating the degree of leaf discolouration, scorching, blackening and ultimately leaf fall, over the ensuing 12 months following treatment. Arsenal� and Roundup CT� induced the highest shrub mortalities across all size classes while mortality rates were consistently higher for E. mitchellii than for E. sturtii. A second experiment involved 5 chemicals (4 arboricides and 1 dessicant) applied in a similar manner to 7-year-old seedlings of Cassia nemophila (syn. Senna nemophila) (punty bush). Significant damage to foliage (> 80% leaf effect) of all 3 shrub species was recorded 2 months after treatment with either Roundup CT� or Roundup� (i.e. either 450 or 360 g/L glyphosate respectively), as well as with Arsenal� (250 g/L imazapur + 60 g/L isopropylamine) but only at the highest concentration (i.e. 100% of the 'recommended' rate). In some lower concentration treatments, leaf effect was still increasing 6 months after treatment. In a second series of screening experiments involving 1- and 2-year-old coppices sprayed in autumn and spring, significant interaction occurred between coppice age and season of spraying when averaged over both Eremophila species. At lower concentrations (i.e. 12.5 and 25% of maximum recommended rate), autumn application of Roundup CT� was more effective than spring application, especially once regeneration was 2 years old. Gramoxone� was also most effective at all rates above 12.5% of the maximum when applied in the autumn to two-year-old coppice. However, Garlon� (600 g/L triclopyr) and Tordon 50-DA(r) (50 g/L picloram + 200 g/L 2,4-D) were more effective when applied to 1-year-old coppice in the spring. Overall, the most effective low-concentration treatment was Roundup CT� applied in the autumn to two-year-old coppice. Low-concentration treatment of one-year-old coppice with Roundup CT� and Arsenal� was also consistently more effective when carried out in the autumn (80–90% leaf effect). The probability of shrub mortality was inversely related to coppice biomass with smaller coppices clearly more vulnerable to the added pressure imposed by secondary chemical treatment, independent of application rate.

Runoff and micromorphological properties of a grazed haplargid, near Cobar, NSW, Australia

We investigated the effects of 2 different grazing regimes on the surface soil properties of a dunefield land system in the semi-arid woodlands of eastern Australia. Sandy siliceous, thermic Xeric Haplargids (Siliceous Sands, Uc1·23) occur on the sandy, 2–4-m-high longitudinal dunes. Fine-loamy, siliceous, thermic Xeric Haplargids (Massive Red Earths, Uc2·13) occur in the swales between the dunes. We compared very high-intensity grazing (approx. 1 year) by feral goats with low-intensity grazing (approx. 4 years) by sheep. A rainfall simulator, applying water at the rate of 30 mm/h, measured the hydraulic properties of the surface soils formed under the 2 different grazing regimes. We examined undisturbed samples of the upper 5-cm layer of the soil surface using micromorphological techniques. In the swales, there were no differences in the effects of the 2 grazing regimes on soil properties. At low-intensity sheep grazing (0·2–0·3 sheep/ha), the soil surface on the dunes remained in an excellent condition. The surface had a good vegetative cover and consisted of either loosely packed sand grains, or areas where the sand grains were bonded together by clay and organic matter to form an organic crust. The total carbon content of the 0–2 cm depth of soil was 0·86%. Both soil surfaces have a high infiltration rate (i.e. >30 mm/h) and also appear to contain stable microaggregates of parna material distributed among the eolian sand grains. Very high-intensity goat grazing (up to 4·0 goats/ha) rapidly depleted the perennial grasses, killed most of the shrubs, and converted the soil surface on the dunes to one highly susceptible to erosion by wind. The low total carbon content (depth 0–2 cm) of 0·3% and absence of iron-stained clay coatings on the sands further support this view. The surface soil on the dunes in the very high-intensity goat-grazing plots consisted of either loosely packed sand grains or areas where poorly orientated clays coated the sand grains to form a strong, physical crust. We suggest that the physical crust may cause a change in the hydrology of the land system which may enhance the conditions for recruitment of unpalatable shrubs in the dune–swale interface. This increase in unpalatable shrubs and decrease in perennial grasses caused by the very high intensity goat grazing is therefore detrimental to the long-term productivity of these semi-arid lands.

Spatial organisation of landscapes and its function in semi-arid woodlands, Australia

The spatial organisation of three major landscape types within the semi-arid woodlands of eastern Australia was studied by a detailed analysis of gradient-oriented transects (gradsects). The aim was to characterise the spatial organisation of each landscape, and to account for that organisation in functional terms related to the differential concentration of scarce resources by identifiable processes. Terrain, vegetation and soils data were collected along each gradsect. Boundary analysis was used to identify the types of landscape units at a range of scales. Soil analyses were used to determine the degree of differential concentration of nutrients within these units, and to infer the role of fluvial and aeolian processes in maintaining them. All three major landscape systems were found to be highly organised systems with distinctive resource-rich units or patches separated by more open, resource-poor zones. At the largest scale, distinct groves of trees were separated by open intergroves. At smaller-scales, individual trees, large shrubs, clumps of shrubs, fallen logs and clumps of grasses constituted discrete patches dispersed across the landscape. Our soil analyses confirmed that these patches act as sinks by filtering and concentrating nutrients lost from source areas (e.g., intergroves). We suggest that fluvial runoff-runon and aeolian saltation-deposition are the physical processes involved in these concentration effects, and in building and maintaining patches; biological activities also maintain patches. This organisation of patches as dispersed resource filters (at different scales) has the overall function of conserving limited resources within semi-arid landscape systems. Understanding the role of landscape patchiness in conserving scarce resources has important implications for managing these landscapes for sustainable land use, and for the rehabilitation of landscapes already degraded.

Changes in Grass Density in Australian Semi-Arid Woodlands.

Changed grazing regimes since European settlement have been widely proposed as the cause of a decline of indigenous perennial grasses in the semi-arid woodlands of eastern Australia. A five year experiment using exclosures examined the effects of grazing on densities of perennial grasses. Short- lived Stipa spp. and Aristida spp. were dominant at most sites. Their densities fluctuated greatly with season and reached over 200 plants/m2 during climatically favourable periods. The long-lived Eragrostis eriopoda occurred at densities that were generally less than 5 plants/m2 and its populations were relatively stable. The response of Enneapogon avenaceus was distinctive. Though its density fluctuated considerably, successive peaks in density were higher and the species increased more in ungrazed areas than in destocked or unfenced areas. The differences between grazed and ungrazed populations became greater with successive peaks in density. Within the short periods that pastoralists are likely to be willing or able to apply such treatments, destocking or even removing all herbivores is unlikely to have a large effect on the density of many palatable perennial grass species. The rate of response to resting pastures will depend on seasonal conditions.

Using BURNECON to evaluate the economics of fire management in semi-arid Woodlands, Eastern Australia

This paper describes a general economic analysis package, BURNECON, for evaluating the benefits and costs of using management burns to improve grazing potential in the semi-arid woodlands of eastern Australia. BURNECON is a user-friendly, microcomputer-based decision support system; it provides the land manager with detailed economic analyses for choosing cost-effective options in using prescribed fire to control encroaching shrubs which reduce forage, animal gains and cause numerous management problems. An economic evaluation of three alternative livestock spelling strategies, after treating a paddock with fire, indicates that redistributing stock to other paddocks located on the property is more profitable than agistment off the property or selling with subsequent repurchase of replacement stock. The sell/repurchase option is the poorest strategy because of income lost for the year of spelling.