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Accueil du site → Doctorat → Australie → 2007 → Integration of Brigalow (Acacia Harpophylla) Vegetation with Dryland Cropping in South-East Queensland : Agronomic, Hydrological, Ecological and Goethean Perspectives

Griffith University (2007)

Integration of Brigalow (Acacia Harpophylla) Vegetation with Dryland Cropping in South-East Queensland : Agronomic, Hydrological, Ecological and Goethean Perspectives

Bradley, Melanie

Titre : Integration of Brigalow (Acacia Harpophylla) Vegetation with Dryland Cropping in South-East Queensland : Agronomic, Hydrological, Ecological and Goethean Perspectives

Auteur : Bradley, Melanie

Grade : Doctor of Philosophy PhD 2007

Université de soutenance : Griffith University. Griffith School of Environment

Résumé
Large tracts of brigalow (Acacia harpophylla F. Muell. ex Benth. Mimosaceae) vegetation once covered the Queensland Brigalow Belt. Over the last century, a significant reduction in brigalow coverage has taken place (> 90%), primarily a result of land clearing for agricultural development. Despite significant loss of brigalow and its associated biodiversity, potential exists for reversing the loss, avoiding further land degradation and promoting sustainable agricultural production, through the establishment of brigalow-cropping systems. Such land-use systems are characterised by deliberate integration of stands of regrowth and remnant brigalow vegetation with areas of dryland cropping. This landscape ecology study increased knowledge and understanding of brigalow-cropping systems using two complementary investigative approaches : (1) a conventional, reductionist, quantitative approach ; and (2) a holistic, qualitative approach. The first approach was used to investigate brigalow-cropping systems from agronomic, hydrological and ecological perspectives. These perspectives provided insight into aspects of brigalow-cropping system function, structure and composition. The second approach looked at brigalow-cropping systems from the Goethean perspective. This latter perspective sought to understand brigalow-cropping systems on a ‘whole-of-paddock’ basis. Each perspective constituted a discrete study, with the studies done on properties located in the Tara Shire of south-east Queensland. Specifically, the agronomic perspective looked at tree-crop competition adjacent to stands of regrowth and remnant brigalow. It found that young brigalow regrowth (< 5 years since last disturbance) did not have a negative impact on adjacent cropping. However, older brigalow regrowth (> 10 years since last disturbance) and remnant brigalow vegetation had a significant, detrimental impact on adjacent production, with tree-crop competition zones ranging from 12–47 m. Both field measurements and simulation modelling (using the Agricultural Production Systems Simulator, APSIM) suggested that reduced yield adjacent to brigalow stands was mainly due to competition for soil water. It was concluded that from a short-term economic viewpoint, integration of brigalow vegetation with dryland cropping generally incurs a production cost. The hydrological perspective investigated determinants of deep drainage and evidence for increased deep drainage in brigalow-cropping systems. Based on plant water extraction patterns, plant available water capacities, and the temporal distribution of active green leaf material, it was reasoned that the highest rates of deep drainage are likely to occur under annual cropping ; followed by regrowth brigalow and remnant brigalow vegetation. Supporting evidence for relative differences in deep drainage rates was found in soil chloride data. At one site in particular, chloride leaching had occurred following the clearing of brigalow vegetation and an eleven-year period of annual cropping. On this basis it was inferred that rates of deep drainage under cropping were greater than those occurring under adjacent regrowth and remnant brigalow. The ecological perspective looked at the floristic and structural characteristics of regrowth and remnant brigalow vegetation, in order to improve understanding of brigalow community development and ecological values in brigalow-cropping systems. It found that brigalow communities tend to follow the inhibition model with respect to their floristic development, with Acacia harpophylla being the dominant woody species. Structural development in these communities is best characterised by changes in tree cover, shrub cover, grass cover and litter cover. Overall, a considerable difference was observed between regrowth and remnant brigalow in terms of floristics and structure. At least 100 years of post-disturbance vegetation development is required before 90% of the floristic and structural characteristics of remnant brigalow will be recovered. It was concluded, however, that the ecological value of regrowth brigalow stands should not be discounted ; as they provide important habitat for both flora and fauna and are a means for increasing native vegetation cover in agricultural landscapes. The Goethean perspective looked at brigalow-cropping systems as a ‘whole’, rather than focusing on one particular element. This perspective sought to appreciate and describe a brigalow-cropping system (at the paddock scale), in qualitative, holistic terms ; based on a method developed by the German poet, playwright and natural scientist Johann Wolfgang von Goethe. The insights and understanding obtained from application of the Goethean method complemented the quantitative, analytical understanding of brigalow-cropping systems acquired from the agronomic, hydrological and ecological studies. In particular, holistic, qualitative understanding was used to redesign a brigalow-cropping system in a single paddock in a manner that was sensitive to the ‘character’ of the landscape. The two approaches to scientific investigation (quantitative and qualitative), and the insights they generated, were brought together in brigalow-cropping scenario analysis. Eight different brigalow-cropping scenarios were developed and evaluated at the paddock scale. These scenarios were derived from the present-day status quo, a conventional approach to land-use redesign and the Goethean method. A series of analyses were done to investigate economic versus ecological trade-offs for the different scenarios. These analyses were based on results from the agronomic, hydrological and ecological studies. The economic variable calculated for each scenario was total paddock yield (tonnes) ; while the ecological variables were deep drainage minimisation area (ha), native habitat area (ha) and total number of woody species (weighted index). Although deep drainage minimisation area is, strictly-speaking, a hydrological variable, for the trade-offs analysis it was considered an ecological variable, given its importance in terms of promoting the long-term health of agricultural landscapes. Using simple numerical analysis to evaluate the four variables altogether, it was found that a brigalow-cropping design directed at enhancing hydrological and ecological outcomes produced the ‘best’ result in terms of economic versus ecological trade-offs. With this particular design, a decrease in total paddock yield was offset by substantially greater gains in deep drainage minimisation area, native habitat area and total number of woody species. In contrast, a design directed at enhancing agricultural production produced the ‘worst’ trade-offs result. In the case of this latter design, an increase in total paddock yield was offset by relatively greater reductions in deep drainage minimisation area, native habitat area and total number of woody species. Overall, this thesis shows that production losses due to the integration of brigalow vegetation with cropping, can be substantially offset by ecological gains. Hence, it is worthwhile for landholders to investigate opportunities for integrating regrowth and remnant brigalow stands with dryland cropping. It is recommended that in the design of brigalow-cropping systems, conventional socio-economic and ecological considerations should be borne in mind. However, it is also important to look towards a broader range of qualitative landscape values and novel land-use conceptualisations. It is suggested that a multi-faceted approach, based on both conventional and Goethean methods for land-use redesign, will assist with the development of brigalow-cropping systems that promote the revitalisation, long-term health and sound management of fragmented landscapes ; throughout the Tara Shire and the greater Brigalow Belt.

Mots clés : brigalow • Acacia harpophylla • Brigalow Belt • biodiversity • dryland cropping • brigalow-cropping systems • South-East Queensland

Présentation

Page publiée le 12 septembre 2007, mise à jour le 30 mai 2017