Fire regimes and biodiversity: the effects of fragmentation of southeastern Australian eucalypt forests by urbanisation, agriculture and pine plantations

Abstract

Fragmentation of eucalypt forests has been common in southeastern Australia. Urbanisation, agriculture and the establishment of plantations of the exotic tree Pinus radiata are major agencies of fragmentation. The study of the effects of these agencies on adjacent forested land has lacked a suitable framework. By constructing generalized trophic-level diagrams for each fragmenting system—farm, urban area and pine plantation—the major potential impacts on adjacent forested land can be examined. Urban areas, for example, have a relatively large non-native predator biomass (especially cats and dogs) compared with the original forest, whilst farms support a relatively large biomass of exotic herbivores. In pine plantations, by way of contrast, the biomass of native or exotic herbivores and predators is relatively small. Landscape fires are an integral part of the ecology of native eucalypt forests but are kept out of suburban areas, farms with improved pastures and standing plantations as much as possible. To explore the potential impacts on biodiversity of fire regimes in forests at the edges of urban areas, farms and plantations, we constructed and sought evidence for, a series of scenarios (each a compound hypothesis). Urban interface scenario: ‘There is a low frequency of unplanned fire in forest remnants. To prevent losses of life and property in adjacent urban areas, regular frequent prescribed burning is practiced. Regular frequent prescribed burning reduces biodiversity’. Support for the first two parts of this scenario was strong although the frequency of fires, prescribed or unplanned, may be a function of distance from the urban edge, the size of management unit and the nature of the tuels. Urban predators may be expected to reduce vertebrate biodiversity, especially after fires. Agricultural interface scenario: ‘Clearing for agriculture leaves only small forest remnants with become fire free. Fire-free fragments eventually decline in plant species biodiversity’. Forest fragments in rural areas vary widely in size and occur as roadside remnants, farm woodlots, Travelling Stock Reserves, State Forests and designated conservation reserves. The circumstances of burning vary widely. Grazing from domestic stock, especially combined with fire, may negatively affect biodiversity. Absence of fire may also reduce biodiversity. Pine interface scenario: ‘Pines spread from plantations to neighbouring forest areas, reducing native plant and animal species diversity. This situation can be reversed by a prescribed burning regime that has fires intense enough to cause pine death and frequent enough to prevent pine seed set’. There was considerable support for this scenario although at this stage the spread of pines may be only in areas peripheral to plantations. Because fires have effects on biodiversity, crops, lives and property in fragmented forests and adjacent areas, integrated management—across landuses and jurisdictions—is recommended.