Jeremy Russell-Smith

Savanna vegetation-fire-climate relationships differ among continents.

Surveying Savannas Savannas are structurally similar across the three major continents where they occur, leading to the assumption that the factors controlling vegetation structure and function are broadly similar, too. Lehmann et al. (p. 548) report the results of an extensive analysis of ground-based tree abundance in savannas, sampled at more than 2000 sites in Africa, Australia, and South America. All savannas, independent of region, shared a common functional property in the way that moisture and fire regulated tree abundance. However, despite qualitative similarity in the moisture–fire–tree-biomass relationships among continents, key quantitative differences exist among the three regions, presumably as a result of unique evolutionary histories and climatic domains. Evolution cannot be overlooked when aiming to predict the potential global impacts on savanna dynamics in a warming world. Ecologists have long sought to understand the factors controlling the structure of savanna vegetation. Using data from 2154 sites in savannas across Africa, Australia, and South America, we found that increasing moisture availability drives increases in fire and tree basal area, whereas fire reduces tree basal area. However, among continents, the magnitude of these effects varied substantially, so that a single model cannot adequately represent savanna woody biomass across these regions. Historical and environmental differences drive the regional variation in the functional relationships between woody vegetation, fire, and climate. These same differences will determine the regional responses of vegetation to future climates, with implications for global carbon stocks.

A LANDSAT MSS-derived fire history of Kakadu National Park, monsoonal northern Australia, 1980-94 : seasonal extent, frequency and patchiness

A 15-year fire history (1980-94) was assembled for Kakadu National Park, a 20000 km 2 World Heritage property in monsoonal northern Australia, based on interpretation of LANDSAT MSS imagery sampled at least three times over the 7-month dry season. Detailed ground-truthing was undertaken at the end of the early dry season period (May-July) for both 1993 and 1994; ground-truth data were not available for previous years. Overall agreement was greater than 80% in both years. In sum, these data inspire a relatively high degree of confidence in the interpreted fire history of the Park for any one year, at least at the landscape and habitat scales examined here. An average of 46% of the Park was found to be burnt each year over the 15 years of records, with 25% burnt in the early, and 21% burnt in the late, dry season. The data indicate a pronounced shift from a fire regime dominated by late (typically more intense and potentially extensive) dry season fires up until the mid-1980s, to one dominated by early (typically of low intensity and patchy) dry season fires subsequently. Whereas an average of 55% of lowland savanna habitats has been burnt annually, 28% of habitats occupying both sandstone plateau and riverine landforms have been burnt each year. The great majority of burning in relatively fire-sensitive sandstone habitats continues to be in the late dry season. Data indicate a marked increase in the extent of burning on floodplains, and in associated fire-sensitive Melaleuca forests and lowland rainforests, from 1990; this increase is attributed to increased herbaceous fuel loads associated with the removal of feral Asian water buffalo. Proximity analyses indicate that slightly more early dry season burning has been undertaken close to roads, and at greater distances from settlements; no proximity differences were discernible for fires late in the dry season. Burning has been concentrated close to lowland drainage lines, both in the early and late dry season. Lowland savanna sites are burnt on average 3 out of 5 years. In contrast, the majority of sandstone plateau and riverine floodplain sites have burnt on average 0-4 times and 0-3 times, respectively, over the 15 years of records. The median size of contiguously burnt areas (patches) has been declining steadily over the 15 years of records, from upwards of 300 ha initially to c. 60 ha in 1994. It is concluded that although the assembled data are imperfect in that they under-represent wet season burns and very small fires, are prone to positional errors of up to c. 300 m, and because little confidence can be placed in the sequential fire histories for any 1-ha site, they afford an effective means by which the Parks fire management programme can be monitored, its problems identified, and its needs assessed. The demonstrable utility of this programme has led to its recent extension on other significant conservation properties in monsoonal northern Australia; the procedure offers potential for wider adoption in savanna regions generally.

Bushfires 'down under': patterns and implications of contemporary Australian landscape burning

Australia is among the most fire-prone of continents. While national fire management policy is focused on irregular and comparatively smaller fires in densely settled southern Australia, this comprehensive assessment of continental-scale fire patterning (1997-2005) derived from ∼ 1k m 2 AdvancedVery High Resolution Radiometer (AVHRR) imagery shows that fire activity occurs predominantly in the savanna landscapes of monsoonal northern Australia. Sta- tistical models that relate the distribution of large fires to a variety of biophysical variables show that, at the continental scale, rainfall seasonality substantially explains fire patterning. Modelling results, together with data concerning seasonal lightning incidence, implicate the importance of anthropogenic ignition sources, especially in the northern wet-dry tropics and arid Australia, for a substantial component of recurrent fire extent. Contemporary patterns differ markedly from those under Aboriginal occupancy, are causing significant impacts on biodiversity, and, under current patterns of human popula- tion distribution, land use, national policy and climate change scenarios, are likely to prevail, if not intensify, for decades to come. Implications of greenhouse gas emissions from savanna burning, especially seasonal emissions of CO2, are poorly understood and contribute to important underestimation of the significance of savanna emissions both in Australian and probably in international greenhouse gas inventories. A significant challenge for Australia is to address annual fire extent in fire-prone Australian savannas.

Aboriginal Resource Utilization and Fire Management Practice in Western Arnhem Land, Monsoonal Northern Australia: Notes for Prehistory, Lessons for the Future

This paper considers traditional resources and fire management practices of Aboriginal people living in a near-coastal region of western Arnhem Land, monsoonal northern Australia. The data illustrate that before the arrival of Europeans freshwater floodplains and riverine habitats provided the major proportion of food resources over much of the seasonal cycle. By contrast, the extensive lowland woodlands and open forests, the sparser vegetation of the Arnhem Land escarpment and plateau, and the generally small patches of rain forest (“jungle”), provided relatively few resources, although jungle yams were of critical importance through the relatively lean wet season. The paper then considers burning as a management tool through the seasonal cycle. In broad terms, burning commenced in the early dry season and was applied systematically and purposefully over the landscape. Burning in the late dry season was undertaken with care, and resumed in earnest with the onset of the first storms of the new wet season, particularly on floodplains. These general patterns of resource use and fire management are shown to have applied widely over much of near-coastal northern Australia. The implications of these data for prehistory and for contemporary land management practices in the region, are considered. It is suggested that pre-European patterns of fire management in the region are likely to have been practiced only over the past few thousand years, given the development of abundant food resources in the late Holocene. It is shown that traditional burning practice offers a generally useful, conservative model for living in and managing a highly fire-prone savanna environment.

Conservation of monsoon rainforest isolates in the Northern Territory, Australia

The reservation status and condition of monsoon rainforest vegetation in the Northern Territory, Australia, are assessed based on geographically comprehensive survey data from a total of 1220 sites. It was found that five of 16 described rainforest assemblages are not currently reserved. One-third of survey sites were found to be severely disturbed by fire; 20% were severely disturbed by introduced cattle and Asian water buffalo; 10% by pigs; 7% by storm damage; 5% by flood damage; and 22% were infested with weeds. The impact of fires, introduced animals, and weeds is shown to apply generally across the three major forms of land tenure operative in the Northern Territory, namely: Aboriginal land, Crown land (mostly pastoral leasehold), and national parks and reserves. The implications of these data for conservative land management practice in the region are considered. It is concluded that effective conservation of significant rainforest patches requires active fire and feral animal management. Equally, however, the long-term genetic viability of small scattered patches and populations requires effective conservation at the landscape scale. Such considerations point to the necessity of setting priorities and practical objectives.

Improving estimates of savanna burning emissions for greenhouse accounting in northern Australia: limitations, challenges, applications

Although biomass burning of savannas is recognised as a major global source of greenhouse gas emissions, quantification remains problematic with resulting regional emissions estimates often differing markedly. Here we undertake a critical assessment of Australia’s National Greenhouse Gas Inventory (NGGI) savanna burning emissions methodology. We describe the methodology developed for, and results and associated uncertainties derived from, a landscape-scale emissions abatement project in fire-prone western Arnhem Land, northern Australia. The methodology incorporates (i) detailed fire history and vegetation structure and fuels type mapping derived from satellite imagery; (ii) field-based assessments of fuel load accumulation, burning efficiencies (patchiness, combustion efficiency, ash retention) and N : C composition; and (iii) application of standard, regionally derived emission factors. Importantly, this refined methodology differs from the NGGI by incorporation of fire seasonality and severity components, and substantial improvements in baseline data. We consider how the application of a fire management program aimed at shifting the seasonality of burning (from one currently dominated by extensive late dry season wildfires to one where strategic fire management is undertaken earlier in the year) can provide significant project-based emissions abatement. The approach has wider application to fire-prone savanna systems dominated by anthropogenic sources of ignition.

Fire regimes and the conservation of sandstone heath in monsoonal northern Australia: frequency, interval, patchiness

Abstract Heathlands are scattered across fire-prone monsoonal northern Australia mostly in dissected sandstone terrain. Such communities, although floristically depauperate by comparison with heathlands in southern Australia and southern Africa especially, share in common a relatively high proportion of fire-sensitive, obligate-seeder shrub species. The paper explores the implications of frequent fires, and associated short inter-fire intervals, on populations of obligate-seeder shrub species occurring in extensive heathlands occupying the western rim of the Arnhem Plateau, in the Northern Territory. Two studies are presented. With reference to published data concerning the maturation times of regional obligate-seeder shrubs, the first study reports on minimum and maximum intervals between fires determined from a 16-year fire history, 1980–1995, for the Plateau landform unit in Kakadu National Park, interpreted from LANDSAT MSS imagery. While species with maturation times of 5 or more years are common in the regional heath flora, minimum fire interval data for each 1 ha pixel indicate that 69% of heath habitats had been burnt at least once by fires recurring within 3 years, and 64% had a maximum fire interval of 5 years; 11% burnt only once or remained unburnt. The second study reports on the effects of an unreplicated experimental fire, involving observations on ca. 4000 individual shrubs, on ensuing heath floristic composition and abundance, undertaken 3 years after a wildfire had burnt the same site. Despite the experimental fire being highly patchy, substantial declines in the occurrence and density of many obligate-seeder shrub species were attended by increases in many herbs, including flammable grasses. Three years after the experimental fire the number of obligate-seeder shrubs was still less than half that pre-fire despite significant recruitment of some species in latter years. Collectively, these and other published data indicate that minimum fire return intervals of at least 4–5 years are required for conserving rapidly maturing tropical sandstone heath obligate-seeder shrubs, and longer still on sites comprising species with longer maturation times. For conservation management purposes individual fires should be small (especially in relation to the extent of any one tract of heath), patchy, and recurring intervals between fires should be varied as far as practicable.

Big fires and their ecological impacts in Australian savannas: size and frequency matters

Savannas are the most fire-prone of the earth’s major biomes. The availability of various broad-scale satellite-derived fire mapping and regional datasets provides a framework with which to examine the seasonality, extent and implications of large fires with particular reference to biodiversity values in the tropical savannas of northern Australia. We document the significance of savanna fires in the fire-prone ‘Top End’ region of the Northern Territory, Australia, using 9 years (1997–2005) of National Oceanic and Atmospheric Administration (NOAA)-Advanced Very High Resolution Radiometer (AVHRR)-, Landsat Thematic Mapper (TM)- and Enhanced Thematic Mapper (ETM+)-derived fire mapping. Fire (patch) sizes from both AVHRR- and Landsat-scale mapping increased through the calendar year associated with progressive curing of grass and litter fuels. Fire frequency data at both satellite sensor scales indicate that regional fire regimes in higher rainfall regions are dominated by large (>1000 km2) fires occurring typically at short (~2–3 years) fire return intervals. In discussion, we consider the ecological implications of these patch size distributions on regional fire-sensitive biota. Collectively, assembled data illustrate that many northern Australian savanna flora, fauna and habitats embedded within the savanna matrix are vulnerable to extensive and frequent fires, especially longer-lived obligate seeder plant taxa and relatively immobile vertebrate fauna with small home ranges.

Managing fire regimes in north Australian savannas: applying Aboriginal approaches to contemporary global problems

Savannas constitute the most fire-prone biome on Earth and annual emissions from savanna-burning activities are a globally important source of greenhouse-gas (GHG) emissions. Here, we describe the application of a commercial fire-management program being implemented over 28 000 km2 of savanna on Aboriginal lands in northern Australia. The project combines the reinstatement of Aboriginal traditional approaches to savanna fire management – in particular a strategic, early dry-season burning program – with a recently developed emissions accounting methodology for savanna burning. Over the first 7 years of implementation, the project has reduced emissions of accountable GHGs (methane, nitrous oxide) by 37.7%, relative to the pre-project 10-year emissions baseline. In addition, the project is delivering social, biodiversity, and long-term biomass sequestration benefits. This methodological approach may have considerable potential for application in other fire-prone savanna settings.

Seasonality and fire severity in savanna landscapes of monsoonal northern Australia

Ten years of photo and associated data records from an extensive fire and vegetation effects monitoring programme established in two large north Australian National Parks were used to (1) develop a simple-to-use semiquantitative fire severity index based on observed fire impact on vegetation, particularly leaf-scorch height; and (2) explore relationships between seasonality and fire severity in different landform and vegetation types. Using a three-tiered fire severity scale, data for 719 fires recorded from 178 plots over the period 1995–2004 indicate that the great majority of early dry season (pre-August) fires were of very low severity (fire-line intensities <<1000 kW m–1), whereas fires later in the dry season were typically of substantially greater severity. Similar trends were evident for vegetation occupying all landform types. The utility and limitations of the fire severity index, and implications for ecologic, greenhouse inventory, and remote sensing applications are discussed.