Productivity and Connectivity in Tropical Riverscapes of Northern Australia: Ecological Insights for Management

Flow regimes are fundamental to sustaining ecological characteristics of rivers worldwide, including their associated floodplains. Recent advances in understanding tropical river–floodplain ecosystems suggest that a small set of basic ecological concepts underpins their biophysical characteristics, especially the high levels of productivity, biodiversity and natural resilience. The concepts relate to (1) river-specific flow patterns, (2) processes ‘fuelled’ by a complex of locally generated carbon and nutrients seasonally mixed with carbon and nutrients from floodplains and catchments, (3) seasonal movements of biota facilitated by flood regimes, (4) food webs and overall productivity sustained by hydrological connectivity, (5) fires in the wet/dry tropical floodplains and riparian zones being major consumers of carbon and a key factor in the subsequent redistribution of nutrients, and (6) river–floodplains having inherent resilience to natural variability but only limited resilience to artificial modifications. Understanding these concepts is particularly timely in anticipating the effects of impending development that may affect tropical river–floodplains at the global scale. Australia, a region encompassing some of the last relatively undisturbed tropical riverine landscapes in the world, provides a valuable case study for understanding the productivity, diversity and resilience of tropical river–floodplain systems. However, significant knowledge gaps remain. Despite substantial recent advances in understanding, present knowledge of these highly complex tropical rivers is insufficient to predict many ecological responses to either human-generated or climate-related changes. The major research challenges identified herein (for example, those related to food web structure, nutrient transfers, productivity, connectivity and resilience), if accomplished in the next decade, will offer substantial insights toward assessing and managing ecological changes associated with human alterations to rivers and their catchments.     

In the Age of Mabo: History, Aborigines and Australia

In the Age of Mabo : History, Aborigines and Australia, Bain Attwood. ed. Sydney: Allen and Unwin, 1996.193 pp.     

The Burning Question: Aborigines,Fire and Australian Ecosystems*

The 'fire stick farming model, which suggests that Aborigines changed the frequency and nature of fires in order to manipulate animal and plant resources, is now widely accepted in Australian phehistory. A re-examination of the biological evidence suggests that Aboriginal use of fire had little impact on the environment and that the patterns of distribution of plants and animals which obtained 200 years ago would have been essentially the same whether or not Aborigines had previously been living here. It is further suggested that 'fire stick farming’, had it been attempted, would in fact have been counter productive economically because of the adverse effects it would have had upon small species of animals. Aborigines observed and made use of an existing natural fire regime in Australia, they did not attempt to develop a new one.     

Optimal Fire Regimes for Soil Carbon Storage in Tropical Savannas of Northern Australia

Modification of fire regimes in tropical savannas can have significant impacts on the global carbon (C) cycle, and therefore, on the climate system. In Australian tropical savannas, there has been recent, large-scale implementation of fire management that aims to decrease Kyoto-compliant non-CO₂ greenhouse gas emissions by reducing late dry season intense fires through strategic early dry season burning. However, there is no accounting for changes to soil C stocks resulting from changes to savanna fire management, although impacts on these pools may be considerable. We present a hypothesis that soil C storage is greatest under low intensity fires with an intermediate fire return interval. Simulations using the CENTURY Soil Organic Matter Model confirmed this hypothesis with greatest soil C storage under a fire regime of one low intensity fire every 5 years. Key areas of uncertainty for CENTURY model simulations include fine root dynamics, charcoal production and nitrogen (N) cycling, and better understanding of these processes could improve model predictions. Soil C stocks measured in the field after 5 years of annual, 3 year and unburned fire treatments were not significantly different (range 41–58 t ha⁻¹), but further CENTURY modelling suggests that changes in fire management will take up to 100 years to have a detectable impact (+4 t ha⁻¹) on soil C stocks. However, implementation of fire management that reduces fire frequency and intensity within the large area of intact savanna landscapes in northern Australia could result in emissions savings of 0.17 t CO₂-e ha⁻¹ y⁻¹, four times greater than reductions of non-CO₂ emissions.     

Ecological response syndromes in the flora of southwestern Western Australia: Fire resprouters versus reseeders

Two fire-response syndromes can be described for species of the vegetation of Mediterranean-climate, southwestern Western Australia. Resprouters survive fires as individuals. Reseeders are killed by fire and must reestablish through germination and establishment of seedlings. Of the Western Australian plant families analyzed for fire-response strategies, 50% of the Proteaceae, 50% of the Restionaceae, 45% of the Orchidaceae, and 25% of the Epacridaceae are resprouter species. Within genera of the Proteaceae, the proportions of resprouters includeAdenanthos (56%),Hakea (52%),Dryandra (35%), andGrevillea (31%). WithinBanksia, 49% are resprouters, and it appears that the reseeding syndrome is the derived character in this genus. The proportion of resprouters within southwestern Western Australian plant communities ranges from 66% to 80%. These percentages are generally higher than in more arid parts of Western Australia and in comparable plant communities from other Mediterranean-type climates of the world. The relatively high proportion of resprouters within plant families and within plant communities probably indicates that the Western Australian vegetation experiences a harsher fire stress regime than do other Mediterranean-type climate areas. Western Australian plant communities have their highest diversity in the early years after fire, when the vegetation contains a higher number of reseeding species and individuals. Seed banks are dominated by the seeds of reseeders. There are no basic differences in mean seed mass, viability, or germinability of seeds between resprouting species and reseeding species, but reseeders tend to have narrower optimum germination temperature regimes. Establishment success is related more to seed mass, seedling size, and leaf ecophysiology and morphology than to fire-response strategy. Reseeder seedlings tend to grow faster than do resprouter seedlings. Basic shrub morphology differs, with reseeders generally being umbrella shaped and resprouters urn shaped. Reseeding species most commonly have a shallow, fibrous root system. Resprouters have a massive, deeply penetrating root system. Shoot:root ratios of first-year seedlings and mature plants are higher for reseeders. Resprouter seedlings store starch in root tissue at a much greater rate than do reseeder seedlings. Although the concentrations of essential nutrients in seedlings are not different between fire-response types, reseeders tend to conserve nutrients to a greater extent through leaf retention. Reseeders tend to produce greater numbers of flowers and greater amounts of floral rewards, but the breeding systems, which lead to the higher seed set in reseeders, can vary between strict outcrossing and considerable selfing. Reseeding species are not likely to be wind pollinated. Species survival in a fire-prone environment can involve a wide range of combinations of attributes. It appears that in Western Australian reseeder species the lack of an ability to resprout is compensated for by a number of other structural and functional features. Knowledge of the fire-response strategies of species of southwestern Western Australia can influence fire-regime management, conservation of rare species, and restoration of vegetation after disturbance. Further knowledge of the fire-response strategies of species of the southwestern Western Australian flora should result in better management of natural and restored plant communities of the region.     

Fire weather in the wet-dry tropics of the World Heritage Kakadu National Park,Australia

Abstract Seasonal changes of weather and fuels in the wet-dry tropics are dramatic; fires follow suit. In this paper, we examine quantitatively rainfall, evaporation, wind, temperature and humidity information, and indices derived from them, for Kapalga Research Station and nearby Jabiru in World Heritage Kakadu National Park, Northern Australia. At Kapalga, the average annual rainfall of about 1200mm mostly falls during a 6 month wet season. Grasses, green in the wet, begin to desiccate during the early dry season. Perennial grasses cure more slowly than the annuals, and grasses in drainages cure later than those on ridges. Fire weather is usually most severe in September-October (late dry season) and least severe in January-February (late wet season). As the dry season progresses to its peak, daily wind patterns change, daily maximum temperatures increase to an average of 36°C, dew points drop to a minimum, and soil moisture is severely depleted. In the early dry season (cf. later), fires have a greater tendency to go out at night compared with later perhaps because winds then are calmer, fuels are more discontinuous, and relights from burning logs are less likely to occur. Fire weather in the north of Australia appears less severe than that in the southeast of the continent where socially disastrous fires occur periodically.     

Ecological gradients in forest communities on Groote Eylandt,Northern Territory,Australia

Vegetational changes over a series of geological boundaries through an area of managanese mineralization on the western side of Groote Eylandt, have been analysed using pattern analysis techniques. Hierarchical classification of data on the frequency of occurrence of plant species in the transects through each of the vegetational zones gave five ecologically identifable groups. The degree of interrelatedness between the sites was emphasized by ordination of the groups, incorporating minimum spanning analysis. The vegetational groups represent developments under different environmental conditions in which differences in water relations and the presence of an impermeable barrier in the soil profile were important. Comparison of the soil nutrient contents of the sites indicated major differences in the total manganese concentration in the topsoil. Even though manganese levels in Eucalyptus tetrodonta leaf tissue were high, no evidence could be found for an iron-chlorosis induced by a low Fe/Mn ratio. The levels of other nutrients in eucalypt foliage were similar to those measured elsewhere. In each vegetational grouping the frequency of occurrence of termite species varied according to site and food type. The role of termites in nutrient cycling needs to be further studied. The closed-forest maybe the climatic climax on the acid red earth soils since in the absence of fire it extends into the surrounding Eucalyptus tetrodonta open-forest. On the Sandstone and Manganese ares the stable state of the vegetation is likely to be composed of the Australian element of the vegetation modified to various extents by fire.     

Fire,storm, flood and drought: The vegetation ecology of Howards Peninsula,Northern Territory,Australia

Twenty different communities, characteristic of the coastal vegetation of the northern portion of the Northern Territory, were recognized by photo-interpretation and ground truthing a 20 × 20 km area near Darwin. Data collected from 161 quadrats placed throughout the study area showed a strong relationship between subjective and objective measures of vegetation structure. Photopattern is best explained by a combination of structural, lifeform and floristic data. The mangrove, grassland, forest and woodland communities form a complex spatial pattern. Variations in moisture regime, through the interaction of topography and soils with temporal fluctuations of salt and/or fresh water supply, appear to best explain vegetation distribution. Three anomalously open eucalypt communities are interpreted as being the consequence of a severe tropical cyclonic storm which destroyed much of Darwin in 1974. These communities are frequently burnt by the nearby urban population and possess layers of sapling size eucalypts, although the height and density of the regrowth appears to vary with site quality. It is argued that fire is a characteristic of the seasonal climate, and that the fire pattern largely reflects the vegetation pattern. Occasional severe tropical storms also affect vegetation types differently. The greatest amount of windthrow occurs in moist monsoon forests while the eucalypts on well drained soils are more windfirm, but remain obviously damaged for over 10 years.     

Efficacy of Integrating Herder Knowledge and Ecological Methods for Monitoring Rangeland Degradation in Northern Kenya

The world-wide debate on land degradation in arid lands, usually linked to local land use practices, does not reflect methodological advancements in terms of assessments and monitoring that integrate local communities’ knowledge with ecological methods. In this paper, we evaluated the efficacy of three different methods related to herder assessments and monitoring of land degradation; herder knowledge and ecological methods of assessing impacts of livestock grazing along gradients of land use from settlement and joint monitoring of selected marked transects to understand long-term vegetation changes in southwestern Marsabit northern Kenya. The performance of each method was carefully evaluated and interpreted in terms of the indicators used by herders and ecologists. Herder interpretations were then related to ecologists’ empirical analysis of land degradation. The Rendille nomads have a complex understanding of land degradation which combines environmental and livestock productivity indicators, compared to conventional scientific approaches that use plant-based indicators alone. According to the herders, the grazing preference of various livestock species (e.g., grazers versus browsers) influences perceptions of land degradation, suggesting degradation is a relative term. The herders distinguished short-term changes in vegetation cover from long-term changes associated with over-exploitation. They attributed current environmental degradation around pastoral camps, which shift land use between the alternating wet and dry seasons, to year-round grazing. We deduced from long-term observation that herders interpret vegetation changes in terms of rainfall variability, utilitarian values and intensification of land use. Long-term empirical data (23 years) from repeated sampling corroborated herder interpretations. Land degradation was mostly expressed in terms of declines in woody plant species, while spatial and temporal dynamics of herbaceous species reflected the effects of seasonality. The efficacy of the three methods were inferred using explanatory strengths of ecological theory; insightfulness of the methods for describing land degradation and the likelihood of using the methods for promoting local community participation in the implementation of the UN Convention on Combating Desertification (CCD) and the Convention on Biological Diversity (CBD).     

Indigenous Knowledge of Ecological Variability and Commons Management: A Case Study on Berry Harvesting from Northern Canada

Common property arrangements govern the subsistence harvest of berries in the Gwich’in region of the Northwest Territories, Canada. Some of these arrangements, including rules for resource access, sharing information and harvest sharing, enable the Gwich’in to deal with ecological variability. The rules change in response to year-to-year variations in the abundance and distribution of the species, spatially and temporally across the region. This paper illustrates the interrelationships between ecosystem dynamics and local institutions, a neglected area of commons research.An erratum to this article can be found at     

Simulating the Effects of Fire Reintroduction Versus Continued Fire Absence on Forest Composition and Landscape Structure in the Boundary Waters Canoe Area, Northern Minnesota, USA

The Boundary Waters Canoe Area (BWCA) Wilderness of northern Minnesota, USA, ememplifies how fire management and natural disturbance determine forest composition and landscape structure at a broad scale. Historically, the BWCA (>400,000 ha) was subject to crown fires with a mean rotation period of 50–100 y. Fires often overlapped, creating a mosaic of differently aged stands with many stands burning frequently or, alternatively, escaping fire for several centuries. The BWCA may never have reached a steady-state (defined as a stable landscape age-class structure). In the early 1900s, a diminished fire regime began creating a more demographically diverse forest, characterized by increasingly uneven-aged stands. Shade-tolerant species typical of the region began replacing the shade-intolerant species that composed the fire-generated even-aged stands. Red pine (Pinus resinosa) stands are relatively uncommon in the BWCA today and are of special concern. The replacement of early-to-midsuccessional species is occurring at the scale of individual gaps, producing mixed-species multiaged forests. We used LANDIS, a spatially explicit forest landscape model, to investigate the long-term consequences of fire reintroduction or continuing fire absence on forest composition and landscape structure. Fire reintroduction was evaluated at three potential mean fire rotation periods (FRP): 50,100, and 300 y. Our model scenarios predict that if fire reintroduction mimics the natural fire regime (bracketed by FRP = 50 and 100 y), it will be most successful at preserving the original species composition and landscape structure, although jack pine (Pinus banksiana) may require special management. With limited fire reintroduction, all of the extant species are retained although species dominance and landscape structure will be substantially altered. If fire remains absent, many fire-dependent species will be lost as local dominants, including red pine. The landscape appears to be in a state of rapid change and a shift in management to promote fire may need to be implemented soon to prevent further deviation from historic, presettlement conditions.