Arnhem Land tropical savanna

• Category: Grassland, Tundra, and Human Biomes.
• Geographic Location: Australia.
• Summary: The Arnhem Land tropical savanna is a vast, relatively intact expanse of tropical savanna embedded with monsoon rainforests, large river systems, extensive freshwater floodplains, and rocky heathland communities.

Arnhem Land is a large region in Australia's Northern Territory. Not sharply defined, Arnhem Land is variously considered to range from 37,000 square miles (95,000 square kilometers) to 56,000 square miles (145,000 square kilometers). It is dominated by vast tracts of tropical savanna—a discontinuous layer of trees over a continuous layer of grasses—across the central northern coast of Australia. Embedded within the savanna matrix are small patches of monsoon rainforest, large river systems with extensive freshwater floodplains, and rocky heathland communities.

The climate of the region is classically monsoon tropical, with year-round warm temperatures and extreme rainfall seasonality; abundant rain falls during the austral summer monsoon season (December to April) with very little during the winter dry season (May to November). Rapid grass growth in the wet season and grass curing in the dry season promote high frequencies of low-intensity grass fires, with around 40 percent of the region burning in a typical year, making it one of the most frequently burned regions on Earth. Although the majority of fires are lit by humans, natural fire frequencies would most likely have been similarly high, though concentrated later in the year following ignition by lightning strikes during the buildup (premonsoon) season. Only a small proportion of the region—around 3 percent—has been cleared for agriculture or other high-impact developments.

Fire has played a prominent role in shaping the biota of the region. The tree layer of the savannas is strongly dominated by eucalypts (genera Eucalyptus and Corymbia), which are extremely fire-tolerant and able to resprout prolifically from burned stems. Eucalypt densities are typically high, though with open canopies that allow a dense grass layer to form underneath and provide fuel for savanna fires. Despite the fire-prone nature of the savannas, fire-sensitive plant communities such as monsoon rainforests are able to persist, mostly on perennially wet sites or those with some degree of topographic fire protection, such as rocky cliffs and gorges.

Topography

The region is dominated by three broad topographic units, each with distinct biota: undulating lowlands, rocky sandstone plateaus, and freshwater floodplains. The lowlands are almost entirely dominated by savannas, an important habitat for a range of species, most of which are widespread across northern Australia.

The heavily dissected Arnhem Plateau, in the center of the region, contains several fire-sensitive plant communities, including large tracts of rainforest along its western margin and extensive areas of heathland. It is an important center of plant and animal endemism, most likely due to its high level of isolation from similar habitats, long-term geomorphological stability, and high topographic complexity. Notable endemic species include the dominant rainforest tree Allosyncarpia ternata, the black wallaroo (Macropus bernardus, a small kangaroo), the Oenpelli python (Morelia oenpelliensis), the white-throated grass-wren (Amytornis woodwardi), and the black-banded fruit-dove (Ptilonopus alligator).

Extensive floodplains occur where the major rivers approach the sea and support grasslands, sedgelands, and Melaleuca woodlands. The floodplains support massive numbers of a range of waterbirds, most notably the magpie goose (Anseranus semipalmata); for this reason, the floodplains of Kakadu National Park are on the Ramsar List of Wetlands of International Importance.

Aboriginal Land Use

Australia has a very long history of human occupation, and well-dated human remains from Kakadu National Park suggest that humans have occupied the Arnhem Land region for at least 50,000 years. Despite European colonization in the nineteenth century and the consequent widespread and sometimes catastrophic disruption of Aboriginal society, parts of the region remain strongholds of traditional Aboriginal culture. Perhaps most notable is the Arnhem Land Aboriginal Land Trust, a vast, 37,000-square-mile (95,000-square-kilometer) area of Aboriginal freehold land in the northeastern part of the region.

More than 80 percent of the region is Aboriginal land, the majority administered by local land councils, but with large parts also incorporated into national parks jointly managed by local Aboriginal people and government agencies. The largest, at 7,645 square miles (19,800 square kilometers), and most notable of such parks is Kakadu National Park, established in 1979 and inscribed on the United Nations Educational, Scientific, and Cultural Organization (UNESCO) World Heritage List in 1981 for both its outstanding natural and cultural values.

Other than traditional Aboriginal land uses and conservation, the predominant land use in the region is cattle-based pastoralism. Although pastoralism typically leaves the woody vegetation largely intact, there is clear evidence that high stocking rates can have negative impacts on biodiversity, as can exotic grasses deliberately introduced as fodder for cattle.

Biodiversity Decline

Despite the absence of large-scale land clearing, a significant biodiversity decline is occurring in the region and across the wider savanna landscapes of northern Australia. Most notable is the dramatic decline in the abundance of small to medium-size mammals in the past few decades, even in relatively intensively managed national parks such as Kakadu. Species such as the golden bandicoot (Isoodon auratus), black-footed tree rat (Mesmbriomys goudlii), and common brushtail possum (Trichosaurus vulpecula) have experienced large range contractions and are now rarely encountered in most areas. Alarmingly, this pattern appears to mirror the catastrophic loss of a diverse suite of mammals in central Australia in the early to middle twentieth century.

A similar decline, though much less pronounced, is also occurring in granivorous bird species across northern Australia. The causes of these declines remain unclear, but ecologists strongly suspect two events: changes in fire regimes in recent decades (specifically, the increasing frequency, intensity, and size of vegetation fires), and predation by house cats, introduced during the nineteenth century. It is now well understood that increases in the frequency and intensity of vegetation fires are having negative effects on components of the region's vegetation. Well-documented examples include the northern cypress pine (Callitris intratropica), a fire-sensitive conifer that has declined substantially over the past century, and the sandstone heath communities that contain a high proportion of shrub species that are killed by fire and need relatively long fire-free periods to regenerate.

The most severe threat to the region's biodiversity in coming decades is likely to be the spread of the exotic gamba grass (Andropogon gayanus). This species rapidly invades savanna, resulting in fuel loads more than four times that observed in non-invaded areas. Such fuel loads produce extremely intense fires that can kill mature savanna trees. Recent research has shown that gamba can convert a savanna woodland to grassland monoculture within a decade or two of initial invasion. Gamba is predicted to spread throughout the region in the coming decades, accompanied by severe effects on savanna biodiversity. Given that removal of gamba, once established, is effectively impossible at large scales, preventing further spread must be a key objective of efforts to effectively conserve the biodiversity values of the region.

Fire Management

The need to better manage destructive fire regimes on the Arnhem Plateau has led to the establishment of an innovative fire management project known as the Western Arnhem Land Fire Abatement (WALFA) project. Operating since 2005, WALFA covers 9,266 square miles (24,000 square kilometers) of mostly savanna woodland and heathlands, and aims to improve fire management on the plateau. The project uses income from greenhouse-gas abatement based on fire management. Recent research has demonstrated that savanna fire regimes that are destructive to biodiversity (such as frequent, intense fires) are also major contributors to greenhouse-gas emissions.

WALFA is the world's first major carbon-abatement project based on fire management and was developed by a group comprising government agencies, Aboriginal Traditional Owners, and land management groups. Funding for the WALFA project is provided by Darwin Liquefied Natural Gas (part of one of the world's largest energy companies, ConocoPhillips), which is providing about $1 million annually for 17 years in return for an annual abatement credit of greenhouse gases equivalent to 100,000 tons of carbon dioxide. This approach to funding improved fire management in tropical savanna landscapes, with accompanying biodiversity benefits, is being rapidly adopted in other regions of northern Australia, including several national parks. It is hoped to be one element in a broader effort to sustain resources such as those found in Arnhem Land against the encroachments fed by climate change and global warming. Australia has already begun to feel the effects of climate change with higher temperatures, extreme droughts and fire, increased flooding and overall more extreme weather.

Bininj Rock Art

The Bininj are an Indigenous people who are the traditional owners of western Arnhem Land. In 2021, they gave a presentation to the Australian Archaeological Association about the progress of a project to locate and preserve the rock art of their culture. The rock art has been carved over hundreds of years on the walls of about 40,000 sites, mostly places where the Bininj have lived for thousands of years. With a grant of about $800,000, mostly from the Ian Potter Foundation, the Bininj have hired their own support staff to live in their communities. Members of this staff work to locate and record rock art sites, many of which have yet to be found. The goal of the project is to find, record, and preserve these sites. The information is then stored digitally for future generations.

Bibliography

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