TS-CRC Student project - Long-term landscape-scale woody vegetation dynamics in an Australian tropical savanna

Oxford University, UK

Ben Sharp

Background | Alluvial floodplains | Lowland habitats | Sandstone plateaus |Future Directions |

Background

Savanna environments worldwide are notoriously dynamic; the identities and relative abundances of woody plant species have been known to change dramatically over time in response to a whole range of management and environmental factors, such as fire, grazing, climate, and natural cycles of die-back and recovery. Vegetation changes may be natural or they may be caused by human activity; regardless of the causes such changes often have serious implications for land use. Pastoral productivity, conservation value, landscape-scale carbon storage, and physical access to the landscape are all affected by changes in woody vegetation.

Anecdotal historical evidence (see Savanna Links, Issue 14, link below.) suggests that significant and in some places dramatic changes may be occurring in savanna landscapes in the Victoria River District.

This was the first study to systematically examine patterns of woody vegetation change across a range of savanna environments in the Victoria River region with explicit consideration of the burning and grazing history of the area.

The study was located on Bradshaw Field Training Area, near Timber Creek, Northern Territory. Historical (1948) and recent (1993-7) aerial photographs were digitally processed and analysed to assess woody vegetation change over the past 50 years, and a variety of vegetation surveys were conducted on the ground to verify the results of the air-photo mapping and to identify the mechanisms responsible for observed changes. There are clearly different change mechanisms operating on three distinct savanna habitats within the study area:

Alluvial floodplains

Within the seasonally flooded alluvial zone near the Victoria River and tributaries there has been a clear and dramatic increase in woody vegetation across the full range of floodplain habitats since 1948. In some locations formerly treeless plains have become virtually closed woodlands since that time.

Invasion and proliferation by two identified tree species is responsible for all observed vegetation increases. Evidence strongly suggests that these increases are a direct consequence of extreme overgrazing, most likely in the 1970s, which rendered the landscape not flammable and permitted the rapid increase of woody plants in the absence of fire mortality. The change is clearly irreversible and almost certainly implies reduced pastoral productivity.

Lowland habitats

By comparing edaphically-similar habitats that have been grazed intensively for different lengths of time, the impact of grazing on lowland savanna vegetation was assessed. It is clear that multiple-change mechanisms are occurring simultaneously on different portions of grazed lowland habitats. Low-lying areas and drainage areas are experiencing woody vegetation increase as the result of proliferation by a single species of melaleuca.

This effect is apparently climate-driven, but fire/grazing interactions may mediate the extent of the climatic effect. On lowland rises dominant bloodwood (genus Corymbia) woodlands are in decline on heavily-grazed areas as a consequence of invasion by several species of fire-sensitive shrubs following fire-exclusion by cattle. The invasive shrubs presumably competitively exclude bloodwood regeneration, and may even drive established adults to an early death where grazing pressure has been consistently high. In contrast, ungrazed/ frequently-burnt lowland savannas are stable and continuously-regenerating. Corymbia woodland instability under heavy grazing has implications for pastoral productivity, conservation, and landscape-scale carbon storage, but research suggests that the effect may be reversible with proper periodic application of fire.

Sandstone plateaus

Sandstone plateaus were surveyed to assess vegetation dynamics in habitats remote from the effects of ungulate grazing. The condition of fire-sensitive Callitris intratropica populations clearly indicates that there has been a widespread increase in the frequency and/or intensity of fire on sandstone plateau habitats within the study area, likely as a consequence of the cessation of traditional Aboriginal burning activities.

While areas of both vegetation increase and vegetation decline are evident in the study area, it is apparent that all dominant savanna woodland species on this habitat (genera Eucalyptus and Corymbia) are resilient to the altered fire regime. What changes have occurred are the result of natural cycles of die-back and regeneration in response to stochastic variables that vary on a small scale, such as patchy early-wet-season rainfall events. Total net woody vegetation cover has been relatively unchanged in the past 45-50 years.

Future directions

It is hoped that this research will inform better decision-making and predictive capability for the sustainable use and management of grazed savanna landscapes in northern Australia.

*Operational funding.

Contacts

Dr Ben Sharp