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Keywords:

  • Indigenous fire management;
  • landscape change;
  • tropical savanna;
  • woody vegetation dynamics

Abstract

Aim  To compare woody vegetation of Eucalyptus tetrodonta savanna in an area that has had near continuous management by Aboriginal people with that of an area managed by neither Aborigines nor Europeans for the last 50 years. In particular, we tested the hypothesis that Aboriginal landscape burning increases the abundance of tree species, particularly those that bear edible fruit. We also examined whether Aboriginal burning increases tree density of the mid-layer using measurements of the density and size distribution of all trees.

Location  Central Arnhem Land, a stronghold of traditional Aboriginal culture, in the Australian monsoon tropics.

Methods  Previous remote sensing analyses have shown that over the decade prior to field sampling, fires in the unmanaged area had produced a coarse-scale mosaic of patches burnt between one to three times, in contrast the area under Aboriginal management was characterized by very high spatial and temporal variability of fire activity. In the study reported here, we assessed density, size distribution and taxonomic identity of trees in 78 and 79 plots (each 79–1385 m2 in area) in the managed and unmanaged areas, respectively. Habitat attributes such as cover of rocks and bare ground and cover of various life forms were visually assessed for each plot, and density of grass and leaf litter was measured.

Results  Despite the contrasting fire regimes there were only minor differences in the fine fuels between the two areas, both of which had a low biomass of grass (dry weight was 35–221 g m−2). The composition and overall density of woody species was similar in the two areas but densities of fruit tree species and eucalypts were significantly higher, particularly in middle of the size class distribution of stems, in the unmanaged area while a few species, such as the fire sensitive conifer Callitris intratropica, were more abundant in the managed area.

Main conclusions  Our findings suggest that current Aboriginal landscape burning has a significant impact on vegetation structure, specifically stems in the middle of the size class distribution, but little effect on either species composition or total tree density. Such burning reduced the abundance of fruit bearing trees but had little effect on grass or fine fuel biomass. The vegetation structure in the area managed by Aborigines was similar to that reported for many savannas subjected to European fire regimes; a core difference, however, was lower biomass of tall grasses and greater abundance of C. intratropica. These data provide important empirical corroboration of previous descriptive ethnographic and historical analyses that vegetation structure at the time of settlement was strongly influenced by indigenous peoples’ use of fire. Thus our results not only inform contemporary land management but also provide insights into the role humans played in the evolution of Australia's remarkably flammable flora. Given our inability to use a replicated experimental design the generality of these conclusions require testing by comparing the findings of allied studies conducted elsewhere in northern Australia.