Contracting Tasmanian montane grasslands within a forest matrix is consistent with cessation of Aboriginal fire management

Abstract

The persistence of treeless grasslands and sedgelands within a matrix of eucalypt and rainforest vegetation in the montane plateaux of northern Tasmania has long puzzled ecologists. Historical sources suggest that Tasmanian Aborigines were burning these treeless patches and models seeking to explain their maintenance generally include fire, soil properties and Aboriginal landscape burning. We aimed to provide a new historical perspective of the dynamics of the vegetation mosaics of Surrey Hills and Paradise Plains in north-west and north-east Tasmania, respectively, and used vegetation surveys and soil sampling to explore the role of vegetation and soils in these dynamics. Sequences of historical maps (1832 and 1903) and aerial photography showed that many treeless patches have persisted in the landscape since European settlement and that forests have rapidly expanded into the treeless patches since the early 1950s. Stand structure and floristic data described an expanding forest dominated by Leptospermum, which is consistent with vegetation succession models for the region. Soils under expanding forest boundaries did not have higher soil nitrogen or phosphorus than those under stable boundaries, signalling a lack of edaphic limitation to forest expansion. The magnitude of forest expansion at Paradise Plains (granite), Surrey Hills (basalt) and south-west Tasmania (quartzite) appears to follow a nutrient availability gradient and this hypothesis is backed by differences in soil phosphorus capital between the three systems. Given that existing vegetation boundaries in northern Tasmania do not coincide with soil nutrient gradients, we suggest that treeless vegetation was maintained by Aboriginal landscape burning and that the recent contraction of treeless vegetation is related to the breakdown of these fire regimes following European settlement. The observed rates of forest expansion could result in a substantial loss of these grasslands if sustained through this century and therefore our work supports the continuation of prescribed burning to maintain this high conservation value ecosystem.

Soil or fire: what causes treeless sedgelands in Tasmanian wet forests?

“…Such slow growth rates suggest that the rate of conversion of unburnt sedgelands to eucalypt forest over the hundreds of years assumed by Jackson is highly unrealistic, and is actually more likely to occur over millennia, thereby involving consideration of longer term climatic changes such as those that have occurred through the Holocene (e.g., Fletcher et al 2014a). This inference is supported by analysis of historical aerial photography showing that conversion of sedgelands to forest is extremely slow, and occurs through colonisation of woody pioneer plants (Leptospermum) that form shrublands close to the forest margins (Ellis 1985;Wood and Bowman 2012;Bowman et al 2013). Repeat field surveys also show that vegetation boundaries are mostly stable on granite, basalt and quartzite substrates, but that localised expansion of Leptospermum shrublands into sedgelands occurs along unburnt forest edges in these settings (Brown et al 2002;Bowman et al 2013).…”

“…This inference is supported by analysis of historical aerial photography showing that conversion of sedgelands to forest is extremely slow, and occurs through colonisation of woody pioneer plants (Leptospermum) that form shrublands close to the forest margins (Ellis 1985;Wood and Bowman 2012;Bowman et al 2013). Repeat field surveys also show that vegetation boundaries are mostly stable on granite, basalt and quartzite substrates, but that localised expansion of Leptospermum shrublands into sedgelands occurs along unburnt forest edges in these settings (Brown et al 2002;Bowman et al 2013). Dendrochronological analysis shows that the chemical fertility of the underlying substrate influences the growth rate of Leptospermum trees into sedgeland.…”

“…The resolution of this debate is severely constrained by the dearth of archaeological sites and research in western Tasmania sedgelands and rainforests. However, a landscape ecological study (Bowman et al 2013) at one site where intensive archaeological surveys have also been conducted (Murray et al 1990;Pickering 1992) found that sedgelands were infilling, albeit slowly, with woody vegetation since this disruption of Aboriginal fire-use (Bowman et al 2013).…”

“…Tree growth, whether determined using dendrochronological techniques (Bowman et al 2013) or via classical field and pot-trial experimentation, must be related to edaphic factors. Collectively, these data can be used in dynamic spatial models (such as that presented here) to determine if growth rates are sufficient to allow forest species to regenerate and escape the firetrap under some, but not all, fire frequencies.…”

A framework for testing the influence of Aboriginal burning on grassy ecosystems in lowland, mesic south–eastern Australia

“…A nested, two-scale treelessness model is proposed on the basis of the Pleistocene or Holocene relict hypotheses developed for montane grasslands in southeastern Queensland and Tasmania (Ellis 1985;Fairfax 1996, 2006;Bowman et al 2013). Under this model, grasslands represent small to large patches in a woodland-forest matrix, resulting from the dynamic interplay of (1) broad-scale, rare or infrequent disturbances (extreme drought, frost, storms/ tornados, wildfire) and (2) frequent, fine-scale disturbances (Aboriginal burning, native herbivore grazing, dense grass sward competition), mediated by soil and terrain (Fig.…”