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

Abstract

Background Ecologists fiercely debate the role of soil conditions and fire regimes in controlling forest -savanna boundaries. A prominent component of this debate centres on the plausibility and existence of fire-mediated alternative stable state dynamics (FMASS), a model first proposed by the Tasmanian ecologist WD Jackson in 1968. The FMASS model asserts that increased or decreased landscape fire activity, often due to human intervention, can overwhelm physical environmental (e.g. topography and edaphic factors) controls of forest and savanna mosaics, thereby creating landscape dynamism. Many FMASS models include fire-vegetation-soil interactions (FVS), in which changes in fire frequency can change soil fertility and hence tree growth. This interaction, in turn, affects the capacity offorests to recover from disturbance and long unburnt savanna to convert to forest. Scope We evaluate support for the FMASS-FVS model in the context of the dynamics of the Tasmanian forests that have recently been drawn into wider, global debates surrounding the cooccurrence of tree and treeless vegetation states. We develop a simple spatial simulation model to illuminate the difficulties in analysing landscape pattern to draw inferences about the existence of FMASS-FVS. Conclusions Our review of the Tasmanian evidence shows that FMASS-FVS cannot unambiguously explain all tracts of sedgelands in Tasmanian wet forests, and hence Tasmania should not be used as an exemplar of these theories globally. Our simulations highlight that soil sampling that targets forest boundaries risks erroneously concluding that the distribution of forest and savanna boundaries is decoupled from edaphic factors. We describe a structured methodological pathway that can identify the role of FMASS-FVS in Tasmanian forest dynamics, and elsewhere in the world. This approach use cross-scale temporal and spatial analyses, and targeted experimental tests, to illuminate the interplay of fire, vegetation and edaphic factors in controlling tree establishment and growth and forest boundary dynamics.

Increased aridity drives post‐fire recovery of Mediterranean forests towards open shrublands

“…Specifically, vegetation–fire feedbacks have been proposed as a driver of alternate stable states in tropical forests and humid savannas (Langevelde et al , ; Hirota et al , ; Staver et al , ; Higgins & Scheiter, ; Staver & Levin, ; Accatino & De Michele, , ; Dantas et al , ; D’Onofrio et al , ), boreal (Johnstone et al , ; Rogers et al , ; Abis & Brovkin, ) and temperate forests (Kitzberger et al , , ; Tepley et al , ). However, other potential drivers of alternate stable states, mostly related to edaphic conditions, also have been proposed (Fletcher et al , ; Bowman & Perry, ; Veenendaal et al , ). The hypothesis that in the Mediterranean Basin forests and open shrublands are alternative stable states is motivated by observations of succession that has stalled in shrublands (Baeza et al , ; Acácio et al , ; Santana et al , ; Acácio & Holmgren, ) and of loss of resilience in oak and pine forests after repeated fires (Diaz‐Delgado et al , ; Mayor et al , ).…”

Mapping Tasmania's cultural landscapes: Using habitat suitability modelling of archaeological sites as a landscape history tool

“…This mosaic is governed by a complex interaction of soil fertility, soil drainage and fire, with the relative importance of these factors still a major topic of debate (e.g. Bowman & Perry, ).…”

“…Key issues include the extent to which Tasmanian Aboriginal hunting, gathering and fire use influenced the structure, function and distribution of modern plant and animal communities (Bowman, Wood, Neyland, Sanders, & Prior, ; Folco & Kirkpatrick, ; Fletcher & Thomas, ; Jackson, ; Mariani et al, ; Thomas & Kirkpatrick, ). Indeed, where and how Aboriginal people burned the landscape, and the extent to which this had landscape‐scale impacts on the island's biota, has long placed Tasmania at the centre of global archaeological and fire ecology debates (Bowman, Perry, & Marston, 2015; Jackson, ; Jones, ; McWethy et al, ). Past human impacts on Tasmania's environment thus remain a question with significant and urgent implications for conservation and landscape management today (Bowman & Perry, ; French, Prior, Williamson, & Bowman, ; Marris, ).…”

“…Indeed, where and how Aboriginal people burned the landscape, and the extent to which this had landscape‐scale impacts on the island's biota, has long placed Tasmania at the centre of global archaeological and fire ecology debates (Bowman, Perry, & Marston, 2015; Jackson, ; Jones, ; McWethy et al, ). Past human impacts on Tasmania's environment thus remain a question with significant and urgent implications for conservation and landscape management today (Bowman & Perry, ; French, Prior, Williamson, & Bowman, ; Marris, ).…”

“…This lack of knowledge has been a particular issue in the rugged south‐western interior of the Tasmanian World Heritage Wilderness Area (TWWHA). In this area, a protracted debate over the extent and biogeographical distribution of Holocene Tasmanian Aboriginal occupation has fuelled uncertainty in cultural heritage management, and the extent to which anthropogenic burning is responsible for the extensive tracts of buttongrass moorland in this perhumid climatic zone (Bowman & Perry, ; Cosgrove, ; DPIPWE, ; Fletcher & Thomas, ; Mariani et al, ).…”