Edaphic factors control fire-prone sedgeland and Eucalyptus forest mosaics in southwestern Tasmania

An unresolved, key question in plant ecology and biogeography is the relative importance of fire disturbance and edaphic factors in controlling treeless-forest boundaries. We investigated the similarities and differences in soil physical and chemical characteristics in Sedgeland, Scrub and Eucalyptus Forest sites in and around a large treeless area (Blakes Opening) situated in the densely forested Huon Valley in southern Tasmania. In each community, fire risk was also assessed over 12 months using microclimate measures of above-ground temperature and humidity, and water tables were monitored seasonally. Sedgeland soils and the upper layer of Scrub soils were organic (18–38 % C) and sand rich (c. 75 % of the mineral fraction), and differed fundamentally from the inorganic Forest soils (c. 5 % C and 55 % sand). Sedgeland soils had a much lower bulk density and a significantly lower concentration and capital of most plant nutrients (except C, N and S) in the rootzone. Ordinations of both soil and rocks in the profiles discriminated the Sedgeland and Forest, with the Scrub being intermediate. In winter, water tables were higher in Sedgeland than Forest and Scrub. However, Sedgeland has a drier microclimate, which makes its fine fuels available to burn more frequently and for a longer period over the summer compared with Forest. This was confirmed by radiocarbon dating of soil charcoal, which showed that fire was a regular feature of this environment throughout the Holocene. The strong edaphic differentiation of Sedgeland-Forest boundaries at Blakes Opening calls into question a prominent theory that vegetation patterns in this region are shaped primarily by landscape fires.