A fire‐driven shift from forest to non‐forest: evidence for alternative stable states?

Abstract

We test the validity of applying the alternative stable state paradigm to account for the landscape‐scale forest/non‐forest mosaic that prevails in temperate Tasmania, Australia. This test is based on fine‐scale pollen, spore, and charcoal analyses of sediments located within a small patch of non‐forest vegetation surrounded by temperate forest. Following nearly 500 years of forest dominance at the site, a catastrophic fire drove an irreversible shift from a forested Cyperaceae–Sphagnum wetland to a non‐forested Restionaceae wetland at ca. 7000 calibrated (cal) yr BP. Persistence of the non‐forest/Restionaceae vegetation state over 7000 years, despite long fire‐free intervals, implies that fire was not essential for the maintenance of the non‐forest state. We propose that reduced interception and transpiration of the non‐forest state resulted in local waterlogging, presenting an eco‐hydrological barrier to forest reestablishment over the succeeding 7000 years. We further contend that the rhizomatous nature of the non‐forest species presented a reinforcing eco‐physical barrier to forest development. Our results satisfy a number of criteria for consideration as an example of a switch between alternative stable states, including different origin and maintenance pathways, and they provide insights into the role of threshold dynamics and hysteresis in forest–non‐forest transitions.