Abstract

Forest systems are dynamic and can alternate between alternative stable states in response to climate, disturbance and internal abiotic and biotic conditions. Switching between states depends on the crossing of critical thresholds and the establishment of feedbacks that drive (and maintain) changes in ecosystem functioning. The nature of these thresholds and the workings of these feedbacks have been well-researched, however, the factors that instigate movement toward and across a threshold remain poorly understood. In this paper, we explore the role of species composition in initiating ecosystem state change in a temperate landscape mosaic of fire-prone and fire-sensitive vegetation systems. We construct two 12-kyr palaeocecological records from two proximal (230 m apart) sites in Tasmania, Australia, and apply the Alternative Stable States model as a framework to investigate ecosystem feedbacks and resilience threshold dynamics. Our results indicate that, in this system, invasion by pyrogenic Eucalyptus species is a key factor in breaking down negative (stabilising) feedbacks that maintain pyrophobic sub-alpine rainforest. We conclude that the emergence of an alternative stable pyrogenic state in these relic rainforest systems depends on the extent of pyrophytic species within the system. These findings are critical for understanding resilience in forest ecosystems under future climate and land management changes and are relevant to fire-adapted cool-temperate ecosystems globally.

Highlights

  • Critical transitions, or catastrophic regime shifts between ecosystem states are of profound importance, given that they are often sudden, unpredicted and can substantially alter important ecosystem functions and services (Folke et al 2004)

  • We conclude that the emergence of an alternative stable pyrogenic state in these relic rainforest systems depends on the extent of pyrophytic species within the system

  • Long-term vegetation development at both sites follows the same trajectory in the ordination space between ca. 14.5-6 ka, with a clear separation of the two sites commencing at ca. 6 ka and culminating in distinct populations within the ordination space after ca. 2.6 ka, with Lake Osborne pollen spectra at this time notably higher in Eucalyptus than Lake Perry

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Summary

Introduction

Catastrophic regime shifts between ecosystem states are of profound importance, given that they are often sudden, unpredicted and can substantially alter important ecosystem functions and services (Folke et al 2004). The emergence of alternative stable ecosystem states is induced by feedbacks between the state of the system and limiting environmental factors, such as climate, disturbance, soil moisture conditions or nutrient loading (Borgogno et al 2007). 2009, Wood and Bowman 2012), and on the various thresholds that must be crossed to tip an ecosystem into a new state (Scheffer and Carpenter 2003, Scheffer et al 2012). Once an environmental and/or disturbance threshold has been crossed, factors such as the presence or absence of species can initiate a positive feedback switch that modifies the local environment in such a way that favours the new community/species (Wilson and Agnew 1992, Borgogno et al 2007)

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