Characterising Ecosystem Composition, Structure and Function of Alternative Stable States in Temperate Forests of South-Eastern Australia.

Abstract

Increasingly frequent and intense wildfires threaten to push ecosystems beyond their ecological thresholds. Fire-adapted forests can be vulnerable to ecosystem conversion under shortening fire-return intervals. To evaluate the impacts of fire regime change, we contrasted Eucalyptus forests that retained canopy cover under tolerable regimes (reference forests) with alternative states of markedly reduced canopy. These alternative states established following short-interval wildfires and have persisted through subsequent fires. Alternative states were dominated by more uniform distributions of small trees (< 20 cm diameter) than reference states, with 97% fewer large trees (> 20 cm diameter). These changes in structure translated into declines in several key ecosystem functions. The loss of large, hollow-forming trees and limited Eucalyptus regeneration indicate long-term constraints on habitat availability for hollow-dependent fauna. Coarse woody debris mass was 38% lower in alternative states, indicating decreased resources for detritivores, fungi, and ground-dwelling mammals. Aboveground carbon stocks were reduced by 59%, with a much higher proportion (98%) stored in small trees than reference states (20%). Lower fire tolerance of small trees suggests a greater vulnerability of carbon stocks to subsequent fires in alternative states compared to reference states. These findings highlight the long-term ecological consequences of altered fire regimes. In addition, emphasising the management and restoration of forest structural integrity is essential to maintain ecosystem functions in fire-prone landscapes that are increasingly vulnerable under changing climates.