Short-interval fires homogenise the structure of diverse temperate forests

Abstract

Changing fire regimes can drive significant transformations in forest landscapes, including shifts to alternative stable states. However, these states have rarely been quantitatively described, limiting understanding of impacts on ecosystem services, biodiversity, and fire behaviour. Using aerial LiDAR data, we quantified multiple structural attributes of ‘reference’ and ‘alternative’ forest states across three structurally diverse forest types, dominated by either obligate seeder or resprouter eucalypt trees. Alternative forest states which have persisted for at least 70 years, arose from short-interval wildfires in the early 20th century; while return intervals of reference states were considered within the ecological tolerance. Results indicate significant transformation across all three forest types. Alternative states were structurally distinct and more homogeneous than reference states. Homogenisation was evident in mean and maximum canopy height, light penetration index, rumple index, and LiDAR leaf area index. Depending on forest type, percentage cover of large overstory trees was significantly lower in alternative states (4–7 %) compared with reference states (39–50 %). Mean height was between 71 % and 78 % lower, and maximum height 52–60 % lower than reference states. Measures of horizontal structural heterogeneity including rumple index (canopy ‘roughness’) and foliage height diversity were lower, suggesting a simplified canopy surface in alternative states, with scattered emergent overstory trees, and decreased vertical differentiation of strata. Moreover, the LiDAR-derived leaf area index and light penetration index indicated a more open environment than reference states. Our findings suggest short-interval wildfires drive widespread and persistent homogenisation of forest structure, potentially degrading ecosystem functions that support biodiversity and ecosystem services.