Shifts in functional group community diversity of threatened mesic forests with changing fire regimes

Changing fire regimes, including increased fire severity, may impact plant community recovery, altering structure and composition, potentially causing a state change. For threatened wet sclerophyll forests, mesic forests which contain both dry sclerophyll and rainforest elements, this could mean a shift to a more fire-prone species composition. We investigate how species and functional group diversity responses differ across a gradient of fire severity and how recovery changes with time since fire in a mesic forest community. We hypothesize that increased fire severity can lead to reduced diversity and altered composition. We surveyed plant species cover and abundance three years post-fire at sites with different fire severities (moderate, high, or extreme) during the 2019/2020 Australian megafires, and sampled adjacent unburnt sites as controls. We calculated species and functional type beta diversity indices across a fire severity gradient. Our data showed a hump-shaped relationship between plant diversity and fire severity three years post-fire. Species richness was highest at moderately burnt sites and lower in unburnt and extremely burnt sites. Species composition also differed, with unburnt sites containing more rainforest-restricted species. Increased fire severity may reduce community-level diversity. The distinct compositional difference between recently burnt and long-unburnt sites suggests how fire regimes may drive shifts in wet sclerophyll forest states. The differences identified three years post-fire indicate that recovery may be slow, with extremely burnt sites potentially taking the longest.