Prescribed Burns in Heathy Swamps Favour Establishment of Threatened Flora

Abstract

Long fire-free periods cause heathy swamps to transition into late-successional age classes which are characterised by low species richness and a depauperate understorey. The majority of the critically endangered heathy swamps on the Fleurieu Peninsula are long unburnt and lack ‘disturbance dependent’ flora, prompting programs to burn some of the long unburnt vegetation. This thesis investigates the utility of fire as a management tool in heathy swamps. I first assessed the influence of heat and smoke on ex situ seedling emergence from soil samples. Then, I investigated the seed dormancy mechanisms of a critically endangered species in response to fire cues. Lastly, I conducted experimental burns to evaluate in situ plant recovery as a result of fire seasonality and herbivory. Seedling emergence from the soil samples indicated that fire facilitates the regeneration of threatened ground-stratum natives, including promoting seedling emergence, suppressing competition from dominant species, and providing these threatened ground-stratum natives with opportunities to replenish their seedbanks. Of particular importance, seedling emergence of native shrubs and subshrubs more than doubled with the fire treatment, including the critically endangered Hibbertia tenuis which increased 18-fold. By investigating the dormancy mechanisms of H. tenuis I then found that germination was negligible without smoke. Prescribed burns were then employed, which initiated a transient successional period with reduced overstorey cover, promotion of the understorey, and increased species richness (7.4 vs 4.5 species m-2 in unburnt controls). Within the long-unburnt heathy swamps, 29 % of plant species appearing after burning were absent from the above ground biomass. These plants depend on fire to re-appear above ground, grow, reproduce and replenish their propagule reserves. Populations of H. tenuis were declining in the absence of fire and were restricted to a population of 20 plants but have increased to nearly 600 plants since the experimental burns. The decline of many Fleurieu Peninsula swamp species can be attributed to an imbalance of age classes in the landscape, given that approximately 98 % of the swamps are likely in late-successional stages. With infrequent burning, native ferns and taller shrubs outcompete threatened ground-stratum plant species. I argue that for fire-dependent species, such as H. tenuis, local extinctions will occur if the time between fires is greater than the lifespan of the species in both the above- and below-ground vegetation. However, despite potential benefits of prescribed burns, herbivory and fire seasonality can have negative implications for recovery. Herbivory negatively impacts recovery after fire by excluding 12 % of species and slowing the recovery of many others. Native species also had opposing seasonal fire preferences, such that fire in any given season may benefit one species to the detriment of another. In heathy swamps, this thesis supports the utility of prescribed burns for maintaining a balanced spectrum of age classes, but also highlights the importance of mitigating herbivory and considering the effects of fire seasonality.