The effect of seasonal burning on three Australian native orchids
Creators: name: Jasinge, Nuwani
Publisher: name: RMIT University
Year: 2024

description: In Australia, bushfires are common in the seasonally dry parts of the continent and prescribed burning is used to reduce fuel load and fire risk. This project investigated the effects of seasonal burns on Australian native terrestrial orchids to find the best (least damaging) season for a burn in spring-flowering (Glossodia major and Thelymitra pauciflora) and autumn-flowering (Pterostylis revoluta) orchids. Mid seasonal burns in autumn, winter, spring and summer were conducted for each species and empirical data before and after the burns were used to compare against controls as a field standard. The effect of fire was compared based on plant (re-emergence, growth and reproduction) and fungal (colonisation, growth and biodiversity) response. Based on plant responses, the least affected season for a burn was different for each species but coincided with seed dispersal or senescence. The least damaging burn season for the spring-flowering orchids was spring or summer and winter for autumn-flowering orchids, with the number of emergent plants post-burn equal or higher than controls.<br><br>The isolation of Rhizoctonia-like fungi from post-burn plants resulted in 3-fold reductions for spring-flowering orchids but more than 3-fold increases for autumn-flowering orchids with up to 80% pelotons growing as Rhizoctonia-like fungi (summer post-burn). Based on Rhizoctonia-like fungi isolation success, the least damaging fire season for fungal isolation was autumn for spring-flowering orchids and summer for autumn-flowering orchids but was affected by phenology with plants in the vegetative stage (leafing) superior to reproductive stage (flowering). The effect of fire on fungal growth was only observed on G. major isolates that was least affected by spring burns and had similar growth increases as controls coinciding with plant senescence. Glossodia major isolates were sensitive to smoke water with post-burn isolates from summer inhibited at high concentrations and may contribute to declining fungal populations if burns continued in this season. The season of fire and smoke water had little or no changes on fungal growth for T. pauciflora and P. revoluta isolates and may reflect the tolerance of fungi to fires.<br> <br>Fire reduced the diversity of Rhizoctonia-like fungi from G. major and P. revoluta plants, with fungal variants as high as 10 individuals pre-burn reduced to only 2 individuals post-burn for P. revoluta. The season of least damage when post-burn fungi were the same as controls for G. major and P. revoluta was autumn and spring respectively, coinciding with leafing phenology. Fungal diversity for P. revoluta was 5 times greater than G. major and could reflect the advantages of mycorrhizal infidelity in situ to exploit available fungi or multiple sources of fungal reservoirs to re-establish post-fire. The findings of this study are novel and critical for land management and conservation of endemic plants. The overall conclusion was that seasonal prescribed burnings at the correct time of the growth cycle were beneficial for plant responses but detrimental for mycorrhizal fungi. The least damaging season for a fire was different depending on the plant host and a general compromise was late spring (late November) for all three orchids.<br>