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Accueil du site → Doctorat → Australie → Fire, environment, and the shrubby understorey of heathy-woodland

University of Melbourne (2016)

Fire, environment, and the shrubby understorey of heathy-woodland

Chick, Matthew Phillip

Titre : Fire, environment, and the shrubby understorey of heathy-woodland

Auteur : Chick, Matthew Phillip

Université de soutenance : University of Melbourne

Grade : Doctor of Philosophy (PhD) 2016

Résumé
The paradigm of prescribed burning with ecological goals, known as ecological burning, has become common practice in fire-prone regions worldwide. This includes using prescribed fire to create heterogeneous environments to aid in the conservation of vegetation communities in flammable ecosystems. The intensity of these fires is important as it dictates the immediate and interactive effects of fuel consumption and soil heating, both of which can trigger change in vegetation composition and influence the post-fire successional pathway of the vegetation community. Overarching environmental factors interact with ecological burning practices and influence vegetation response post-fire. This thesis aimed to disentangle and understanding interactions and relationships between understory shrub and herbaceous species diversity, climatic and environmental variability, and fire-disturbance in a fire-prone heathy-woodland vegetation community. It studies the theories of plant diversity pattern and succession currently incorporated into ecological burning practices in southeast Australia and examines how current practices are influencing a fire-prone vegetation community.
Chapter 2 tested successional theory in the soil seedbank of heathy-woodland and explored its relevance to the currently used growth stage management paradigm across a climatic gradient. No consistent post-fire pattern was found and the classic model of succession was not evident in the soil seedbank. Environmental and spatial variability across the region was found to influence variation in soil seedbank composition more than time since fire.
Chapter 3 investigated factors that influenced heathy-woodland understory plant diversity at different scales to test the theory that climatic factors influence diversity at larger scales and fire history at smaller scales. It also tested Dynamic Equilibrium Model (DEM) predictions that ecosystem productivity interacts with disturbance at small scales to govern species competition. Aboveground diversity followed expected patterns as predicted by the DEM, whilst belowground diversity did not at some scales. The sensitivity of above- and belowground diversity to fire history increased in regions of higher productivity, which suggests that the patterns of plant diversity in the region align with the DEM theory of scale dependent plant diversity influences. How plant communities respond to prescribed fire is a function of fire intensity, which influences fuel consumption, soil heating, and the recruitment of species through seed germination post-fire. To understand how prescribed burning practices influence these processes chapter 4 combined an experimental approach with observational studies of soil heating following fire to understand the link between fire, soil heating and seed germination cues. Soil temperatures during prescribed fires predominately failed to reach temperatures found to trigger germination of species with physical dormancy.
Experimental heating of soil cores collected from the field found that moisture content likely governs heat transfer in soil cores, which suggests that current burning practices in the spring and autumn when soil moisture is higher will not maximise recruitment of species requiring heat to break their physical dormancy. This may have positive and negative effects as it may conserve seeds in the seedbank for future recruitment events ; for example, following an unexpected wildfire soon after prescribed burning, but it may also drive changes in aboveground composition by favouring species without physical dormancy.
A key question of this thesis and a key knowledge gap for forest managers is determining what the optimal growth stage distribution required to conserve current heathy-woodland biodiversity is and what the appropriate prescribed burning regime likely to achieve this distribution is. Chapter 5 estimated the optimal growth stage distribution for this ecosystem by modelling the geometric mean of the relative abundance for above- and belowground species diversity data. It then utilised the LANDIS-II landscape simulation model to conduct a scenario analysis of varying prescribed burning regimes to determine the best option for achieving the optimal growth stage distribution. _ The outcomes of this analysis found that burning 5% of the heathland per year should maximise the geometric mean abundance of heathy woodland species over time. The outcomes of this research suggest that the heathy-woodland vegetation community in southeast Australia is resilient to current prescribed burning practices. The present management paradigm does not appear to be having a negative influence on current understory diversity. This research does however highlight the importance of incorporating regional climatic and environmental variation into ecological burning paradigms as the current homogenous application of prescribed burning across the heathy-woodland system studied does not incorporate the role these drivers have at broader spatial scales.

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