Date of Award

2018

Degree Type

Thesis

Degree Name

Master of Science (Environmental Management)

School

School of Science

First Advisor

Professor William D Stock

Second Advisor

Dr Eddie Van Etten

Field of Research Code

05, 0502

Abstract

There is growing understanding of the importance of landscape mosaics and heterogeneity for biodiversity outcomes in Western Australia. However, there is limited information on the patchiness (spatial configuration of unburnt and burnt patches which occur at a range of spatial scales) within the perimeter of a single prescribed burn. Of particular concern is the idea that prescribed burning operations, carried out under very restricted weather and environmental conditions, can lead to structural and floristic homogenization of the area within a burn perimeter. This may be evident as reduced post-fire vegetation patchiness. Western Australian Jarrah (Eucalyptus marginata) forests are managed to reduce fuel loads to protect life and property. Additionally, specific biodiversity and fuel reduction programs are completed by means of low intensity controlled burns carried out by the Parks and Wildlife Service, part of the Department of Biodiversity, Conservation and Attractions. The northern Jarrah forest is thus an ideal study site to test whether prescribed burning homogenizes a within-burn area. The objective of this study was to determine the post-fire patchiness of vegetation following prescribed burning to assess the potential for homogenisation of the within-burn area of Jarrah forests. To achieve this, a set of post-burn surveys were created to determine the patchiness of the burnt areas within 18 prescribed burns. The quantitative post-burn assessment designed in this study was evaluated to determine if it is suitable to be used as a possible future management option. A survey was undertaken to obtain data on 29 environmental variables which capture the variability across an individual prescribed burn area to determine the within-burn fire characteristics. A Pearson correlation matrix table was constructed to determine the significant correlations between variables as well as the strength of the relationships (coefficient of determination, r2). The patchiness between seasons autumn and spring was compared. Generalised Linear Modelling (GLM), and the application of Akaike were used to identify which variables were essential and most significant in predicting patchiness, to produce a list of candidate models. The AICc indicated which models were the most parsimonious or plausible candidate models (AICc < 2). Structured Equation Modelling (SEM) was completed to determine which variables in the ‘most parsimonious’ model were the greatest contributors to the model.

It was found that the number of vegetation patches decreased with an increase in fire intensity and percentage of area burnt. Autumn and spring burns were not significantly different in terms of overall patchiness and the majority of patches were found to be small, between 1 – 10 m in length. Surface Moisture Content (SMC) was the only pre-fire condition variable that negatively influenced prescribed burn fire intensity and subsequently patchiness, while time since last burn was found to increase the number of vegetation patches and SMC. The post-fire survey data obtained in this study on the 18 post fire sites was also compared to the post-burn assessments completed by Parks and Wildlife Service on the same sites. GLM and AICc showed that the variables Fire Danger Index, ash cover (%), area burnt (%), month of fire and the number of vegetation patches, are the most parsimonious and ‘best’ fit at predictors of patchiness within a prescribed burn area. While SEM showed that area burnt (%) was the most important predictor of patchiness. Within-burn patchiness appeared to be low in the study sites. Results from the prescribed burns in the northern jarrah forests showed that in most prescribed fires, 90 to 100% of the vegetation area was burnt with a limited number of unburned patches (up to 12 km-1) remaining. Although successful for fuel reduction burning, such large scale vegetation loss has been found to lead to structural homogenisation within a burn area, which in turn can result in long-term impacts on biodiversity. This study thus showed that prescribed burns in the northern Jarrah forest can homogenise the within-burn area and the implications of these findings are discussed in the broader context of landscape patchiness. This study also shows that a quantitative post-fire patchiness survey similar to that designed here should be developed as a future management option.

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