Year

2012

Degree Name

Doctor of Philosophy

Department

School of Biological Sciences

Abstract

Complex structure has been identified as an important resource within forested landscapes globally, because it provides critical habitat resources for a diverse array of fauna. Fire regimes have a strong influence on forest structure, and frequent fire, in particular, has been associated with the simplification of forest structure. In turn this may lead to the decline of many animal populations. There is some available evidence that topographically driven variation in fire behaviour can play an important role in preserving high quality habitat and animal populations within fire prone landscapes. The aim of this thesis was to assess whether the effects of fires on forest structure and resident populations of small mammals are dependent upon landscape context in the eucalypt forests of south eastern Australia. This was achieved by: (i) examining spatial patterns in fire severity, and identifying topographic features which consistently mitigate these effects, and (ii) assessing whether the impacts of fire frequency on elements of forest structure and animal populations varied with topography. The study focused on small ground dwelling and arboreal mammals.

Remotely sensed measures of fire severity were used to examine spatial variation in the severity of four large wildfires which burnt concurrently across a gradient of mean annual rainfall in the Greater Sydney Basin. Fire severity was strongly driven by fire weather and mean annual rainfall, with topography and time since fire (fuel age) having secondary effects. Fire weather was the primary determinant of fire severity, with severe crown fires typically only occurring under extreme weather conditions. Interestingly, the effects of time since fire, weather and slope were all influenced by mean annual rainfall. In areas of low rainfall, sites burnt within the preceding five years were more likely to experience fires confined to the understory. However, in areas of high rainfall, time since fire had no apparent effect on fire severity, and this probably reflected the much faster rates of fuel accumulation in these more productive areas. The effectiveness of non-extreme weather and steep slopes at reducing fire severity decreased with increasing annual precipitation, presumably in response to greater fuel availability. Topographic position had a consistent effect on fire severity across the study region, with gullies and deep valleys typically experiencing lower fire severity than ridges, thus supporting the hypothesis that gullies and valleys may be providing important fixed refugia in the landscape for a range of fauna.

A set of 49 field sites spanning a range of combinations of fire frequency and topographic position (gully, ridge) were used to assess forest structure and small mammal populations. The severity of the most recent wildfire and logging intensity were also measured at each site, as these were potentially influential disturbances that varied spatially across the study area. The surveys of forest structure focused on several habitat elements of key importance to biodiversity, namely tree and log hollows, log volume and vegetation complexity. Frequent fire reduced vegetation complexity and the occurrence of hollows in logs, and increased the risk of collapse for large potentially hollow bearing trees by increasing the occurrence of fire scars. However, gullies retained high quality habitat irrespective of fire frequency, as a result of inherent differences in forest structure and the tendency for these areas to experience patchier burns or remain completely unburnt. The severity of the most recent wildfire generally had little impact on forest structure, suggesting that the cumulative impacts of multiple fires have a greater effect on forest structure than the intensity of the most recent fire. Surveys also revealed that intensive timber removal reduced the abundance of hollow bearing trees, and increased log volume and the number of hollow logs, through the removal of trees and input of logging residue, respectively. Longer term deficits in log volume and log hollows are likely to occur in the future on sites that have experienced intensive timber removal. This will result from a reduction in the current volumes of logging residue due to continued decomposition and consumption by fire, and a decreased input of large logs in the future as a result of past removal of large trees.

Small ground dwelling and arboreal mammals were surveyed at 26 and 44 sites respectively, spanning a gradient of topographic complexity. In general, fire frequency had little effect on fauna abundance, with only the abundance of male Antechinus stuartii showing a significant positive response to increasing length of fire interval. Gullies supported a greater abundance of Rattus fuscipes, while the abundance of Petaurus breviceps was greatest in flat areas. Wildfire severity and logging intensity did not significantly affect the abundance of the study species. In general topography appeared to have more influence on fauna populations than fire frequency. It is likely that the complex topography of the study area, internal variation of fire intervals within the frequency categories used in the study and the generally flexible habitat requirements of the species are the main reasons for the absence of a fire frequency effect.

The results of this study support the hypothesis that gullies play an important role in mitigating fire effects and preserving structurally complex habitat within the fire prone temperate eucalypt forests of south eastern Australia. Protecting gullies from land clearing and timber removal will be an important step in maintaining the resilience of animal populations dependent on structurally complex forest in these landscapes. Ensuring that prescribed burns are conducted during periods when there is a high moisture differential between ridges and gullies will reduce the likelihood of prescribed fires burning down into gullies, and further protect complex forest structure in these locations. The need for active fire management aimed at preserving complex forest structure may be greater in topographically homogeneous landscapes, due to the lower density of gullies within these areas. These findings are likely to have broader implications across dry sclerophyll eucalypt forests, which constitute the bulk of the forested biome in south eastern Australia.

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