Fire frequencies for Western Sydney’s woodlands: indications from vegetation dynamics Penelope J. Watson June 2005 Submitted for the degree of Doctor of Philosophy at the University of Western Sydney, Sydney, Australia SUMMARY Although the importance of fire management for biodiversity conservation is increasingly being recognised, little is known about the relationship between fire regimes and plant diversity in Australia’s temperate grassy woodland ecosystems. This project sought to address this gap in the woodlands of Western Sydney’s Cumberland Plain. Aspects of vegetation dynamics were investigated through six studies, mostly in shale-based Cumberland Plain Woodland (CPW) remnants. Results indicate that fire frequency profoundly affects both vegetation composition and structure. The influence of fire cycles was most readily apparent in the shrub layer. A survey in CPW remnants with differing fire histories found a markedly higher abundance of Bursaria spinosa in sites where fire frequency was low, to the point where this species dominated the landscape. Other native shrubs, particularly obligate seeders, were most abundant in sites burnt once or twice a decade. Findings were consistent with predictions based on fire-related attributes of individual shrub species. Themeda australis dominated the ground layer in high and moderate fire frequency sites, but not where fire frequency was low. A study of woodland microhabitats found that fire frequency did not affect ground layer species richness or composition directly, however open patches, patches around trees and patches under Bursaria varied in species composition. Thus fire frequency is likely to affect ground layer composition indirectly, through its influence on the shrub layer. Findings from the six studies were synthesized into a state and transition model which allows exploration of management actions. Interfire intervals between 4 and 12 years are predicted to maintain Themeda woodland with both Bursaria thickets and open areas, and obligate seeder shrubs. Variable intervals across time and space within these thresholds should maintain much of the landscape at fuel levels compatible with property protection; fuel loads in CPW peak well below those in woodlands on sandstone. Low fire frequency remnants dominated by Bursaria retain many conservation values, but are likely to support lower abundances of obligate seeder shrubs and open patch herbs, and to be more weed-prone, than remnants burnt once or twice a decade. Experimentation with one or two short interfire intervals may be appropriate in long unburnt CPW. i TABLE OF CONTENTS Summary .........................................................................................................................i Table of contents ............................................................................................................ii Acknowledgements ....................................................................................................... ix Abbreviations ............................................................................................................... xii Chapter 1 Introduction to the project...................................................................... 1 1.1 Project goal and focus ....................................................................................... 1 1.2 Temperate woodlands........................................................................................ 3 1.2.1 Description, location, conservation........................................................ 3 1.2.2 Abiotic influences .................................................................................. 4 1.2.3 Introducing Western Sydney’s woodlands ............................................ 5 1.3 Plant species responses to burning .................................................................... 6 1.3.1 Post-fire regeneration modes ................................................................. 6 1.3.2 Reproduction and fire............................................................................. 7 1.3.3 Life history stages .................................................................................. 8 1.3.4 Plant responses to fire on the Cumberland Plain ................................... 9 1.3.5 Study question concerning plant responses to fire............................... 10 1.4 Fire frequency and woodland shrubs............................................................... 10 1.4.1 Effects of frequent fire ......................................................................... 10 1.4.2 Effects of infrequent fire...................................................................... 12 1.4.3 Shrubs and fire frequency on the Cumberland Plain ........................... 12 1.4.4 Study question concerning shrubs and fire frequency ......................... 13 1.5 ‘Encroachment’ in grassy vegetation .............................................................. 13 1.5.1 Encroachment across the planet........................................................... 13 1.5.2 Encroachment on the Cumberland Plain.............................................. 14 1.5.3 Study questions concerning encroachment.......................................... 14 1.6 Fire and woodland trees................................................................................... 14 1.6.1 Eucalypt recruitment............................................................................ 14 1.6.2 Tree survival ........................................................................................ 15 1.6.3 Fire and trees on the Cumberland Plain............................................... 16 1.6.4 Study question concerning fire and trees ............................................. 16 1.7 Fire, grasses and herbs..................................................................................... 16 1.7.1 Grassland research ............................................................................... 16 1.7.2 Grassy woodland research ................................................................... 17 1.7.3 Fire, grasses and herbs on the Cumberland Plain ................................ 18 1.7.4 Study questions concerning fire, grasses and herbs............................. 19 1.8 Guidelines for the use of fire in conservation management ............................ 20 1.8.1 Recommended fire frequency thresholds............................................. 21 1.8.2 Guidelines for management of grassy woodlands ............................... 22 1.8.3 Guidelines for managing fire on the Cumberland Plain ...................... 22 1.9 Fire management at the urban fringe............................................................... 23 1.9.1 Fuel accumulation on the Cumberland Plain ....................................... 24 1.10 Methodological approach .............................................................................. 24 1.11 Thesis outline................................................................................................. 25 ii Chapter 2 The study area: Western Sydney’s Cumberland Plain.......................27 2.1 Location and landform.....................................................................................27 2.2 Geology and soils.............................................................................................28 2.3 Climate .............................................................................................................30 2.3.1 Climate averages...................................................................................30 2.3.2 Rainfall during the project....................................................................31 2.4 Vegetation ........................................................................................................32 2.4.1 Cumberland Plain Woodland................................................................34 2.4.2 Castlereagh Woodland..........................................................................36 2.5 Management history.........................................................................................38 2.5.1 Aboriginal management .......................................................................38 2.5.2 European management .........................................................................40 2.6 Current status of vegetation .............................................................................41 2.7 Study sites ........................................................................................................43 2.7.1 Locating remnants ................................................................................43 2.7.2 Identifying fire history..........................................................................44 Chapter 3 Shrub vital attributes .............................................................................45 3.1 Introduction......................................................................................................45 3.1.1 Vital attributes and plant functional types............................................45 3.1.2 Interval sensitivities..............................................................................46 3.1.3 Study aims, boundaries and questions..................................................46 3.2 Methods............................................................................................................48 3.2.1 Documenting post-fire regeneration modes .........................................48 3.2.2