The effect of fire regime on tropical savannas of north-eastern Australia: interpreting floristic patterns through critical life events. Thesis submitted by Paul Richard Williams BSc (Hons) University of New England in June 2002 for the degree of Doctor of Philosophy in Tropical Plant Sciences School of Tropical Biology James Cook University Statement of access. Statement of Access. I, the undersigned, the author of this thesis, understand that James Cook University will make it available for use within the University library and, by microfilm or other means, allow access to users in other approved libraries. All users consulting this thesis will have to sign the following statement: In consulting this thesis I agree not to copy or closely paraphrase it in whole or in part without the written consent of the author; and to make proper public written acknowledgement for any assistance which I have obtained from it. Beyond this, I do not wish to place any restriction on access to this thesis. …………………………………………. ………………………. Paul Williams i Statement on sources declaration. Statement on Sources Declaration. I declare that this thesis is my own work and has not been submitted in any form for another degree or diploma at any university or other institution of tertiary education. Information derived from published or unpublished work of others has been acknowledged in the text and a list of references is given. …………………………………………. ………………………. Paul Williams ii Acknowledgements. Acknowledgements. This thesis greatly benefited from the complete support and expert advice provided throughout the project by my three supervisors, whom I thank sincerely for their friendship and dedication: Dr Bob Congdon (James Cook University), Dr Tony Grice (CSIRO) and Dr Peter Clarke (University of New England). This research would not have occurred if were not for the enthusiastic support of the Townsville staff of Queensland Parks and Wildlife Service, who implemented the fires at the Cape Cleveland, Mt Elliot and Many Peaks Range sites, and maintained the fire breaks at Cape Cleveland: Col Adams, Steve Brooker, Graeme Bulley, Michael Dicketts, Jim Denison, Brett Galloway, Rob Graham, Doug Harrison and Denis Masters. Mike Lloyd (Kings Park Botanic Gardens, Perth) kindly provided advice on the design of the apparatus to produce the smoke exposure treatments and Brett Galloway (Queensland Parks and Wildlife Service) expertly welded the smoke equipment together. James Cook University generously provided funding for field equipment, travel to Christchurch to present some of the results of this research at the Bushfire 2001 conference, and to finalise the completion of the thesis. Queensland Parks and Wildlife Service gave permission for the research to be undertaken at Bowling Green Bay National Park and Town Common Conservation Park, and allowed me time during work to implement the fires and undertake the annual floristic surveys (Chapter 2). Townsville City Council, with particular thanks to Greg Bruce, provided permission to survey plant regeneration on Castle Hill. Thanks to Drs Malcolm Gill and Gay Crowley for providing valuable comments on a draft of Chapter 1, and to the Queensland Herbarium staff, particularly Russell Cumming, for taxonomical advice and identifying several specimens for me. Drs Mike Steele (James Cook University) and Bob Mayer (Queensland Department of Primary Industries) provided expert statistical advice. I appreciate the input from the many friends with whom I discussed this project and fire management of savannas in general, including Col Adams, Grant Anchen, Graeme Bulley, Greg Calvert, Gay Crowley, Tim iii Acknowledgements. Devlin, Leasie Felderhof, Adam Felton, Annika Felton, Malcolm Gill, Rob Graham, Dave Green, John Hunter, Janice Jackson, Shayne Kimlin, Ken McMahon, Mark Parsons, Jeremy Russell-Smith and Eddie Staier. Thanks also to Adam Felton for producing the scanning electron microscope images of the Crotalaria montana seed (Figure 4.1 and the inside-cover image), to Kate O’Donnell (CRC Tropical Savannas) for producing Figure 1.1 and to Sharon King (Environmental Protection Agency) for producing Figure 2.1. I give special thanks to my wonderful girlfriend, Eleanor Collins, for your full support throughout the project, assistance in the field, skill in scanning all photographs, as general thesis editor, and for enduring my absence at Cape Cleveland during weekends and holidays for 5 years. Most importantly of all, you kept things in perspective so that I could continue to enjoy life while producing this thesis. Thank you. I dedicate this thesis to the memory of my late father, John Cambell Williams, for his commitment as a father and because his 48 years service with the one company demonstrated to me perseverance. iv Abstract. Abstract. The degree to which fire influences plant species composition and abundance (referred to in this thesis as floristic patterns) in Australian tropical savannas has been the subject of debate. Available information emphasises floristic patterns rather than the mechanisms that determine patterns. The assessment of critical events in a plant’s life cycle (referred to in this thesis as critical life events) is necessary to understand population dynamics and therefore the processes that determine floristic patterns. Fire may affect population size through its influence on one or more critical life events including seed dormancy release, seed bank dynamics, seedling emergence, seedling survival and the survival and growth of juvenile and mature plants. This thesis presents data on floristic patterns in response to fire regimes in savannas of north-eastern Australia, a region with limited quantitative information on fire response at a community level; and contributes to the broader understanding of savanna function, through an examination of life cycle processes. The aims of the research were to examine the effect of fire regime on: a) floristic patterns in eucalypt savannas in the vicinity of Townsville, north-eastern Australia; and b) critical life events, to investigate mechanisms determining floristic patterns. To achieve these objectives, laboratory and shadehouse experiments were carried out and surveys of seedling emergence and vegetative post-fire regeneration were undertaken at four sites in the Townsville region. At one of these sites, Cape Cleveland, floristic patterns and critical life events were studied in detail using a manipulative fire experiment which examined three fire regimes between 1997 and 2001: 1. Control: savanna burnt in the mid dry season (July) 1997 only. 2. Early burnt: savanna burnt in the mid dry season 1997 and early dry season (May) 1999. 3. Late burnt: savanna burnt in the mid dry season 1997 and late dry season (Oct) 1999. v Abstract. Five annual surveys of permanent plots at Cape Cleveland demonstrated a high degree of stability in species composition, however the abundance of several species was affected by fire regime. A significant increase in the abundance of various subshrubs, ephemeral and twining perennial forbs, and grasses was detected in the first year after fire, particularly following late dry season burning. The abundance of these species declined towards pre-fire levels in the second year after fire. The dominant grass Heteropogon triticeus significantly declined in abundance, and tree density increased, with fire intervals of four years. Investigations of critical life events detected mechanisms that determine floristic patterns. A pulse of seedling emergence of subshrubs and herbaceous species occurred in the post-fire wet season, with greatest seedling density following late dry season fires. This pulse of seedling emergence explained the increase in plant abundance documented in the year following fire. It resulted from seed dormancy release, triggered by multiple fire-related agents, including the removal of herbaceous cover, which increased germination of a range of species; and the exposure to: heat-shock, which increased the germination of a range of forbs; smoke, which promoted the germination of upright perennial forbs and grasses; and nitrate, which interacted with heat shock to increase germination of at least two forbs. The germinable soil seed bank was dominated by grasses and forbs, and seed banks of trees and shrubs were scarce and short-lived. Seasonal fluctuations were evident in the seed bank, which increased through the dry season to a maximum density in the late dry season, and declined to a minimum by the mid wet season. The high intensity late dry season fires reduced seed production by the dominant tree Corymbia clarksoniana in the year following burning, however the effect of fire on soil seed bank dynamics was limited due to the annual wet season decline in seed reserves that occurred irrespective of fire. These data indicate that late dry season fires promoted the emergence of a greater density of herb seedlings, compared with early dry season fires, by stimulating seed dormancy release in a higher proportion of seeds, and because more soil seed reserves were available in the late dry season to be affected by fire. vi Abstract. Seedling survival of herbaceous species was higher for those that emerged from recently burnt rather than unburnt savanna. High rates of seedling mortality of perennial species, and the short lifespan of ephemeral forbs, explained the decline in plant abundance documented in the second year following fire. A moderate proportion of seedlings of common sprouter species developed the capacity