Fire Severity and Plant Community Dynamics in Montane Ecosystems
Total Page：16
File Type：pdf, Size：1020Kb
Fire Severity and Plant Community Dynamics in the Australian Alps, Southeastern New South Wales by Michael Damien Doherty Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy of the Australian National University March 2021 ii ilt Gand idate's Declaration This thesis contains no material which has been accepted for the award of any other degree or diploma in any university. To the best of the author's knowledge, it contains no material previously published or written by another person; except where due reference is made in the text. Michael Damien Doherty Date: +(u /[i,Kcq AoLl iv v Acknowledgements First and foremost, I dedicate this thesis to my parents John Doherty and Marie Patricia Doherty. Many thanks go to my supervisory panel comprising Dr. Geoff Carey (Chair), Dr. Mike Austin and Dr. Malcolm Gill, for guidance, fruitful discussions and infinite patience! Thanks also to Dr. Hwan-Jin Yoon for specialist statistical advice on modelling, and also to Dr. Julian Reid for fruitful discussions on the use of multivariate approaches to ecological data analysis. Dave (now Dr.!) Taylor provided part-timer camaraderie over the concurrent terms of our PhDs and as we both dealt with the emerging realisation that we had made life somewhat difficult for ourselves by engaging on our PhD research paths while still working and, at least in my case, being a bit long in the tooth. Enormous thanks to NSW Office of Environment and Heritage, Queanbeyan, for providing monetary and in-kind support for many of the fieldwork components of this research over a number of years both before and after the 2003 fires, and in particular, special thanks to Genevieve Wright and Geoff Robertson, for invaluable support with field work and logistics, as well as many discussions about plant taxonomy, plant geography and fire ecology over many years. Thanks also to many others who have provided assistance with field work over the years including Mal Stanger, Mark Clayton, Brendan Baker and Guy Barnett (pre-fire sampling), and Karen Hosking, Bronwyn Horsey and Jacqui Meyers (post-fire sampling). CSIRO provided study leave support for part of my candidature. And last but by no means least, much love and thanks to my wife Paula, patiently supportive over the many years I have taken to complete my research, and to my children Yarran and Tessa, who thought there would be no end to this PhD. Well, here it is, finally – but there is always more research to be done! vi vii Abstract The effects of fire on plant communities are a subset of a broader range of spatial and temporal influences on plant community dynamics. Widespread unplanned fires in the Australian Alps in 2003 provided a unique opportunity to investigate the response of vascular plant species to high severity fire and to test whether there was a differential response to variations in fire severity that may affect long-term vegetation dynamics within and between subalpine, montane and tablelands plant communities. A series of pre-existing sites established in the late 1990s in the northern parts of the Australian Alps in Brindabella National Park, Bimberi Nature Reserve and Burrinjuck Nature Reserve were resurveyed and monitored from 2003-2012. Two research questions were posed: 1) what is the response to high severity fire of vascular plant species found in subalpine, montane and tablelands plant communities? 2) does high severity fire result in significant or permanent change to vascular plant species richness and plant community composition as compared to low severity fire in subalpine, montane and tablelands plant communities and does the recovery rate vary with severity? The fire responses of over 400 vascular plant species in 21 plant communities were documented, with 24% of species being fire-killed and responding via canopy and soil seed banks and 76% via resprouting. There was no loss of plant species from the study areas and all vascular plant species recovered rapidly post-fire, but some fire-killed species exhibited slow recovery rates. Although vegetation structure is affected by fire severity, it is fire severity interacting with fire frequency that has the potential to change vegetation structure in the long term, particularly via the loss of structurally dominant obligate seeder species. The overall trend in vascular plant species richness was an increase in species richness post-fire irrespective of fire severity, with a time since fire decline. Significantly for forest ecosystems, this peak and decline response of vascular plant species richness post-fire is modified and varies over time in relation to rainfall, decreasing in drier years. This fluctuation in vascular plant species richness is driven by both native and introduced species. Habitat complexity, vegetation type, understorey cover and season of sampling are also major influences on overall vascular plant species richness with variation over space due to environmental gradients and variation over time both within and between vegetation types in relation to time since fire and rainfall. The results have implications not only for pre- and post-fire sampling but also for vegetation sampling in general, as any initial sample in a sequence of samples over time will not reflect the total vascular plant species richness of a site. The full expression of species richness at a point can only be measured as the result of resampling over multiple years, where variations in climatic and disturbance variables cause a wider array of species to become apparent on site. While there was an initial difference in floristic compositional dissimilarity between low and high severity sites in the Brindabella sites and greater dissimilarity in the short to medium term, this viii dissimilarity diminished over time and there was no evidence of permanent compositional change in high severity sites. In high severity sites, there was an increase in the density of many dominant shrub species leading to differences in the short-term post-fire. As shrub layers started to senesce, both the changes in composition and the changes in abundance reduced and by approximately 10 years post- fire any compositional difference due to severity was not detectable. The initial significant response immediately post-fire was partially due to differences in species composition pre- and post-fire but was also due to changes in the abundances of species pre- and post-fire. While there were some initial differences post-fire between low and high severity sites, these differences diminished over time. The Burrinjuck study area did not exhibit these differences in relation to fire severity but did exhibit differences in richness and composition based on season of sampling. Because of the interaction of time since fire and rainfall, species richness and community composition are not constant over time. However, the results of this research show that sites maintain a strong compositional self-similarity over time after both low and high severity fire and that the plant communities investigated are buffered against long-term shifts in composition, even after a large infrequent disturbance event such as the 2003 fires. Although fire influences plant species richness and plant community composition, it sits within a matrix of other major environmental influences such as rainfall, and post-fire effects are transient rather than permanent. Infrequent high severity fire events in the Australian Alps should be regarded as natural disturbances, and not as disruptive perturbations, as they do not lead to permanent changes in the plant communities investigated. However, if high severity fire events become more frequent under climate change with multiple fires per decade, then resultant changes in structure, composition and richness are likely to occur. This research has vastly improved our understanding of how subalpine, montane and tablelands plant communities respond to high severity fire and provides reserve managers with detailed species and community level responses to better inform fire management. The research also provides a baseline with which to compare the effects of future changes to fire regimes in these communities under climate change. More broadly, the research has shown how long-term data sets and ongoing monitoring play a crucial role in understanding how species richness, community composition and vegetation structure vary over time in relation to the combined influences of both climate and fire. ix Table of Contents Candidate's Declaration .............................................................................................................. iii Acknowledgements ....................................................................................................................... v Abstract ....................................................................................................................................... vii Table of Contents ......................................................................................................................... ix List of Figures ........................................................................................................................... xii List of Tables ........................................................................................................................... xiv List of Boxes ............................................................................................................................ xiv List of Equations .....................................................................................................................