Plant traits and their effect on fire and decomposition Saskia Grootemaat BSc Forest and Nature Conservation MSc Forest and Nature Conservation Submitted November 2015 for the degree of Doctor of Philosophy in Biological Sciences Department of Biological Sciences Macquarie University North Ryde, Sydney, NSW 2109 Australia Principal supervisor: Dr. Ian J. Wright Associate supervisors: Prof. Peter M. van Bodegom and Prof. Johannes H.C. Cornelissen Table of contents Table of contents Summary 9 Statement 11 Contribution 13 Acknowledgements 17 1. General introduction 21 The importance of studying wildfires 21 Fuel availability 24 Litter dynamics 25 Fuel flammability and plant traits 27 Research objectives and thesis outline 29 References 32 2. Burn or rot: leaf traits explain why flammability and decomposability are decoupled across species 39 Summary 40 Introduction 41 Material and methods 44 Site and species selection 44 Material types 46 Leaf trait measurements 47 Decomposition experiment 48 Burning experiments 48 Calculations and statistical analysis 50 Results 51 Leaf traits as drivers of decomposition 51 Differences in flammability among species 52 Correlations between flammability parameters 54 Leaf traits as drivers of flammability 55 Decomposition and flammability 58 Table of contents Discussion 60 Flammability decoupled from decomposability 60 Trait relationships unravelled 61 Implications 63 Acknowledgements 64 Data Accessibility 64 References 65 Supporting information 71 3. Towards a better understanding of fuel bed flammability; scaling up from individual leaves 77 Summary 78 Introduction 79 Material and methods 83 Species selection and leaf collection 83 Trait and fuel bed measurements 84 Experimental fuel bed burns 85 Data analysis 86 Results 87 Flammability of the fuel beds 87 Which fuel properties drive variation in fuel bed flammability? 89 Does the ranking in species’ flammability hold for individual leaves versus fuel beds? 92 Discussion 95 Putting the conceptual framework to the test (Figure 3.1) 95 Individual leaf flammability versus flammability in fuel beds 98 Conclusions 98 Acknowledgements 99 References 100 Supporting information 103 4. Bark fates explored: decomposition and flammability of 10 woody species from the Sydney region (south-eastern Australia) 109 Summary 110 Introduction 111 Table of contents Material and methods 113 Site description and species selection 113 Material collection and trait measurements 114 Decomposition experiment 117 Experimental burns 119 Statistical analyses 119 Results 121 Bark versus leaf decomposition 121 Bark versus leaf flammability 122 Decoupling of decomposition and flammability as explained by different drivers 124 Discussion 129 Slow decomposition of bark 129 Flammability of bark is lower than that of leaves 130 Decoupling of decomposition and flammability 132 Plant organ coordination and ecological strategies 132 Concluding remarks 134 Acknowledgements 134 References 135 Supporting information 140 5. Models for leaf ignitibility based on leaf traits 145 Summary 146 Introduction 147 Drivers of leaf ignitibility 149 Towards a better prediction of leaf ignitibility 151 Materials and methods 152 Dataset specifications 152 Trait selection for models 156 Calculations and model selection analysis 157 Model validation 158 Results 159 What are the best predictive drivers for leaf TTI? 159 Evaluation of the leaf ignitibility model 161 Discussion 164 Table of contents The best predictors for leaf ignition delay time 164 Significance of this work for fire behaviour modelling 165 Concluding remarks 166 Acknowledgements 167 References 168 Supporting information 175 Standardising test temperatures 175 6. General discussion 179 What have we learned? 179 The role of traits for interspecific variation in flammability of individual leaves 179 The role of leaf traits for the flammability of fuel beds 180 Flammability of bark chunks 181 Decomposability of bark chunks 182 Decoupling of flammability and decomposability 183 Implications 185 Fuel management planning 185 Estimation of carbon and nutrient fluxes 186 New research directions 189 References 192 Summary Summary Wildfires are a major disturbance worldwide with large effects on ecosystem functioning, species composition and nutrient cycling. A fundamental factor in wildfires is the fuel, namely, live and dead plant material. Plant species differ in their flammability, but the role of plant traits in this remains largely unknown. The decomposition rates of different plant materials (and species) can strongly affect the availability of fuel for potential wildfires. While the influence of leaf traits on litter decomposability is reasonably well studied, it has never been compared to the drivers of litter flammability. In this thesis I focused on these two important turnover processes of plant material, i.e., fire and decomposition. By comparing a wide range of species from south-eastern Australia, I investigated the existence of general relationships between plant traits, flammability and litter decomposability. In experiments on individual leaves (Chapter 2) I found that morphological leaf traits (such as specific leaf area or dry mass) were most strongly correlated with interspecific variation in flammability, while decomposability was mainly driven by chemical traits. Similar results were found for bark, another important litter component of the Australian forests (Chapter 4). Bark ignitibility of smooth bark species was driven by bark mass per area, while decomposition was strongly associated with initial lignin concentration. Consequently, fire and decomposition, as two alternative fates for leaves or bark, were unrelated. Next, I demonstrated that leaf traits which affect the flammability of individual leaves (e.g. specific leaf area) continue to affect flammability when scaling up to fuel beds (Chapter 3). Can we use these findings on interspecific variation in leaf trait – flammability relationships to improve predictions of fire behaviour? In Chapter 5 I showed that the inclusion of leaf traits (especially leaf thickness) improved the prediction of individual leaf ignitibility. Altogether, this suite of studies increased our understanding of trait-effects on leaf and bark flammability and decomposability. Including plant traits in future analyses could improve the estimation of fuel loads and the prediction of wildfires. 9 10 Statement Statement I, Saskia Grootemaat, declare that this thesis titled “Plant traits and their effects on fire and decomposition” is my own work and has not been submitted in any form for another degree at any other university or institution. I also certify that this thesis is an original piece of research and has been written by me. Any help or assistance that I have received has been appropriately acknowledged, and the contribution of co-authors has been indicated in the contribution section. All sources of information and literature used are cited in the thesis. Saskia Grootemaat November 2015 11 12 Contribution Contribution Chapter 1. General introduction I, Saskia Grootemaat (SG), performed the literature review and writing of this chapter. Constructive feedback was given by my supervisors, Ian J. Wright, Peter M. van Bodegom and Johannes H.C. Cornelissen. Chapter 2. Burn or rot: leaf traits explain why flammability and decomposability are decoupled across species Ian J. Wright, Peter M. van Bodegom, Johannes H.C. Cornelissen and William K. Cornwell were involved in concept development and gave constructive feedback on draft versions of the manuscript. Decomposition data were kindly provided by William K. Cornwell. Concept: SG 70% Data collection: SG 80% Data analysis: SG 100% Writing: SG 100% (with feedback from co-authors) Chapter 3. Towards a better understanding of fuel bed flammability; scaling up from individual leaves Johannes H.C. Cornelissen was involved in concept development. Peter M. van Bodegom helped with some statistical analysis. Ian J. Wright, Peter M. van Bodegom and Johannes H.C. Cornelissen all provided constructive feedback on draft versions of the manuscript. Concept: SG 80% Data collection: SG 100% Data analysis: SG 90% Writing: SG 90% (with feedback from co-authors) 13 Contribution Chapter 4. Bark fates explored: decomposition and flammability of 10 woody species from the Sydney region (eastern Australia) Original concept by SG. Ian J. Wright, Peter M. van Bodegom and Johannes H.C. Cornelissen provided suggestions for experimental set-up and gave constructive feedback on draft versions of the manuscript Veronica Shaw contributed enormously by helping me with the extensive field and lab -work. Concept: SG 100% Data collection: SG 80% Data analysis: SG 100% Writing: SG 100% Chapter 5. Models for leaf ignitibility based on leaf traits The concept was developed as a collaboration between SG and Philip Zylstra. Half of the data came from experimental burns by SG. Additional data was provided by Philip Zylstra. Peter M. van Bodegom contributed with useful statistical advice. Concept: SG 60% Data collection: SG 50% Data analysis: SG 70% Writing: SG 90% Chapter 6. General discussion I, (SG), performed the literature review and writing of this chapter. Constructive feedback was given by my supervisors, Ian J. Wright, Peter M. van Bodegom and Johannes H.C. Cornelissen. 14 15 16 Acknowledgements Acknowledgements So here it is, my PhD thesis. I am happy but also sad that it has come to an end. It has been such an adventurous journey! Moving to the other side of the world, starting