Testing Vegetation Flammability: Examining Seasonal and Local Differences in Six Mediterranean Tree Species

Institute of Landscape and Plant Ecology University of Hohenheim Plant Ecology and Ecotoxicology, Prof Dr rer nat Andreas Fangmeier Testing Vegetation Flammability: Examining Seasonal and Local Differences in Six Mediterranean Tree Species Dissertation submitted in fulfillment of the regulations to acquire the degree "Doktor der Agrarwissenschaften" (Dr.sc.agr in Agricultural Sciences) to the Faculty of Agricultural Sciences presented by Zorica Kauf born in Brežice, Slovenia Stuttgart, 2016 This thesis was accepted as a doctoral thesis (Dissertation) in fulfillment of the regulations to acquire the doctoral degree “Doktor der Agrarwissenschaften” by the Faculty of Agricultural Sciences at University of Hohenheim on 06.06.2016 Date of the oral examination: 30.06.2016 Examination Committee Chairperson of the oral examination Supervisor and Reviewer Co-Reviewer Additional examiner Prof Dr Stefan Böttinger Prof Dr Andreas Fangmeier Prof Dr Manfred Küppers Prof Dr Reinhard Böcker Table of contents Fundaments of combustion…………………………………………………… 1.1 Definition of combustion………………………………………………… 1.2 The course of combustion……………………………………………… 1.3 Fuel characteristics and combustion…………………………………… 1.3.1 Intrinsic fuel properties…………………………………………… 1.3.1.1 Moisture content…………………………………………… 1.3.1.2 Extractives…………………………………………………… 1.3.1.3 Ash content………………………………………………… 1.3.1.4 Cellulose and lignin………………………………………… 1.3.1.5 Physical intrinsic properties………………………………… 1.3.2 Extrinsic fuel properties………………………………………… 1.3.2.1 Quantity…………………………………………………… 1.3.2.2 Compactness………………………………………………… 1.3.2.3 Arrangement………………………………………………… 1.3.2.4 Particle size and shape……………………………………… Environmental factors influencing fire……………………………………… 2.1 Ignition………………………………………………………………… 2.1.1 Natural ignition sources……………………………………….… 2.1.1.1 Lightning…………………………………………….……… 2.1.1.2 Spontaneous ignition………………………………….…… 10 2.1.1.3 Volcanic activity and meteorites……………………….… 11 2.1.2 Anthropogenic ignitions……………………………………… 11 2.2 Climate and weather………………………………………………… 13 2.2.1 Climate…………………………………………………….…… 13 2.2.2 Weather…………………………………………………….…… 14 2.3 Topography……………………………………………………….…… 15 Development and types of fire………………………………………….… 16 3.1 Fuel stratification…………………………………………………….… 16 3.2 Development and types of fire……………………………………….… 18 3.2.1 Fire in grasslands and low-density woodlands…………….….… 19 3.2.2 Fire in shrublands…………………………………………….… 19 3.2.3 Fire in forests……………………………………………….…… 20 3.2.4 Ground fires………………………………………………….… 22 Defining and measuring plant flammability/fire behaviour……………… 23 4.1 Definition of vegetation flammability……………………………….… 23 4.2 Measuring vegetation flammability/fire behaviour…………………… 24 4.2.1 In situ experimental fires……………………………………… 24 4.2.2 Stand/fuel recreating experiments…………………………… 25 4.2.2.1 Wind tunnel tests………………………………………… 25 4.2.2.2 Large-Scale Heat Release (LSHR) apparatus…………… 26 4.2.2.3 Burning tables…………………………………………… 27 4.2.3 Small-scale experiments (disturbed samples)……………….… 28 4.2.3.1 Experiments employing the burning table principle…….… 28 4.2.3.2 Small scale wind tunnel…………………………………… 29 4.2.3.3 Measurements based on oxygen consumption calorimetry 29 4.2.3.4 Epiradiator-based tests…………………………………… 32 4.2.4 Single-leaf testing…………………………………………….… 33 4.2.4.1 Exposing leaves above a known heat source……………… 34 4.2.4.2 Muffle furnace tests…………………………………….… 35 4.2.5 Testing ground samples……………………………………….… 35 4.2.5.1 Gross heat of combustion measurements……………….… 35 4.2.5.2 Thermogravimetry (TGA) and differential thermogravimetry (DTG)………………………………… 36 4.2.5.3 Relative limiting oxygen index (RLOI)………………….… 36 4.2.6 Testing undisturbed samples…………………………………… 37 4.2.6.1 Flaming combustion of undisturbed litter beds………… 37 4.2.6.2 Smouldering combustion of undisturbed duff samples….… 38 4.2.7 Determining flammability by combining results of different measurements………………………………………………… 38 Anthropogenic influence and fire in the Mediterranean Basin………….… 40 Species of interest and their adaptations to cope with drought…………… 42 6.1 Strawberry tree (Arbutus unedo L.)………………………………….… 43 6.2 Carob (Ceratonia siliqua L.)………………………………………… 45 6.3 Olive (Olea europaea L.)…………………………………………… 47 6.4 Aleppo pine (Pinus halepensis Mill.)………………………………… 50 6.5 Pomegranate (Punica granatum L.)………………………………… 53 6.6 Holm oak (Quercus ilex L.)………………………………………… 55 Overview of the study…………………………………………………… 57 7.1 Field study…………………………………………………………… 59 Articles…………………………………………………………………… 61 8.1 Testing Vegetation Flammability: The Problem of Extremely Low Ignition Frequency and Overall Flammability Score………………… 61 8.2 Seasonal and Local Differences in Leaf Litter Flammability of Six Mediterranean Tree Species…………………………………………… 73 8.3 Seasonal and local differences in fresh leaves ignitibility of six Mediterranean tree species…………………………………………… 89 Discussion………………………………………………………………… 113 9.1 Epiradiator-based flammability testing and flammability score…… 113 9.2 Relationship between plant traits and flammability………………… 117 9.3 Seasonal and local differences in ignitibility………………………… 119 9.4 Do laboratory experiments make sense? …………………………… 121 9.5 Management implications of the presented work…………………… 122 10 Conclusions………………………………………………………………… 124 11 References……………………………………………………………….… 127 12 Summary…………………………………………………………………… 151 13 Zusammenfassung………………………………………………………… 154 Curriculum vitae…………………………………………………………… 157 Fundamentals of the combustion process 1.1 Definition of combustion Before one can start measuring and discussing the fire-related characteristics of a material, a basic understanding of the combustion process is necessary According to Quintiere (2006), combustion can be defined as an “exothermic chemical reaction that results from a runaway rate caused by temperature or catalytic effects” This extensive definition may be applied to any combination of fuels, oxidisers and physical environments Nevertheless, in environmental sciences, the main area of interest lies in the combustion of vegetation fuels under physical conditions commonly present on Earth Due to the complexity of combustion, further explanation will focus only on this situation, and it will be presumed that (i) the fuel of interest predominantly consists of polymeric organic compounds; (ii) atmospheric oxygen is the common oxidiser; and (iii) an external heat source is needed for initiation of the chain reaction of combustion With these presumptions in mind, combustion can be defined as “the rapid exothermic oxidation of pyrolysate hydrocarbon vapours released from the surface of the fuel and slower oxidation of char” (DeBano et al 1998) Conceptually, perfectly complete and efficient combustion of vegetation fuels can be seen as a reaction opposite to photosynthesis, in which organic compounds are oxidised to carbon dioxide (CO2) and water (H2O), with release of energy (Byram 1959) In practical terms, combustion of vegetation fuels is neither perfectly complete nor efficient Furthermore, vegetation fuels contain elements other than carbon, hydrogen and oxygen As a result, numerous compounds are formed during combustion (Aurell et al 2015; Faria et al 2015; May et al 2015), and the released amount of energy is always lower than the amount of energy chemically bound in the fuel (Pyne et al 1996; Jenkins et al 1998; Ward 2001) Four elements mentioned in the presumptions (fuel, oxygen, heat, and chain reaction) represent the fundamental fire/combustion tetrahedron They govern the initiation, behaviour and persistence of combustion, and removal of any of them results in the termination of combustion (Pehrson 2004) 1.2 The course of combustion Upon exposure of plant materials to an external heat source, preignition has to take place before sustained combustion can be achieved Preignition begins with preheating of the fuel As the fuel temperature increases, extractives are gradually volatilised (Pyne et al 1996; DeBano et al 1998) Once 100°C is reached, dehydration is initiated and water is expelled from the fuel Release of hot gases reduces oxygen concentration in the heated zone, and further increase in temperature leads to pyrolysis of long-chain organic molecules Pyrolysis of vegetation fuels can follow either of two competing pathways One pathway predominantly produces volatiles and tars, whereas the other mainly yields char and water Both pathways compete for the same initial substance, as well as for some of the intermediate products, with higher temperatures promoting production of volatiles If the first pyrolysis pathway predominates, ignition—the transition point from preignition to combustion—results in flaming (Pyne et al 1996) Flaming represents combustion of the gas phase of the fuel For flaming combustion to occur, combustible volatiles must be emitted from the solid surface, mix with surrounding air, and produce a flammable mixture that is ignited either spontaneously once a high enough temperature is reached (auto-ignition), or with the help of an external spark or a flame termed a “pilot” (forced or piloted ignition) Furthermore, for flaming to be sustained, the amount of heat released by the nascent flame must be high enough to overcome convective heat losses and ensure a sufficient and continuous supply of combustible volatiles to the reaction zone (Atreya 1998) High exothermicity of flaming combustion results in a pronounced increase in temperature (from 300 to 500°C at ignition to higher than 1,400°C), acceleration of the pyrolysis, and increased rate of production of combustible gases, resulting in fires that can potentially move with the wind as masses of burning gases Once combustible volatiles of a fuel are exhausted and the production rate of flammable gases decreases to a point where it can no longer sustain flaming, smouldering combustion starts In fuels in which the second pyrolysis pathway predominates and the production rate of the combustible volatiles is insufficient for sustained flaming, smouldering forms the initial phase of combustion In comparison to flaming, smouldering is much slower (