Evolution of flammability
Example of a thick, protective bark. Source: Charles Darwin Univ.
Figure 1: Distribution of fire-dependent, fire sensitive and fire independent ecoregions. Shlisky et al, 2007. The evolution of flammability is an intricate time progression of ancient plant adaptation to the changing disturbance regimes of natural wildfires. Wildfires are a powerful disturbance driving vegetation distribution and structure, the nutrient and carbon cycling of many ecosystems, and climate. Throughout the world, many ecosystems are fire-dependent, meaning they rely on fire for their persistence and excluding fire can severely alter these systems. Fire reduces vegetation accumulation, which can inhibit plant growth, it reduces the accumulation of insects and pathogens, stimulates growth and reproduction of plants and provides habitat for a wide variety of wildlife. In this sense, fire can be deemed an agent for biodiversity conservation.
The fuel for these fires is provided by the plant species, but plants vary widely in their flammability, even within the same climate. Different vegetation will differ not only in flammability, but also in their contribution to fuel continuity (the degree or extent of continuous or uninterrupted distribution of fuel particles in a fuel bed) and to total fuel load (the oven dry weight of fuel per unit area) (Scarff and Westoby, 2006; Schwilk, 2003; van Wilgen et al., 1990), and this diversity in vegetation composition substantially modifies fire regimes (Scarff and Westoby, 2006). Almost all climate-change scenarios point to increased temperatures and extended periods of drought in most ecosystems, accompanied by increased lightning frequency, which will in turn lead to periods of extreme fire weather (IPCC, 2007). As species composition changes under these predictions, understanding the burning characteristics of mixed stands will prove very useful for future assessments of fire behaviour. Numerous [[plant] species] are prone to fire, due to their intrinsic morphological and physiological traits. Thus, the need to find which species are less flammable becomes crucial in the quest to minimize fire consequences in fire-prone areas (Ormeño et al., 2009).
Contents
What is flammability?
Figure 2. Resprouting from dormant basal structure. Source: USFS In a broad sense, flammability can be defined as the ease with which a substance is set on fire. This term is used in common English to avoid safety hazards. The correct wording should be Inflammability, where the prefix “in” means “inside” rather than “not”, but it can be incorrectly perceived as “non-flammable”. Flammability was initially characterized by Anderson (1970) in three components: ignitability, sustainability, and combustibility. Ignitability is the time until ignition (when the particles catch on fire) once exposed to a heat source. Sustainability is the stability of burning rate, or the ability to sustain fire once ignited. Combustibility is defined as the rate of burn after ignition. The definition of flammability has since been expanded to include consumability, the proportion of mass or volume consumed by fire (Martin et al., 1994). Flammability components of individual plants can be related to fire characteristics at an ecosystem level. Ignitability of individual plants drives the sequence of ignition in an ecosystem. Sustainability is related to the rate of fire spread, and combustibility is associated to fire intensity. The consumability of individual plants is analogous with the amount of fuel available for fire consumption on the ecosystem level (Martin et al., 1994).
Scaling flammability
Figure 3. Serotiny exhibited by closed cone at left and open cones at right.
Fire is a process that acts on a global scale. The Global Fire Assessment found that fire-dependent ecoregions cover 53% of the globe, fire-sensitive ecoregions cover 22%; and fire-independent ecoregions cover 15% (Figure 1), with a distribution that spans across major habitat types (Shlisky et al., 2007). The fire-dependent habitats are the Mediterranean forests, woodlands, and scrubs (Mediterranean Basin, California, Chile, South Africa, Chile), the boreal forests of North America, Europe and Asia, the coniferous forests of the American West and Mexico, and the vast plains, grasslands and savannas of North America, Africa, Australia, South America and East Asia (Allen, 2008).
Within the aforementioned habitats, some associations of trees and shrubs are more flammable than others. Plants exhibit numerous traits that can be related to fire. Some have developed structures to protect growth, such as thick bark (promoting survival) as seen in larches and pines (Lead image). Other plants use the heat produced by the fire in different ways. Some resprout (re-growth from dormant vegetative elements, like buds) from burned structures, as Longleaf pine, scrub oak, chamise, and eucalyptus (Figure 2). Others use heat as a way to melt the wax or resin that keeps cones shut, thus allowing the release of seeds, called serotiny, an example being Lodgepole pine and Jack pine (Figure 3). Another adaptation is heat or smoked-induced germination, like in Mountain ash. These traits can be seen as adaptive responses to fire, either to its direct effects or to the post-fire environment. These fire adaptation traits seem to be correlated with traits that promote flammability. For instance, retention of dead branches, canopy architecture (prolific branching, with fine branches) (Figure 4) and the presence of volatiles (resins and oils) exemplify such flammability-promoting traits.
Figure 4: Example of prolific branching. www.bilsdalettreeservices.co.uk The fact that some trees have retention of dead branches is most interesting, since it does not seem to provide an apparent benefit for the tree. In fact, it can be a vector of infection, hence leading to decreased fitness. Yet it is very common among fire-prone species, especially in the genus Pinus. Dead branches increase the likelihood of a fire reaching the canopy of the tree, generating enough heat to release the seeds from serotinous cones, enabling the seeds to reach the soil and establish themselves (recruitment). Where serotiny has been selected for, it seems likely that traits that ensure canopy ignition, like canopy architecture and dead branch retention, should be selected for, thus making the case for the evolution towards increased flammability (Bond and Midgley, 1995; Schwilk and Ackerly, 2001).
The Mutch hypothesis
Flammability is an important contributor to fire regimes (the pattern, frequency and intensity of a fire that prevails in a given area), increasing both the frequency and intensity of a fire, and as such has important ecological and economic impacts. By changing their environment, flammable plants influence fire regime and severity, affecting not only the current but also the future generation.
Many plant species, like pines, firs, cypress, cedars, junipers, gums, sages, depend on fire for their continued persistence and possess traits that provide them the potential for persistence following fire (Bond and Midgley, 1995; Pausas and Bradstock, 2007; Schwilk and Ackerly, 2001). These traits can be viewed as an evolutionary adaptation to fire (Pausas and Bradstock, 2007; Schwilk, 2003).
But are plants adapting to fire? The question is as pertinent now as it was in 1970, when Richard Mutch proposed that fire-prone communities are more easily burned than non fire-prone ones. Mutch innovatively indicated that some reproductive or anatomical traits in plants had evolved towards increased flammability, perpetuating a given fire environment (Mutch, 1970). Mutch's hypothesis suffered a great deal of criticism. Its detractors proposed that differences in flammability might be due to chance or other selective pressures, meaning it was relevant to distinguish the importance of fire from selective pressures on other traits. It was also argued that such a proposal at a community level is unlikely (Bond and Midgley, 1995: Schwilk, 2003). No attention was given to alternative explanations concerning chemical composition, as in the relationship with herbivory, in the way they can act to attract or repel insects, for instance. Nonetheless, recent work has pointed out that interactions between flammability traits and fire-related traits in a fire-prone environment are more than likely (Schwilk and Ackerly, 2001).
A flammable individual in a community would constitute a dangerous neighbour, as stated by Bond and Midgley (1995). The consequence of an individual's self-sacrifice by dying during the fire can be the high mortality of neighbouring vegetation, which clears gaps and creates the opportunity for the recruitment of the next generation. Arguably, the cost of the individual that dies is offset by the increased fitness of the offspring. This self-sacrifice is analogous to altruistic behaviours in animals.
Selection for flammability might have been a by product of selection for other traits. For example, the presence of volatiles, commonly associated with flammability, may have been selected as defence against herbivory. Similarly, scleromorphic leaves (small, hard, with thick cuticles), which are relatively flammable, might have been selected for drought tolerance (Kerr et al., 1999). Kerr et al. (1999) defend that, even if this is the case, flammable traits are still acting as a source of selection on different loci, a sort of piggy-backing known as genetic hitch-hiking, where an evolutionary neutral allele may spread by virtue of being linked to a gene being selected for. This means that, independently of how they came to be, flammability enhancing traits have an influence on their environment, creating new selective pressures (Schwilk and Kerr, 2002). These new selective pressures, in turn, can lead to selection of fire-surviving strategies, such as resprouting, smoke-induced germination or serotiny.
Although past research has demonstrated the effect of community composition on fire regimes (e.g., Philpot, 1977), elucidating the specific traits that influence flammability has yet to be accomplished. The study of plant functional traits over the past decade has led to increased understanding of ecological evolution and the process of community assembly (e.g. the “leaf economic spectrum”, Wright et al., 2004). Research on how these well-studied leaf trait patterns influence how leaf litter behaves during fire, however, is still under early investigation.
References
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