Flammability as a biological concept

Studies examining plant flammability descriptors as fire-adaptive traits (e.g. Schwilk, 2003; Scarff & Westoby, 2006; Cowan & Ackerly, 2010; Saura-Mas et al., 2010; Pausas et al., 2012) are normally formulated within the framework of inclusive fitness theory. In such a framework, flammability-enhancing traits are considered to favour individuals if the elevated flammability confers fitness benefits. In post-fire seeder species (i.e. those with fire-stimulated germination from a persistent seed bank), higher flammability could increase the recruitment opportunities for the offspring of the individual with enhanced flammability by increasing the chance of opening spaces and by producing the necessary cues for triggering germination from the seedbank (‘kill thy neighbour’ hypothesis; Bond & Midgley, 1995). This might be particularly relevant in nonresprouting (obligate) seeders with strong spatial population structure (e.g. with short-distance dispersal). Because there is some evidence of heritability for both seed dormancy (e.g. Baskin et al., 2000; Huang et al., 2010) and flammability-enhancing traits (e.g. Sampedro et al., 2010), we would expect a selection for higher flammability with repeated fires. In a recent paper, we provided evidence that individuals of an obligate seeder species (Ulex parviflorus, Fabaceae; a shrub from the Mediterranean Basin) growing in populations recurrently burnt (HiFi populations) were more flammable than individuals of the same species in populations arising from old-field colonization that did not suffer any fire (NoFi populations, i.e. with fire-independent recruitment; Pausas et al., 2012). Specifically, twigs of plants from HiFi populations ignited quickly, burnt slowly and released more heat than twigs of NoFi plants. In addition, HiFi plants had higher bulk density than NoFi plants. Previous studies have showed that bulk density in U. parviflorus (Fig. 1), as well as in other shrub species (e.g. Bradstock & Auld, 1995; Tachajapong et al., 2008), is associated with higher temperatures and longer residence time of high temperatures in the soil during a fire. Thus, the results at the twig and the whole-plant scale were in agreement and suggested that HiFi plants should ignite easily and reach higher temperatures and produce higher heat doses in the soil than NoFi plants (Fig. 1). This higher probability of ignition and higher heat in the soil would increase the chance of recruitment of U. parviflorus from the soil seed bank by opening spaces and by enhancing seedling emergence (the heat shock from fire breaks seed dormancy and stimulates germination in this species; Baeza & Vallejo, 2006; Moreira et al., 2010). Thus, these results are in agreement with the kill thy neighbour hypothesis. By including our data on U. parviflorus bulk density in a fire behaviour model, Fernandes & Cruz (2012; this issue pp. 606–609) predict lower fire spread rates in HiFi populations, which implies higher fire residence time and thus higher heat dose in the soil and in the seed bank (Bradstock & Auld, 1995; Gagnon et al., 2010). Their simple modelling approach inadvertently provides further support for our results, although a modelling framework accounting for variability and uncertainty would have been much more appropriate. That is, the conclusions by Pausas et al. (2012) remain firm: in U. parviflorus shrublands there is a divergence on flammability traits between populations living in different selective environments, and the mechanism by which plant fitness would be enhanced is driven by the increase in both the probability of ignition and the heat released to the soil.