Fires enhance flammability in Ulex parviflorus.

Wildfires have been occurring in terrestrial ecosystems since the early evolution of plants (Bowman et al., 2009; Pausas & Keeley, 2009; Bond & Scott, 2010) and have shaped many species and ecosystems worldwide (Keeley et al., 2011). Plant flammability is an important driver of fire in terrestrial ecosystems and thus has a fundamental role in ecosystem dynamics and species evolution (Belcher et al., 2010; Bond & Scott, 2010; He et al., 2011). Many traits enhance plant flammability (Cornelissen et al., 2003), including structural traits such as high surface area to volume ratio and retention of standing dead biomass (Papió & Trabaud, 1990, 1991; van Wilgen et al., 1990; Schwilk, 2003), and chemical traits, such as high cellulose : lignin ratio and high levels of volatile compounds and waxes (Philpot, 1970; Rundel, 1981; Dimitrakopoulos & Panov, 2001; Alessio et al., 2008). Indeed, it has been suggested that flammability-enhancing traits have evolved to maintain fire-prone ecosystems. This hypothesis was first proposed at the community level, where it was stated that ‘fire-dependent plant communities burn more readily than nonfire-dependent communities because natural selection has favoured development of characteristics that make them more flammable’ (Mutch, 1970). However, this hypothesis was criticized, mainly for being group selectionist and for lacking an explanation on how increased flammability of an individual would increase its fitness (Snyder, 1984; Christensen, 1985; Troumbis & Trabaud, 1989). Later, the hypothesis on the evolution of flammability was reformulated at the individual level within the framework of inclusive fitness theory (Bond & Midgley, 1995). It suggested that flammability-enhancing traits could be favoured in individuals if the elevated flammability resulted in increased mortality of neighbours, and thus opened up space for recruitment opportunities of the flammable individual offspring (‘kill thy neighbour’ hypothesis, Bond & Midgley, 1995; also called ‘born-to-burn’ hypothesis, Zedler, 1995). More recent modelling studies have suggested different genetic mechanisms for the evolution of flammability (Kerr et al., 1999; Schwilk & Kerr, 2002). Indeed, the correlation between post-fire regeneration strategy and flammability across different species provides some evidence for the ‘kill thy neighbour’ hypothesis (Keeley & Zedler, 1998; Schwilk & Ackerly, 2001; Cowan & Ackerly, 2010; Saura-Mas et al., 2010; He et al., 2011). However, rigorous field evidence on whether fire enhances plant flammability within populations of a particular species is lacking. If natural selection enhances flammability traits in fire-prone ecosystems through an increase in opportunities for post-fire recruitment (Bond & Midgley, 1995), the hypothesis should be better tested in obligate seeder species, that is, in species that lack resprouting ability and for whom post-fire regeneration relies only on germination from the seed bank (Bond & Van Wilgen, 1996; Pausas et al., 2004). This is because obligate seeders have shorter and nonoverlapping generations and higher population turnover than resprouters (Verdú et al., 2007), and thus should be more sensitive to fire regime changes. Previous experimental studies on plant flammability in fire-prone ecosystems have focused on differences between species and, in particular, between regeneration strategies (e.g. Cowan & Ackerly, 2010; Saura-Mas et al., 2010), but none has studied the intraspecific variability and its relation to fire regime. In this study, we predict that individuals of seeder species growing in populations subject to recurrent fires will be more flammable than individuals of the same species growing in populations that rarely burn. In addition, because of this directional selection and the different population dynamics in contrasted fire regimes, we also predict that seeder species living in different fire regimes should differ in the variance structure of flammabilityenhancing traits (Endler, 1986). We tested these predictions in Ulex parviflorus (Mediterranean gorse, Fabaceae), a shrub species of the Mediterranean Basin lacking the ability to resprout and whose germination is triggered by fire. We quantified, at the individual level, flammability-enhancing traits in populations with contrasted fire regimes.