Do the recurrently burned vegetation communities become more flammable? The example of shrublands and Quercus suber woodlands | Les communautés végétales fortement perturbées par les feux deviennent-elles plus inflammables et plus combustibles ? Un exemple dans les maquis et formations à chêne liège

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Inglés. Communities dominated by Quercus suber and Cistus-Erica maquis are recurrently disturbed by wildfires in the Maures massif (southern France). The fire regime (i.e. the number of fires, the interval between fires and their intensity) is hypothesized to impact the composition and structure of vegetation communities, then subsequently their flammability and combustibility. A hypothesis (Mutch 1970; Bond & Midgley 1995) indicates that fire repetition would favour species with a high flammability, which could maintain by eliminating their neighbours. We tried to assess vegetation flammability and combustibility along a gradient of fire recurrence. We selected 51 plots along a gradient of fire recurrence and date, including control unburned plots. We described the vegetation composition and structure (vertical, horizontal), and we simulated fire behaviour using a semi-empirical model. Vegetation flammability has been assessed by burning experiments in the lab. Our results indicate that flammability and combustibility do not vary simply with the number of fires. The fire regime favours some highly-flammable species but it almost has an impact on vegetation structure, and this subsequently alters the intensity and the rate of spread of fires. The time since the last fire influences accumulation of vegetation fuel, and thus fire intensity and spread rate. The flammability is mainly controlled by the accumulation of dead fuel in the forest understorey, in relation to the life cycle of vegetation. In total, plant traits are likely to be less determining than external factors (climate, human practices) and the location of vegetation communities within the landscape to explain the repetition of fires.