Assessing post-treatment vegetation flammability under laboratory conditions: At what point does the fuel treatment lose its effectiveness?

Wildfires are a major risk worldwide, particularly in Mediterranean areas. One of the best ways to mitigate fire behaviour is to reduce vegetation load and continuities using different fuel treatment methods. While these methods are widely studied through ecological and environmental approaches, the evolution of the treated vegetation flammability over time is overlooked. In southeastern France, fuel treatments are implemented regularly every three years along forest firefighting tracks. It is crucial to assess the short-term evolution of this flammability to ensure that the treatment is effective in maintaining a mitigating effect between two treatment sessions. We focused on the two most common treatment methods in the area, prescribed burning and mechanical shredding, and we sampled vegetation treated at three periods since the treatment implementation: T0 (six months after), T1 (18 months after), and T2 (30 months after). We assessed the effect of time-since-treatment on flammability during laboratory burnings, recording variables that encompassed the four flammability components (ignitability, sustainability, combustibility, and consumability). We found that combustibility varied the most among the studied periods, most variables increasing from 30 months after treatment for both methods, in contrast to sustainability and consumability. Ignitability varied only for mechanical shredding, increasing just after treatment before decreasing onwards. This study, provided new insights on the temporal evolution of treated vegetation flammability, highlighting the short-term effect of prescribed burning and mechanical shredding, but also suggesting a loss of effectiveness, mainly regarding combustibility, between two and three years after treatment, especially for mechanical shredding, as the vegetation regrows.