Measuring the postfire resilience of a bird-vegetation system: a 28-year study in a Mediterranean oak woodland.

Despite numerous studies on the response of Mediterranean ecosystems to fire, few have measured the respective resilience of vegetation and fauna compartments. For 28 years, we conducted an annual monitoring of avifauna composition and vegetation structure (cover profile)following a severe wildfire in a holm oak (Quercus ilex) stand in southern France. Our aim was to estimate the time necessary for this bird-vegetation system to return to a state analogous to its pre-fire state. In the burned plots, low herbaceous and shrub layers were gradually replaced by higher, woody layers of vegetation. Neither bird species richness nor inter-annual bird species turnover showed significant differences from one year to the next over the study period. In contrast, bird species composition did change steadily, leading to an almost complete replacement of early-successional species by late-successional ones. Using the first axes of multivariate analyses as 'proxy variables' of vegetation or avifauna recoveries, we estimated by extrapolation the recovery times of these two ecosystem components at ca. 50 and 35 years, respectively. Towards the end of the study period, the rate of change in avifauna composition decreased comparatively to that of vegetation structure. Our results show that holm oak woodlands are highly resilient and seem to tolerate an approximately 50-year fire interval, even if it remains to be assessed how resilient they would be in the case of increased fire frequency. More generally, our multivariate approach, which allows comparative estimations of resilience in different components of an ecosystem using qualitative as wellas quantitative criteria, could be applied to various case studies in disturbance and restoration ecology.