Résumé
Between 2005 and 2008, a group of 10 research teams studied vegetation, litter, earthworms, microfauna, bacteria, soil physics, nutriments, organic matter (quality and quantity) and toxic compounds in a multidisciplinary research program dealing with the impact of repeated forest fires on biodiversity and the soil in mediterranean forests. Plots were laid in old growth forests and in sites burned between 1 and 4 times in the last 50 years. For each fire regime, fires dating 1, 3, 5, 15 and 25 years were chosen. The goals were: (1) to look for biodiversity indicators for the assessment of the ecosystem status and functioning, (2) to understand interactions between biotic and abiotic factors in ecosystem resistance and resilience, (3) to look for critical fire frequency thresholds. Taking advantage of the exceptional drought which occurred during the experiment, we studied the interaction between fire and drought. Two thresholds appeared in fire frequency (1 and 4 fires in 50 years) leading to deep changes in the ecosystem biodiversity and functioning. Few differences were observed 3 and 15 years after fire between plots burned 1, 2 or 3 times, the ecosystem being used for millenniums to such fire frequencies. Earthworms and bacteria diversity and activity appeared critical for ecosystem functioning and recovery after fire. Due to drought, this activity was severely reduced and strong interactions were found between fire and drought, each of these disturbances amplifying the impact of the other. High disturbance frequency by fire or drought reduced biological diversity, increasing the resistance but reducing the resilience of communities. Some obligate seeders tree species disappeared when fire occurred at less than 15 to 20 years interval, changing the ecosystem structure. Trees mortality due to drought was far higher in recently burned areas, degrading vegetation from forest to matorals. The quantitative resilience of several physical and chemical parameters is achieved within 15 to 25 years after fire, but 50 years are necessary to fully recover the qualitative values and balance for these parameters as well as for most of the biological factors. Most of the critical biological, physical and chemical processes were concentrated in the topsoil (0-5 cm), deeper horizons being impoverished by a long fire history, except in old growth forests. These ecosystems are fragile and their dynamics may shift to desertification with increasing drought due to climate change.