The Mediterranean Old-Growth Forests: Anomalies or Relicts? The Contribution of Soil Charcoal Analysis

Old-growth forests are of high interest for biodiversity conservation, especially in the disturbance-prone Mediterranean landscapes. However, it remains unclear whether the survival of patches of old-growth forest in the degraded Mediterranean landscapes results from local anomalies or from past, larger forests. Therefore, in this study, we assessed (1) the origin, (2) the long-term ecological trajectory, and (3) the mechanism(s) that explain the survival of a Mediterranean old-growth forest, the Sainte-Baume forest. To achieve this, we used soil charcoal analysis. We opened fifteen soil profiles in the forest and five in its surrounding areas for soil description and sampling. The soil descriptions enabled us to highlight in situ soil horizon and colluvial layers. A total of 1656 charcoal pieces from different soil samples were taxonomically identified to characterize the composition of past forests. Selected charcoal pieces (n = 34) were dated to obtain chronological data. Our investigations indicate that the survival of the Mediterranean old-growth forest, in the context of the semi-open/open Mediterranean landscapes, is the result of a combination of biotic and abiotic factors, which reduced the influence of past forest disturbances. Thus, the resistance and resilience of the forest areas are preserved over a long-term ecological trajectory. Therefore, the potential of Mediterranean old-growth forests as baseline reference points for the conservation of biodiversity is related to the identification and maintenance of the local biotic and abiotic factors which allowed the survival of the old-growth forest.