Past land use effect on forest understory vegetation in the Mediterranean region: should we use ancient maps or in situ land use remnants? | Effet de l'utilisation antérieure des terres sur la végétation de sous-bois forestier en région méditerranéenne: comparaison des cartes anciennes et des vestiges in situ de l'utilisation des terres ?

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN [ADD1_IRSTEA]Dynamique et fonctionnement des écosystèmes

International audience

英语. The importance of including a historical approach in ecological studies is now acknowledged, as history can help understand structure and functioning of present ecosystems but also assess conservation objectives. In temperate regions, several studies showed ecological differences according to forest temporal continuity, including soil physicochemical properties, vascular plant composition and traits. The definition of ancient and recent forests usually relies on a threshold date, provided by historical data. The best tools for studying the impact of forest continuity on forest plants are historical maps. Those maps should be ancient and precise enough. In France, those conditions are best met by the "Etat-Major" map. Indeed, unlike the Cassini map (18th century), small woodlots were not overlooked, the spatial resolution and precision is reasonable and that map was achieved at the national scale, which allows scientists to compare response patterns among regions in a homogeneous fashion. It has been digitized on about one third of France and is more and more used by scientists. However, is it that simple? Indeed, in lowlands, there are no ambiguities between land uses, and boundaries are very easy to identify. In a semi mountainous region, things are a bit trickier: relief may challenge the identification of land uses and boundaries; some land uses are in fact a combination of two land uses which are distinguished by a criterion that has not yet been clearly defined; in Mediterranean regions, long history of use of forests and particularly sylvo-pastoralism has probably smoothed the difference between ancient forests and recent forests developed on rangelands or pasturelands. We here question the relevance of the "Etat-Major" map as an absolute reference for studying the ecological effect of past land use and particularly in a Mediterranean and semi-mountainous context: the Natural Regional Park of Luberon. A hundred sites were sampled according to past land use (forest, "pâquis" and arable lands) on the "Etat-Major" map. In each plot, we also recorded past land use remnants: terrace, stone removal or none. Understory vegetation and soil physico-chemical properties were recorded and analysed according to past land use, and in situ past land use remnants. We showed that soil properties and understory vegetation significantly responded to past land use remnants, but not to past land use according to the "Etat-Major" map. This suggests that ancient maps may not be sufficient to detect the effect of past land use in the Mediterranean area.