Sandrine Chauchard

Patterns of Land-use Abandonment Control Tree-recruitment and Forest Dynamics in Mediterranean Mountains

A bstractMediterranean ecosystems have been impacted for millennia by human practices, particularly agricultural and pastoral activities. Since the middle of the nineteenth century, land-use abandonment has lead to scrubland and forest expansion, especially in mountain areas of the northern Mediterranean basin. This study aimed at analyzing how grazing history affects subsequent forest dynamics at a site located in the limestone foothills of the Southern Alps (France). The approach combines archival documents and dendroecology to investigate the origin, establishment and development of forest following land-use abandonment. Scots pine (Pinus sylvestris) started to colonize quickly in the 1870s, with the recruitment rate increasing during the first decade of the 1900s, associated with a decline of the local human population and regional livestock. Since the 1960s, European beech (Fagus sylvatica) and silver fir (Abies alba) have regenerated in the understorey of Scots pines. Regeneration is controlled by a threshold of grazing pressure. Noticeably, the rate of reforestation differs according to the former land-use, with pastures being colonized more quickly than ploughed areas. Different previous land-uses leading to different times of grazing cessation, combined with variable herbaceous competition explain the contrasting micro-scale regeneration patterns. Agricultural land-use and abandonment are both significant driving forces of vegetation dynamics. Knowledge of these factors is thus necessary to understand present patterns and to predict future forest pathways in the Mediterranean mountains.

Interactive effects of land-use change and distance-to-edge on the distribution of species in plant communities at the forest-grassland interface

Questions What are the patterns of plant communities in terms of richness and floristic composition along stable forest–grassland interfaces? Are there specific species at the edge and/or specialist species transgressions between adjacent habitats? How does edge displacement following land-use change alter distance-to-edge plant diversity patterns? What are the relative influences of the edge effect and the edge displacement in forests and grasslands? Location Forests and grasslands in northeast France. Methods Floristic surveys were conducted in three types of forest–grassland interface, which can be stable or the result of edge displacements due to afforestation or deforestation. The sample comprised a total of 132 plots in 22 forest–grassland interfaces. Point-biserial correlation coefficients were used to classify the species into several groups in stable interfaces: forest, edge or grassland specialists and generalists. Using LMMs and Tukeys HSD tests, the total species richness and that of each species group were compared between the plot positions (i.e. distance-to-edge) and between the stable interfaces and cases of afforestation and deforestation. To evaluate the relative influence of the edge effect (plot position) and edge displacement (afforestation/deforestation) on plant community composition, CCAs and variation partitionings were performed. Results The largest difference in both plant community richness and composition was found between the forest and grassland edges. The highest species richness was found at the grassland edges. In addition to a high number of generalist species, a mixture of forest and grassland specialist species were predominant at the edges, and the forest specialists were almost the only transgressive species. Afforestation interfaces showed an extinction debt of grassland and edge specialist species and a colonization credit of forest specialists in recent forests. The effects of edge displacement were larger than those of the edge effect in forests, and the opposite was observed in the grasslands. Conclusions Forest and grassland plant communities differ strongly even over very short distances. However, the edges host a large number of forest and grassland specialist species, which explain higher species richness than in adjacent habitats. This results from interactive effects of distance-to-edge and edge displacement following land-use change.

Network connectivity value

In order to unveil the value of network connectivity, we formalize the construction of ecological networks in forest environments as an optimal control dynamic graph-theoretic problem. The network is based on a set of bioreserves and patches linked by ecological corridors. The node dynamics, built upon the consensus protocol, form a time evolutive Mahalanobis distance weighted by the opportunity costs of timber production. We consider a case of complete graph, where the ecological network is fully connected, and a case of incomplete graph, where the ecological network is partially connected. The results show that the network equilibrium depends on the size of the reception zone, while the network connectivity depends on the environmental compatibility between the ecological areas. Through shadow prices, we find that securing connectivity in partially connected networks is more expensive than in fully connected networks, but should be undertaken when the opportunity costs are significant.

Quelles sources cartographiques pour la définition des usages anciens du sol en France

Un travail de cartographie historique sur un territoire réduit, ou un massif forestier, nécessite naturellement un récolement exhaustif des sources à disposition, de l’Ancien Régime aux photographies aériennes récentes. Mais à plus petite échelle, il est préférable d’opérer à partir d’une source unique disponible pour la totalité de l’espace étudié. À la limite, la carte de Cassini, déjà intégralement digitalisée pour ce qui concerne les forêts, pourrait être prise comme référence, mais sa précision est médiocre, et sa période de réalisation est trop précoce. En effet, le déroulement de la transition forestière implique de fixer une date de référence dans la première moitié du XIXe siècle, correspondant au minimum historique présumé des surfaces forestières métropolitaines françaises, ce qui suppose que les documents d’archive utilisés pour la définition des forêts anciennes aient été réalisés dans cette période, ou au plus près de cette période. La carte d’état-major, bien qu’incomplètement digitalisée à ce jour, paraît correspondre aux objectifs fixés, tant en raison de sa date de réalisation qu’en raison de sa bonne précision. Encore faut-il connaître ses limites, et les risques inhérents à son utilisation. Nous avons donc cherché à la comparer à un corpus cartographique supposé plus fiable et plus précis, réalisé pendant la même période, mais difficile à utiliser comme référence dans des études à petite échelle en raison de la lourdeur du travail nécessaire à sa digitalisation : le cadastre napoléonien. Le cadastre sert ainsi de référence pour estimer la valeur de la carte d’état-major et préciser la localisation des surfaces forestières de la première moitié du XIXe siècle.