Twenty five years of observation of the old-growth Massane forest dynamics: a unique permanent monitoring system

Located in southern France and bordering Spanish Catalonia, the Massane beech forest is one of the last old-growth forest of the Mediterranean basin and represents one of the southern populations of the European beech Fagus sylvatica in Western Europe. As such, it deserved special attention and a fine monitoring of its dynamics appeared especially relevant in the context of the ongoing climate change.  As the team responsible for managing the Massane National Nature Reserve, we initiated a comprehensive inventory of its unmanaged forest stands in late 1998. This article aimed at presenting the methodology and monitored traits in order to shed light on this exhaustive and long-term monitoring approach. This survey encompassed all standing trees—both living and dead—as well as fallen dead wood within a 9.4 hectares area known as the integral reserve. The study extended along a 5.25 km stretch of the Massane River, covering an additional 19.4 hectares. The primary objective was to establish a detailed baseline of the forest composition, predominantly consisting of European beech (Fagus sylvatica).  Collected data included dendrometric measurements, phenological stages, health assessments, and the presence and characteristics of various tree-related microhabitats (TreMs). These data were systematically stored in a Geographic Information System (GIS) and are regularly updated—semi-annually for tree decline and approximately every 12 years for den­drometric data.  To date, 70,794 items have been cataloged across the 28.8 hectares study area, comprising 61,532 standing trees (alive or dead) and 9,262 pieces of fallen dead wood. This extensive data collection provides valuable insights into forest structure, dynamics (such as growth, intra- and interspecific competition, regeneration and decay), and the presence of parasitic and saprophytic fungi. It also allows for a nuanced understanding of dead wood distribution and various TreMs, including cavities and brown rot. By correlating these observations with data from the on-site weather station, we can finely assess the impacts of climatic related events, such as drought stress and phenological anomalies. The weather station, operational since 1959, has documented a nearly 2°C increase in mean temperatures and a significant decline in precipitations since 2021, which may explain the changes observed in the stands, such as the decline of many trees. This ongoing monitoring initiative, underpinned by a robust cartographic database, forms the core of a unique forest observatory. The choice to conduct comprehensive inventories over substantial areas ensures a thorough representation of the forest’s richness and complexity. Although initially labor-intensive, the endeavor has yielded significant results, fostering numerous scientific collaborations and encouraging continued commitment to this invaluable research.