Aim The distribution of overmature forests in metropolitan France is poorly known, with only a few well‐studied prominent sites, and has never been evaluated countrywide. Here, we modelled French forest reserves' time since the last harvesting operation—a proxy for forest maturity—then inferred the current statistical distribution of overmature forests (i.e., forests over 50 years without harvesting) in France. Location Metropolitan France. Methods We used inventories from forest reserves and managed forests to calibrate a generalised linear mixed model explaining the time since the last harvesting with selected structural attributes and environmental variables. We then projected this model on the independent National Forest Inventory dataset. We thus obtained an updated estimation of the proportion and a rough distribution of overmature forest stands in metropolitan France. Results We found that high basal area of very large trees, high volumes of standing and downed deadwood, high diversity of tree‐related microhabitats and more marginally diversity of decay stages best characterised the time since the last harvesting. Volumes of stumps and high density of coppices translating legacy of past forest management also distinguished more overmature plots. Our projection yielded an estimated 3% of French forests over 50 years without harvesting mostly located in more inaccessible areas (i.e., mountainous areas). Main conclusions Our study showed that the time since the last harvesting could be derived from a combination of key structural attributes characterising overmature temperate forests. It gives the first robust statistical estimate of the proportion of overmature forests in France and may serve to report on their status. Our method could be extended in countries with accessible National Forest Inventory and calibration data, thus producing indicators at an international level.
Aim : The global knowledge of the distribution and proportion of forests with high conservation values such as mature forests in metropolitan France is scarce, focuses only on a few specific well-known sites, and has never been tackled countrywide and evaluated with a robust multivariate statistical approach. Here, we modelled French forest reserves' time since the last harvesting operation, then inferred the current statistical distribution of mature forests (i.e. forests over 50 years without harvesting) in metropolitan France. Location : Metropolitan France Methods : We used inventories of forest stand structure issued from forest reserves and managed forests to calibrate a generalised linear mixed model explaining the time since the last harvesting with selected stand structure attributes of maturity, combined with environmental variables. We chose the time since the last harvesting as a proxy for forest maturity, assuming older unmanaged plots to host older trees. We then projected this model on an independent nation-wide dataset issued from the National Forest Inventory. Thus, we obtained an updated estimation of the proportion and a rough distribution of mature forest stands (i.e. abandoned for over 50 years) in metropolitan France. Results : We found that the structural variables that best characterized the time since the last harvesting were high basal area of very large trees, high volumes of both standing and downed deadwood, high diversity of tree-related microhabitats and more marginally diversity of decay stages. Variables translating legacy of past forest management also characterised more mature plots, which displayed lower volumes of stumps and high density of coppices. Our projection gave an estimated 3% of French forests over 50 years without harvesting and a promising proportion of French forests that could reach interesting levels of maturity in the near future. Main conclusions : Our model showed that the time since the last harvesting is a good proxy for forest maturity in French temperate forest. It highlighted the stand structure attributes that are key in characterising those forests with high conservation values. We found that forests with higher levels of maturity seem to be located in more inaccessible places notably mountainous areas. Our projection gives the first robust statistical estimate of the proportion of mature forests in metropolitan France and may serve to report on the status of mature forests.
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