Key message. French forests exhibit the fastest relative changes across Europe. Growing stock increases faster than area, and is greatest in low-stocked private broadleaved forests.Past areal increases and current GS levels show positive effects on GS expansion, with GS increases hence expected to persist.Context. Strong increases in growing stocks (GS) of European forests for decades remain poorly understood and of unknown duration. French forests showing the greatest relative changes across Europe form the investigated case study.Aims. The magnitudes of net area, GS, and GS density (GSD) changes were evaluated across forest categories reflecting forest policy and land-use drivers. The roles of forest areal changes, GS and GSD levels on GS changes were investigated.Methods. National Forest Inventory data used to produce time series of area, GS and GSD across forest categories over 1976-2014, and exploratory causal models of GS changes.Results. GS (+57%) increased three-times faster than area, highlighting an advanced stage in the forest transition. Low-stocked private forests exhibited strong changes in GS/GSD, greatest in private broadleaved forests, stressing the contribution of returning forests on abandoned lands. Regressions demonstrated positive effects of both past areal increases and current GS, on GS expansion.
Conclusion.Aerial C-sink in French forests is expected to persist in future decades.
Forest tree species strongly influence the dynamics and management of forests. French forests show the greatest compositional diversity in Europe, and constrains the quantitative analysis of associated wood resources. The aim was hence to develop a partition of French forests' area according to their dominant compositions in tree species and stratified by biogeographical regions (GRECO) in order to organize this diversity.The partition relies on forest composition as measured by the national forest inventory (2006-2015, >65,000 plots). It builds upon the J-shaped distribution of elementary composition abundance, identifies dominant compositions describing a minimum 50% of GRECO's forest area, minor compositions being gathered. An ecological assessment of this partition and its application to the analysis of the growing stock are developed. This partition describes 61,4% of the forested area (66% of the growing stock) according to 29 dominant compositions, demonstrating its efficiency. These compositions revealed the importance of broadleaved mixtures, and of ignored forest strata (pine species in Northern France). Growing stock densities appeared lowest in broadleaved compositions (Mediterranean oaks), and greatest in some coniferous compositions (mixture Silver fir/Norway spruce in mountains). They highlight the role of ecological contexts and historical forest management of these forests.
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