Résumé
In order to improve the estimation of carbon stored in the French forest biomass from National Forest Inventory data, we developed six species-specific equations for estimating the total aboveground volume of trees, including merchantable volume, branches and twigs. Equations use circumference at breast height and total height as independent variables. They were built from even-aged forests of the Landes massif, the northern half and the eastern mountain regions of France. The sample was selected within archives of volume measurements taken in growth and yield permanent plots between 1920 and 1955. It is made up of 4619 trees belonging to seven important species: 1222 Sessile Oak, 1293 Common Beech, 347 Douglas Fir, 309 Norway Spruce, 389 Scots Pine, 297 Maritime Pine and 762 Silver Fir. These trees were felled and measured in 26 different forests (62 stands). Tree form factor was analysed, rather than volume, to remove heteroscedasticity, and height was substituted by a hardiness coefficient to remove diameter–height correlation. The analysis identified species-specific modes of variation of tree form with respect to developmental stage and tree hardiness. Maritime and Scots Pine did not differ statistically, despite large differences between ecological and silvicultural situations of both species. This suggests the possibility to use identical volume equations for species belonging to the same genus. Regional variations of tree form were explored by a cross-validation technique. Prediction biases did not exhibit a clear geographic structure. A 5% overestimation for hardwoods in southern sites is possible, and would deserve further testing. The hypothesis that recent growth changes may have slightly altered tree form is also discussed. Finally, a national-scale application provided Biomass Expansion Factors consistent with former studies and suggested that these volume equations behave well in extrapolation to coppices, uneven-aged or mixed stands.