Lois de croissance en biomasse du taillis: le robinier dans le Val de Loire

L’évolution de la masse ligneuse totale sur pied (ou production utile) est étudiée dans le cas des taillis traditionnels de robinier (Robinia pseudacacia L.) en Val-de-Loire. Des tarifs > sont établis sur un échantillon de 217 individus. Les relations sont du type : (formule voir document ci-joint) pour les brins de plus de 1,30 m de hauteur (b est la masse ligneuse totale du brin, c sa circonférence à 1,30 m, HD la hauteur dominante du peuplement d’appartenance) ; et : (formule voir document ci-joint) pour les brins de moins de 1,30 m (h est la hauteur du brin). Ces tarifs, appliqués aux inventaires de 31 placettes échantillons, âgées de 1 à 25 ans, permettent de reconstituer l’évolution de la masse ligneuse sur pied. Un modèle de croissance de la lorme : (formule voir document ci-joint) (B : masse ligneuse totale du peuplement ; Ip : indice de productivité défini à partir de la hauteur dominante ; t âge du peuplement) permet d’expliquer 97,5 p. 100 de la variance de B. Les courbes de croissance ainsi définies décrivent la production utile en masse ligneuse. La production courante est maximale à 5 ans, juste à la suite de la fermeture du couvert. La production moyenne culmine à 10 ans, et vaut alors 6 tonnes de matière sèche par hectare et par an, pour un peuplement moyen de la région.Our aim is to study the biomass production potentialities of traditional coppice, for various species, sites and managements. This contribution deals with Black Locust (Robinia pseudacacio L.) in the « Val-de-Loire » region. This species, widely introduced in the region, is interesting in coppice management, because of its nitrogen-fixing and rootsuckering ability. Our measures concern the evolution of standing woody mass during the classical coppice-cycle (25 years), without separating living and dead parts of trees. The studied coppice stands are 1 to 25 years old, and have been cut a variable unknown number of times. They have grown on alluvial sandy soils, under oceanic climate. 31 circular sample plots lrom 100 to 400 m2 have been surveyed in pure coppice.In each plot, girth at breast-height of every shoot, and dominant height of the plot have been measured. In young stands, height of small shoots - smaller than 1.30 m - has also been measured. From a sample of 217 shoots of various ages, mass prediction equations were built. Two equations were applied on the different inventories :(formule voir document ci-joint) (b : total woody mass : c : girth at breast height ; HD : dominant height of the stand), for shoots higher than 1.30 m, and : (formule voir document ci-joint) (h : height of the shoot) for shoots smaller than 1.30 m. Different growth curves have been fitted to the computed values of total standing woody mass. The ones finally adopted are : (formule voir document ci-joint) (B = standing woody mass, t = age of the stand), were Ip is defined as a ratio of the observed dominant height to the mean dominant height, computed with a similar model. These growth curves describe the variations of potential crop with age and fertility. They show that maximum current annual increment is reached at the age of 5 years, when the stand has just closed. The maximum mean annual increment is reached at the age of 10 years, and at this time, the crop could yield 6 metric tons per hectare and per year, for a mean stand of this region. Aditional measurements are suggested to improve the accuracy of the model, and to help describe the yield along successive coppice-cycles