Biomass increment and carbon balance of ash (<i>Fraxinus excelsior</i>) trees in an experimental stand in northeastern France

Abstract

In this study, we compared the annual biomass increment of ash trees to their annual carbon balance calculated from the end of the growing season in 1994 to the end of the next one in 1995. In 1995, three trees of variable competitive status and aged 25 were studied. Stem, branch and root biomass increments were derived from detailed measurements. Tree crowns were divided vertically into three layers. In each crown layer, the foliage biomass and area were determined, net CO 2 assimilation (A n ) and global radiation (R g ) were measured regularly throughout the growing season. Outside this period, R g was estimated from global radiation measured above the canopy and from estimations of light transmittance. Net assimilation of trees (A N ) was obtained by scaling leaf A n to the tree level, using relations established between A n and R g for each crown layer, the distribution of foliage area, and measured climatic data. Above-and below-ground tree respirations, not measured, were estimated. A n was correlated to R g and potential evapotranspiration. It decreased from the upper to the lower crown layers, but was independent of tree competitive status. Total estimated respiration of trees accounted for about 37% of gross assimilation. The proportion of carbon allocated to the stem was more than 45%. Net productivity of trees obtained from simulated annual carbon fluxes compared reasonably well with the biomass increment of trees. biomass partitioning / carbon assimilation / respiration / scaling / leaf area Rsum -Accroissement en biomasse et bilan de carbone du frne dans un peuplement exprimental du nord-est de la France. Cette tude avait pour but de comparer l'accroissement annuel en biomasse de frnes leur bilan de carbone tabli par estimation de la photosynthse et de la respiration sur une anne complte (depuis la fin de la saison de vgtation 1994 la fin de celle de 1995). En 1995, trois frnes de diffrents statuts concurrentiels gs de 25 ans ont t tudis. Les accroissements en biomasse de la tige, des branches et des racines ont t obtenus partir de mesures dtailles. Les houppiers des arbres ont t diviss verticalement en trois strates. Pour chaque strate, les variables suivantes ont t mesures: la biomasse et la surface foliaires, la photosynthse et le rayonnement global (R g ) pendant la priode de croissance. En dehors de cette priode, le rayonnement global a t estim partir du rayonnement global hors couvert et de valeurs estimes de la transmittance pour chaque strate. L'assimilation nette des arbres (A N ) a t obtenue en extrapolant l'assimilation nette foliaire (A n ) partir des relations tablies entre A n et R g pour chaque strate du houppier, de la distribution du feuillage par strate et des donnes climatiques mesures. La respiration des arbres, non mesure, a t estime. L'assimilation nette A n est lie R g et l'vapotranspiration potentielle et dcrot du haut vers le bas du houppier ; elle est indpendante du statut concurrentiel des arbres. La respiration totale des arbres reprsente environ 37 % de leur assimilation brute annuelle. La proportion de carbone alloue la tige reprsente en 1995 plus de 45 % du carbone total stock par les arbres. Le bilan de carbone des arbres obtenu par estimation des flux de carbone est en assez bon accord avec leur accroissement en biomasse. rpartition de la biomasse / assimilation carbone / respiration / changement d'chelle / surface foliaire