Répartition de la biomasse entre organes végétatifs et reproducteurs chez le hêtre européen (Fagus sylvatica L), selon le secteur de la couronne et l'âge des arbres

Les allocations de ressources chez les végétaux peuvent être estimées pour étudier la répartition de la biomasse entre différents organes lors d'un cycle de végétation annuel. Quatorze hêtres (Fagus sylvatica L) ont été étudiés : 8 arbres très âgés (150 ans environ) et 6 arbres adultes de 100 ans environ. La couronne de chaque arbre a été divisée en 2 secteurs, supérieur et inférieur. Le hêtre produit 2 types de pousses annuelles, longues et courtes. Elles peuvent être végétatives ou florifères. Des pousses de chaque type situées sur des axes de premier ordre dans la ramification ont été analysées pour décrire la répartition de la matière sèche entre différentes fonctions végétatives et reproductrices. Les stratégies de développement changent selon l'âge des arbres, le secteur et les types de pousses. Les fonctions végétatives sont favorisées dans les pousses longues des arbres jeunes, surtout dans le secteur supérieur de la couronne. Les fonctions reproductrices mobilisent une biomasse plus importante chez les arbres âgés que chez les arbres plus jeunes. Chez tous les arbres, l'allocation de matière reste élevée pour assurer la maintenance dans les pousses végétatives courtes, ce qui confirme leur rôle essentiel d'exploitation du milieu.Biomass distribution in vegetative and reproductive organs of the European beech (Fagus sylvatica L), according to crown sector and tree age. Resource allocation in plants can be estimated to study biomass distribution in the various organs during an annual vegetative cycle. Fourteen beech trees were chosen on the Aigoual mountain (Cévennes, France, 44° 20'N, 3° 60'E, alt 1 400 m): 8 were very old trees (150 yr), and 6 were vigorous adult trees (100 yr old). The tree crown was divided into upper and lower sectors. Beech produces long (L) and short (C) annual shoots, which are vegetative (V) or both vegetative and floriferous. Shoots of each type located on first-order axes in the ramification were analysed to describe the distribution of dry matter among various vegetative and reproductive functions. In each tree, 40 terminal buds were chosen at random: 20 on the upper part of the crown and 20 on the lower sector; approximately half seemed floriferous. During April these buds were enclosed in situ in gauze bags permeable to pollen and light. The 640 shoots produced from these buds were collected during the following November; of these only 524 were in good condition and were analysed. After desiccation, shoot length (V2L) was measured and shoot organs were weighed: leaves and stipules (V1), axes (V2P), buds (V3), female production (R1) and male inflorescences (R2). Development strategies vary according to tree age, tree sector and shoot type. Vegetative functions are favoured in long shoots of the youngest trees particularly in the upper sector (except V1). In short shoots, biomass is also more important in the upper crown sector. There was no biomass difference in hermaphrodite shoots according to the sector. But the age effect is considerable: female acquisitions are much greater in old trees (p < 0.001), whereas male acquisitions tend to be a little lower (p < 0.05). There are great differences in vegetative biomass according to shoot type, but, in hermaphrodite shoots there are few differences in reproductive functions according to age and sector. Biomass allocation (% of the shoot biomass) is often different according to tree age: (i) female allocation in hermaphrodite shoots is much greater in the oldest trees, whereas it is the opposite for vegetative functions; and (ii) vegetative long-shoot allocation is greatest for the V1 function in old trees, whereas vegetative short shoot allocation is very high for the V1 function whatever the age (this confirms their essential role in exploiting the environment). There are significant positive biomass correlations in vegetative functions, above all in long shoots. Concerning biomass allocations, correlations among vegetative functions vary according to age and shoot type. Generally, there is a negative correlation between female allocation and the various vegetative allocations. All these results show: (i) a partial development of each shoot type; (ii) a synergy between vegetative functions in hermaphrodite shoots (biomass and allocations), whereas a strong competition appears for allocations principally between female and vegetative functions; and (iii) a tree age effect: in 100 yr-old trees, vegetative development is still very important, principally at the upper part of the crown. In very old trees, vegetative development tends to become stable and female function is preponderant