L'influence de divers caractères architecturaux sur l'apparition de la fourche chez le Fagus sylvatica, en fonction de l'absence ou de la présence d'un couvert

Abstract

A qualitative architectural analysis conducted on young beech (Fagus sylvatica L.), benefiting from the protection of a leaf canopy or exposed to full sunlight, revealed architectural characteristics susceptible of influencing the arrangement of vertical lateral axes entering in direct competition with the trunk and contributing to the formation of forked axes. The distinct structures of the forked trees are dependent on the absence or the presence of a superior forest canopy. Forks are formed when the following events occur in nature: monocyclic growth (formation of a single growth unit per year) and polycyclic growth (formation of numerous growth units per year) of the terminal meristem originating from the main axis, destruction of this terminal meristem, and large increases or decreases in the annual growth rate. None of these characteristics alone will determine the arrangement of a forked branch. Nonetheless, the annual fluctuations in growth rate seem to have the strongest influence. A quantitative analysis conducted on two 18-year-old groups exposed to either full sunlight (no canopy) or shade (under dense high forest) revealed that the frequency in vertical lateral axes is greater in full sunlight (0.18 – 0.45) than in shade (0.03 – 0.1). Similarly, the percentage of polycyclic shoots is greater in full sunlight (51%) than in shade (14%). However, this polycyclic growth does not seem to have a direct influence on the formation of vertical lateral axes. On the other hand, there may be an intermediary influence on apical mortality since in full sunlight the percentage of seasonally traumatized stems is greater (31%) than in shade (21%). The apical mortalities seem to stimulate the organization of the vertical branches. However, this formation of vertical lateral axes seems mainly controlled by the fluctuations in annual growth rates and more specifically by the rate of growth reached by the plant. In full sunlight, the annual stems have an average size (40 – 75 cm) notably superior to 50 cm. It is beyond this threshold of development that the observed trees produced greater annual frequencies in the appearance of vertical lateral axes. In the shade, the average size of annual stems ranged from 20 to 55 cm, but seldomly surpassed 50 cm. The annual frequency in the appearance of vertical lateral axes is negligeable. Key words: beech, Fagus sylvatica, architecture, forked branch, growth, forest canopy. [Journal translation]