Ontogenetic strategy shift in sapling architecture of Fagus crenata in the dense understorey vegetation of canopy gaps created by selective cutting

Abstract

Ontogenetic architectural variation is a key aspect of the light foraging of saplings under canopy gaps, where the interactions with the surrounding understorey plants are intense and change as saplings grow. Therefore, relationships between the allometry of 58 Fagus saplings (15-184 cm tall) and both the surrounding vegetation and light environment were investigated in the dense understorey of canopy gaps. The light available to the saplings exhibited a positive exponential relationship to their height, and at a given sapling height, it was inversely related to the height of the surrounding understorey. The slenderness of the saplings' stems increased as their height increased, and their crown architecture shifted from horizontally spread monolayer to vertically extended multilayer forms, indicating an ontogenetic strategy shift from shade tolerance to shade avoidance. These changes in sapling architecture were independent of the height of the surrounding understorey and, therefore, light availability, although the length of their current-year shoots (reflecting the rate of architectural development) was inversely related to the surrounding understorey height. Thus, ontogenetic architectural variation in Fagus saplings probably has the character of a developmentally programmed growth trajectory preadjusted to the most likely environments; the surrounding environments mainly affect the rate at which saplings move along this trajectory.Résumé : La variation architecturale ontogénétique est un aspect clé de la recherche de la lumière pour les semis situés dans des trouées de la canopée où les interactions avec les plantes de sous-bois environnantes sont intenses et changent à mesure que croissent les semis. Ainsi, les relations entre l'allométrie de 58 semis de Fagus (de 15 à 184 cm de hauteur), d'une part, et la végétation environnante et les conditions lumineuses, d'autre part, ont été étudiées dans la végétation dense du sous-bois de trouées dans la canopée. La lumière disponible pour les semis augmentait de façon exponentielle avec leur hauteur et, pour une hauteur donnée des semis, elle était inversement reliée à la hauteur de la végétation de sous-bois environnante. L'élancement de la tige des semis augmentait avec leur hauteur et l'architecture de leur cime passait d'une forme monoétagée s'étendant horizontalement à une forme multiétagée s'étendant verticalement, ce qui indique un changement de stratégie ontogénétique allant de la tolérance à l'ombre à l'évitement de l'ombre. Ces changements de l'architecture des semis étaient indépendants de la hauteur de la végétation de sous-bois environnante et, par conséquent, de la disponibilité de la lumière, même si la longueur des pousses de l'année courante (reflétant le taux de développement architectural) était inversement reliée à la hauteur de la végétation de sous-bois environnante. Par conséquent, la variation ontogénétique architecturale des semis de Fagus adopte probablement une trajectoire de croissance dont le développement est programmé et qui est préaju...

Retrospective analysis of tree architecture in silver fir (Abies alba Mill.): ontogenetic trends and responses to environmental variability

“…During ontogeny, it would appear that tree populations show similar trunk increment behaviours in common ontogenetic years (Heuret et al 2006). This growth trajectory may correspond to the genotype's development programme (Yagi 2009). A retrospective analysis of growth can identify three main kinds of variation around this trajectory: (1) breaks due to the establishment of relay axes replacing dead axes; (2) inter-annual growth fluctuations linked with climate: see following section; (3) speed of the move along the trajectory (i.e.…”

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Deciphering the developmental plasticity of walnut saplings in relation to climatic factors and light environment

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