Sugar maple (Acer saccharum Marsh.) shoot architecture reveals coordinated ontogenetic changes between shoot specialization and branching pattern

摘要

Key message Trees display contrasting specialized annual shoots during their life-span and along with ontogeny-driven modifications in their branching pattern, they can fulfill different combinations of light exploitation and space exploration functions Tree ontogeny is related to major changes in tree structure and function at different scales, from individual organs to the whole tree. Yet, little is known about the direct effects of tree ontogeny on shoot specialization and branching patterns. Such specific architectural changes occurring with tree growth and aging are of critical importance for understanding the response of trees to their environment. The uppermost branching system of 0.1- to 23-m-tall sugar maple trees was sampled at the end of the growing season. Measurements were made at both the branching system (n = 40) and annual shoot scales (n = 803). An algorithm for automated shoot typology was developed to characterize branching pattern variations. Sugar maple shoots were divided into four types with contrasting sizes and levels of foliage (i.e., relative biomass allocation into leaves, LMF). These morphological differences were interpreted as functional specializations for light exploitation (high LMF) or space exploration and support (low LMF). Only annual trunk shoots exhibited trait value changes during ontogeny such as a minimum allocation to foliage in the current-year shoots for the 5-m-tall trees, which is related to lower light interception capabilities but higher space exploration abilities. However, this relative loss of light interception function is compensated by ontogenetic changes at the branching system scale, which are associated with higher rates of ramification to produce lateral shoots. This study reveals how branching system and annual shoot traits change simultaneously during tree ontogeny to fulfill different functions, particularly light exploitation and space exploration.