Modeling qualitative and quantitative elements of branch growth in saplings of four evergreen broad-leaved tree species growing in a temperate Japanese forest

Abstract

Factors influencing branch growth differed between a qualitative element (whether a branch grows) and a quantitative element (the amount of biomass accretion). We measured branch growth in saplings of four evergreen broad-leaved tree species growing in a temperate Japanese forest. A hierarchical Bayesian approach was used to model two elements of branch growth: a qualitative component [whether branches produce new annual shoots (ASn)] and a quantitative component (total ASn mass). The two components were influenced by somewhat different factors. The probability that a branch would produce ASn was affected by the total mass of previous-year annual shoots (total ASn–1 mass), branch age, relative branch height, canopy openness measured at the branch tip, maximum canopy openness within a single sapling, and the interaction between canopy openness and maximum canopy openness. The total ASn mass was influenced by the total ASn–1 mass, relative branch height, branch inclination, and maximum canopy openness. These two components of branch growth should be considered separately when tree architecture is modeled. In addition, we detected interactive responses among branches within individual saplings. The relative importance of interactive modular response and branch autonomy may differ depending on the status of plants, such as individual age, environmental conditions, and the timing of the measurements. We found considerable similarities in the responses of branch growth to the internal and environmental factors among the species studied. We also found some among-species differences in branch growth responses to the explanatory variables measured.