Height-dependent changes in shoot structure and tree allometry in relation to maximum height in four deciduous tree species

Abstract

1. Tree allometry often varies among coexisting species of different maximum height (Hmax) in forests. Although shoot growth patterns directly influence overall tree architecture, the structures of current-year shoots at the tops of crowns have not been directly related to differences in tree allometry across species. 2. I investigated height-dependent changes in structure and biomass allocation patterns in current-year shoots of four coexisting tree species differing in Hmax in a cool-temperate forest in Japan. The relative importance of total biomass, biomass allocation, shoot allometry, and shoot angle to vertical growth was quantified and compared with tree allometry. 3. Height-dependent changes in total biomass of current-year shoots varied across species. In contrast, stem length per unit mass, shoot angle, and total leaf area per unit stem cross-sectional area decreased, and leaf mass per unit area increased with height in all species. Vertical growth rate consequently declined with increasing height in all species. Sensitivity analyses revealed that the primary determinant of declining vertical growth rate was change in stem length per unit mass; shoot angle was a secondary determinant. In contrast, increases in total shoot mass with height modulated declining vertical growth rates. 4. Vertical growth rate was greater in two canopy species than in two sub-canopy species at given heights at the shoot level, and this pattern coincided with allometry between tree height and trunk diameter. In contrast, vertical growth rate was greater in sub-canopy species than in canopy species near their maximum heights. These patterns suggest that allometric differences between species may be useful for evaluating crown-development patterns, but not for estimating Hmax of species.