Effects of the spatial arrangement of aerial stems and current-year shoots on the demography and growth of Hydrangea hirta in a light-limited environment

Abstract

This study investigates the effects of the spatial arrangement of aerial stems and current-year shoots on patterns of herbivory, competition and on the requirements for mechanical support in Hydrangea hirta Sieb. et Zucc. growing in a light-limited environment. Stem demography was followed for three consecutive growing seasons. Stem mortality rates decreased with increasing stem age. Age-specific mortality rate was highest for new stems owing to the high incidence of herbivory. Mortality due to herbivory was low in older stems and was attributed to the dispersion of the risk of herbivore attack as a result of the increase in number of current-year shoots per stem. Clumping of stems led to intense intra-clonal competition as inferred from size-dependent mortality and density-dependent turnover of stems, and higher maximum stem length in clumps with higher stem density. Analysis of the spatial distribution of current-year shoots within the stem crown revealed a uniform, mono-layered crown architecture. The horizontal distribution of buds within the mono-layer was aggregated as a result of their opposite position, but as current-year shoots grew, their apices became uniformly distributed, resulting in efficient leaf display for light capture and shading out competing stems. However, limits on stem growth were observed in crown expansion, shoot production rate and current-year shoot size. Crown width increased with increasing basal diameter to reach a maximum. Both shoot production rate and current-year shoot size decreased with stem growth and a minimum current-year shoot size was defined. These limits on stem growth were attributed to suppression from intra-clonal competition and to the requirements for mechanical support of the mono-layered crown.