Neighbourhood composition determines growth, architecture and herbivory in tropical rain forest tree seedlings

Abstract

Summary Interspecific and intraspecific competition in seedling banks may be important determinants of regeneration success in some groups of canopy trees, e.g. dipterocarps in south‐east Asian tropical rain forests, and the outcomes of these competitive interactions may contribute to the maintenance of the high levels of plant diversity. We present the results of an experiment examining the effects of seedling bank diversity within canopy gaps on seedling growth, architecture and levels of insect herbivore defoliation over 18 months. Within each of six experimental canopy gaps, we planted two conspecific blocks of Shorea leprosula (Dipterocarpaceae) seedlings and two heterospecific blocks of S. leprosula seedlings amongst four other dipterocarp species with differing growth rates and palatability to herbivores. The growth allocation patterns of S. leprosula differed according to whether they were planted among conspecifics or heterospecifics. Although S. leprosula seedlings grew taller in conspecific stands, their above ground biomass did not differ between planting treatments, due to increased branching and leaf area in seedlings in the heterospecific stands. Shorea leprosula displayed the lowest levels of antiherbivore defences and sustained the greatest levels of defoliation of all species studied. Despite an increase of almost 60% in leaf damage in conspecific vs. heterospecific stands, neither biomass nor survival of S. leprosula seedlings differed between stands over the 18‐month growth period. The important effects of neighbourhood composition on the growth and herbivory of S. leprosula seedlings, suggest that diversity of seedling banks can be an important determinant of regeneration success and, thus, forest dynamics. Neighbourhood composition may have similar impacts on competitive outcomes and biomass allocation patterns in other ecological communities.