Photosynthesis in Relation to Growth and Seedling Ecology of Two Dipterocarp Rain Forest Tree Species

Abstract

1 We report growth and photosynthetic characteristics of seedlings of two contrasting climax tree species (Shorea leprosula Miq. and Dryobalanops lanceolata Burck., Dipterocarpaceae) growing in three differing forest light environments (ranging from 2.4-21.8 mol photon m-2 day-') in lowland rain forest in Sabah, Malaysian Borneo. 2 Our aim was to determine whether interspecific differences in seedling ecology reflected differences in the ability of the photosynthetic apparatus to acclimate to light (assessed by measuring photosynthetic light response curves in the field). 3 S. leprosula has been described as a fast growing light-demander whose seedlings will not survive in deep shade whereas D. lanceolata is slower growing with seedlings that can survive a range of light conditions from deep shade to large canopy gaps. 4 Light environments were characterized by analysis of hemispherical photographs taken in the position of each leaf used for photosynthesis measurements, such that we could analyse both within and between site variation. Growth, seedling architecture and mortality were recorded. 5 D. lanceolata showed a greater ability to acclimate to shade, where it had the greater net carbon assimilation rate of the two species at low instantaneous photon flux densities (PFD). Conversely, S. leprosula had a greater ability to acclimate to high light and had greater assimilation rates at high instantaneous PFDs in all environments. 6 Photosynthetic performance, growth and light availability showed strong correlation across environments. Although there were small interspecific differences in photosynthesis within each forest environment, differences in growth and survival were more clearly related to the differing carbon allocation patterns of the two species. 7 D. lanceolata showed an increasing allometric shift in low light in favour of the growth of long branches which are likely to increase sunfleck interception. S. leprosula maximizes height growth and thus has a competitive advantage in more open sites. 8 Although photosynthetic performance paralleled that of growth, we conclude that rates of carbon dioxide fixation alone cannot define the seedling ecology of a species in the absence of information on photosynthate allocation.