Interaction between light and drought affect performance of Asian tropical tree species that have differing topographic affinities

We studied two pairs of congeneric species (Dipterocarpus hispidus, Dipterocarpus zeylanicus, Mesua ferrea, and Mesua nagassarium). These species are canopy trees of rain forest in south-west Sri Lanka that exhibit differing topographic affinities. We hypothesized that topographic affinity is related to successional status and species ability to endure drought. We tested for these changes by measuring morphology of seedlings growing with each other in differing combinations of light and water. We constructed shade houses with a range of photosynthetic photon flux densities (PFD) and red:far red (R:FR) ratios. Two watering regimes within shade treatments created soil conditions that were either evermoist or periodically dry. Seedlings of the four species were inter-planted at equal spacing within large flats. They were allowed to grow amongst each other for a 2-year period. The more shade-intolerant Dipterocarpus spp. exhibited greater morphological responsiveness to increases in irradiance than the more shade-tolerant Mesua spp. We also demonstrate that all four species differ substantially from each other when morphological attributes (height, dry mass, leaf area) are compared together. Differences among these four species can be logically explained by their sequential competitive exclusion in relation to increasingly limited resources of light and soil water. In high light and evermoist soil conditions D. zeylanicus performs best. Under deep shade Mesua spp. have greater height than Dipterocarpus spp. When soil water is limiting, and the level of shade intermittent, M. nagassarium has greater height than the other three species. These results conform to species observed topographic distribution in the forest. Further studies are needed of wild populations growing across a range of forest sites to confirm whether these four species differ when grown under natural field conditions. (c) 2005 Elsevier B.V. All rights reserved.