Tree architecture in a Bornean lowland rain forest: intraspecific and interspecific patterns

Intraspecific and interspecific architectural patterns were studied for eig ht tree species of a Bornean rain forest. Trees 5-19 m tall in two 4-ha per manent sample plots in primary forest were selected, and three light descri ptors and seven architectural traits for each tree were measured. Two gener al predictions were made: (1) Slow growing individuals (or short ones) enco unter lower light, and have flatter crowns, fewer leaf layers, and thinner stems, than do fast growing individuals (or tall ones). (2) Species with hi gher shade-tolerance receive less light and have flatter crowns, fewer leaf layers, and thinner stems, than do species with lower shade-tolerance. Sha de-tolerance is assumed to decrease with maximum growth rate, mortality rat e, and adult stature of a species. Two light descriptors (crown position index and available space, but not ca nopy height) indicated higher light conditions for trees with more rapid gr owth, and for trees of greater height. Light levels were higher for species with high maximum growth rates and with greater adult stature. Most intraspecific architectural patterns conformed to the predictions: tot al leaf area and the number of leaf layers increased with increasing height and higher growth rates, and crown length/tree height ratio and stem slend erness respectively increased and decreased with growth rate. Yet, crown wi dth/tree height ratio and relative crown length did not change with tree he ight, nor did they with previous growth. Slow growing (and short) trees may not have the reserves to invest in further horizontal crown growth, and to avoid leaf self-shading sufficiently within their relatively narrow crowns . Predictions on interspecific architectural patterns were not supported by t he data. Species that were expected to be more shade-tolerant (lower maximu m growth, lower mortality, and shorter stature) had deeper crowns, greater leaf areas, and more leaf layers, than did less shade-tolerant species. The se patterns may be explained by lower loss rates of branches and leaves of the more shade-tolerant species. These species avoid leaf self-shading by d istributing their leaves at the crown periphery. The role of lateral light appears to be more important than hitherto realized. The crown width/height ratio and height/dbh ratio were negatively correlated, both intraspecifica lly and interspecifically. It is suggested that trees co-ordinate their cro wn and stem growth so that they maintain their stability at small safety ma rgins in the forest understorey.