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

Intraspecific and interspecific architectural patterns were studied for eight tree species of a Bornean rain forest. Trees 5-19 m tall in two 4-ha permanent sample plots in primary forest were selected, and three light descriptors and seven architectural traits for each tree were measured. Two general predictions were made: (1) Slow growing individuals (or short ones) encounter lower light, and have flatter crowns, fewer leaf layers, and thinner stems, than do fast growing individuals (or tall ones). (2) Species with higher shade-tolerance receive less light and have flatter crowns, fewer leaf layers, and thinner stems, than do species with lower shade-tolerance. Shade-tolerance is assumed to decrease with maximum growth rate, mortality rate, and adult stature of a species. Two light descriptors (crown position index and available space, but not canopy height) indicated higher light conditions for trees with more rapid growth, 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: total leaf area and the number of leaf layers increased with increasing height and higher growth rates, and crown length/tree height ratio and stem slenderness respectively increased and decreased with growth rate. Yet, crown width/tree height ratio and relative crown length did not change with tree height, 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 the data. Species that were expected to be more shade-tolerant (lower maximum growth, lower mortality, and shorter stature) had deeper crowns, greater leaf areas, and more leaf layers, than did less shade-tolerant species. These 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 distributing 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 intraspecifically and interspecifically. It is suggested that trees co-ordinate their crown and stem growth so that they maintain their stability at small safety margins in the forest understorey.