Crown development in tropical rain forest trees: patterns with tree height and light availability

1. Monitoring of two canopy species Dicorynia guianensis and Vouacapoua americana (Caesalpiniaceae) in a tropical rain forest in French Guiana was used to investigate vegetative crown development at five organizational levels: leaf, metamer, extension unit, sympodial unit and whole crown. The effects of light availability and tree height on different traits were evaluated in trees < 25 m in height and compared with taller individuals (25-37 m). Path-analysis is used to illustrate the consequences of trait changes at multiple levels of organization for the whole crown level. 2. Tree height and canopy openness influenced crown development at each organizational level. Crowns in higher light levels had lower specific leaf area, greater leaf spacing, greater extension of all branches, and greater extension of the leader shoot. With increasing tree height, crowns had a lower specific leaf area, greater leaf area index and greater relative crown depth. 3. Vouacapoua showed some responses to light not seen in Dicorynia. In particular, Vouacapoua increased meristem activity with light, but the lack of response in Dicorynia may be due to moderate light levels rather than inability to respond. 4. Low leaf-display costs at low light availability may enable trees to survive light suppression. 5. Light availability cannot explain trait changes with tree height. Alternative explanations for trait changes with tree height are discussed. 6. Several of the relationships between plant traits and tree height or canopy openness became non-linear when taller trees (25-37 m) were included. In these taller trees, vegetative growth was reduced at all organizational levels, particularly in Vouacapoua, which does not grow as tall as Dicorynia. 7. Qualitatively, plant responses to light did not differ between trees of different height, and were similar to seedling and sapling data in the literature. Responses were, however, quantitatively different, suggesting that small saplings cannot serve as model organisms for crown development in taller trees.