The consequences of crown traits for the growth and survival of tree saplings in a Mexican lowland rainforest

1. Many studies discuss the adaptive value of plant architecture, but few have actually measured architectural effects on plant growth and survival. In this study, sapling growth and survival are related to crown traits for two tree species, Trophis mexicana (Liebm.) Bur. and Pseudolmedia oxyphyllaria Donn. Sm., in the Los Tuxtlas lowland rainforest of Mexico. The traits investigated were crown width, crown depth, number of leaves, number of leaves per unit crown area (horizontal self-shading), and number of leaves per unit silhouette area (vertical self-shading). 2. Self-shading indices decreased with crown size, but were unaffected by the number of leaves per tree. Larger crowns thus had more diffuse foliage, with less self-shading. 3. The number of leaves had positive effects on growth and survival, while self-shading indices had no effect. This indicates that shaded leaves do not necessarily have negative carbon balances. 4. Negative effects of crown width on horizontal crown growth, and positive effects on vertical crown growth, suggest that saplings tend to grow towards a shape intermediate between the narrow and wide crown extremes. 5. Survival was positively correlated with crown width in Pseudolmedia , and with the number of leaves in Trophis . Apparently, dependence of survival on crown traits differed among species. 6. Crown traits affected plant growth and survival, but the hypothesis emerging from light-limited carbon acquisition was confounded by other factors, such as tree size and the inherent branching patterns. 7. Crown traits are good and rather simple predictors of future sapling growth and survival, and may help foresters to select potential crop trees.