Growth and morphology of seedlings of ten tropical rain forest species were studied at Los Tuxtlas, Mexico. Seedlings were grown in three environmental conditions: the shaded forest understorey (FU, receiving 0.9-2.3% of the daily photosynthetic photon flux, PF, above the canopy), a small canopy gap of approx. 50 m2 (SG, receiving 2.1-6.1% of daily PF), and a large canopy gap of approx. 500 m2 (LG, receiving 38.6-53.4% of daily PF). The growth of all species was enhanced in gaps, and in LG the effect was stronger than in SG. Plants grown in LG had a sunplant morphology, with a high root-shoot ratio (R/S), a high specific leaf weight (SLW) and a low leaf area ratio (LAR). Plants grown in SG or FU showed a shade-plant morphology, with a low R/S, a low SLW and a high LAR. Growth responses varied from species unable to grow in the shade but with strong growth in the sun, to species with relatively high growth rates in both shade and sun conditions. Shade tolerant species were able to grow in the shade because of a relatively high unit leaf rate. The pioneerCecropia had a high growth rate in LG because of a high LAR. Most species showed a complex growth response in which they resembled the shade intolerant extreme in some aspects of the response, and the shade tolerant extreme in other aspects.
Keywords: Biomass allocation; Canopy gaps; Seedling growth; Shade tolerance; Tropical rain forest.