The variability of 13C values was measured in leaf, stem and root tissues of several tree species growing in closed natural and plantation forests in the Luquillo mountains of Puerto Rico. Results confirm a significant decrease of <513C values from the tree canopy to the forest floor. The values measured in understory plants growing in gaps were not significantly different from the average for plants growing under the forest shade. Seedling leaf values tended to be more positive than those of saplings, probably reflecting the contribution of organic matter from the mother tree. Photosynthetic independence on the forest floor results in a reduction in ?13C value. Stem and root tissue values of seedlings and saplings were less negative than those of the leaves of the same plants. It is suggested that this difference results from the slower change in isotopie composition experienced by the woody tissue, as the seedlings become photosynthetically independent in the forest floor.