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Four annual tree-rings (2 of juvenile wood and 2 of mature wood) were sampled from fast-growing plantations of Gmelina arborea in two climatic conditions (dry and wet tropical) in Costa Rica. Each annual tree-ring was divided in equal parts in a radial direction. For each part, X-ray density as well as vessel percentage, length and width fiber, cell wall thickness and lumen diameter were measured. Wood density and profile patterns of cell dimension demonstrated inconsistency between juvenile and mature wood and climatic conditions. The Pearson correlation matrix showed that intra-ring wood density was positively correlated with the cell wall thickness and negatively correlated with vessel percentage, fiber length, lumen diameter and width. The forward stepwise regressions determined that: (i) intra-ring wood density variation could be predicted from 76 to 96% for anatomical variation; (ii) cell wall thickness was the most important anatomical feature to produce intra-ring wood density variation and (iii) the vessel percentage, fiber length, lumen diameter and width were the second most statically significant characteristics to intra-ring wood density, however, with low participation of the determination coefficient of stepwise regressions.