Natural durability of fast-grown teak heartwood under tropical field exposure

The expansion of short-rotation Tectona grandis plantations has intensified the need to understand whether fast-grown teak heartwood can maintain adequate durability and service performance under tropical outdoor exposure. However, limited field-based evidence exists linking degradation patterns to end-use suitability. This study evaluated physical, mechanical, colourimetric, and chemical changes in 15-year-old plantation-grown teak heartwood after 12, 24, and 36 months of decay field exposure under tropical savanna conditions in central Brazil. While the total mass loss remained below 10%, indicating high durability according to standard classification criteria, the compressive strength parallel to the grain decreased by up to 44%, revealing that structural performance declined disproportionately relative to mass loss. These findings demonstrate that mass loss and visual ratings alone may overestimate service performance when mechanical integrity is critical. Colourimetric alterations were more pronounced in above-ground portions, with progressive reductions in redness and yellowness, reflecting weathering effects. In contrast, extractives, lignin, holocellulose, and NaOH solubility showed no significant variation, suggesting that early degradation primarily affected the microstructural organisation of the cell wall rather than the bulk chemical composition. Overall, fast-grown teak heartwood exhibited performance compatible with non-structural outdoor applications under tropical conditions, but caution is warranted for load-bearing uses in decay-prone environments.