Why do trees grow older in the wet tropics?
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It has been shown that trees get older in the wet tropics, but the mechanisms behind this observation are still not clear. Literature shows that moisture could directly affect longevity by modulating physiological processes of trees. Other studies point to the potential role of water on tree size, which could indirectly affect longevity if one reconciles to the fact that the growth rate of tropical trees doesn’t differ significantly between wet and dry sites. The third group points to the potential role of light competition in the understory of tallest tropical forests where trees may take decades to reach the canopy. Although these hypotheses are intrinsically linked, we evaluated the most critical mechanisms that result in long lifespans in the wet tropics using a tree-ring dataset of 22 population and 450 trees of Hymenaea spp. across South America. We used disturbance analysis to assess the age of the trees during major canopy or gap releases and tested the potential role of precipitation on tree longevity using Path Analysis. The oldest analyzed tree was 372 years old, and it only showed a major release at 237 years old and 34.1 cm of DBH in the tall forests of the eastern Amazon. The path analysis reveals that the direct effect of precipitation on tree longevity is rather weak as is the indirect effect of precipitation mediated by tree size. Still, precipitation significantly modulate canopy height and trees remain longer in understory of the tallest forests. The time to the canopy or gap release is then strongly associated with tree longevity at the site level. Thus, competition for light and the time spend in the understory seem to be the most critical mechanisms resulting in older trees in the wet tropics.