In tropical seasonal semideciduous forests (TSSF), plants face alternating drought or light shortage in dry and rainy seasons, respectively. To understand how emergent trees deal with seasonality across ontogeny, we sampled all individuals of the widely distributed species Astronium graveolens Jacq. (Anacardiaceae) in 100 random plots (10 m x 10 m each) in a TSSF fragment in southeastern Brazil, and conducted greenhouse experiments on photosynthetic and gas exchange responses to drought and shade. We recognized six ontogenetic stages by external structures, and analyzed height-diameter allometry and biomechanical design across ontogeny. Trade-offs were inferred in early life (increasing height, diameter, or leaf number implied Smaller leaves; height growth implied less leaf production) and in mid-life (height or diameter growth implied no branch production). The population did not adjust to a biomechanical model, implying a growth pathway between pioneer and canopy species. All stages were biomechanically stable, and the safety factor decreased with height. In adult trees, the asymptotic height indicated water limitation. In young plants, drought did not affect gas exchange. Light-shadow experiments suggested a trade-off between shoot and root growth. Our results indicate how a light-demanding emergent tree balances trade-offs across ontogeny to deal with seasonally alternating water or light shortage.391207218