XVI International Botanical Congess

We studied the interaction of leaf age and seasonal drought in the control of leaf shedding in two dry forest trees with contrasting phenologies: Bombacopsis quinatum, a deciduous, stem-succulent species, and Astronium graveolens, an evergreen, hardwood species. Experimental defoliations through the rainy season resulted in trees bearing leaf cohorts of 4 known ages at the onset of the dry season. Measurements of leaf water relations and gas exchange, and tree growth and phenology, revealed that the timing of leaf shedding is not directly related to leaf age. Leaf shedding occurred simultaneously in all treatments, irrespective of leaf age, and did not seem to be caused either by reductions in soil water content. Our results indicate that leaf shedding in dry forest trees is controlled by factors different from leaf age or seasonal drought.