Abstract
Psychotria marginata, a shrub common to humid tropical forests in Central America, produces leaves in two seasonal flushes with contrasting morphology and physiology. Leaf production is bimodal with a major peak at the beginning of the annual wet season and a secondary peak at the end of the wet season. Together these peaks account for 75-87% of annual leaf production. Leaves produced just prior to the dry season have higher specific mass and, during drought, have lower stomatal conductances and higher water-use efficiencies. Plants irrigated during two consecutive dry seasons continued to produce leaves with these morphological and physiological differences, indicating that this feature has been strongly canalized by some factor other than water availability in this highly predictable yet seasonal habitat. The bimodal leaf production results in acclimation to changing conditions through co-occurring leaves that lack the physiological plasticity usually associated with acclimation. Heterophylly of this form suggests that the moderate drought of the dry season has been a significant selective factor for understory plants.