Abstract
Dioon merolae (Zamiaceae) is an arborescent, long-lived cycad, which has its trunks curved and inclined, possibly due to phototropism. It grows on shallow sandy soils on alluvial slopes of terraced rocky hills. We estimated the light environment of 144 adults in eight rocky hills using slope angle, hill height and distance, and distance from the plant crown to the top of the hill; we compared it with the null model of a non-photropic vertical plant. Most plants (64%), and those largest, occurred on northeast facing slopes; less (3%) and smallest on southeast facing slopes. Total and main stem length and volume fitted a quadratic regression (P < 0.013; R2 = 0.044-0.193). More female and male plants were associated to intermediate and high differences, respectively, between observed values of inclined plants and values with the model of a non-phototropic vertical plant. We propose that the spatial distribution and trunk inclination of D. merolae may be adaptive on light availability gradients.