Many long-lived perennial plants fluctuate between reproductive and non-reproductive status among years. Additionally, certain long-lived plants are able to change from male to female phenotypes between reproductive bouts. Both of these changes in phenotype are influenced by amount of available stored resources and current environmental conditions. Sex switching at the individual level can also influence the population sex ratio and in turn the number of potential mates. We explored the effects of light environment, current and previous plant size, and previous reproductive effort on current season's gender and flower number in an epiphytic orchid, Catasetum viridiflavum. We found that current size and light environment have strong effects on reproductive status. Following onset of flowering, plants tend to continue to flower in subsequent years. However, how current and prior size influence reproductive status varies significantly among years. Female individuals were found to have higher photosynthetic rates than males, perhaps offsetting the construction and maintenance costs of the larger female flowers and longer floral display time. Since orchids experience both infrequent successful pollinator visitation and high costs of fruiting, labile sex determination may be one way of avoiding the maintenance costs associated with fruit production.

Key words: Catasetum viridiflavum , gender determination, reproductive ecology, sex ratio