The basis for obligate epiphytism in Tillandsia brachycaulos (Bromeliaceae) in a Mexican tropical dry forest

The basis of obligate epiphytism remains poorly understood. In this study we compare mortality, growth and fecundity for two populations of the epiphytic bromeliad Tillandsia brachycaulos in Yucatan. The fate of dislodged individuals of Tillandsia brachycaulos was compared with that of specimens anchored on tree bark in a dry forest in south-east Mexico. Annual percentages of dislodgement differed among location on phorophytes and epiphyte life stage, ranging from 1% on primary branches to 32% on quaternary branches, and from 7% for adult ramets to 64% for seedlings. Fallen versus attached individuals exhibited different mean annual leaf production, 18 and 21 leaves per individual respectively. The same pattern was observed for capsule (0 and 1.04 +/- 1.09 capsules per individual) and ramet production (0.01 +/- 0.08 and 0.54 +/- 0.68 ramets per individual). Annual percentage mortality was different for fallen (92%) and attached individuals (72%). The plants on the ground and those still anchored to hosts performed differently in ways that help explain why this bromeliad is an obligate epiphyte at the study site. More fundamental causes for its obligate status are also considered.