Population dynamics of epiphytic bromeliads: Life strategies and the role of host branches

Epiphytes need to balance the trade-off between long-term survival and colonizing new branches in their resource-limited and high-risk canopy habitats. We censused populations of five bromeliad species over two years and calculated matrix population models to determine the key factors influencing population growth rates. Additionally, logistic regression models were applied to study how demographic features change with microsite characteristics and disturbance. The bromeliads studied are slow-growing, with species preferring thinner branches maturing faster. Population growth rates were below unity in all species except for the drought-resistant Tillandsia juncea. Population growth rates depended almost exclusively on survival, above all of adult plants. Fecundity had only little impact, even in species of the outer canopy where branchfall-related disturbance and mortality are frequent. Survival rates and the probability to become reproductive increased with light availability in most species. Microsite characteristics had the greatest impact on seedling survival, although this contributed very little to population growth rates. We conclude that branchfall-related mortality is a key factor for population persistence of epiphytes dwelling in the outer canopy, and that resource availability constrains the possibility to counteract disturbance with higher fecundity. For species of the outer canopy an increased disturbance by more frequent strong winds could further constrain population growth. Because population persistence strongly relies on the survival of adult plants, harvesting bromeliads for ornamental or other purposes should be restricted to immature individuals. In the future, better models of epiphyte populations in a habitat with highly patchily dynamics should be based on a metapopulation approach.