Defoliation and ENSO effects on vital rates of an understorey tropical rain forest palm

1. Rain forest understorey plants suffer leaf area losses due to natural causes or when leaves are harvested as non-timber forest products. The negative effects of defoliation on plant fitness can be exacerbated during periods with strong water shortage and high temperatures, typical during ENSO (El Niño Southern Oscillation) years in Mexico and Central America. At present, the isolated and combined demographic effects of ENSO events and repeated defoliation on tropical rain forest plants are poorly understood. 2. We studied the consequences of repeated defoliation and an ENSO event on vital rates (mortality, growth, and reproduction) of the dioecious understorey palm Chamaedorea elegans. From March 1997 to March 2000 (including the 1998 ENSO year), we subjected 814 mature individuals to one of five defoliation treatments (0-100% of newly produced leaves were removed twice a year), recording mortality, growth (leaf production) and reproduction (inflorescence and seed production) every 6 months. 3. Increasing defoliation strongly reduced reproduction but had smaller effects on growth and mortality. Among non-defoliated palms, the probability of mortality increased with light availability, likely due to drought stress during the dry season, but this was not the case for the defoliated plants, probably because leaf area removal lowered transpiration and increased the root mass-to-leaf area ratio. During the ENSO year, growth and inflorescence production were stimulated, but survivorship and seed production diminished significantly, independent of defoliation level. 4. Synthesis. Variation in light availability and the occurrence of severe droughts can strongly affect demographic behaviour of understorey plants such as C. elegans, significantly influencing the effects of defoliation. Thus, strong episodic disturbance events (such as ENSO, insect outbreaks, strong storms, fires and landslides) should be taken into account to adequately understand the mechanisms that determine the population dynamics of forest plants and the potential for sustainable utilization of non-timber forest products.