Long-term population dynamics of the epiphytic bromeliad, Werauhia sanguinolenta

The population dynamics of the epiphytic bromeliad, Werauhia sanguinolenta, growing in the moist tropical forest of Barro Colorado Island, Panama, was studied for seven years from 1997 through 2004. In contrast to the generally held notion of the great importance of moisture availability for growth and survival of vascular epiphytes, no demographic process showed a significant correlation with the amount of annual precipitation or the varying number of rainy days per year. Most deaths, for example, were rather related to substrate instability (tree falls, branch breakage, or flaking bark) in all but the smallest size classes. We found evidence for both competition and facilitation. Elasticity analysis revealed that the finite rate of population increase, which invariably exceeded unity, was mostly influenced by survival (stasis) and to a lesser extent by growth, and very little by fecundity. In contrast to earlier reports on disastrous outbreaks of herbivores in this epiphyte species, the long-term impact of herbivory on the population dynamics of W. sanguinolenta was negligible. Being at least facultatively autogamous, reproduction seems to be controlled by resource availability alone: this is suggested by long intervals between reproductive events, and a decrease in size and an increased mortality after reproduction. We conclude that the demography of this epiphytic bromeliad is probably influenced at least as much by biotic factors (i.e. the dynamics of the substratum) as by abiotic limitations.