population dynamics of epiphytes

Epiphyte populations are distributed in a three-dimensional space, which is defined by their supporting trees and presents substantial gradients of microclimate, resources, and substrate. As most epiphytes die when falling to the ground with a breaking branch, their population dynamics is strongly dependent on the dynamics of the trees and branches they grow on. Both, the different micro-habitats in the canopy, resulting in niche separation, and the high rates of disturbance, preventing dominance of a few species, are considered to be important contributors for their high diversity in tropical rainforests. While some aspects of epiphyte population dynamics have been studied in several species and matrix models have been developed for a few, none of these includes the complex relationships between tree zonation and dynamics on epiphytes and on their distribution within the canopy. The project aims to develop population models for three orchid and five bromeliad species in a population in a Mexican humid montane forest studied since 1992, partly drawing on previously recorded data. It will include studies of growth, mortality, flower production, pollination, seed production, and germination as related to the plants` position within the trees. Population genetic analysis will help to identify to what extent the spatial distribution is related to genetic distance and thereby inform about the geneflow within the population. Matrix and individual-based population models will be important tools to compare between species, to identify bottlenecks in a species` life cycle and to understand the complexities of the dynamics in rainforest canopies.

Epiphytes are characteristic plants of tropical rainforests, where they constitute a major component of plant biodiversity and also provide food and shelter for many animals living in the forest canopy. Since epiphytes grow on trees, without directly damaging their hosts, they are distributed in the complex 3-dimensional structure of the rainforest. Most species prefer certain locations on a tree. As any plant, to survive a population has to reproduce (which often depends on pollinators, the number of seeds produced, seeds have to land on a suitable surface, germinate and establish), and the number of plants dying (from abiotic stress, herbivores or because the branch the epiphytes grows on falls to the ground) has to be balanced by the number of new plants. Population dynamics, which describes the stages in a life cycle and analyses the factors responsible for population growth or decline, was studied for three orchid and five bromeliad species in a Mexican montane rainforest by tagging and measuring several thousand individuals and conducting germination and pollination experiments. Ecological theory suggests that leaves of plants living in an environment with strong abiotic stress, such as epiphytic orchids and bromeliads, would be little damaged by herbivores. This was confirmed, but the little biomass that was consumed in the form of flowers, developing fruits and the growing leaf tissue caused disproportionate damage to population, and substantially reduced seed production and increased mortality in some species. In general, epiphytes require many years to produce flower and if their mortality is high, for instance because thin branches tend to break easily, most plants die before being able to reproduce. It was found that species growing on thin branches, which tend to break easily, have adapted with a faster population cycle, investing less in vegetative growth and relatively more in fast fruit production, which makes sense on a labile substrate. Also, while the flowers of most species require pollinators (mostly bees or hummingbirds), those on exposed substrates with high abiotic stress are often self-pollinating, which ensures that seeds are produced even in the absence of pollinators. The strong microclimatic gradient and the differences between branches within the canopy affects many stages in the life cycle, including germination, survival, growth and nutrient relations. This not only helps to explain the observed distribution of epiphytes within the canopy, but should also provide us with tools to predict how changes in the biotic environment (herbivores, pollinators), the microclimate and macroclimate, and host trees (tree species differ in their suitability for epiphytes) can affect epiphyte populations. This is of substantial relevance for the conservation of this diverse group. On the one hand particularly montane rainforests have been and continue to be cleared at alarming rates and on the other several epiphytes are collected in large numbers and sold as ornamental plants, without considering sustainability of this harvesting. Without natural forests, many epiphyte species can survive in secondary vegetation such as shade trees of coffee plantations, but others appear to find no suitable refuge outside natural forests. Results from the project suggest that the survival of different species of epiphytes can be limited by different factors, some probably by the inability to successfully establish in a different microclimate, others because of herbivore pressure or lack of pollinators. Understanding which factors affect the survival of populations should help to manage secondary vegetation such as tree or coffee plantations in a way that ensures the survival of many, but probably not all epiphytic species in the area.