Long-term community dynamics in vascular epiphytes on Annona glabra along the shoreline of Barro Colorado Island, Panama

Despite the ecological importance of vascular epiphytes in the tropics, even basic information on the processes that form epiphyte communities is scarce. This is partially due to an almost complete lack of long-term studies. Here, we present data that were collected in three consecutive censuses over 21 years in a monospecific host tree system that is about 100 years old and interpret them in an empirical and a metapopulation/metacommunity framework. We documented an ongoing increase in abundance and diversity at the level of the entire system (metacommunity) and at the level of communities (all epiphytes growing on individual Annona glabra tree stands). This was also reflected at the population level; >2/3 of the species showed positive population growth rates. Similarly, colonisation of empty Annona trees was still ongoing, with the most abundant species being also the first to colonise empty trees. The community composition of colonised trees became more similar with time. In all three censuses, habitat size of epiphytes explained much of the variation in species richness among stands (groups of trees with individual trees within 25 m of each other) and overall abundant species tended to be more abundant at the stand scale. The species frequency distribution was unimodal, regardless of whether considering all stands or only stands with very large epiphyte communities, indicating that the metacommunity still has not reached an equilibrium phase. Similarly consistent in both census intervals, population growth rates decreased with increasing population size with substantial asynchrony among populations. Synthesis. In contrast to a typical herbaceous plant community, this epiphyte metacommunity is still expanding after one century of development. Our study shows that previously reported short-term patterns of epiphyte community dynamics hold true in the long term. It remains to be shown, however, whether results from this monospecific host tree system are really representative for epiphyte community dynamics in more diverse tropical forests.