More stable forests promote higher epiphytic diversity
Epiphyte life span depends directly on the stability of the host tree. However like many clonal plants, epiphytes have the capacity of regrowth and to perpetuate in the forest. Moreover, epiphytes are very sensible to changes in humidity. Climate changes impact the epiphyte dynamic directly affecting environmental conditions and indirectly affecting forest (tree) dynamics. Specifically, we addressed the following questions: how does the epiphytes mortality rate change spatially? and how are these changes related to climatic factors? Particularly we examine whether mortality rate influence epiphyte species richness and evenness. Vascular epiphytes were studied in 10 sites spanning 2.000 km2 in the north of the Colombian Andes, with changes in elevation from 50 to 3.000 m. Sites are inventoried 1 ha plots where random trees were selected until reaching 35 trees hosting epiphytes. Epiphytes were divided in four contrasting growth forms: rosettes, true-epiphytes, nomads, treelets. More than 7.000 individuals (708 species) were recorded and monitored a year later. Causes of mortality were classified as mechanical (fallen branches and trees, and detached plants) and in situ factors.
Our results showed that mortality is relatively continuous, ranging from 5% yr-1 to 25% yr-1. Mortality caused by mechanical factors was three times superior than in situ factor. True epiphytes and nomads growth forms were particularly more vulnerable than others growth forms. Mortality causes were differentially by growth forms. For treelets and true-epiphytes more than 75% of their mortality was due to mechanical factors, while for rosettes and nomads, it represents less than 70% of their mortality. Multiple regression analysis indicated that climatic, more than forest structural variables, were related with total mortality and mortality due to mechanical factors. There was no relationship between these variables and in situ mortality. Epiphytes richness was negatively correlated with mortality rates.
High temperature and evapotranspiration rates seem to affect forest dynamics and indirectly climatic factors act as a filter for species and growth forms. Our results indicate as well that more stable forests promote higher epiphytic diversity contradicting intermediate disturbance hypothesis. These findings suggest that epiphyte species with slow life cycles have a higher risk of extinction because their mortality is highly conditioned by falling branches and trees. We predict that the increase in temperature and frequency of extreme events in the next years will progressively increase mortality rates for epiphytes.