Epiphytes are characterized by their structural dependence on other plants to persist high in the forest profile and by their ability to survive without a root connection to the ground. Although epiphytes constitute one-third of tropical vascular plant diversity, the ecological trade-offs of this unique aerial growth habit remain largely untested experimentally. Mortality due to falling from the host tree may be an underappreciated driver of epiphyte community dynamics, because less than 30% of fallen epiphytes survive even a year. However, little empirical evidence exists for whyepiphytes do not survive when forced to become terrestrial. Moreover, the variation in mortality rates of fallen epiphytes among epiphytic taxonomic groups, and among strata groups, remains relatively unexplored. Here, we experimentally test two hypotheses regarding the drivers of epiphyte mortality in a cloud forest of central Panama. We test whether simple contact with terrestrial soil is deleterious to epiphytes, preliminarily testing the Epiphyte Enemy Escape Hypothesis. We also test the Vertical Niche Differentiation Hypothesis, wherein epiphytes are specifically adapted for microsites throughout the vertical forest strata. By monitoring leaf loss, health status, and mortality of 270 transplanted epiphytes for nine months, we pinpoint the extent to which distance from the ground regulates epiphyte mortality.
Results/Conclusions
We found support for both of our hypotheses. After nine months, leaf abundance was 40% lower in epiphytes that were in contact with the ground in comparison to those transplanted onto the same trees just few meters above the ground. Likewise, epiphytes in contact with the ground had a 26% lower survival rate versus epiphytes transplanted higher. Mortality rates were also mediated by original height, where the epiphyte occurred in the host tree prior to transplanting. Our results demonstrate that contact with the terrestrial soil regulates early fallen epiphyte mortality, and epiphytes that occur high in the canopy are particularly vulnerable to mortality via falling. These results have important implications both for fundamental ecological theory and for biodiversity conservation management. Follow-up studies should explore the role of terrestrial soil microbes as potential drivers of decreased grounded epiphyte survival. Moreover, the high success and survival rate of epiphytes transplanted above the soil surface highlight an overlooked management technique: moving fallen epiphytes to nearby trees could help increase establishment or population stability of epiphyte communities. This would be particularly useful in relatively epiphyte-poor secondary forests and small forest fragments.