Habitat partitioning of epiphytes within tropical tree canopies

Background/Question/Methods

The tropical canopy is one of the most diverse and complex systems on the planet. A single tree canopy can host up to 65 vascular epiphyte species, which raises the question of how these species coexist. The structural complexity of tropical tree canopies has resulted in a diversity of microenvironments created by gradients in light, temperature (T), vapor pressure deficit (VPD), and the amount of canopy soil that develops from the decomposition of organic material. The co-existence of such a high diversity of vascular epiphytes in tropical canopies may be through habitat partitioning along these gradients. To determine if epiphyte species distributions are related to environmental gradients, we examined the spatial distribution of environmental conditions including relative humidity (RH), T, and VPD along with species composition within the canopies of several individuals of different-sized Virola koschnyii trees at La Selva Biological Research Station in Costa Rica. 

Results/Conclusions

Species richness and abundance increased with tree size and was highest in the inner canopy of large trees. The inner canopy of large trees was significantly different in species composition and environmental conditions relative to the outer canopy of large trees and smaller tree canopies. Species composition of epiphytes in the outer canopy of large trees was more similar to the species composition of small tree canopies than to the inner canopy of large trees. RH was highest and VPD was lowest in the inner canopies of large trees where canopy soil was present. These data suggest that the inner canopies of large emergent trees host a very unique microenvironment that supports a distinct epiphyte community. The most abundant epiphyte species found in the inner canopy were those that require specific substrates, such as the presence of canopy soil, or particular environmental conditions, such as a low VPD. Species found in the outer canopy of large trees were not restricted by the availability of substrate as many of these species grew on bare bark and were, thus, found also in the canopies of small trees. Our research supports our hypothesis that epiphyte distributions are influenced by the gradients in substrate and environmental conditions that exist within tree canopies, and may explain the coexistence of a high diversity of species in such a unique environment.