The distribution of vascular epiphytes over gradients of light and humidity in north-east Australian rainforest

Microclimatic conditions have a strong influence on the distribution of vascular epiphytes, among which orchids often occur in sunnier and more drought-prone situations than ferns. However, very few studies have looked at the distribution of ferns and orchids in Australian tropical rainforests. By using transmitted light measurements at the locations of individual epiphytes and vapour pressure deficit from the canopy and base of host trees, we were able to determine the patterns of light and humidity in the rainforest environment, and the responses of ferns and orchids to variation in the physical environments. We surveyed five sites, ranging from 800 to 1180 m in elevation in the lower montane rainforests of north-east Australia. Data loggers recorded the vapour pressure deficit (VPD) at the forest floor and canopy of each site. Light was correlated with height within the host tree and VPD differed significantly over position in the host tree and elevation. There was a strong partitioning of taxonomic groups over the light and VPD gradients. Orchids occurred in environments that had higher mean light levels and mean daily maximum VPD (27% and 0.43 kPa, respectively) than ferns (21% and 0.28 kPa). There was also strong microclimatic partitioning of species within taxonomic groups, suggesting that microclimatic factors play an important role in the realized niche spaces of epiphytes within the tropical Australian rainforest. Thus, the tested ecological generalizations made on tropical rainforest epiphytes apply in Australia.