Fine partitioning of epiphyte habitat within Johansson zones in tropical Australian rain forest trees

For over three decades, the Johansson zones have been widely used in epiphyte studies as a way of stratifying the host tree into habitat zones. The usefulness of this system, however, has been questioned. We test the effectiveness of the Johansson zones by grouping epiphyte species by the substrate and microclimatic attributes of their individual occurrences and assessing the fidelity of these groups to the Johansson zones. Habitat characteristics were recorded for every individual epiphyte on 30 trees in the lower montane rain forests of northeastern Australia. Twenty-four epiphyte species were agglomerated into four groups using Ward's method. Group 4 was highly distinct and included shade-loving species and nomadic vines from the lower zones of the host trees. Group 3 contained species from the most exposed habitats. Group 1 had higher light levels and lower substrate thickness than Group 2, yet both groups had close to identical distributions over the Johansson zones. This suggests that groups of epiphyte species may utilize different micro-sites within the same zone. While the Johansson zones are a useful tool in epiphyte studies, finer partitioning of habitat within the host tree may be missed.