How arboreal are epiphytes? A null model for Benzing's classifications

Benzing (2004) classified epiphytes as either 'obligate', 'facultative' or 'accidental' to describe variability in the tendency of epiphytic plants to grow arboreally. However, no method currently exists to quantitatively categorize epiphytes according to this classification system. Here, I derive a null model to test whether epiphytes are obligate, facultative or accidental, and then apply it to local epiphyte assemblages in two similar latitude forests on either side of the equator to test for hemispherical asymmetries in the relative abundance of different functional guilds of epiphytes. Results from the null model showed that the northern hemisphere site was comprised mostly of facultative epiphytes, which usually occurred on the forest floor. Conversely, the southern hemisphere site was comprised mostly of obligate epiphytes, which rarely occur on the forest floor, if at all, supporting previous speculation that epiphyte communities are structured in fundamentally different ways in the northern and southern hemispheres.