Phorophyte type determines epiphyte-phorophyte network structure in a Mexican oak forest

Phorophytes are assumed to act as facilitators for epiphytes, but empirical evidence indicates that they also act as amensalists. While few epiphytes can colonize most phorophytes, some epiphytes (such as orchids) appear in few phorophytes and/or have low population sizes, suggesting that are strongly phorophyte-limited. We assume that phorophyte type must determine the structure of the interaction network between epiphytes and phorophytes. In order to identify phorophyte type, we studied their distribution of epiphytes and, using two methods for measuring interaction intensity (raw and standardized data), analyzed the structure of the epiphyte-phorophyte interaction network. We quantified and identified all of the epiphytes and their phorophytes in all of the woody plants (diameter at breath height ≥ 5 cm) found in five 0.1 ha transects (10 × 100 m) in an oak forest of central Mexico. Acacia farnesiana and Bursera copallifera were the preferred phorophytes with greater epiphyte loads than would be expected by chance, while Quercus magnoliifolia and Q. candicans were limiting, with less epiphytes than would be expected by chance. In the network analyses, the preferred phorophytes concentrated interactions. These included B. copallifera, which, despite being infrequent, had the highest intensity of interaction with the most abundant epiphyte (Viridantha atroviridipetala: Bromeliaceae), while the most abundant phorophyte (Q. magnoliifolia) had low interaction intensity. This study reinforces the increasing evidence showing that epiphytes can be phorophyte-limited and shows that the type and importance of phorophytes within the interaction network is not determined by their abundance.