Our study is the first to suggest that lianas may have an impact on plant species other than trees, namely epiphytic ferns. Previous studies have shown that lianas can negatively affect their tree hosts, especially in disturbed forests where they become hyper-abundant (e.g. Putz 1984; Laurance et al. 2001; Schnitzer & Bongers 2002, 2011; Paul & Yavitt 2011). In disturbed environments, lianas may affect trees negatively by loading heavily on their crowns (Putz & Mooney 1991), competing for light, water (Pérez-Salicrup & Barker 2000; Pérez-Salicrup et al. 2001) and soil nutrients (Schnitzer et al. 2005), reducing tree growth and reproduction (Lowe & Walker 1977; Putz 1984; Schnitzer & Bongers 2002), suppressing tree regeneration in canopy gaps (Schnitzer et al. 2000) and accelerating tree turnover rates (Laurance et al. 2001). Our analysis suggests that following disturbance, trees become an increasingly aggregated resource for climbing and epiphytic plants, grouped in clusters of roughly 2–3 m in diameter, particularly dense in forest interiors. The lower numbers of trees and the large increases in liana abundances in edge environments (twice as abundant as in the interior), together with the probable increase in liana re-sprouting near edges in response to increased light levels, is apparently translated into clumped distributions of lianas and epiphytic ferns. The lower abundances of epiphytic ferns in edge environments may very well reflect a response to abiotic drivers such as humidity, which is lower in these areas. However, the presence of ferns within plots with less canopy cover suggests this is not the only feature determining their distribution, and that a lack of suitable hosts for colonization is also limiting their distribution. Indeed, our finding that in edge plots, epiphytic ferns were less abundant near lianas (within ca. 1 m), suggests the existence of a high interspecific competition between them. Certainly, the prodigious re-sprouting ability of lianas enhances the creation of massive clumps of them around small groups of trees (Benítez-Malvido & Martínez-Ramos 2003). This, coupled with their high specific leaf area, allows them to allocate large amounts of canopy leaves above their hosts, competing aggressively with trees (Schnitzer & Bongers 2011) and leaving relatively little space available for other lianas and epiphytic plants. And such a pattern is supported by our data at distances <2 m. Specifically, we found a clustering of host trees with high liana loads at edge plots, which is probably leading to a high intra- and interspecific competition at local scales. However, although epiphytic ferns were largely excluded from using trees colonized by lianas in our study area, the reverse could actually be true as in certain other systems, where ants inhabiting epiphytic ferns have been found to actively exclude lianas from attaching to their host trees (Tanaka & Itioka 2011; see also Fayle et al. 2011).
Our results suggest that lianas with tendril climbing mechanisms are more abundant than the other guilds. Although tendril climbers are generally thought to require smaller (<10 cm diameter) supports (Putz 1984), many species found in our study area initially use their tendrils to attach to small trees but, when fully grown, drop their tendrils and sometimes twine completely around the branches or trunks of trees. Lianas bearing these kinds of climbing mechanisms are probably the first to colonize small trees, acting then as facilitators for other lianas to further colonize host trees and contributing to the creation of clumps of lianas (Pinard & Putz 1994). Therefore, re-sprouting and potential facilitation processes between types of liana seems to create groups of trees with large amounts of lianas in close vicinity, hindering the colonization of epiphytic ferns, especially so near forest edges.
A key caveat of our study is that we measured morphological plant guilds rather than individual species (see Fayle et al. 2009 for differences in the distribution of species of Asplenium ferns). However, we observed a clear pattern of fern displacement in relation to trees with lianas, despite the large spatial heterogeneity of lianas in edge plots. This strong response (especially apparent when analysing the presence of epiphytic ferns in host trees with lianas) would probably be even stronger if species were analysed separately. Larger sample sizes for both ferns and lianas would be needed to assess species-specific analyses, and such studies with a taxonomic component will help in understanding the mechanisms behind the patterns observed in this paper.
In summary, our results suggest important differences in the composition of epiphytic and climbing plants between the edges and interiors of forest fragments in our study area. Edge ecotones seem to be the domain of lianas with detrimental effects upon epiphytic ferns. The vital importance of ferns in tropical forests in Australia (Cummings et al. 2006) and their apparent decline in fragmented forests might have cascading impacts on animals and plants that depend on them.