Epiphytic vascular plants

Epiphytic plants are by essence plants that are not self-supporting, that spend their entire life cycle on the support on which they germinate and that have no interactions other than physical with the supporting tree. There are other forms of plant that depend on a support to thrive, but which, unlike ‘true’ epiphytes (or holo-epiphytes), are grounded at some point in their life cycle: firstly, those that germinate on the ground and retain this connection while colonising the support (herbaceous or woody lianas); secondly, those that germinate on the support and later root to the ground (primary hemi-epiphytes); and finally, those that germinate on the ground and free themselves from this terrestrial connection at a later stage in their growth, with degeneration of the proximal part of the stem (secondary hemi-epiphytes). Some epiphytes can also be considered ‘accidental’ when these plants grow mainly on the ground and are only rarely seen in trees.

The structure of the epiphytic plant community in a tree is strongly influenced by vertical gradients in abiotic conditions (inter-crossing and partially opposing gradients in light, temperature, humidity and nutrient supply), but also by horizontal gradients, from the inner crown (shaded, older and less dynamic) to the outer crown (sun exposed, younger and more dynamic). Dependence on the supporting tree implies that epiphytes are also strongly affected by the three-dimensional structure of the tree and its own dynamics.

Generally speaking, there is a positive relationship between the abundance and species richness of epiphytes and tree size. This reflects both a temporal effect, as epiphytes are limited in their dispersal capacity, and a size effect: taller trees make larger ‘targets’ and also offer a greater diversity of micro-habitats. The large horizontal branches in the centre of the crown accumulate organic matter which is transformed into humus, an essential substrate for the development of certain epiphytic plants such as araceae and most ferns. Large, old trees also display greater environmental heterogeneity, from the forest floor to the upper canopy.

At the forest scale, the very uneven distribution of epiphytes, whatever the environmental conditions, is still poorly explained. A high proportion of ‘empty’ trees has been reported in various lowland forest sites in Asia, Africa and the Americas. Mountain sites are clearly different, although there is much less data available. But even on mountain sites, it is not uncommon to find a number of trees without epiphytes. This absence of epiphytes on many trees is partly due to the fact that certain tree species are poor hosts, such as those with smooth or peeling bark, limiting the possibility of establishment. But the main reason put forward by Gerhard Zotz[1] is the very slow colonisation of any tree by epiphytes. Large, old trees, offering their crowns to potential colonisation over many decades, generally host a disproportionate share of all the epiphytes present locally. Such a concentration of epiphyte abundance and diversity in large trees therefore has important implications for conservation, particularly in the current context of global decline in large and old trees.

[1] Plants on plants – The Biology of Vascular Epiphytes. Gerhard Zotz, Springer 2016.

Among the ‘true’ epiphytes inventoried on trees in the Life On Trees programme, two families largely dominate: orchids and, to a lesser extent, bromeliads. This should come as no surprise; orchids form a highly diverse family (25,000 known species) that dominates the world of epiphytic plants on tropical trees. Bromeliads are a family of flowering plants found only in South America in neotropical climates. Of the 3,200 species listed, almost half live as epiphytes. Other families of flowering plants also thrive on the trees studied, and around twenty are represented in our collections.

Another abundant group of epiphytic vascular plants is the pteridophytes. Pteridophytes refer to the artificial grouping of all vascular plants that produce and disperse by means of spores. They represent two of the six evolutionary lineages of terrestrial plants or embryophytes (the other lineages are liverworts, anthocerota, mosses and seed plants). These two lineages are the ferns, the sister group to present-day seed plants, and the lycophytes, the sister group to all other present-day vascular plants. Pteridophytes represent around 12,000 species worldwide, with maximum diversity at tropical latitudes. In tropical rainforests, pteridophytes abound both on the ground and as epiphytes from the base of trees tò the canopy. In some neotropical regions, pteridophytic diversity can account for more than 10% of the diversity of the vascular flora.

Unlike small plants, such as mosses or liverworts, the abundance and distribution of vascular epiphytic plants are not continuous on the trunk and branches. As a result, random sampling by quadrats – as carried out for mosses – is not suitable. As a result, we increased the number of surveys in all parts of the tree. The inventory is carried out without worrying about a predefined area in which to sample, but by maximising the collections in the entire tree, in order to see as we go along a decrease in the new plants collected compared with the previous days.