Skyrail Nature Diary: January 2013

This month we’ll be looking at the life of the unusual plants known as hemi-epiphytes. These interesting plants have a most unusual life cycle. Most plants stick to one lifestyle throughout their lives whilst hemi-epiphytes go through two. One of the lives is always spent as an epiphyte, either at the beginning or later in life. An epiphyte is a plant that grows on a tree to have easy access to sunlight without having to start from the forest floor. They don’t harm their host, which sets them apart from a parasite. A parasite harms its host by taking something from it. A good example of a plant parasite is the mistletoe. An epiphyte however, does not benefit its host either. Too many epiphytes on a single tree can cause it to be a bit top heavy and topple over in windy conditions.

A hemi-epiphyte changes its lifestyle at some point. Usually the epiphytic stage turns into a tree or a vine. Here in Australia there are no good examples of a hemi-epiphyte that becomes a vine after spending time as an epiphyte. In the Amazon there is a relative of the Native Mangosteen, the Clusia Vine, that starts life as an epiphyte on the middle of a horizontal branch before dropping the root to the ground and becoming a thick, woody vine in the process. Hemi-epiphytes that become trees here in Australia include Strangler Figs, The Umbrella Tree and Cape Jitta. Sometimes a hemi-epiphyte starts out as a vine and becomes an epiphyte. The best example of this in Australia is the Native Monstera. So why does this lifestyle exist? A possible answer is to reap the benefits of being an epiphyte without all of the hazards and trials an epiphyte normally has to put up with. A Strangler Fig can grow as a normal tree if it wins the race for the light, which is a rare event. This has happened once at Skyrail's Red Peak station. If the fig starts life as an epiphyte on the other hand, this low survival rate is in part, missed. The future tree would already be in the light and avoid the tedious race all other canopy trees have to endure.

Strangler Figs make up about a tenth of all of the figs of the world. They start life as a seed encased in a closed structure know as a head. Since the seeds are on the inside of the fig, which is actually made up of many fruits, and the figs have separate sexes, the figs need help to reproduce. This is achieved by being in a symbiotic relationship with a special kind of wasp known as fig wasps. These wasps are tiny and hatch from eggs laid in special gall flowers inside the fig. The females generally hatch in a male fig and have to pass over the stamens on the way out, picking up pollen in the process. The female wasp flies to a female fig and forces her way inside, usually losing her wings and antennae in the process. They deposit the pollen by walking over the female flowers and proceed to mate with the resident blind and flightless male. The eggs are laid in the special gall flowers provided for this purpose. When the fig ripens, birds and flying foxes eat them and expel the seeds throughout the canopy. Those that land in a Basket Fern often germinate in the thick compost and grow as an epiphyte for a few years. When they reach a certain size they send their roots down the sides of the tree trunk, fusing as they go. The tree tends to give up after a few years due to a combination of lack of sufficient light, no space to grow thicker and lack of sufficient minerals. If the Strangler Fig survives (some species turn into banyans), stranglers that kill more trees and have multiple trunks as a result. The world’s largest Banyan covers a massive 20 acres of land!

Native Monstera is an unusual hemi-epiphyte that starts life as a vine. The vine climbs toward the canopy using roots that cling to the tree trunk. When the vine reaches the canopy, the roots are incredibly severed and the vine continues life as an epiphyte. During very dry spells, the roots are occasionally temporarily re-established until wet conditions return. The leaves are large and lobed. The flowers start out as a small leaf that turns yellow and roll around a central spike like a sarong. When the flower is fertilized, the flat, hexagonal fruits ripen along the stalk.

By Tore Lien Linde