The fight for light

Skyrail Nature Diary: October 2010

A rainforest's canopy is extremely dense, comprising of many different layers. Because of this, plants have adapted intriguing processes in order to reach sunlight for survival. Hemi-epiphytes are a select group of unrelated plants that have developed a unique way of doing this.

Hemi-epiphyte plants spend part of their life-cycle as epiphytes. An epiphyte is a plant that lives on top of a tree to reach the sunlight without intentionally harming its host. Examples are orchids and several species of ferns. The rest of their life-cycle is lived as either a tree or a vine.

Those plants that grow as trees start out as epiphytes in over ninety percent of cases. The remaining ten percent start out as trees and remain that way. Those that grow as vines normally start out as vines and become epiphytes. An exception is the Clusia Vine (Clusia sp: Clusiaceae) from the Amazonian rainforest in South America that starts as an epiphyte on the middle of a large branch and becomes a snaky vine.

Birds are often the main disperser of these unusual plants, eating their fruits and spreading them about while on the wing. The ones that become trees usually need a pile of compost in order to germinate, which is usually a large fern in most cases. The Basket Fern (Drynaria rigidula: Polypodiaceae) is a good example of this (image 1).

Strangler Figs (Ficus sp: Moraceae) are without a doubt the best known hemi-epiphytes (image 2). Of the world's 500 species of figs about 50 are stranglers. Strangler Figs kill other trees to take their place. This strategy is so successful that in some places nearly twenty percent of the area's biomass is made up of figs.

Figs have separate sexes and rely on special symbiotic wasps for their pollination. Female wasps hatch inside the male fig and gets coated with pollen from the enclosed male flowers. They leave the fig and seek out the female figs. Once inside, they deposit their load of pollen before mating with the flightless male wasps. They then lay their eggs in special gall flowers. Birds eat the ripe figs and spread the seeds around the canopy.

The fig grows as an epiphyte for a few years until it gets too big. It then sends its roots down the host tree's trunk and fuses all the roots into a confining cage, slowly cutting off the host tree's circulation. Less than ten percent of the figs are strong enough to support themselves once the tree rots away.

Umbrella Trees (Schefflera actinophylla: Araliaceae) start their lives as epiphytes in large epiphytic ferns on top of the host tree (image 3). When the Umbrella Tree gets too big for the fern it sends its roots down along the host tree's trunk and wraps around it. Unlike the strangler figs the Umbrella Tree only rarely kills its host tree, most likely due to its relatively short life span. The roots are quite invasive and have been known to block drains, sewerage pipes and water lines in their quest for water. The umbrella shaped leaves and the multiple trunks covered in triangular scars make these trees easy to identify. The flower stalks occur in whorls on top of the tree and produce copious amounts of nectar. The bark was used by the Djabugandjii as a poultice for infected wounds, while pulped trunks were used to stun fish.

Native Monstera (Epipremnum pinnatum: Araceae) is an unusual vine found in small numbers in Barron Gorge National Park (Queensland, Australia). They have strongly lobed leaves and climb the host tree with adventitious roots found in regular intervals along the stem. When the vine reaches the light on top of the tree canopy, they incredibly sever the root and continue on as a trunk epiphyte. During periods of drought, the root reforms again until the rain returns when they sever yet again. This hemi-epiphyte's fruit is edible, but unripe fruit will burn the mouth and throat.