In the gloom of the rainforest: Plant survival

Skyrail Nature Diary: April 2011

In an undisturbed rainforest, the amount of light that reaches the forest floor can often be as little as 1%. Very few plants can survive let alone grow under such conditions.

Most plants survive on a chemical process called photosyntheses (a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight). Some plants that live in the under layers of the rainforest floor amazingly bypass this process, getting all their nutrients from other plants, two of which include the Balanophora (Balanophora fungosa: Balanophoraceae) and some little known orchids.

The Balanophora is a plant parasite (causes harm to its host) that for most of its life resembles a mat of threads living underground, feeding on the sap of tree roots. At certain times of the year they produce mushroom-like structures that harbour the flowers. Its structure looks like a soft acorn and has a pink bulb. The bulb is covered in female flowers, while the collar underneath is covered in male flowers. Ironically the flowering structures have a ring of leathery brown leaves at the base, remnants of a photosynthetic past.

Ground orchids are an unusual species of plant. They are completely yellow and have no chlorophyll (a green pigment found in almost all plants). Another unusual characteristic is that they use fungus as their host; it's usually the other way around. Ground orchids have double onions that are toxic underground. Leaf litter is their source of minerals and is the main reason rainforest trees are able to start out as saplings on the ground.

Ferns are the most common plant, apart from seedlings and saplings, that grow in the gloom of the rainforest. All ferns have underground stems and have two stages in their life cycles - one asexual and one sexual. The asexual stage, the sporophyte, is what we associate with the visible fern itself. The sexual stage, the gametophyte, is a tiny heart shaped plant, virtually hidden from view, both male and female and short-lived. Ferns don't have seeds and reproduce with spores.

The Robber Fern (Pyrrosia rupestris.: Polypodiaceae) and the Native Wishbone Fern (Nephrolepis obliterata: Lomariopsidaceae) are commonly found in the Wet Tropics Rainforests of Tropical North Queensland (Australia). Another common fern that loves shady conditions is the Black Tree Fern (Cyathea rebeccae: Cyatheaceae). In shade they grow as skinny tree-like plants, up to 5m reaching for the light. When growing in the open, they tend to clump and look more like a collection of ferns rather than a tree.

The most impressive fern growing in the Barron Gorge National Park (Tropical North Queensland, Australia) is the King Fern (Angiopteris evecta: Marattiaceae). Apart from being one of the most primitive ferns still around, they are credited with having the world's largest leaves. The leaves sometimes grow up to 7m, however heights of up to 3m is more commonly encountered in the rainforest. This fern grows its leaves at ground level.

Tulipwoods (Harpullia spp.: Sapindaceae) prefer shady conditions and usually grow as bushes or small trees. At least two of the known species have winged leaf stalks, the wings being fleshy growths on the sides of the leaf stalk. The tiny, inconspicuous flowers are terminal, meaning they grow on the very top of the plant. The fruits are fleshy pods, coloured orange or red, with up to four seeds. The open pods have a slight resemblance to a butterfly.

Candleberry (Pothos longipes: Araceae) is an unusual vine that doesn't require the full, harsh exposure to the light at the top of the canopy. Just below the canopy, light levels can reach 10%, which is just enough to allow the Candleberry sufficient growth. These vines have small leaves that look like candle lights. The Candleberry climbs trees using adventitious roots. The flowers are tiny and inconspicuous and its berries are red and have a slightly spicy aftertaste.