Adaptations to Light

Skyrail Nature Diary: February 2014

This month we will be looking at various adaptations to differing light conditions in the rainforest. There are four areas of interest here - the forest floor, the sub-canopy, the canopy and the forest edge.

Plants on the forest floor are few and far between. At Red Peak the two most obvious are Native Wishbone Fern and the Robber Fern. Both of these ferns prefer moist areas and can tolerate as little as 1% sunlight for their growth. The leaves contain lots of extra pigments which allows them to absorb more from the light spectrum than many other plants. Growth is not surprisingly, rather slow in the dim conditions. These ferms frow on rocks and tree trunks. 

Native Wishbone Fern is very similar to Boston Wishbone Fern which is a popular indoor plant around the world. Native Wishbone Fern is a lot more tolerant of low humidity levels than its better known relative and as such, is more suited as an indoor plant. Red Peak seems to be this fern’s most southerly distribution in Australia. They are incidentally also found in the Kimberley district of Western Australia where they are known as Kimberley Queen Fern. The Robber Fern also grows on rocks and tree trunks. The ones that grow on tree trunks are noticeably much smaller than those that grow on rocks, possibly due to the lack of moisture. When exposed to light, like the specimens next to the King Orchid at Red Peak, the leaves grow quite large and take on a bleached appearance. Of course, tree saplings make up a significant proportion of the vegetation on the forest floor, but they never have any intention of staying down there.

The sub-canopy is the area just under the topmost trees. The light levels are often around 10% of the light levels in the canopy itself. The leaves of plants that prefer this area have very dark green leaves due to increased levels of chlorophyll and various other pigments. Most vines only stay in this area temporarily with the exception of the Candleberry. These unusual small vines have leaves that look like candles, the flame being the actual leaf. The fruits of these vines are clusters of red berries. Glossy Laurel is a very common small tree which loves growing in the shade. The leaves are dark green and glossy with the veins shaped like a pitchfork. The flowers are tiny and barely noticeable and are followed by wrinkly red drupes that are laced with a strong nerve toxin. These handsome trees only grow up to seven metres.

Another typical subcanopy tree at Red Peak is the unusual Lethedon Tree. This tree has thick dark green leaves with a hairy underside reminiscent of corduroy. The flowers are tiny yellow saucers that mainly attract clumsy beetles. The fruits most of all look like miniature hairy pumpkins and have irritant hairs. The name Lethedon means “forgotten gift” and refers to the fact that the original specimen was left in the herbarium in Brisbane for many years before a botany student finally examined it and found out that it was a kind of missing link between the world’s first flowers and the more modern ones of today. Lethedon grows up to 15 metres and is a nice looking tree.

Another primitive tree in the sub-canopy is the Winterwood. The thick oblong leaves have a layer of white wax on the underside and reddish bases. The fruits are black berries. Despite being the second most primitive flowering plant in the world, it is very common in Barron Gorge. They grow up to 12 metres.

The canopy is the most obvious part of the rainforest for us here at Skyrail. The trees growing here receive the full brunt of the sun’s energy for better or worse. In other words they have enough sunlight, sometimes too much. Several trees in this zone have adaptations that limit reflection that can cause dehydration. Often the leaves point upward and are smaller than the leaves of their saplings for this reason. Kauri and Rose Silky Oak are the two most obvious examples of this strategy. Kauris have medium sized dark green leaves when growing in the sub-canopy that shrink and bleach when reaching the canopy. The leaves are arranged in several clusters that point upward. This is also the area where the cones are borne. Rose Silky Oaks have large, leathery dark green leaves with massive lobes when growing in the sub-canopy. When they reach the canopy the lobes disappear and the leaves take on a lance shape with rounded edges that all point upward.

Edge plants are generally fast growing shrubs or trees that have relatively short life spans. They produce huge amounts of seeds that respond to sudden strong light by germinating in large numbers. These plants have simple structures and flat canopies. Referred to as pioneers, these plants are important in the early stages of the re-vegetation of disturbed areas of rainforest. Sooner or later however, they are shaded out by longer lived climax trees, often referred to as Oscars. This is one of the reasons they are generally only found at the edges of well -developed rainforests where there is no obvious disturbance. Examples are numerous; Celerywood, Pink Ash, Brown Macaranga, Red Kamala, Mango Bark etc.

By Tore Lien Linde