Decomposition in the Rainforest

Skyrail Nature Diary: May 2012


April this year seems to have followed the "rules" regarding weather. The temperatures were slightly down and the rain has ceased. This means that there will be fewer flowers than we have had so far in the rainforest. Even fruits are less common on the trees. There are some pleasant surprises however. The Northern Sassafras is perhaps the nicest one. These trees grow up to 40 metres and may have buttressed roots. The leaves are dark green and glossy with serrated edges and a distinct intramarginal vein along the leaf edge. The flowers are tiny, white and arranged in small almost inconspicuous clusters. Both the crushed leaves and the flowers have a very pleasant aroma and the timber also smells quite nice. The timber resembles that of the "real" sassafras and is a useful if dull short grained timber. These trees tend to grow along creeks and in gullies and are not found on ridge tops and are the food plant for the larvae of Macleay’s Swallowtail and Blue Triangle Butterflies. The Black Wattle is still flowering in small numbers along the cableway. The dull yellow flowers are borne on short racemes. The fruits are light grey half moon shaped pods with several small seeds along the inner edges, each seed with a thin trail of flesh that attracts ants attached. Black Wattle grows up to 30 metres and is usually found as revegetation in disturbed areas. Although often plagued with borers, the timber is quite useful and is occasionally used as a substitute for teak.

Rose Alder is currently flowering in small numbers on both sides of Red Peak. These trees grow up to 30 metres, particularly along creeks and gullies. The flowers of these trees are white with a woolly texture. The fruits are tiny rust red pods with metallic blue seeds. The leaves are serrated and slightly askew whilst the bark is dark red under the surface and very thick. The roots generally grow on the surface without penetrating the soil much. If there is a young Orania Palm nearby with some mulch in its basket shaped crown, the roots of the Rose Alder will grow into this mulch and tap the nutrients within. When the palm grows taller, the roots of the Rose Alder grow with it upwards. The timber of these moderately "primitive" trees is both general purpose and utility cabinet timber.

Rose Silky Oak is now fruiting in small numbers along the Kuranda line. The fruits are woody pods that look like two small bells pressed against one another. The flowers are dark pink and borne on pedicels between the large leaves. The leaves are long and strap shaped with a leathery texture. When young, the leaves like those of most silky oaks, are strongly lobed as an adaptation to low light conditions. These lobed juvenile plants are becoming popular as indoor plants. The timber is a useful general purpose timber with no conspicuous oak grain despite the tree’s name and classification.

This month we will also be looking at the process of decomposition in the rainforest. All tropical rainforests of the world have very thin soils that are often poor in nutrients. This leads to a layer of leaf litter that regularly is deposited onto the ground when the trees drop their leaves for various reasons. These leaves and twigs in addition to rotting logs of dead trees are the basis for the entire food web of all tropical rainforests. Everything is basically recycled. The rate of recycling varies but is generally faster the closer you get to the equator. The most important recyclers are the fungi and the bacteria. The fungi are masses of tiny threads known as hyphae that spread out searching for food. They break down their food by releasing enzymes into the environment. Fungi are the most important decomposers as far as plant material is concerned. All plants contain the complex chemical cellulose in their cell walls and many trees are also loaded with virtually indigestible chemicals known as lignins. Only fungi can break down the latter and fungi are generally more efficient at breaking down cellulose than bacteria are. The main reason for this is their special growth form which allows them to penetrate timber with relative ease. Bacteria are very simple one celled organisms with very litter in the way of obvious structures in their cells. Unlike more advanced organisms though, bacteria generally consume almost any organic chemical. There are bacteria which have been discovered that eat petroleum oil, some bacteria can even grow without the presence of oxygen. So the bacteria’s main role in decomposition is the breakdown of animal corpses and tricky chemicals as well as decomposition in anoxic environments. An example of the latter is the production of methane gas often seen in lakes. You see it as bubbles coming up to the surface. Methane is, of course, quite explosive but luckily there are bacteria that eat methane. It’s quite common in the leaf litter to find groups of bacteria living in a loose kind of symbiosis where one of the partners eats the other’s waste products while producing chemicals the other needs for its survival. These symbiotic relationships can be quite complex with multiple partners involved. The end result is the recycling of dead organisms so the locked up nutrients are made available for consumption rather than piling up uselessly.

Insects are very common in the leaf litter but are often overlooked since most of them are quite small. The largest are the giant cockroaches that spend all their time digging in the soil and eating rotting vegetation. They have no wings like their pesky relatives and hiss when threatened. Next in size are the mole crickets, relatives of the grasshoppers that dig through soil and eat rotting vegetation. The front legs are modified into digging organs while the hind legs are smaller than in other crickets. Most leaf litter insects are very tiny and are usually regarded as quite "primitive". The most common are the springtails. These tiny insects have a fork held under tension under their bellies. When this tension is released they spring into the air. Springtails have special tubular appendages that can be everted, possibly for attaching themselves to the ground. Most springtails have no tracheae, the standard system of breathing tubes among the insects. Instead the springtails breathe through their cuticles and this is the main reason they are so tiny.

Millipedes are perhaps one of the most obvious creatures in the rainforest leaf litter, partly because of their sometimes large size. The name millipede means "thousand feet", but the number of feet actually varies from 30 to about 300. The most diagnostic feature of the millipede is the fact that each joint on the long body has exactly two pairs of feet. They share their ability to roll up with the unrelated pill bugs, which are land crustaceans that inhabit the same habitat as the millipedes. Millipedes feed on rotting vegetation and help recycle dead plants. Centipedes are occasionally mistaken for millipedes but are generally larger and move by undulating their bodies rather than "glide" over the surface. Centipedes are carnivorous and subdue their prey with venomous fangs on the underside of their heads.

By Tore Lien Linde