Roots of the rainforest

Skyrail Nature Diary: June 2010

There are a number of unusual types of roots found in the rainforest, many of which have adapted over time to the climate of the rainforest and function using unique processes.

Most would be unaware of the importance of the process involved with roots and fungi in the rainforest. This process is called symbiosis and is vital for a rainforest's existence.

For a tree to survive, its roots collect minerals and water from the ground. Intriguingly, it's not the roots themselves that carry out this task. On the root tips of over 90% of all trees in the Wet Tropics, there are symbiotic fungi known as Mycorrhizae that feed the tree. The remaining tree species, many of them Silky Oaks, have a symbiotic relationship with a fungus-like bacteria known as Actinomycetes.

Buttress roots (image 1 & 2) are unique to rainforests. Apart from being a beautiful feature of the tree, there are many theories around why trees would develop such roots. It is thought by many, that buttress roots assist the tree in staying upright in shallow soils, typical of most tropical rainforests. However, there is another theory that makes this an unlikely proposition. Most trees growing on mountain tops like Red Peak (Barron Gorge National Park - Skyrail mid-station), don't have buttress roots and those that do, have only small ones, hardly suitable for any support function.

This leaves the theory that these roots work as channels for water coming from the canopy. Even though it rains substantially in the rainforest, most of the water is captured in the dense foliage of the rainforest canopy. For water to reach ground level, it will drip down the tree trunk, channelling down the exposed buttress roots to where the water is needed. Additional to this theory, on rocky ground, like on hills and mountain tops, water tends to run off, making collection of water difficult for trees.

Aerial roots are a feature of most figs (image 3), especially those that grow as stranglers and banyans. It's believed that these roots help supplement the fig's requirement for minerals when they are sending their roots down to the ground from a Basket Fern on top of the canopy. A feature of aerial roots is that they look to be dangling in thin air. In order for the tree to get its nutrients, fungi and bacteria inside the roots extract nutrients from windblown debris. Aerial roots are also advantageous in nutrient depleted soils.

Legume nodules are a feature of bean, mimosa and cassia families. Black beans and wattles (image 4) both share this feature. These trees live in extremely impoverished soils, with little nitrates available. Their way of surviving is by producing legume nodules on their roots which can turn gaseous nitrogen into usable minerals to feed the tree.

Coralloid roots are unique to cycads (image 5), ancient relatives of the conifers that look like small palms. All cycads have coralloid roots, called so because of their coral-like appearance. These specialized roots contain symbiotic cyanbacteria and are normally found near the surface of the ground, under a thin layer of leaf litter. Unique to this species, the symbiotic cyanbacteria feeds the plant by using complex chemical reactions.

Although roots end up feeding a tree its minerals, there are helpers and complex processes that aid a tree's roots in order for them to carry out the task. This demonstrates that there is an intrinsic network of processes taking place in rainforests, imperative for their survival.