Rainforest Succession: The pioneers

Skyrail Nature Diary: April 2013

This month we will be taking a closer look at the ecology of rainforests. As anyone who has visited Skyrail will know, the main message of our boardwalk tour is the constant 'fight for the light' in rainforests. The canopy is extremely dense and does not allow much sunlight to reach the forest floor which is where most seeds fall and where there is soil for roots to establish. This creates a big problem for recruitment of new rainforest trees because some seeds can either not germinate at all, or they just manage to pop out a little root and turn into a seedling less than a foot tall. Such seedlings are then doomed to wait until an old tree falls to get their chance to grow to a tall tree and reach the canopy level. Those conditions create a process called succession which nearly every forest has to go through in order to reach its full potential. Ecologists talk about different stages and even different communities of plants that you will find in our forest, depending on how much time it had to regenerate since the last disturbance. Firstly you have the pioneer stage, then the secondary stages and finally the mature stage.

Disturbance can happen through a number of events. Tall trees can die of disease or just old age, or a land slide can bring down a whole swathe but the most common causes for disturbances in our region, are cyclones. This is reflected in the general appearance of the rainforest and the species along our cableway which is why you will always find different stages adjacent to one another in a patchwork pattern which gives it a pretty rugged appearance in places.

Whenever people talk about an impenetrable jungle it mainly refers to quite open areas along water courses, or places where the canopy is broken allowing sunlight into the lower levels of the forest. This is what is called a 'gap situation'. These are the only spots where the rainforest can rejuvenate. The dense undergrowth makes it very difficult to walk through. This photo, taken around tower 16 illustrates what a gap looks like and shows the types of plants that take advantage of such a bright situation.

Now let’s look at how individual plant species deal with this. These plants called pioneers and amongst them are gingers, bananas, stinging trees, the slender Celery Wood, the Ivory Basswood, the climbing ‘Wait-A-While’, our rainforest wattles, tree ferns and even our Kauri Pines. These plants have all sorts of adaptations to make the most out of the sunlight which may only be available for a few years. Vines and lianas are very common plants in rainforest for exactly that reason. These skinny plants that can quickly outgrow the surrounding trees by simply using them as a support structure. One of the reasons why rainforests are full of vines and lianas is because they are adapted to exactly this process of succession.

Pioneer trees grow quickly by taking nitrogen out of the air and can produce seeds from very early age on. They are mostly short lived and when they die, their decaying biomass creates the kind of soil conditions that favours the next generation of trees of the secondary stage.

Another interesting feature of pioneer trees is that most of them are slender with huge leaves but hardly any branches. This is to save energy as it makes more sense for them to grow tall first before the gap closes again - growing sideways branches would only slow them down. On top of tha,t the pioneer trees are the ones that have to heavily compete with vines like the ‘Wait-A-While’ in the gaps. A good example is the area around Innisfail which was almost smothered by vines after cyclone Larry. It suffered another set-back when cyclone Yasi hit during the recovery stage.

Studies have shown that the longer the branch-free trunk sections of a tree, the less likely it is to be pulled over by vines as they have little to grab onto with their hooks or tendrils. Just like the fight for the light, there is a parallel struggle going on between the trees and the skinny and fast vines that use the trees as support structures. Both of them adapt constantly and find new ways to give each other a hard time to reach the sunlight.

By Mike Gailer