The rise and fall of eucalypts in a tall, wet eucalypt forest

The rise...

After a eucalypt seedling establishes itself, it will go through three distinct growing stages. These growing stages reflect the need of the plant to obtain optimal sunlight by quickly gaining height, developing a mature crown of at full height and then gradually losing height and gaining trunk thickness until the tree dies or falls over. These three stages of growth outlined by Wardlaw et al. (2009) are described and shown below.

This young stand of Eucalyptus obliqua saplings are growing quickly towards the sky in order to outcompete the surrounding understorey species for sunlight.

These Eucalyptus obliqua trees have long, straight trunks, with deepening crowns, that are relatively free of branches.

At an estimated 300 or more years old, these two old growth Eucalyptus obliqua have already reached their maximum height. The crowns have retreated and the impact of past lightening strikes have reduced the original height of the trees.

0 - 100 years

Crown Lifting phase
Young saplings begin a race to the sky towards sunlight and above any surrounding light blocking vegetation. For the first one hundred years, my crown begins to rise, and they enter the Crown Lifting phase, where many branches growing further below along the trunk, become shaded and will eventually die and fall to the ground below. This leaves most of the leaves at the top.

100 - 200 years

Crown Deepening phase
At around 70-100 years after disturbance (eg: fire), the tree begins the Crown Deepening phase. The tree has grown taller and there is more available light for the tree to use in photosynthesis. The light also helps the tree branches on the trunk to stay longer, and the tree becomes more dense with branches and leaves over the next one to two hundred years where it will reach its maximum height.

200 - 450 years

Crown Retraction phase
As the eucalypt tree continues to age, it will eventually reach the Crown Retraction phase where it will eventually become shorter due to impacts including lightening strikes, top breakage, and dieback. By the time the eucalypt tree reaches a possible maximum age of 450 years, the trunk may become much wider in girth.

The fall...

Temperate wet eucalypt forests contain some of the highest amounts of coarse, woody debris (CWD), on the forest floor in the world! CWD in these forests can contribute more than 800 cubic metres per hectare, providing a huge banquet of wood for saproxylic (rotting wood), organisms such as beetles and wood decaying fungi (Wardlaw et al. 2009).

The bulk of coarse woody debris is made up of large fallen trees, including eucalyptus species, that may take 200 years to be decomposed fully, to where the original tree is difficult to see amongst the moss and ferns (Wardlaw et al. 2009).

There are five stages of decay on CWD logs (Wardlaw et al. 2009) described and shown below.

Decay Class 1

Decay Class 1
After being freshly deposited onto the forest floor, the fallen tree shows few signs of decay and is mostly still intact.

Decay Class 2

The tree trunk is still very solid, but the outer bark is mostly gone and there are signs of wood-decay.

Decay Class 3

Decay Class 3
The fallen tree has softened and may be hollow in some areas. Establishing fungi and moss.

Decay Class 4

The fallen tree is mostly soft and hollow. Well established moss fungi and invading roots of other plants.

Decay Class 5

The outline of the fallen tree is now much harder to see on the forest floor. The tree has now been almost completely decomposed.

References:

Wardlaw, T., Grove, S., Hopkins, A., Yee, M., Harrison, K. & Mohammed, C. 2009. The uniqueness of habitats in old eucalypts: contrasting wood-decay fungi and saproxylic beetles of young and old eucalypts, Tasforests, Nov ed., vol.18, pp. 17-32.