Tree Buttresses

Large sinuous buttreses on fig

Moreton Bay Fig with sinuous buttresses


When walking in tropical and sub tropical rain forests it is common to see trees with large flange-like expansions extending several metres up tree trunks. They are generally encrusted with other life forms including ferns, mosses, and lichens larger epiphytes and in some cases bracket fungi. The expansions are called buttresses.

Various unresolved theories exist for their function such as –
  • a mechanical brace for the tree against strong winds or a reaction to strain.
  • compensation for restricted tap-root development in shallow soils with poor drainage.
  • allowing greater exploitation of the surface soil layer which contains the greatest concentration of nutrients relative to the lower layers.
Two types are recognised -
  1. Spur Buttresses Thick, rounded, buttresses extending not more than 0.5 metres up the trunk. They may appear as thickened roots.
  2. Plank Buttresses Very thin parallel sided expansions extending 1-2 metres up the tree trunk. In some species the plank buttresses can reach 10 metres. These are generally found in warm moist forests.

Not all trees show the tendency to form plank buttresses but it appears to be genetically fixed within a number of plant families. Some common genera include Sloanea (Yellow Carabeen), Elaeocarpus (quandongs) both in the same family, and Ficus (figs).

In some species the roots extend for quite a distance from the base of the tree as wave-like expansions. (figs and quandongs).

Out and About

All forms exist in subtropical and tropical rainforests but plank buttresses are absent from warm and cool temperate rain forests. Some particular areas in the south east Queensland-Northern New South Wales include –
  • Palm Grove walk Tambourine NP has excellent examples of very tall plank buttresses on Sloanea woollsii (Yellow Carabeen). These buttresses are spectacular extending several metres up the trunks.
  • Witches Falls walk Tambourine NP has large Moreton Bay Figs (Ficus macrophylla) with extensive wave-like extensions to their plank buttresses.
  • Bunya Mountains NP on several walks has examples of plank buttresses on White Quandong (Elaeocarpus kirtonii) and spur buttresses on Bunya Pines (Araucaria bidwillii) and Giant Stinging trees (Dendrocnide excelsa),
  • Mt Warning NP New South Wales in the lower part of the forest walk has a spectacular example of wave like expansions on a Blue Quandong (Elaeocarpis grandis) encircling rocks.

If you are in the Brisbane area the Palm Grove walk at Mt Tambourine provides good examples of all buttress types.  In particular spectacular examples of plank buttresses on Sloanea woollesii and extensive wave-like extensions on a Moreton Bay fig (Ficus macrophylla) in and around palms. This huge fig is on the right at a small set of steps but you need to walk across to the back of the tree at the top step level to see the spectacular root development.

Things to look for with buttresses include their –
  • shape and size (plank buttresses often have side buttresses themselves)
  • growth progression by comparing buttresses on saplings and more mature trees
  • attached life forms (ferns, mosses, lichens larger epiphytes and in some cases bracket fungi)
In the Bunya Mountains we noticed the White Quandong buttresses often had a resident large bracket fungus in one of the inner angles.

Buttress Examples

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