While cyclones pose an enduring threat to this region, this does not mean yards and public spaces should be without large trees. Indeed if it were not for the fact that large trees endure through cyclones and storm events there would not be any large trees in this region. All large trees in this region are sufficiently mature to have survived as least one cyclone.
Many people are highly fearful of what trees might do if we get a cyclone. This is an unknown quantity and in reality the neighbours tiled roof or back shed is more likely to do more damage during a cyclone than are most trees. While tree debris accounted for a lot of the clean up requirements after Cyclone Larry, in the scheme of things trees really did not do most of the property damage. In fact many houses may have been prevented major damage by fallen trees holding the roof on.
A trees height does not directly correlate to its likelihood of falling down in a cyclone or any other storm event, as many factors are involved with tree failure under high winds. The wind speed and direction, and the level of soil saturation are all critical. As cyclone behaviour is so unpredictable it is impossible to determine which way a tree would fall or even how far tree debris would spread.
Trees and palms that move or bend a lot are actually less likely to snap than are inflexible specimens. If however, during the storm event, the tree lifts the surrounding ground and roots are obviously snapped on one side then get it out and do so quickly.
To have an entire tree fail (as opposed to a tree loosing branches) is more related to the level of soil saturation and any history of damaged or defective roots. Chopping off roots under a tree’s canopy is one of the key things that people do that increases a trees likelihood of failure wether during a storm or just during a drought. Planting root-bound trees, using plastic sheeting under mulch and watering to only a shallow soil depth (thus not encouraging any deep roots) all predispose a tree to being blown over.
The canopy of a heavily foliaged tree can also act as a sail and cause it to be blown over or have sections torn out. Many native species like Sea Almond (Terminalia spp.) tend to drop a lot of debris in storms as their way of reducing this sail effect and so the tree survives to regrow new branches and leaves. It seems that some trees especially mangos are so dense that the wind just passes over them affording some protection to structures on the lee-side.
As a response to concern over trees many people seek to have their trees lopped; cut off to main trunk or branches. While the height of a tree is reduced by lopping, the branches that arise from the lopping wounds are very weakly attached and easily snapped off under windy conditions and actually increases risk of branch failure.
The Australian Standard for Pruning of Amenity Trees (AS 4373) does not recommend lopping nor does the Arborist Association of Queensland. In most circumstances removing dead and weakened wood is the first priority for minimising cyclone damage. To then thin out the branches, remove overhanging limbs and re-balance the canopy is the most appropriate pre-cyclone treatment for trees.
Also, if you have groups of trees they are much less likely to be blown over than are trees in isolation. Be particularly careful if considering removing healthy trees from established groups and leaving isolated individuals.
As wet soil has very little ‘hold’ on plant roots, to reduce the risk of cyclone damage ensure water can exit the property and does not pool in areas where trees are growing.
One of the major root damaging fungi in this region is Ganoderma spp.
Trees with this root disease are much more likely to fail in a storm event than are healthy trees. Check for white spongy “puff balls” near the trunk or under the trees canopy, especially during warm, humid weather which is ideal for the production of the fruiting bodies for this any many other fungi species . These initially creamy white spongy fungi with age turn hard with a shiny, lacquered upper-side and stem. This fungi attacks the roots of a range of trees from Poincianas, Cassias and citrus through to eucalypts and tea trees. Trees with these fruiting bodies are to be regarded as having an incurable disease that will eventually kill them and greatly increases their chance of being felled in a cyclone or storm event.
So while cyclones pose an enduring threat to this region, well maintained trees are long term assets and provide numerous benefits including shade and cooling.