A few recent wind ‘events’ sets the mind to ponder. Trees and strong winds are not a good mix but we do need to put things into perspective a little.
Up to date Australian figures for the risk of being killed by a falling tree are difficult to locate. Obviously if you shelter under a tree in a windstorm you will raise the chances. In 2017, Hellis Tree Consultants in the UK stated that the chance of being killed by a falling tree in that country was estimated at 1 in 10 million, an insignificant risk in most terms.
In 2012, hurricane Sandy toppled 8,500 trees in New York City alone, killed 233 people in eight countries and cost the USA government $69 billion. Sandy’s maximum wind speed was 185km/h.
The strongest wind speed, (gust), in Melbourne this year to date, 18th March, (remember?), was a mere 92 km/h. Melbourne’s windiest month is September and the average wind speed for September over the years from 1955 to 2010 is just over 15km/h.
Eucalypts, and in fact many other species of large trees do not have a deep root system. 90% of the roots of most eucalypts are less than 35cm below the surface.
|Drouin South, July 2017 - large tree/shallow roots|
|Alex Goudie Reserve, August 2017 - a relatively large Mountain Grey Gum|
|Same tree as above - shallow roots, no tap root, wet ground.|
So, tall trees with shallow root systems along with strong winds will mean the likelihood of ‘windthrow’ occurring. Windthrow is the phenomenon of the trunk of a tree acting as a lever during a windstorm and uprooting the tree. The windthrow effect is greatest for tall trees.
Other factors that may influence uprooting during high winds is the moisture content of the soil – trees growing near creeks, etc are more vulnerable, and the type of root system – trees with tap roots are less vulnerable to windthrow.
The intertwining of the roots of a group of trees helps to support individuals within the group. Windthrow, uprooting of a tree, is far more likely to affect an isolated tree rather than a group of trees. If one tree in a group fails, it can affect other neighbouring trees. (Link to a video of windthrow occurring on the Black Spur Victoria in December 2014 - worth a look!).
|Clusters of trees help support one another in strong winds|
We need our trees for all the benefits they provide and it seems, at times, the trees just like us need one another!