Tuesday, April 17, 2018

Blowing In the Wind

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.
 (To protect the shallow roots of many trees during construction and other work, local authourities require a Tree Protection Zone (TPZ) to be observed. The radius of the TPZ is calculated as 12 times the diameter of the trunk of the tree at breast height (DBH). Thus, a fairly large tree with a DBH of 1m requires a TPZ radius of 12m. Many of Drouin’s large eucs have trunk diameters greater than 1m!)

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!

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