Monday, 24 March 2008

Who Has Seen The Wind?

The title of this article pays homage to W.O. Mitchell's tale of life on the prairies during the dust bowl years. I don't think that there could be a more fitting title for a frank discussion of the visible effects of strong winds. We really don't see the wind, what we do see daily is its dramatic effects on the trees in our landscapes.

Trees are long lived stationary organisms that deal with wind on a day to day basis.

The events that lead to dramatic and some times tragic failures of trees, branches or entire forests often depend on changes to the the trees normal environment.

Take for instance wind, both its direction and strength. For any given location winds will come from a prevailing direction. Trees grow roots and crowns in patterns that compensate for this type of wind. A wind coming from any direction other than the prevailing direction, will cause more damage. Winds also blow at predictable strengths for most locations. Unusually strong winds from any direction will cause extensive damage to trees.

People have been watching the effects of wind long before hi-tech devices to measure wind speeds were available. Knowing common indicators of wind speed can give you some insight into tree damage and failure. One system of measuring wind speed related to its action on sailing ships was developed by Sir Francis Beaufort, a British admiral. The scale was updated over the years to include wind indicators for those of us on land. Beaufort's scale indicates that little tree damage occurs below 22 mph ( 35 km/h). At 27 mph, ( 44 km/h) umbrella use becomes impossible and large branches begin to sway. As wind increases up to 42 mph ( 68 km/h ) twigs begin to break off of the larger branches. Damage continues to increase and when you reach 60 mph ( 96 km/h) whole trees begin to topple over. Beyond this point damage increases dramatically.

What Beaufort's scale fails to take into account are the site conditions of the trees. Trees that are in full leaf will fail at much lower wind speeds. My own personal observations indicate that at 42 mph ( 68 km/h) trees in full leaf in saturated soils will topple readily. Soils that are drenched with rain or melt water do not hold roots with the same strength as when they are dry. Trees that have lost their leaves and are in frozen soil can survive much stronger winds. Drought stressed trees lose fine roots that anchor the support roots of the tree. The result can be increased potential for wind throw.

The answer to the question “ Why did this tree fail?” can be complicated. The correct answer may include the following: Root loss due to drought. Unstable soil conditions due to rain or thawing. Wind blowing from an unusual direction. Wind blowing at unusually high speeds. Rot or decay in the root system. Decay in the trunk of the tree. Recent or past construction in the area. One has to take all the factors into consideration to determine when a tree will fail. The answer to the failure question may be quite simple. Once trees or limbs are dead they begin a slow countdown to the moment when they break off and succumb to gravity. This is often chaotic and unfortunately can be tragic. A properly trained arborist can identify potential hazards an hopefully avert unfortunate failures of trees and branches. When was the last time your trees had your trees inspected?