A Nigerian based in Maryland, USA, who wants to remain anonymous, is thankful that nobody was inside his BMW sedan car when a huge tree in his Maryland residence’s parking lot came crashing down, crushing his car and partially damaging another car. Strong wind and rain storm are to be blamed for this accident.
In a report presented by Mary Knudson and published on Scientific America, Mary wrote “Why do some trees fall in a storm, while most do not? To find out, I asked three experts to explain the science behind falling trees: David R. Foster, Director, Harvard Forest at Harvard University, a Long-Term Ecological Research Site funded by the National Science Foundation; Kevin T. Smith, a plant physiologist with the U.S. Department of Agriculture Forest Service; and William E. de Vos, a registered consulting arborist who is president of Treeworks, a tree preservation company in Montpelier, VT. Be forewarned that Smith admits he’s “been brainwashed by foresters” and says “stem” when he means “trunk” of a tree.”
Why do some trees fall during storms and others don’t?
The answer from Smith is not very comforting: “The first thing to know is that all trees have the potential to fail at some level of force from wind, snow, ice, either singly or in combination,” he says. One main reason, all three experts agree, is the phenomenon known as “windthrow” which uproots a tree. “The tree trunk acts as a lever and so the force applied to the roots and trunk increases with height,” says Foster. “Taller trees are more susceptible to windthrow.”
“The roots of trees can extend 1-2.5 times the radius of the branches and many urban areas do not allow this extensive development,” answers de Vos . “The problem lies mostly with trees that have been developed around and had roots cut, crushed or torn in the process. There may be ensuing decay.”
Smith explains further: “Wood is a very strong and wonderful structural material. Wood, however, is not homogeneous or consistently strong at all places in the stem (trunk). Wood decay caused by fungi can weaken wood structure. However, the mere presence of decayed wood or even a hollow does not mean that the tree is more vulnerable to failure.” What he says next is of some comfort. “Strength comes from the quality and quantity of wood that is present, not what might have been degraded.” An equally big factor in tree falls, he says, is bark between two trunks or between branches and the trunk, and wounds from past injuries which make a tree vulnerable when high winds bend its branches or even cause the trunk to sway.
de Vos adds “generally trees tend to uproot more than break off during wind events, although poor structure in the crown will result in limb breakage, splitting and tearing as well.”
Other risk factors: Large trees growing in shallow soil or in a rocky area and trees that were accustomed to living in a forest. More on that below.
Source of Scientific Report: