One-Question Interview: Why Trees Fall

Norman Christensen, is professor and former dean of the Nicholas School of the Environment and Earth Sciences

Q. With so many trees on the ground throughout the region, it's hard to talk about how effective trees' defenses against storms are, but in fact most trees usually survive even during the severest storms. Why is it some trees do come down during storms?

A. Three factors are most important in determining whether or not a tree will fall in an ice storm: the amount of ice that accumulates on branches and leaves; the character of the wood; and soil moisture content. Because they present enormous leaf area for ice accumulation, evergreen trees such as pines and red cedar are at particularly high risk for ice damage. Evergreen magnolias and hollies are also susceptible to ice damage. All other things being equal, the greater the leaf area the greater the risk, and ecologists have speculated that the restriction of longleaf and loblolly pines (species with relatively long needles) to the eastern half of North Carolina is related to the higher frequency of ice storms in the west.

The arrangement and chemistry of the fibers and water-conducting cells in wood also influence the risk of breakage. The branches and boles, or trunks, of trees such as pine and red cedar have rigid wood and vertically arranged fibers and are susceptible to splitting and breakage. On the other hand, species with more limber wood such as tulip trees are able to bend under the weight of ice. The wood fibers of some trees, like sweet gum and sourwood, are highly interwoven, giving their branches the strength to resist breakage. Anyone who has tried to split logs from these species has experienced this resistance first hand. Damage and rot can also influence the character of wood. For example, when red maple branches or boles are bruised, the wood beneath the wound tends to rot, making the tree more susceptible to future breakage.