snow send the limbs sagging, I worry, and run to the center of the house when I hear a loud crack. Who is next? How do we know if our birch, elms, maples, oaks, pines, cedars, willows, hemlocks, and other urban trees that grace our landscape throughout the seasons, will safely co-exist with our families and our homes when the weather rages. 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