Correct answer: 3, they all behave the same. Susanna, the windstorm that hit Switzerland last Tuesday, left broken trees in its wake. In some areas, such as Chasseral, winds reached peak speeds of more than 150 km/h. That indeed is the critical wind speed that causes trees of all species to break, according to an article published in early February in Physical Review E. It’s no surprise, then, that Lothar, with winds reaching 272 km/h, knocked down 10 million spruce, beech, fir and oak trees in Switzerland alone back in 1999. To get a better sense of how trees hold up in high winds, researchers at Ecole Polytechnique in Paris and ParisTech mapped out the damage visited upon trees by windstorm Klaus, which pummeled France in January 2009. They observed, unsurprisingly, a high correlation between the maximum wind speed in a given region and the number of trees left damaged. But they also noticed that a tree’s hardness and thickness had little to do with its strength. Oaks and fir trees were equally affected, regardless of their girth. In an effort to resolve this apparent paradox, the researchers ran tests on logs of different material, length and diameter. They loaded them with tubs of water to test their strength. But the logs did not obey Hooke's Law, according to which the more ‘elastic’ an object is, the more it will bend before breaking. Digging into other materials research, they were able to corroborate their hypothesis. The answer has to do with the number of imperfections in a tree’s trunk: the larger the diameter, the more imperfections there are. Knots, cracks and other irregularities are weak points when a tree is battered by a storm. In addition, while the trunk of a larger tree may appear sturdier, it is also subject to greater aerodynamic forces because it has more surface area exposed to the wind. Researchers tested the logs by putting them into a wind tunnel and training a constant level of wind at all parts of the trunk. It turned out that all the logs broke at about the same time, when the wind reached 151 km/h. The ability of trees to withstand high wind thus has little to do with the stiffness of the wood, air density, the shape of the tree and the length of the trunk. Article paru dans Physical Review E.