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A clockwork pine - Now we know why some trees get so wound-up

点击量:   时间:2017-12-08 01:01:08

By Charles Seife CYCLONES spin in opposite directions in the northern and the southern hemispheres—and, oddly enough, so do trees. Two Norwegian foresters believe they have found out why the trees get in a twist. In many trees, the grain lines wrap around the trunk rather than going straight up and down. For instance, the radiata pine, a conifer from the northern hemisphere, eventually winds up with a right-handed spiral. In the southern hemisphere, trees like Agathis palmerstonii reverse the pattern, ending up with a left-handed spiral. “The patterns of left-handed and right-handed conifers are something every forester knows,” says Sondre Skatter, a forester and physicist at the Norwegian Forest Research Institute in Hoegskoleveien. “It’s something mysterious.” To try to explain this, Skatter and forester Bohumil Kucera measured the amount of foliage on conifers throughout Norway. They found that the trees had more branches and pine needles on the southern side. This is no surprise, since biologists suspect that plants grow more leaves and branches in the direction of the Sun, which is to the south in the northern hemisphere. But Skatter also noted that in most places where conifers live, the wind blows from the west. “If you look at the occurrence of conifers, they tend to live in the west-wind belt, the areas from 30 degrees north and south to the poles,” he says. When a west wind pushes on southern leaves, the tree trunks are stressed with a counterclockwise twist, something that could partly explain the spiralling grain. The twisted grain may be beneficial to the trees, the researchers add. During storms, wood is vulnerable to forces that twist the trunk and pull layers of wood apart. “That’s very reasonable,” says Karl Niklas, a plant biomechanist at Cornell University in Ithaca, New York. “Where trees broke in recent storm damage, there was very good evidence of shear stress—layers of wood were prised apart.” A tree whose grain spirals in the same direction as the twisting force won’t shear apart so easily and has an evolutionary advantage. Skatter and Kucera are convinced that evolution and genetic factors are helping to create the twist. When radiata pines were grown in New Zealand, they kept their right-handed spiral, even though the trees were in the southern hemisphere. This suggests that the trees have evolved to resist the wind. “That does fly in the face of conventional wisdom,” says Steven Vogel, a plant biomechanist at Duke University in North Carolina. “The fact that twist is preserved is wild and exciting.” Skatter is hoping to test his hypothesis by seeing if conifers that grow in areas with different wind patterns have evolved different spiral patterns. “We made a field trip down to Croatia, where the prevailing winds are easterly,” says Skatter. Unfortunately, the local conifers turned out not to have a strong spiral pattern at all,