Why trees fall

    Although our region was spared more than others, driving or walking through the neighborhoods of Philadelphia it’s apparent that Sandy left an unwelcome calling card when she stopped by earlier in the week.

    Before the storm hit, I feared that my trees were doomed. They’re all old and misshapen, one has a lopsided top, and another is succumbing to disease. But when the storm cleared they were all still upright, minus a few dead branches I had been meaning to get removed. Others weren’t so lucky, and nearby I saw trees that had appeared much more vigorous than mine toppled. In some cases the whole root plate lifted, and in others the trunk was snapped like a toothpick.

    Trees can usually withstand extremely high wind gusts without damage- with some exceptions. In Philadelphia the prevailing winds come from the Northwest. As trees grow, the roots and branches spread in a way that counterbalances the pressure of the Northwest wind. As we know from those radar pictures, Sandy did not arrive from the Northwest but the Southeast, slamming into trees on their more vulnerable sides.

    It didn’t help that this storm hit while the trees still had leaves. Each leaf acts like a mini parachute and creates resistance, which puts the trunk under a lot of pressure. This is why most evergreen trees have needles (which don’t create as much resistance in severe winter weather) instead of true leaves.

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    Before Sandy hit, our region was about six inches under typical rainfall for the year. This probably helped some trees survive the storm, and resulted in fewer trees that toppled by the roots. Dry soil is hard, which you’ve noticed if you’ve ever tried to dig in it. Its cement-like consistency holds the roots securely in the ground. If we were to get a second Nor’easter now that the ground is saturated (perish the thought) the mucky soil wouldn’t be able to hold the roots down so effectively against the driving wind.

    If your trees were damaged, I’m sorry. If they damaged anything as they fell, I’m even sorrier. When these weather events happen we question the wisdom of placing big trees and houses in close proximity. But it’s worth remembering that except during rare, unprecedented weather events, trees add value to neighborhoods, provide cooling shade, support wildlife, and greatly increase the beauty of our city. They’re biologically engineered to fall down, and we can often interrupt this natural process. But not always.

    As for my trees, maybe there were enough nearby buildings and bigger trees to act as buffers. Perhaps the wind gusts weren’t quite as strong as they were a mile away. Even though they won’t win any beauty contests, maybe they’re still going to hang on for awhile longer.

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