A tree doesn't need to make contact with a conductor to create a short circuit. Also, it might be several years before the utility can get back through the area to trim the trees again, so they trim them enough to allow several years of growth without interference. Tree contact was one of the major causes of the 2003 blackout. One of the lines that tripped out due to tree contact had a crew working to clear vegetation along its corridor, and they were just several hundred feet from where the line contacted a tree that had yet to be trimmed.
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