Something in all of us loves a disaster, if only as a way to measure our own fortunes. So it was to no one's surprise that thousands of citizens came out in the wake of last week's massive wind storm to ogle the scene. The principal agents (and objects) of the destruction they witnessed were trees. In the storm's most torn-up path, around Lake Nokomis in Minneapolis and east to Highland Park in St. Paul, linden, hackberry, Colorado spruce, elm, and even great oak trees splintered and thundered to the ground. They'd crushed cars (in one case, a new Jaguar) and crashed through roofs. For every one of the nearly 4,000 trees down, it seemed a dozen gawkers appeared on the street to gasp and gape.
But as is so often the case with acts of God, it turns out most of the tree damage was caused by human hands and was quite preventable. In the forest, 70 mile-per-hour straight-line winds still make a dent. But the city is another story--and a particularly treacherous place for a tree to grow up. Pollution, weed whackers, lawn mowers, automobiles, street salt, sign-staplers, tree-fort-builders--all take a toll on the urban forest. Then there's the "crummy, compacted, anaerobic city soils," says Don Willeke, the former head of the National Urban Forest Council. There's also, he adds, the "arrogant stupidity, and I emphasize the word stupidity, on the part of the people designing sidewalks."
Among those still out gawking late last week was a tree scientist from the University of Minnesota, Gary Johnson. "My heart bleeds when we lose trees," he says, "but part of me gets kind of excited." The Saturday night wind storm provided him with perfect laboratory conditions to explore one of the central questions in his field. Put simply: why does one tree stay standing and another tree go down? For answers, Johnson performs his own brand of autopsy on the fallen trees.
By way of example, he points to a hackberry lying in ruin along the parkway near Lake Nokomis. Before the storm hit, the tree looked just like the rest to an untrained eye. But, Johnson says, "an arborist could have come through and known it was doomed." Today, the tree is a wreck. Two enormous branches, each weighing 1,000 pounds or more, lie cracked on the ground, and a third, smaller branch has peeled off from the trunk.
Johnson finds what he's looking for on what's left of the tree trunk: a dark smiley-face above the bare wood, marking where each branch broke off. The smiley face is "included bark," Johnson explains. "These branches came up at narrow angles and pinched the bark between them and the tree trunk." The pinched bark began to decay years ago, and the rot undermined the strength of the branch. This malady, Johnson says, "is the number one cause of branch failure. Thirty years ago, somebody could have seen that problem, cut the branches off, and we wouldn't see this." Included bark begins to develop even at tree nurseries, where full, bushy trees with more sharply angled branches catch buyers' eyes. Johnson says you should hack off all the branches before you plant a sapling, or make a point of purchasing spindly young trees.
Down the block, several annihilated lindens litter the pavement and grass. Classic wind-throw, he says, that takes the branches and pulls a tree up by its roots. But a few feet from the downed lindens an elm stands unscathed. In fact, amid a dozen wind-thrown trees stand here and there an elm, an oak, even other lindens that were buffeted but remain unbowed.
Johnson unravels the mystery: the toppled trees grew in the boulevard, the thin strip of grass with street on one side and sidewalk on the other. There's a common myth that trees' roots sink as deep as the branches are high. Not so. In their natural habitat trees will form a sort of "pad" of roots a foot or two deep and two to four times as wide as the trunk is high. So, he explains, in the forest a 20-foot linden might stand on a root system growing 40 or 50 feet around it on all sides. But in the city between curb and sidewalk there's only three to ten feet for a tree to spread its roots in. Your average linden or elm sends out roots to the curb in one direction and under the sidewalk in another. One year street workers come along and hack out the roots to put in a new curb. The next year the sidewalk buckles and cracks. More crews come along and carve out a hole in the roots to put in a new sidewalk, and so on until, Johnson says, "the trees are perched on one foot." He pulls up a leg to demonstrate, balancing himself precariously, arms out like the crown of a tree suffering under the assault he's been describing. "You could push me over with one hand."
A third variety of tree death the Cities saw last weekend takes the form of "stem failure." Johnson finds an example nearby, a tree that's rotten to the core and snapped about ten feet up its trunk. Next to it lies the corpse of a tree that appears to have been plucked like a mushroom; it's roots are still buried, but the stem is snapped at the ground level: A typical case of "girdling roots," Johnson explains. He pulls out a pocket tape measure and marks off a dark brown line at soil level. It's some 20 inches from the soil down to the roots. "This is a tree that's been planted too deeply," he concludes. "Roots have to have oxygen." If a tree is planted too far down in the soil, its roots will come up to the surface for air. When that happens, Johnson says, "they get confused." Instead of spreading out like a normal tree's roots would, these roots tangle around and around the trunk under the soil, tightening over the years like a vise. Even a relatively mild storm can tip over a tree with girdling roots.