At least once a week for more than 20 years, I’ve walked past a row of five or six identical boulevard trees in Minneapolis’ Longfellow neighborhood. I guess I never really bothered to look up, since I so busy managing what the dogs were doing below.
Two or three weeks ago we rounded the corner on our walk only to come face-to-face with a road blocked by orange street barriers. There lay one of the big trees. There had been a short, windy storm the night before, and the trunk had snapped off about four feet from ground level. The linden tree fell the only direction it could without damaging property, landing smack in the middle of the intersection.
I stopped to look at the splintered trunk. This is what I saw:

The lesson: we look, but we do not see.
The remaining trunk shows the classic signs of a stressed tree. Fungi growing is a sign of internal decay. The many holes on the bark indicate woodpeckers were feeding on insects, possibly linden borers that feed in the lowest part of the trunk.
I have no excuses for not paying attention. As a Master Gardener, I’m supposed to be observant. I’ve taken tree classes at the U of M . Yet in a city like Minneapolis, where trees are everywhere, many of us simply don’t pay attention. We plant trees too deep, don’t water them during drought, gash bark with lawn mowers and slice trunks with string trimmers. We allow fly-by-night tree trimmers to “top” trees instead of prune them. As trees age or sicken, we ignore the messages that fallen branches and dead wood high in canopies send us.
Here’s some of what I saw on walks around the neighborhood this weekend:



Our most recent storm was a wake-up call. While a lot of the tree damage in that storm was due not to neglect but to sodden ground and freakish bursts of high wind, it was a reminder to pay attention to our trees and to care of them. If you hire someone to examine a tree, make sure it’s a licensed arborist who knows their stuff.
On my walk last Friday, I ended up on that block of linden again. The fallen tree I’d seen before had been reduced to a stump. But one of its siblings had come down in the big storm. Like the other tree, it had snapped off several feet above the ground, and the remaining stump had mushrooms growing on the trunk and holes in the bark.
Here’s a pretty good briefing on hazard trees, some of it written by Gary Johnson of the U of  M:

And here's another useful site: