The City of New York obviously doesn't remove its street trees lightly!
|Upper East Side, Manhattan (click to enlarge)|
This American Sycamore (Platanus occidentalis) was pretty old and had been seriously rotted out before it finally got the chop.
Most of the strength of a tree is in the outer wood (think hollow pipe) so you can often leave them even when they look badly compromised.
But the cross section on this one shows that even this healthy outer wood was starting to rot (the yellow and white sections right and rear) so it was obviously time for it to go.