Sometimes trees are sick and they need to be cut down. Sometimes they are taken down preemptively because an insurance company does not want to pay for damage in the future even though a tree is healthy and it is not realistic that it might fall on a house or a car. Sometimes trees are cut down for no apparent reason. And more often than not they are not being replaced with a new tree but with a block of concrete in the sidewalk. In this case, there was enough time to leave a note. More pavement writing here.