Late September loosens summer's grip on Cambridge, and the big street maples along Mass Ave start letting go of the limbs they can no longer carry — which is when Chilel's Tree Company is out early with the ropes and the saws. This is close work in a close city: pruning a silver maple that leans over a triple-decker roof, taking down a storm-split trunk in a narrow dooryard, grinding a stump flush so a small back garden behind a brick sidewalk gets its light back. They clear land, trim and cut, prune for structure and health, and answer the calls that come at odd hours when a tree has come down across a driveway. What's left when the chipper pulls away is the plain satisfaction of it — a canopy lifted clean off the shingles, the curb swept, a yard opened again to the low river light coming off the Charles.