This sculpture sits outside FDNY Squad Company 1, which lost about a dozen members on 9/11. According to one of the firemen there:
This chainsaw sculpture was made by an artist in Oregon, who drove it to NYC in his truck. The city refused to take it, however, because they didn't want to be liable for people running into it and getting hurt. It eventually found its way here to Park Slope, and the Food Coop agreed to keep it on their property next door to the firehouse. The bottom of it is pretty badly rotted — it was only placed up on those blocks after a considerable amount of water damage had already occurred.
(This article tells a slightly different story.)