Kuusentainta is the partitive case of kuusentaimi, meaning a spruce seedling. It's a compound of kuusi (a spruce), and taimi (a seedling). So the whole sentence translates to something like The girl passed the statue of Habarovsky, who was bearing a pathetic spruce seedling.
To me kuusentainta sounds a bit oldish and/or dialectical, I would say kuusentaimea instead. But it's really just a matter of style and taste.