I assumed that this memorial grove in Prospect Park would consist of trees planted around the time of the grove's dedication in 2003, so the scarcity of young trees in the area (you're looking at just about all of them) made me wonder if some others had died and been removed over the past decade. This page leads me to believe that the grove's creation mostly involved the restoration of an existing treed area of the park, however, which makes more sense given the surroundings. You can take a look around the grove here.