Actually, only tree is real. "Forest" is a mental construction - just region of space with the density of trees being high enough. Play with that "high enough" variable, and a meaning of "forest" will drift.
"Tree" is also a mental construction. It is just a bunch of particles that form a recognizable pattern in the features and scale we've evolved to discern in the world. Examine two trees in enough detail, and the vast differences may make the tree-scale similarity seem utterly insignificant. But alas, "particle" is also just a mental construction...