This heart requires you to grab the six soul jars.  They can be found in these locations:

1. Near the big waterfall.  Jump from the cliff by the sundial to grab it.
2. On a ledge along the path between the entrance to Timberdale and the Hall of Heroes.
3. Between the bridges along the same path.  Climb the wall to the top and you find it on a ledge.
4. On the rotating bridge near the big tree.  You can't miss it.
5. Behind the tree in the water.  Make your way around the tree, drop into the water, and then climb onto the bounce pad.
6. At the top of the poles by the big tree.  Follow the path that goes up the tree until you see a pole.  Jump from each pole to the next until you reach the one that you can climb all the way up.  You'll enter a room with the jar.

After you have all six, the heart will appear along the path that goes up the tree.