Heart Piece 14
This can only be done late in-game, after defeating the Imprisoned for the third time. Behind the temple, speak with Gorko to learn about Goddess Walls and how to use them. He’ll ask you to draw a specific shape on the one nearby – do so for a Piece of Heart. (This may be the second time around; if so, do it correctly once and come back at a later time.)
Heart Piece 15
To the southwest of the Great Tree, there should be a tree branch in the air that is a viable walking path. Go onto it and left, towards the Great Tree. At the other end of the path, cross the tightrope there to find the Piece of Heart.
Heart Piece 16
Once you have access to the Bomb Bag, go south of the Great Tree, in the mushroom-filled portion of the forest. Look to the north and look around there to a rocky wall. Use a Bomb to blow down this rock blockade, which reveals a Piece of Heart.
Heart Piece 17
Go to the large room with the dome-shaped structure in the middle. This is the same room in which you obtained the Beetle inside that same structure, but stay outside of the structure. Go around to the north end, on the ground, to find the Piece of Heart behind some bars. Using the Beetle or another object, hit the pink crystal switch far above to raise the bars, permitting you to grab this Piece of Heart.