First a note from the National Parks Service:
"You may notice some dead trees within the eroded bowl of the dune. This is called a "ghost forest" and tells a story of alternating stability and change. After an initial phase of active sand accumulation, a period of stability followed when trees began to grow on the dune. Later, more sand moved in and buried the trees. Two layers of buried soil within the dune indicate that there was a second period of stability and growth followed by another period of sand build-up and then the final growth of the trees and shrubs that now cover the sheltered portions of the dunes."
Just bits of trees in the sand. But, amazingly gorgeous nonetheless.