It's the fact that the trees or not as densely packed, so there is more light penetrating the tree crown. A dark thick forest with no light penetration is what everyone associates with a "real" computer game forest, so I expect that's why.
I don't know if you all know it, but that area will not be included in the final version of the game. That was just for the E3 demo. I found this info at zeldadungeon.net