Plants limited by light availability.

Considering that it is well known that plants are dependent on light from the sun it only makes sense that in minecraft all plants should die if they go too long without some source of light and should never appear in perpetually unlit areas. In trees death could be shown by a loss of leaves and it wouldn't need to happen all at once. This change would be small enough to make little impact on the feel of the game while still acting as a nice touch of detail for plants.