Shade is death--slow death to the life of trees.
These little aspens are fightin' for place in the sunlight.

It is a merciless battle.

They push an' bend one another's branches aside an' choke them.

Only perhaps half of these aspens will survive, to make one of the larger clumps, such as that one of full-grown trees over there.
One season will give advantage to this saplin' an' next year to that one.

A few seasons' advantage to one assures its dominance over the others.