In the Ecology world these trees are a biotic limiting factor.  A biotic limiting factor is a living, growing feature that limits the population in the ecosystem.  These trees limits the sunlight that plants and grass needs to grow.  However, these trees also provide a source of shelter and food to living animals.  For example, birds and squirrels get acorns from these trees.  In addition, birds make their nest in these trees. This is why trees are biotic limiting factors in the ecology world.

This picture was took by WhiteKnight83