Trees contribute to our environment and well-being in a wide variety of ways. The Trees Are Good website of the International Society of Arboriculture describes the many benefits that trees offer, including societal, communal, environmental and economic.
This snappy little YouTube video sums it up:

This article “What is a tree worth?” shows how scientists are discovering how they play a crucial role in the green infrastructure of American cities.
Trees’ role in energy conservation in the home is well illustrated by this video from This Old House.
