Trees are made up of three main components: the roots, the leaves and the woody structure that connects them. The function of the roots is to bring the water and minerals to the rest of the tree. The leaves also serve to feed the tree. They absorb carbon dioxide from the air and use sunlight to combine this gas with the moisture brought up from the roots, making the simple sugars which feed the tree giving off oxygen as a byproduct.
That is the true magic of trees; they feed off of a toxic gas and provide clean oxygen in return. They are truly the earths air filter. According to David Nowak of the USDA Forest Service a persons oxygen needs could be supplied by two trees. To make up for the carbon dioxide created by the average household with a single car would take about 1/6th of an acre of trees (so start planting).
The woody structure, including the trunk, branches and twigs hold the trees leaves in position to receive the life-giving sunlight and carbon dioxide; they also act as a means of carrying the raw materials and nutrients back and forth between the roots and the leaves. The moisture taken up by the roots is pulled up by a process of capillary attraction and the osmotic action induced by the evaporation of water from the leaves. This loss of water through the leaves is called transpiration.
On a warm summer day, a single birch tree may transpire as much as 900 gallons of water. This enormous flow of water causes a continuous flow of tree sap from the roots of the tree to the uppermost leaves.
When moving a tree or working around an existing tree that you wish to preserve, the highest priority is to protect the root structure of the tree itself. The larger roots at the trunk anchor the tree to the ground and stabilize it, while the small root-hairs at the ends of the rootlets absorb the water from the ground.
The trunk of a tree is made up of the bark, the wood and the pith. The pith is the middle section surrounded by the wood. Between the wood and exterior bark is a thin layer that creates new wood on the inside and bark on the outside. This layer is known as the cambium layer. When the cambium ring is severed the tree is killed, such as when a fence wire is wrapped around a tree and wears through the bark. Damage to the cambium layer also makes a tree vulnerable to insects and disease, so anything driven into it can wound a tree severely.
Besides man himself, trees have many natural enemies. There are more than 200,000 known insects that attack trees. Diseases, such as blight, rust, and rot, just to name a few can cause tremendous amounts of damage to trees or groupings of trees. High winds, ice storms and droughts can also create a great deal of havoc with trees. Fortunately, trees have several thing going for them. They are extremely resilient and can survive even serious damage, storms and droughts are not terribly common and birds ally themselves with tree to keep most of the insects in check.