Redwood trees are the tallest plants on the earth,reaching heights of up to 100 meters.They are also known for their longevity,typically 500 to 1000 years,but sometimes more than 2000 years.A hundred million years ago,in the age of dinosaurs,redwoods were common in the forests of a much more moist and tropical North America.As the climate became drier and colder,they declined to a narrow strip along the Pacific coast of Northern Califomia. The trunk of redwood trees is very strong and usually forms a single straight column.It is covered with a soft bark.This bark can be pretty thick,well over two feet in the more mature trees.It gives the older trees a certain kind of protection from insects,but the main benefit is that it keeps the center of the tree harmless from moderate forest fires because of its thickness.This fire resistant quality explains why the giant redwood grows to live that long.While most other types of trees are destroyed by forest fires,the giant redwood actually prospers because of them. Moderate fires will clear the ground of competing plant life,and the rising heat dries and opens the ripe fruit of the redwood,releasing many thousands of seeds onto the ground below. New trees are often produced from shoots,little baby trees,which form at the base of the trunk.These shoots grow slowly,fed by the root system of the "mother'' tree.When the main tree dies,the shoots are then free to grow as full trees,forming a "fairy ring" of trees around the original tree.These trees,in turn,may give rise to more shoots,and the cycle continues. How does a "fairy ring'' form according to the passage?
By killing an old tree. By connecting root systems. By inserting holes into old trees. By surrounding a mature tree.