The first trees were actually ferns, some reaching heights of 100 feet (30 meters). During the Devonian period, about 408 to 360 million years ago, ferns spread across the land.
During the succeeding Carboniferous period (360 to 286 million years ago) huge forests were established, containing a variety of plants and trees.
Plants evolved just as animals evolved, and for the same reason—to enhance their chances for survival. Some adapted to living in the shade of trees. Some grew completely along the swampy forest floor or climbed up the trunks of trees to reach the sun.
Trees with highly developed patterns of foliage evolved. Tree branches figuratively fought for the most exposure to sunlight, the plant life’s source of energy. The shape, number, and pattern of leaves evolved in order to catch the most sunlight possible. In the same way, root structures developed in competition for soil nutrients and moisture.
Dead and decaying plant life returned nutrients to the soil in the dense forests. Layers of this rotting material, called peat, were so plentiful that much of it was buried before fully breaking down into soil. Mud encased the peat and, over time, more peat and more mud compressed—as a result of gravity, lack of oxygen, and chemistry—into coal.
The age of great forests was in the Carboniferous period, which means the “coal-bearing” period.