The history of forests

The first forests developed in marshlands about 365 million years ago, toward the end of the Devonian Period. They consisted of tree-sized club mosses and ferns, some of which had trunks nearly 40 feet (12 meters) tall and about 3 feet (1 meter) thick. These forests became the home of early amphibians and insects. By the beginning of the Carboniferous Period-about 360 million years ago-vast swamps covered much of North America. Forests of giant club mosses and horsetails up to 125 feet (38 meters) tall grew in these warm swamps. Ferns about 10 feet (3 meters) tall formed a thick undergrowth that sheltered huge cockroaches, dragonflies, scorpions, and spiders. In time, seed ferns and primitive conifers developed in the swamp forests.

When plants of the swamp forests died, they fell into the mud and water that covered the forest floor. The mud and water did not contain enough oxygen to support decomposers. As a result, the plants did not decay but became buried under layer after layer of mud. Over millions of years, the weight and pressure on the plants turned them into great coal deposits.

Later forests. As the Mesozoic Era began, about 248 million years ago, severe changes in climate and in the earth's surface wiped out the swamp forests. In the new, drier environment, gymnosperm trees became dominant. Gymnosperms are plants whose seeds are not enclosed in a fruit or seedcase. Such trees included seed ferns and primitive conifers like those that grew in the swamp forests. They also included cycad and ginkgo trees, which became widespread. Gymnosperm trees formed forests that covered much of the earth. Amphibians, insects, and large reptiles lived in these forests.

The first flowering plants appeared during the early Cretaceous Period, sometime after 145 million years ago. Flowering plants, which are called angiosperms, produce seeds enclosed in a fruit or seedcase. Many angiosperm trees became prominent in the forests. They included magnolias, maples, poplars, and willows. Flowering shrubs and herbs became common undergrowth plants.

At the start of the Cenozoic Era, about 65 million years ago, the earth's climate turned cooler. Magnificent temperate forests then spread across North America, Europe, and Asia. The forests included a wealth of flowering broadleaf trees and needleleaf conifers. Many birds and mammals lived in these forests.

Modern forests. The earth's climate continued to turn colder. By about 2.4 million years ago, the first of several great waves of glaciers had begun to advance over much of North America, Europe, and Asia. By the time the last of these glaciers had retreated-about 11,500 years ago-the ice sheets had destroyed large areas of the temperate forests in North America and Europe. Only the temperate forests of southeastern Asia remained largely untouched.

The forests of the world took on their modern distribution after the last of the glaciers retreated. For example, the great boreal forests developed across northern Europe and North America. But the world's forest regions are not permanent. Today, for instance, temperate forests are invading the southern edge of the boreal region. Another ice age or other dramatic environmental changes could greatly alter the world's forests.