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Needleaf forests

Needleleaf forests grow mainly in regions that have long, cold winters. These forests, which are also called softwood forests, stretch across Canada, northern Europe, and Siberia. Many firs, larches, and spruces grow in these northern forests, along with a few broadleaf trees, such as birches and willows. Some willows grow even farther north than needleleaf trees do. But they seldom reach more than shrub size. Needleleaf forests also blanket slopes in such mountain ranges as the Alps and the Rocky Mountains. The Canadian needleleaf forests extend southward into the Western United States, where they include many of the world's largest trees. Many California redwoods tower over 300 feet (91 meters). Tall Douglas-firs also grow in the Western United States.

A few needleleaf forests grow in warmer regions. For example, the Southeastern United States has large forests of pines, such as loblolly pines and longleaf pines. These forests provide great quantities of wood for lumber and wood pulp.

How forests spread. Many forests did not always grow where they are growing now. These forests have spread from other areas. For example, broadleaf forests grow today in parts of the Northeastern United States where only needleleaf forests grew several thousand years ago. The spread of forests from one area to another is called migration. The wind helps trees migrate by carrying their seeds beyond the forests. Animals also help spread the seeds. Trees that grow from these seeds produce their own seeds, which may be spread in the same ways. Over hundreds or thousands of years, a particular kind of tree may thus spread to surrounding areas if the climate and soil are suitable.

Several hundred thousand years ago, glaciers moved down across much of North America and Europe. These glaciers caused the forests of needleleaf and broadleaf trees to migrate south. Thousands of years passed, and the ice began to melt. As the glaciers retreated northward, forests of needleleaf trees grew up again on the land that the glaciers had covered. The glaciers moved still farther north, and the climate became warm enough for broadleaf trees. Broadleaf trees usually crowd out needleleaf trees in areas where both are able to grow. As a result, broadleaf forests replaced needleleaf forests in many regions.

Forests can migrate over fairly level land but not across oceans or mountain ranges. Yet similar types of forest trees grow in areas separated by oceans or mountains. For example, the United States has oak trees much like those that grow in Europe. Most scientists believe that many millions of years ago, all the continents were connected. Needleleaf trees developed and spread across much of the earth. Broadleaf trees developed next and also spread. Over millions of years, the continents became separated-along with their trees and other forms of life--by the oceans. Mountain ranges rose up on the continents and separated the trees on each side of the mountains. In time, many of the trees on each continent and on each side of the mountain ranges developed into different species.