Reading 2

TROPICAL AND TEMPERATE RAINFORESTS

1                                  Earth’s oldest living ecosystems, rainforests are areas of high humidity, abundant rainfall, and lush vegetation. Tropical rainforests cover a large part of the planet, while temperate rainforests occur in only a few regions. In the Western Hemisphere, tropical rainforests are found in equatorial Central and South America, where rainfall is greater than 250 centimeters per year. Temperate rainforests occur in the mid–latitudes on the western edge of North and South America, where moist air from the Pacific Ocean drops between 150 and 500 centimeters of rain in a year. Tropical rainforests are characterized by a warm, wet climate with an even distribution of rainfall annually, often two wet seasons and two dry seasons, with dry periods lasting only a few months. There is only slight temperature variation throughout the year. Temperate rainforests, in contrast, have seasonal variation: typically one long wet winter or spring, followed by a dry summer. In the American temperate zones, a range of mountains traps moisture from the ocean, protecting the forests from severe weather extremes. Winter temperatures rarely drop below freezing, and summer temperatures seldom exceed 27 degrees Celsius.

2                                  The tropical rainforests of Central and South America are characterized by luxuriant vegetation: tall, dense jungle, with thick vines covering the trees. The most common trees are broad–leaf evergreens including palm, bamboo, and tree ferns. Tropical rainforests are typically divided into four layers, each with different plants and animals adapted for life in that particular area. At the top is the emergent layer, which contains a small number of very tall trees. Next is the canopy, a nearly continuous cover of foliage formed by the treetops. The canopy is the densest area of biodiversity, home to around half of the forest’s plant species, including large trees 30 to 45 meters in height. Below the canopy is the understory, filled with large–leaf plants, vines, and shrubs, and home to many insects, birds, reptiles, and mammals. The bottom layer, the forest floor, is relatively clear of vegetation because only a small amount of sunlight penetrates the forest’s upper layers.

3                                  The temperate rainforests of North America have a less complex ecology. The topmost layer is dominated by a few species of tall coniferous evergreens such as spruce, hemlock, cedar, and fir, along with deciduous big–leaf maples. The understory consists of small, shade–loving trees such as dogwood. The temperate forest has a jungle–like appearance, but ferns and mosses are more common than vines. The forest floor is littered with needles, leaves, twigs, and fallen trees—all of which lie on and under a thick carpet of mosses, lichens, grasses, and small plants. The low–growing plants are shade tolerant because little sunlight extends down to the forest floor. The mild climate has produced an ecosystem in which change occurs slowly, giving the illusion that time itself moves slowly. Trees in the temperate rainforest live very long lives and grow to immense sizes. When fully grown, some trees reach heights of 85 meters.

4                                  When trees and plants die, their nutrients are recycled to maintain the forest. Nutrients in rainforests are recycled at various rates, due to differences in rates of decomposition and variations in soil chemistry. In tropical rainforests, the warm temperatures and abundant precipitation cause most organic material to decompose in a few months to a few years. Because decomposition is so rapid, relatively little organic material accumulates on the floor of tropical forests. As the vegetation dies and decays, the roots of the trees quickly absorb the nutrients. Most of the nutrients supporting the rainforest—about 75 percent of those in the ecosystem—are stored in the woody trunks of trees, while only about 10 percent are contained in the soil. The relatively low concentrations of nutrients in the soil are a result of the fast cycling time rather than a shortage of these elements in the ecosystem.

5                                  In temperate rainforests, decomposition is much slower, taking four to six years. More dead plant material accumulates as litter on the forest floor, where it is slowly digested by fungi, insects, and bacteria. The abundance of organic matter on the ground creates soil rich in nutrients, with the soil containing as much as 50 percent of all the nutrients in the ecosystem. The nutrients in the forest litter and soil may remain there for long periods of time before being absorbed by the trees. The nutrient–rich soil makes the temperate rainforest less vulnerable to the effects of destruction and the recovery period faster than it is for the tropical rainforest.