There are many reasons to be thankful for trees. Besides being beautiful and giving shade, they provide habitats for birds, insects, and other animals, and they are essential for the production of oxygen, which is vital for life on Earth. Additionally, they supply important products like wood, paper, fruit and nuts. The livelihoods of more than 1.5 billion people worldwide—about 20 percent of the global population—depend on trees.
Unfortunately, trees are not present everywhere—only about one-third of Earth’s surface is forested. Forests tend to fall into one of three types based on their location: boreal, temperate, and tropical. Boreal forests are located the farthest north, temperate forests grow in the mid-latitudes, and tropical forests are found closer to the equator. Countries with the largest forested area include Russia, Canada, Brazil, China, and the United States.
Forests are concentrated in particular places because trees, like other plants, require specific conditions to thrive. Fertile soil, sufficient nutrients, sunlight, and adequate rainfall are all important for tree growth. In places where the soil is of poor quality or lacking in nutrients, tree growth may be stunted or not occur at all. For instance, trees atop mountains are typically much smaller than those at lower elevations, because soils tend to be poorer on steep slopes. Likewise, places that do not receive adequate sunlight or rainfall may be largely devoid of trees. Temperature also matters—most types of trees do not do well in extremely hot areas like deserts or perpetually frozen landscapes. Trees are a bit like Goldilocks: They want conditions that are just right.
Even when trees do have the necessary climatic conditions, they can be hurt by natural processes such as pest infestations. One serious pest, for instance, is the mountain pine beetle (Dendroctonus ponderosae), a black insect roughly the size of a grain of rice. This menace has ravaged more than 260,000 square kilometers (100,000 square miles) of forest in the western United States and Canada. The mountain pine beetle kills trees by clogging the trees’ connective tissues.
Another threat to forests is fire. Lightning strikes from thunderstorms can set entire forests ablaze, and heavy winds can quickly spread a fire. Forest fires have become an increasingly frequent occurrence in the western United States, though they are often started by people—sometimes even intentionally by arsonists. Forestry experts point out that fires have always been part of the natural cycle in forests. However, as drought and high temperatures have become more common, forest fires are becoming larger and more dangerous.
Less common phenomena affecting forests include landslides, earthquakes, and volcanic eruptions. In May of 1980, the explosive eruption of Mount St. Helens in the U.S. state of Washington sent out a shockwave that toppled thousands of trees and stripped others of their branches. The eruption also triggered a series of volcanic mudflows that ripped trees from the ground and scattered them across the landscape.
Human activities take a serious toll on forests. Some forests are clear-cut—completely cut down—for timber, to make room for new trees, or to simply clear the land for another purpose. When new trees are planted, they are often selected because they grow rapidly and because their resources—wood or fruit, for example—can be harvested and sold. In other cases, clearcutting is done to make room for grazing livestock. Clearcutting is occurring in many regions, from the northwestern United States to South America to Africa, and it can be devastating for forest ecosystems in many ways. In addition to being an eyesore, swaths of razed forest are more susceptible to erosion because tree roots are no longer holding the soil in place. Furthermore, clearcutting reduces the diversity of animals because a large portion of their habitat has been destroyed and they must flee to find new shelter. Clearcutting is also harmful to the millions of indigenous people around the globe who live in or near forests, and many of these people rely on forests for their food, shelter, and even their livelihoods.
Because trees are not distributed equally around the planet, some regions possess more forest resources than others, and this inequality has important economic and social effects. To begin with, in areas that lack forests, there are no forest products to be harvested for financial gain. People in those areas also miss out on secondary financial benefits such as revenue from tourism and fees from hunters going after large game animals like deer and moose. Aside from financial considerations, forests serve as cool and calming refuges—for people as well as animals.