Tree Image Gallery

National Geographic/­Getty Images
Great forests like these once greeted newcomers to North America. See more tree pictures.

­When European settlers arrived in North America in the 1600s, they found vast forests stretching from the Atlantic Ocean to the Mississippi River. N­early 40 percent of the land north of Mexico lay beneath tree cover [source: Maycock]. Canada and Alaska held much of this forestland, but even below the Great Lakes, trees dominated the landscape.

While the primeval forests of the New World presented challenges, they offered a resource -- timber ­-- that would help the colonies grow and prosper. Wood harvested from forestland provided light and heat. It yielded homes and their furnishings. It became the building block of tools and machines. And it formed the framework of wagon and ship, enabling the efficient transportation of goods both across the country and across the sea.

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­Now, even though log cabins are a historical footnote and fossil fuels reign supreme, forests a­nd forest products remain critical natural resources. And not just in North America. Approximately 30 percent of the planet's land area is forestland, with Russia and Brazil holding the largest tracts [source: Food and Agricultural Organization of the United Nations]. Globally, the forest industry employs 13 million people and produces 123.6 million cubic feet (3.5 million cubic meters) of roundwood [source: Food and Agriculture Organization of the United Nations].

Like any global enterprise, the timber industry is complex and has spawned its share of idiosyncratic vocabulary and technology. This article will examine the world of trees -- from top to bottom, you might say -- to understand exactly how timber works. We'll focus on the forests of the United States, but the principles of planting, growing and harvesting trees is consistent regardless of location. So too are the major threats, such as deforestation, invasive species and wildfires.

Let's get started with a basic question: What's the difference between forest and timber?