while much forest land has been con

while much forest land has been converted to other uses, the net area of forest land has remained rela￾tively stable since the 1920s (Alig et al. 2003). As shown in Figure 1.1, about 70% of the original amount of forested land still remains as forest, although much of it is likely modified from its struc￾ture and composition in 1600. Since 1932, however, as farmed land acreage decreased, forest area in the United States has been increasing. Forests have been the beneficiary of the conversion from ani￾mal power to mechanical power in farming. An estimated 20 million acres of grain fields and pas￾tures were no longer needed when gasoline tractors replaced horses and mules. As agricultural productivity per acre increased, as a result of plant breeding, fertilizers, and pesticides, forests have reclaimed many acres back from farm fields.Wood has remained an important substance throughout history because of its unique and useful properties. Wood is recyclable, renewable, and biodegradable. Many species are shock resistant, bendable, and stable (although all wood changes dimensions as it loses or gains moisture). Density among species varies greatly; the balsa popular with model-airplane builders can weigh as little as 6 lb/ft3, while some tropical hardwoods weigh more than 70 lb. Wood and lignin can be converted to many useful industrial chemicals, such as ethanol and plastics. Wood can be treated to resist decay, and with proper construction techniques, and stains or paint, wood buildings can last hundreds of years. The oldest surviving wood structure is an Asian temple, built in the seventh century. Today, wood is used in tools, paper, buildings, bridges, guardrails, railroad ties, posts, poles, mulches, fur￾niture, packaging, and thousands of other products.Wood’s versatility makes many wood products recyclable. Perhaps the earliest and simplest recy￾cling was the burning of used wood for heat, whether in a wood stove, fireplace, or furnace. New technologies are improving the efficiency with which used or scrap wood can generate electricity and heat. The paper and paperboard industry has recycled paper to augment virgin wood pulp for decades. At first, recycled paper generally found its way into newsprint and other low-grade prod￾ucts, but recent advances in recycling technology permit used paper to go into the manufacture of higher quality papers, where appearance, texture, and consistency are important. Other products, whether railroad ties or structural timbers from 60-year-old buildings, find second lives as lumber. Affluent consumers, especially in the United States, have long been willing to pay a premium for