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Threats

United States forests face challenges that threaten their health and even their existence. The major threats are insects and diseases, invasive species, fire risk, urban growth, and climate change.

Insects

Insects and Diseases By 2006, approximately 58 million acres (8 percent of US forests), were at significant risk from insect and disease mortality. Several major forest insects currently affect US forests:

• Gypsy Moth
• Southern Pine Beetle
• Mountain Pine Beetle
• Western Spruce Budworm
• Spruce Budworm
• Emerald Ash Borer
• Mortality of Pinon Pine
• Asian Longhorned Beetle
• Hemlock Wooly Adelgid
• Sudden Oak Death

Invasive Species

Plants that are considered invasive species are also major threats to forests. Invasive alien species have a major impact on many native species and ecosystems and are one of the most important threats to biological diversity. According to the US Forest Service, 14 percent of the plant species on non-crop lands in the Midwest are now non-native invasive plants. Estimates show that some 3.5 million acres of US forestland is infested with invasive weeds, with about 2,000 non-native invasive plant species concentrated mostly in California, Florida, and Hawaii. The species of particular concern are leafy spurge, knapweeds and starthistles, saltcedar, non-indigenous thistles, purple loosestrife, and cheatgrass in the West, and garlic mustard, kudzu, Japanese knotweed, tree-of-heaven, purple loosestrife, cogon grass, and hydrilla in the East.

Forest Fires

In the past decade, the number of wildland forest fires in the United States has increased, especially in the West. Between 1999 and 2006, fires affected an average of 5.8 million acres per year. Catastrophic forest fires continue to be one of the major forest health challenges today because of the increased undergrowth and tree density in some areas. In response, the US Forest Service in 2000 created the National Fire Plan to respond to severe wildland fires and their effects on communities, and Congress adopted new legislation, the Healthy Forest Restoration Act of 2003, to prevent future large, catastrophic fires by reducing hazardous fuels through fuel treatment programs, such as thinning and prescribed burning.

Housing Development

Today, housing development is the cause of land-use conversion from both forests and agricultural lands. Although in recent years the number of forest acres has remained relatively constant and even increased in some years, a recent study estimated that more than 44 million acres of private forestland might be converted to housing development in the next three decades. The East, the Pacific Northwest, and parts of California will likely be the regions most affected, but the Southeast has the highest rate of urban growth.

Several states have recently experienced a decline in the acreage of forestland near many urban centers as the land is permanently converted to residential, commercial, and other non-forest uses. This trend is the result of a growing population that needs land for residential and commercial development as well as highways and other infrastructure. The impact of urban growth on forests can be minimized through thoughtful urban growth planning and incentives to own, manage, and maintain forestland. For the past century, national forest inventory data show a relatively flat trend, with small oscillations, in aggregate forestland area.

Climate Change

Native tree species, and vegetation in general, are adapted to the local climate, atmosphere, soils, and disturbances (both human and natural) in a particular region. Any changes in any of these characteristics will have, to a certain degree, an effect on forest species composition. During the past century, the average US temperature has increased almost 1° F, and precipitation has increased by 5 to 10 percent, mostly coming in heavy downpours. Predictions for the 21st century suggest that changes will be significantly larger. Temperatures could rise 5° to 9° F, and droughts and flash floods are likely to become more frequent and intense if worldwide growth in greenhouse emissions continues its current trend.

Scientific models that use inventory data are predicting forest vegetation response to different climate change scenarios. According to the US National Assessment Synthesis Team on climate change, timber inventories are likely to increase because of increases in atmospheric carbon dioxide and a longer growing season. Hardwood productivity is likely to increase more than softwood productivity. The southeastern forests could potentially break up into a mosaic of forests, savannas, and grasslands. Species composition in the Northeast is likely to change, with a loss of sugar maples. And those ecosystems already constrained by climate, such as alpine meadows in the Rocky Mountains, are likely to face extreme stress and even disappear in some places. Highly managed ecosystems and plantations seemed more resistant to climate change and could even benefit from it.

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