Forest health

Timber increments and removals (million m3/year): North America

North America currently grows some 255 million m3 more timber than it harvests Source: UNECE and FAO 2000

In the past, a forest was deemed healthy if it was free from disease and was growing vigorously (NRC 1999). Over the past 20 years, however, the long-term sustainability of the forest ecosystem has become the primary measure of forest health (UNECE and FAO 2000). A forest may be considered healthy when it maintains biodiversity and resilience, provides wildlife habitat, ecological services and aesthetic appeal, and maintains a sustainable supply of timber and nontimber resources (NRC 1999). In many areas, forests are becoming increasingly fragmented, biologically impoverished, and weakened or stressed (Bryant, Nielsen and Tangley 1997).

Human intervention and demand for timber and paper are the primary drivers of forest modification. Poor harvesting practices, the introduction of exotic species and suppression of natural disturbances have created large forested landscapes with an unnatural tree distribution and age structure, which has increased the forest's vulnerability to drought, wind, insects, disease and fire (USDA 1997).

Air pollution is increasingly recognized as a contributing factor to forest degradation (Bright 1999). It has played a role in the major die-off of spruce-fir forests in the southern Appalachians, a region that has been the focus of concern for the US Forest Service (USDA 1997, Mattoon 1998). Although pollution regulation has reduced acid rain in the northeast, there is evidence that reduced growth in some tree species is linked to the long-term effects of acid precipitation (Driscoll and others 2001).

An emerging issue in maintaining healthy forests is the potential impact of climate change and the connections between climate change and other damaging influences (NRC 1999). North America's forests, particularly its broadleaf ecosystems that appear to have a large capacity for carbon absorption, are unlikely to maintain their absorption attributes in an unhealthy state (Bright 1999). As management practices place greater value on non-timber attributes, as more forested lands are protected from logging, and as a weakened forest's ability to absorb carbon is questioned, it becomes increasingly important to reduce North America's consumption of both wood products and fossil fuels.