Linda S. Heath, James E. Smith, and Richard A. Birdsey. 2002.

Carbon Trends in the U.S. Forestlands: A Context for the Role of Soils in Forest Carbon Sequestration.

The Potential of U.S. Forest Soils to Sequester Carbon and Mitigate the Greenhouse Effect. CRC Press. Chapter 3, pp 35-45.

INTRODUCTION

Forestlands are unlike croplands and grazing lands, in that a large amount of carbon can be sequestered for long periods of time above ground by trees and below ground in coarse roots. Carbon in trees can also be harvested, and some of the harvested carbon can be stored for long periods of time as wood products or as waste wood or paper in landfills. As in croplands and grazing lands, most of the carbon in forests is usually in the soil, with some forest types having a greater percentage in the soil than other types. The density (metric ton per hectar - t/ha) of carbon stock in mature forests is usually greater than the carbon density of cropland or grazing land would be if it occupied the same site. Cultivating land for crops in the long-term, all other things being equal, usually means emitting carbon from the soil in the form of a greenhouse gas; growing forests on cropland usually means sequestering carbon aboveground and perhaps in the soil, and an increase in carbon density. Thus, forests have the potential to increase carbon in soils for a very long time, because of the long residence time of carbon in soils, and they may be the best available option for storing carbon in terrestrial ecosystems (US DOE, 1999). In addition, aboveground components and other nonsoil belowground components of the forest have the potential to sequester a substantial amount of carbon. The purpose of this chapter is to discuss forest carbon budgets of U.S. forests, to provide the context in which to compare the soil carbon component of forests with other components of the forest ecosystem, such as trees. We present historical and current estimates of forest carbon and carbon in harvested wood, summarized by attributes such as region, forest type, and owner. Finally, we discuss uncertainties and needs for future research for national-level estimates. Although we include soil carbon estimates, our focus is on all forest carbon to highlight the importance of forest soils. For specific broad estimates of forest soil carbon, see Johnson and Kerns (2002).