Carbon and water tradeoffs in conversions to forests and shrublands

Abstract

Carbon sequestration programs on land and in the oceans are gaining attention globally as a means to offset increasing fossil fuel emissions and atmospheric carbon dioxide concentrations (e.g., DeLucia et al. 1999; Caldeira and Duffy 2000; Schimel et al. 2000; Pacala et al. 2001; Hoffert et al. 2002; Jackson et al. 2002; Hungate et al. 2003; McNeil et al. 2003). Many industrialized nations now have national plans to foster land-based sequestration; Australia’s Plantations 2020 program is one example of such a national effort (Polglase et al. 2000). Ocean based sequestration, particularly deep ocean pumping of CO2 and iron fertilization, is also receiving considerable attention, although it remains even more controversial than land-based programs (e.g., Chisholm et al. 2001; Lawrence 2002; Buesseler and Boyd 2003; Tsuda et al. 2003). Despite uncertainties about the size and sustainability of sinks and markets, programs for emissions trading and carbon credits are underway, including the Chicago Climate Exchange and the European Union Greenhouse Gas Emission Trading Scheme. On land, many biological sequestration programs emphasize storing carbon in soil organic matter in agricultural fields, in woody encroached sites, and in the soils and wood of plantations. Land-based sequestration in agricultural soils restores all or part of the soil organic carbon (SOC) lost with plowing and intensive agriculture (Gebhart et al. 1994; Lal et al. 1999). No-till and lowtill management are additional approaches proposed for increasing soil organic carbon in croplands. For plantations, the most controversial factors for carbon sequestration and management include the feasibility and permanence of the carbon sequestered, the scale of management needed to offset anthropogenic emissions, and the accompanying biogeochemical changes that would occur. As an example, a carbon sequestration rate of 3 000 kg C ha–1 annually in plantations would require an area the size of Texas or Pakistan to offset 0.2 Pg C yr–1 of emissions; such a rough calculation ignores economic and biophysical limitations to storage and downstream losses of carbon as the wood is processed. A more complete evaluation of the feasibility of carbon storage by vegetation management, both scientifically and economically, is needed, including a more complete biogeochemical accounting of the consequences. The biogeochemical interaction that we examine in this chapter is water availability; other key interactions, such as with nitrogen, are beyond the scope of this chapter (e.g., Vejre et al. 2001; Dalal et al. 2003). Here we will examine some of the potential benefits of biological sequestration programs on land, some of the uncertainties surrounding them, and some unintentional consequences if they are initiated broadly. We will also address a related land-cover change, woody plant encroachment, which has important consequences for carbon and water cycling. Woody plant encroachment differs from afforestation and abrupt land-cover changes because it occurs over many decades. However, its global extent, potential for carbon sequestration, and similarities to afforestation make it important to address. For these land cover and land-use changes we will estimate potential carbon sequestration rates, explore key biophysical interactions, and discuss examples of other biogeochemical and hydrological changes that may occur. For example, plantations may be the most beneficial environmentally when they are used to ameliorate groundwater upwelling, but they may also decrease water yield (defined as the amount of water from a unit area of watershed) (Herron et al. 2002; Farley et al. 2005). Our long-term goal is to identify these biogeochemical and hydrological costs and benefits that accompany sequestration scenarios.