Carbon and Water Tradeoffs in Conversions to Forests and Shrublands 237-246

Chapter 1919.1IntroductionCarbon sequestration programs on land and in theoceans are gaining attention globally as a means to off-set increasing fossil fuel emissions and atmospheric car-bon 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 nationsnow have national plans to foster land-based sequestra-tion; Australia’s Plantations 2020 program is one exampleof such a national effort (Polglase et al. 2000). Oceanbased sequestration, particularly deep ocean pumpingof CO2 and iron fertilization, is also receiving consider-able attention, although it remains even more controver-sial 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 sustain-ability of sinks and markets, programs for emissions trad-ing and carbon credits are underway, including the Chi-cago Climate Exchange and the European Union Green-house Gas Emission Trading Scheme.On land, many biological sequestration programsemphasize storing carbon in soil organic matter in agri-cultural fields, in woody encroached sites, and in the soilsand wood of plantations. Land-based sequestration inagricultural soils restores all or part of the soil organiccarbon (SOC) lost with plowing and intensive agricul-ture (Gebhart et al. 1994; Lal et al. 1999). No-till and low-till management are additional approaches proposed forincreasing soil organic carbon in croplands.For plantations, the most controversial factors for car-bon sequestration and management include the feasibil-ity and permanence of the carbon sequestered, the scaleof management needed to offset anthropogenic emis-sions, and the accompanying biogeochemical changesthat would occur. As an example, a carbon sequestrationrate of 3000 kg C ha–1 annually in plantations would re-quire an area the size of Texas or Pakistan to offset0.2 Pg C yr–1 of emissions; such a rough calculation ig-nores economic and biophysical limitations to storageand downstream losses of carbon as the wood is pro-cessed. A more complete evaluation of the feasibility ofcarbon storage by vegetation management, both scien-tifically and economically, is needed, including a morecomplete biogeochemical accounting of the conse-quences. The biogeochemical interaction that we exam-ine in this chapter is water availability; other key inter-actions, such as with nitrogen, are beyond the scope ofthis chapter (e.g., Vejre et al. 2001; Dalal et al. 2003).Here we will examine some of the potential benefitsof biological sequestration programs on land, some ofthe uncertainties surrounding them, and some uninten-tional consequences if they are initiated broadly. We willalso address a related land-cover change, woody plantencroachment, which has important consequences forcarbon and water cycling. Woody plant encroachmentdiffers from afforestation and abrupt land-cover changesbecause it occurs over many decades. However, its glo-bal extent, potential for carbon sequestration, and simi-larities to afforestation make it important to address. Forthese land cover and land-use changes we will estimatepotential carbon sequestration rates, explore key bio-physical interactions, and discuss examples of otherbiogeochemical and hydrological changes that may oc-cur. For example, plantations may be the most beneficialenvironmentally when they are used to ameliorategroundwater upwelling, but they may also decrease wa-ter yield (defined as the amount of water from a unitarea of watershed) (Herron et al. 2002; Farley et al. 2005).Our long-term goal is to identify these biogeochemicaland hydrological costs and benefits that accompany se-questration scenarios.

19.2Afforestation19.2.1Afforestation: Carbon Storage PotentialFor purposes of this chapter, afforestation is defined asthe conversion of rangelands and agricultural lands totree plantations. Forest plantations grown on rangelandsor agricultural lands have large sequestration potentialsbecause carbon can be stored rapidly in biomass poolsthat are large, have relatively slow turnover times com-pared to the previous vegetation, and have wider carbonto nutrient ratios (e.g., Paul et al. 2002; Halliday et al.

Carbon and Water Tradeoffs in Conversions to Forests and ShrublandsRobert B. Jackson · Kathleen A. Farley · William A. Hoffmann · Esteban G. Jobbágy · Rebecca L. McCulley