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iForest -Biogeosciences and Forestry
IntroductionArid and semi-arid regions cover more than one-third of the surface of the Earth, making these systems the most common types of biomes in the world (Reynolds 2001, Gao et al. 2012. The vegetation and soils in these regions represent approximately 46% of the global terrestrial carbon (C) stock (Verhoef et al. 1996, Lal 2002. Thus, arid and semiarid ecosystems play an integral role in the response of the global C cycle to climate change (Melillo et al. 1993, Housman et al. 2006, Lufafa et al. 2008). However, ecosystems in these areas are particularly vulnerable to environmental constraints and human activities (Puigdefábregas & Mendizábal 1998, Gao et al. 2012. Previous studies showed that rising temperatures and shifting precipitation patterns associated with climate change will lead to the degeneration of community structures and functions in arid ecosystems (Ehleringer & Cooper 1988, Midgley et al. 2004. Elevated atmospheric CO2 concentrations may cause the temperature of arid regions to increase sharply (Schlesinger et al. 1990, Maestre & Cortina 2004. Areas affected by desertification have lost twothirds of their C, mainly through the loss of vegetation and soil organic matter (IPCC 1996).Revegetation is one of the most effective methods for combating desertification and preventing soil C loss in arid and semi-arid regions (Yang & Wu 2010, Amiraslani & Dragovich 2011, Corona et al. 2012, D'Odorico et al. 2013. Shrub species are usually selected for revegetation, and they are therefore the dominant vegetation in such regions (Lufafa et al. 2009, Zhang et al. 2009, Conti et al. 2013. Many studies on the C stock and allocation in shrub ecosystem of arid and semi-arid regions have been conducted on the Mediterranean coast, in Africa, and in Latin America (Farage et al. 2007, Trumper et al. 2008, Corona et al. 2012, RuizPeinado et al. 2013. Nevertheless, studies on this topic are scarce in China. China possesses 334 000 km 2 of desertified or desertification-prone lands, mainly in the north (Feng et al. 2000). Vegetation restoration, including revegetation using sand-binding species in desert regions to reduce the effects of desertification, has been applied to more than 2.4 million ha of degraded land in China (Li et al. 2004, 2007a, Zhang et al. 2009, Gao et al. 2012. Given the large distribution area, accurate and reliable estimates of C stock in the desertification regions are important in the development of effective policies and strategies to mitigate climate change. A number of studies have shown that stand age may have a significant effect on the changes in C stock and allocation among different ecosystem components, such as trees, understory vegetation, forest floor, and mineral soil (Turner et al. 1995, Peichl & Arain 2006, Peichl & Arain 2007, Noh et al. 2010. Tree biomass and C stock increase with stand age, and the allocation and growth rate of tree biomass and C pools vary across stands with different ages (Tobin & Nieuwenhuis 2007, Li et al. 2011, ...