4 DISSCUSSION
4.1 Influence of planting trees on carbon storage
Afforestation and reforestation are regarded as potentially effective strategies for mitigating the adverse effects of global climate change (Canadell and Raupach 2008, Kaul et al. 2010). The restoration of degraded lands in arid and semi-arid tracts by planting appropriate tree species is a method followed worldwide because it protects the soil, arrests desertification by increasing vegetation cover, increases the amount of C sequestered, and supplies natural resources (Kumar et al. 2001, Maestre and Cortina 2004, Nosetto et al. 2006, Grünzweig et al. 2010). The capacity of trees species to store C can be exploited to increase regional C budgets, the choice of tree species for afforestation programmes aimed at greater C sequestration being guided by that capacity (Schulp et al. 2008, Chen et al. 2016). The high potential for C storage by afforestation of desertified grasslands has been established by several researchers. For example, Hu et al. (2008) found a distinct increase in the above-ground C stocks after the former grasslands in the Keerqin sand lands had been planted with Mongolian pines and poplars, and Li et al. (2013) noted an increase in the C sink in the form of the ecosystem’s biomass component.
In the present study, we found that 20 years after planting, the amount of stored C had increased by 27.11 t/ha in the stands of P. × beijingensis , by 22.38 t/ha in those of U. pumila , and by 21.34 t/ha in those of P. sylvestris var. mongolica .Populus thus significantly outperformed the other species. These results support the view that living wood in regrown forests is a dominant sink for atmospheric CO2. We do not have data on the growth rates of the above tree species: those data would have been useful in determining the rates of C sequestration. Nevertheless, the contributions of tree species in restoring desertified sandy grassland by reducing wind erosion and by greater stabilization of surface sand are clearly established by the present study. These benefits accrue while trees sequester C and enhance local biodiversity (Kirby and Potvin 2007). The differences in biomass C among the treatments were probably closely linked to the differences in the tree species, in management intensity, and in soil quality (Giese et al. 2003, Lal 2005, Mills and Cowling 2010).
The grasslands that were enclosed by shelter belts of forest trees and those in which scattered pockets had been planted with trees had greater amounts of biomass C than that in the control (Table 3). Average C stored in biomass amounted to 19.77% of the ecosystem total and was significantly higher in FG than in UG and MG. Vegetation is a very important pool of C in sandy grasslands, where wood acts as a substantial reservoir of C (Dixon et al. 1994). Approximately 50% of C assimilated by young plants may be transferred below the ground, where it is used for building and maintaining the root system and for respiration; some organic matter from roots is lost to the soil through exudation and root turnover (Rees et al. 2005). Trees, shrubs, and perennial grasses can be grown on arid and semi-arid lands to reduce soil erosion; re-establishing vegetation is particularly important for arresting the desertification of grasslands.