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.