4.4 Challenges for afforestation in Horqin sandy land
Degradation of grasslands reduces ground cover, leaving bare surfaces
exposed directly to erosion by strong winds. In these damaged landscapes
where soils are sandy and winds speeds are high, large quantities of C
are lost to wind and to water (Gomes et al. 2003, Li et al. 2004, Su et
al. 2004, Hoffmann et al. 2011). Afforestation, either by planting trees
or by allowing natural forests to regenerate, is effective in restoring
degraded soils and ecosystems, one of the principal advantages of these
measures being that they deploy natural processes instead of potentially
expensive artificial processes (Li et al. 2013). Dryland forests play an
important role in the stabilization of land, reduction of erosion,
protection of watersheds, and the control of desertification. However,
tree planting in drylands remains controversial because the trees
compete with other vegetation for water. Not all trees can be grown
everywhere, and trees should be planted only when and where the water
available in a dryland ecosystem to sustain their growth and survival in
the long term, while permitting the survival and growth of other
vegetation. Trees have also been dying in recent years, and Wang et al.
(2010) and Yan et al. (2011) attribute the deaths partly to the warmer
and drier climate associated with climate change, because the warmer
climate is expected not only to decrease the availability of water to
trees greatly but also to increase transpiration. Therefore, the choice
of suitable tree species is particularly important for the afforestation
of degraded lands in arid and semi-arid regions (Reubens et al. 2011).
Further research will be needed to determine the minimum rainfall
required to support each species used in afforestation, because many
species cannot survive under these conditions (Li et al. 2017). This
work will help managers to choose species that are most likely to
survive in a given area. On the other hand, the need for a more
sustainable trade-off between protecting and improving the local
environment and developing the local economy is becoming increasingly
acute in the Horqin sandy lands.
In the present study, comprehensive analysis shows that grasslands
enclosed by a shelter belt of Populus × beijingensis or
planted with Pinus sylvestris var. mongolica or withUlmus pumila in widely distributed but small pockets result in a
mosaic of pure forests across the landscape, which is dotted with large
trees—a landscape that is different not only from the complex
structure of natural forests but also from the simple structure of
grassland, with different levels of reflection, a unique community
structure that offers protection from strong winds, effectively reducing
wind speeds that affect the herbaceous layer. Populus ×beijingensis, Pinus sylvestris var. mongolica , andUlmus pumila are better adapted to the moderately desertified
grasslands in the south-eastern Horqin sandy land in northern
China. Therefore, afforesting grasslands through establishing
small but widely distributed pockets of trees or by enclosing the
grasslands with a shelter belt is a suitable approach in semi-arid areas
that may be lacking in adequate levels of soil moisture and available
nutrients.