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.