China

A new technique to halt desert
encroachment:
Shelterbelts surrounding
the Xinjiang Oasis
china
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Region Eastern Asia
Capital Beijing
Surface area 9,596,961 km2
Population 1,323,636,000 inhab.
Population density 138 inhab./km2
Infant mortality rate (per thousand births) 35
Fertility rate (births per woman) 1.7
Population growth rate (per annum) 0.6 %
Life expectancy (female-male) 73 – 70 years
Average temperatures (min./max.) 6.5 / 17.7 ºC
Forest area 18 %

Introduction

Surrounded by mountains and far from any sea or ocean, the autonomous region of Xinjiang Uygur in the north-west region of the People’s Republic of China represents the largest stretch of drylands in China.
Xinjiang territory stretches over 1,650,000 km2 of which 49.5% are mountainous zones and 22.5% is desert. This temperate desert is subject to winds uplifted by the Tibetan Plateau and differs from sub-tropical deserts found in other regions of the world in that it is much cooler and is influenced by a continental climate.
The Xinjiang oases are distributed according to the availability of water supplies. To prevent the desert sands from encroaching on the oases, the local populations have developed a shelterbelt system of green hedges: trees and shrubs stabilize the dunes and reduce wind erosion thus halting desert encroachment.

The Xinjiang oases

The most prominent landscape feature of the oases are the vivid green colours of the cultivated and natural woods, in stark contrast to the yellow or grey colours that predominate in oases of other deserts.
The trees of the oases indicate the key role that water plays in the oases ecosystem and the relative abundance of water resources available on surface and underground areas. The mechanical and chemical weathering of rocks, their granular and sandy textures containing low levels of organic matter, form desert soils that are relatively poor in nutrients. On the other hand, soluble minerals are abundant and accumulate on the surface.
The stresses subjected by the inhabitants in Chinese oases can be summarized as follows: severe drought, accompanied by large variations in the quantity of available water, which produce unfavourable conditions for agriculture; severe temperature fluctuations during the year and even during the day; violent winds that frequently generate sand drifts and serious wind erosion. The region equally suffers from intense evapo-transpiration, aridity, excessive soil and groundwater salinity from the lack of organic matter and nitrates in the soil.

C1M1
1. Tree plantations used to stabilize the sand
in the Xinjiang desert.
© Yang Youlin

badCauses and effects of desert encroachment

China is one of the countries in the world that is seriously confronted with the problems of desertification (picture 2.). The affected land surface covers approximately 2,622 million km2, representing 27.3% of the total land territory in China. Despite partial improvement and effective control, desertification is spreading and the land is deteriorating at an estimated rate of 2,460 km2 a year across the whole of the country. It is estimated that 400 million people are suffering from the impact of desertification and the effects of sand dust that attack skin and lungs.

Desertification in China is mainly caused by human induced factors and by extreme climatic conditions. Human factors such as population growth, intensive agriculture, irrational use of steppe for cereal production, overgrazing, unsustainable exploitation of firewood and medicinal plants, poor water resource management,
oil exploitation and excessive mining are all significant factors. These activities, coupled with poor awareness on combating desertification and environmental protection, have destroyed the vegetation cover and accelerated the uncontrolled advance of the desertification process.

goodPossible solutions

There are two basic ways to protect the oases so that they conserve a balanced and stable ecosystem, favourable to human activities. Artificial oases can rely on important energy and technical inputs to create an artificial environment in the form of a closed system. On the other hand, ecological oases can protect themselves from desert encroachment mainly by biological factors. Providing water resources are sufficient, if they are used in a traditional way, the shelterbelt system has shown itself to be efficient in establishing and protecting ecological oases.

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2. In Xinjiang Province in China, due to its continental climate,
the arid zone is characterized by cooler sub-tropical deserts.
© Sisavang Sissombat UNESCO

bladShelterbelt systems

According to the characteristics of the Xinjiang oases, various construction models of shelterbelts have been employed:

Thus, from the perimeter to the interior of the oases, different kinds of shelterbelt systems and forest networks could be introduced according to the specific needs of the area under consideration. The shelterbelts are composed of diverse plantations since it involves constructing a complex ecosystem composed of trees, shrubs and grasses.

The shelterbelt or shrub-grass belt at the perimeter of the oases is designed to control sand movements and to prevent the fringe area from being overwhelmed by desert sand or threatened by wind erosion. Studies show that due to its roughness and friction, the ground wind speed is very much reduced by the presence of a shrub-grass shelterbelt standing 50 cm to 60 cm high.
The effectiveness of the shelterbelt depends on its width, vegetation cover and plant composition.

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3. A shelterbed of shrub-grass in the fringe area of the oasis.
Yang Youlin

In areas threatened by erosion, land degradation can be curbed when vegetation cover reaches 65% and the soil surface becomes stable. In areas where sand accumulates, vegetation can reduce the development of dunes once the vegetation covers 40% of the surface. The wider the barrier, the greater its effectiveness in protecting the oases. In general, the width of the shelterbelt should not be less than 200 metres.

Based on observations of sand movements caused by wind and avalanches, the shelterbelt with a width of 100 metres controls 90% of total sand movements. A belt of 244 metres wide fixes 97% of moving sand.
Moreover, it can be noted that certain species of grass and shrubs supply fodder and food rich in nutrients for cattle in the desert zones.
In the fringe areas of the oases, planting trees especially poplars and elm prevents sand from accumulating around the oases.

In areas of dense plantations, the moving sands accumulate at the edge of the shelterbelt, on the windward side to form high, large longitudinal dunes. The average sand accumulation around the shelterbelt is 12.48 m2 per metre whereas if the structure of the plantation is more sparse, the sand accumulates on the sheltered side of the shelterbelt to form long and flat longitudinal dunes resulting in an average sand accumulation of 9.1 m2 per metre.

In order to conserve irrigation water and minimize reforestation investment, narrow belts composed of two rows of tamarisk species is recommended (as opposed to poplars and elms planted to prevent sand accumulation in fringe areas).
In the interior of the oases, forest networks composed of four to six narrow rows of trees protect agricultural land. The protective role of the forest network is closely related to the distance between the trunks and for this reason, in Xinjiang, the distance between the principal rows has been reduced to increase the effectiveness of the shelterbelt against the spreading of the desert.
The shelterbelt also acts as a biological drainage system that plays an important role in improving the soil in the Xinjiang oases. In Anjiahi, in the northern part of Xinjiang, the groundwater level of farmland could thus be lowered by between 20 cm to 70 cm and the salt concentration of the topsoil could be reduced.

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4. Forest network at the interior of the oasis.
Rows of trees stop the wind and sand and protects the crops.
© Yang Youlin

In addition, shelterbelts protecting farmland also supply a great deal of timber and other wood by-products. The tree network creates a microclimate improving the ecological environment of the farmlands. For instance, the rate of water consumption for one kilogram of wheat or corn has decreased by between 15% to 22.8% with the tree network.

Conclusion

In the Xinjiang oases, the shrub-grass shelterbelt systems and the tree networks inside the oases prevents the fragile ecosystems from degrading. Since the construction of shelterbelts in Xinjiang, it has been observed that:

  • wind speed has decreased.
  • soil surface roughness has increased due to the shrubs and grasses.
  • sand invasion has been reduced and the mobile dunes have stabilized.
  • light reflection rate has decreased due to vegetation cover.
  • the concentration of air humidity has increased due to forest transpiration.
  • the microclimate has improved agricultural productivity.
  • the forest network supplies the inhabitants with firewood, wood by-products and fodder.
  • trees provide shade during the hot season.
  • poverty has decreased thanks to the large plantations of fruit trees and the harvesting of seeds and fruit.

Thus the shelterbelt system to protect the oases in the Xinjiang desert of northern China brings about positive effects for preventing desertification and improving production and living conditions in the desert.

This case study was proposed by Mr Yang Youlin.
For more information, please contact him at the following address:

Mr Yang Youlin
Division Director
National Bureau to Combat Desertification
18 Hepingli Dongjie
Beijing 100714
China

Tel. (+86) 10 8423 8848
Fax (+86) 10 8423 8828
E-mail: yangyl@caf.forestry.ac.cn
E-mail: sds@forrnax.africaonline.com

activities

pencil

The teacher explains
the shelterbelt system
to the class.

pencil

Where is China situated?
What characterizes the climate of China compared to your country? Are the problems of desertification in the Xinjiang oases similar
to those in your region?
What are the differences?
What are the similarities?

smile

Draw the oases of the Xinjiang desert.
Show how the sand dunes
are displaced by the wind. Draw the tree plantations
that protect the oases.
Are there some found
in the interior of the oases
to protect the crops?

smile

What role does each tree play in the shelterbelt? Imagine trees that speak who tell us of its fight against desertification. What would they say? People plant trees around their crops to protect them from the wind and also to harvest fruits and wood.

smile

Imagine that you construct
a shelterbelt in your region. Where do you think such
a system could be useful? Which species would you plant? What would you have to do
to ensure the effectiveness
of the shelterbelt?

smile

Tick the correct answers from the following:

In China, the shelterbelts:

• stabilize the soil.
• act as a windbreak.
• are wooden barriers.
• protect the oasis.
• prevent sand from
covering farmland
and crops.
• combat violent rainfall.


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