Ci
Longjun
(Chinese Academy of Forestry, 100091, Beijing)
Abstract This paper, by applying the theories of landscape ecology,
illustrates the role and mechanism of windbreak system in the establishment and
maintenance of oasis ecosystem on the basis of systematic analysis of
characteristics and ecological crises of China's desert, and especially of
desert zone in western China. Furthermore, direct economic benefits are summarized.
Key Words: oases ecosystem, windbreak
system, ecological landscape effects, economic benefits, desert zone, western
China
The most prominent sign of landscape in
irrigation oases and natural oases on river banks and alluvial fans in desert
zone of western China is the lush cultivated or natural woods m vividly green
color, which is in remarkable contrast with yellow or gray color in other
desert zones. Besides showing the relatively abundant water (surface or
underground) available in the area, this living green sign also plays important
roles in landscape formation and ecological function of oases. This woodland
system in the desert is particularly interactive with surrounding environment, farmland,
plantation, and irrigation system. Residential area, etc., and is also a
significant component of desert Total Human Ecosystem (THE). It is necessary to
provide some background information about desert before discussing the
mechanisms of the system.
1 Basic
Characteristics and Ecological Crises of Desert in China
The major deserts of the world result primarily
from global, or at least from hemispheric wind pattern, and arise from the
whole general circulation (Reitan, C. R., 1968). At the same time, while many
aspects of meteorology are pertinent to and zones, few of them are unique to
deserts. Within the broad expanse of the desert zone on earth, the geographic
characteristics of China's (eastern Central Asia) desert are as follows
(1) Because of the uplifting of the Tibetan
Plateau, the westerlies, which control the formation of the desert zone on
earth, are forced northward, which makes a northward presence of the desert zone
in Central Asia. Thus an expanse temperate desert area was formed to the north
of 35"N in latitude and differed from the subtropical desert in other
areas on Earth (Figure l).
(2) Under the control of the powerful Mongo-Siberian
anticyclone and influence of the Arctic cold current, there is a severe and
long dead winter and a rather hot summer in the eastern Central Asian desert.
Although precipitation mostly concentrates in summer there, it is of no importance
because of very limited amount, thus it significantly differs from the desert
of the Mediterranean climatic type with a more moist winter and spring.
(3) Far away from the oceans in all directions
as a result of being separated by high mountains and plateaus and very little
affected by the moist maritime air currents. The eastern Central Asian desert
has combined multiple factors that cause the formation of deserts with such
features as constant high-pressure patterns, mountain rain shadows, and a
mid-continental location.
The mean annual precipitation of China's deserts
is less than 250mm, even less than 50mm in some extremely arid desert areas.
Only 3.9mm annual falls at Togtoh in Turpan Depression and less than 20mm in
southern Tarim Basin. The aridity (by Penman's equation) can be as high as 60 or
more. The temperature amplitude is serious: the maximum temperature is 47.5℃
(Turpan) and minimum temperature is -43℃ (Junggar Basin). Sunshine is sufficient
in the desert, the annual Sunshine hours are usually more than 3000 hours;
sunshine percentage is 50-80 percent. Wind is frequent and strong, it can raise
dust skywards and form dust storm even shutting out the sunlight, blown
sanddrifts cause serious wind erosion on the ground surface and make barren
gravel ‘Gobi’- a windswept field covered with gravel pavement.
Desert soils are relatively undeveloped. They
are produced almost entirely by mechanical and chemical weathering of rocks.
Despite uniformly low precipitation and high potential evaporation, they are
quite variable in their physical, chemical and biological properties. Their
textures are usually coarse sandy loam, sand, and gravel, in which contents of
organic matters are low and nitrogen concentration is poor, but soluble mineral
salts are abundant which even accumulate on the surface. The pH reaction of
soils is mostly alkaline, high groundwater table and saline crusts covered
large areas in enclosed basins can be formed inadvertently through the
irrational application of irrigation water. The lakes, which develop in the
lowest points of playas or sebkhas, are the regions in which the finest
sediments are deposited and dissolved salts precipitated. Saline soils offer
special problems for subsistence and agriculture.
Under such severe environment, the biological
process in deserts is seriously limited. The development level of desert
ecosystems is relatively low and it is usually an immature and unstable
Systems with relatively low ability in self-regulation.
The physical effects of inorganic factors usually dominate the system; thus the
entropy of the system tends to increase. In the desert ecosystem, the biomass
is low and the diversity of species is rather poor. Its food chain is
relatively short, the environmental factors are always in the margin of their
limits, even a tiny quantitative change of the factors in space or time can
cause them passing the limits and make significant differences in biological
components, even make them die out. Compared with usual ecosystems controlled
by energy, desert ecosystem is water-controlled. All processes of energy
transformation and matenals areulation are determined and carried on by water
transformation in the system (Noy-Meir, 1973). The water flow also determines
and controls the biological process in the desert ecosystem. Because of the
great variance of annual rainfall and lack of guarantee in water supply, the
variability of desert ecosystem is great. Thus it is a rather fragile,
degradable, and destructible System.
There were many ancient ruins of historical
cities buried in oblivion by sands of eastern Central Asian desert, which was a
process of destruction or disintegration for human ecosystems in desert zone.
The abandoned farmland by irrational irrigation or exploitation nowadays is
also the degraded process of the ecosystem in desert areas.
The stresses for humans (including both desert
and oasis) in China's desert could be generally listed as follows:
1)
Serious drought, great variance in animal
water supply which are unfavorable to the biological agricultural activities;
2)
extreme temperature and severe amplitude
of annual or daily temperature;
3)
strong and frequent wind, sand drifts,
and serious wind erosion;
4)
intense evapotranspiration and aridity;
5)
original and secondary salinization of
soil and saline ground water;
6)
lacking of organic matters and nitrogen
in soils; and
7)
coarse texture of soils.
2
Role and Mechanism of Windbreak Systems in the Establishment and Maintenance of
Oasis Ecosystem
There are two basic ways to control and change
unfavorable ecological factors or crises in deserts and to form a highly
productive and stable oasis, which is suitable for biological and agricultural activities
and advantageous to human ecosystem:
(1) an
artificial oasis with intensive energetic and technical inputs or an artificial
environment in the form of a close system (e.g. greenhouse), and
(2) an ecological oasis in which the desert environment
is remolded mainly by biological means.
Under the prerequisite of appropriate supply and
rational regulation of water, the windbreak system is a powerful biological and
mechanical measure for the establishment and maintenance of the ecological oasis.
It is an essential component in the oasis ecosystem and, therefore, it affects deeply
a series of ecological factors. The forest belts transform unused energy into
biomass, weaken or check harmful energy (strong winds, high temperature etc.),
convert wastewater into usable water for transpiration and regulate microclimate
or local climate. Forest belts can also accelerate nutrient circulation in the
soil so that biomass may be replenished (Figure 2).
Windbreak systems consist of the following
components:
(1) Sandbreak forest-shrub-grass belts on the
fringe of oases in seriously sand drift threatened desert area, a sand-fixation
and anti-erosion belt is planted in the front edge of the oasis, and a sandbreak
belt is established along the edge of the oases. In the area with abundant sand
supply, the multi-sandbreak belts with the combination of the trees, shrubs,
and grasses are planted. The species mainly used are Elaeagnus orycarpa, E. Moorcroftii, Ulmus pumila, Populus bolleana, P.
euphratica, Ailanthus altissima, Hippophae rhamnoides, Fraxinus americana. Haloxylon
ammodendron, Alhagi sparsifolia, Nitraria roborowskii, N. Sibirica, Calligonum spp. etc.
(2) Windbreak forest belt networks on the fringe
of and within oases This is the central part of the protective forest belt
system which plays a basic role in improving the oasis climate and maintaining
stability of the oasis ecosystem. The effective “narrow forest belts and small
grids” is dominant in field windbreak system within oases.
(3) Canal/reservoir-protective and
road-protective forest belts: The forest belts follow roads along irrigation
ditches, and are generally composed of narrow forest belts of 4-8 rows of trees
which ventilation or thin-structure.
(4)Protective forest belts and plantations for
residential areas.
(5)Husbandry protective forest belts.
(6)Shrub-grassland on wetland or saline soils.
In addition, the timber woods, plantations of
economic trees, orchards, and fuel woods should be planned and established as a
part of the windbreak system in oases.
The local arrangement and distribution of
windbreak systems in Western China's desert are related closely to the natural
landscape structure. Besides the climatic zonality or natural zonality of
eastern Central Asian desert, which has been menntioned above, three secondary
or geomorphologic zonations of deserts should be considered, namely 1) Concentric
geomorphologic-substratum pattern of desert basins (Figure 3); 2) triangular
diagram for sand desert (Figure 4); and 3) aeolian zonation pattern of desert types
(Figure 5).
Windbreak system can serve as a mechanic defense,
a filter and biological conditioner for environmental stress or crises in the
desert-oasis ecosystem. Its principal roles and functions are as follows.
2.1 As a mechanic defense
for the oasis ecosystem to Prevent or reduce strong wind, sand drifts, cold
current, and radiation
(1) Reducing wind velocity a windbreak forest
belt (shelterbelt) vertical to the prevailing wind reduces wind velocity on
both the windward and leeward sides. When designing a forest belt with optimum structure,
the windbreak effects of the forest belt system. including the total amount of
wind velocity reduction, the protected area, the maximum protective distance possible,
and so on: should be considered integratively.
There are four basic structure categories of
forest belt: l) dense structure; 2) thin structure; 3) high ventilation
structure; and 4) low ventilation structure. Low ventilation structure and thin-structure
are widely used in the construction of farmland protective forest belts in
Western China.
The characteristics of the velocity field for
forest belts with ventilation structure are as follows. Having reached the
forest belt, the airflow is separated into three parts. The first Jet stream
zone appears over the forest belt and its intensity increases with the decrease
of porosity of canopy location and the reduction of trunk height. Hence the
second Jet stream zone forms at the leeward where the tree stems of belts stems
of the belts are located. This jet stream zone gradually weakens due to the
drag friction of the ground. Another low velocity zone is in front of it
successively and then gradually reverts to field conditions. In this way the effective
windbreak distance of this kind of forest belt becomes greater.
In accordance with species composition,
wind-permeability, coefficient and trunk height of grown trees, windbreak
effect of a forest belt with ventilation structure will vary with the trunk height,
structure (permeability) of the windbreak belt as well as the wind speed and
direction. Usually, a forest belt with higher and thinner structure has better
function for reducing wind velocity in a longer distance. A very dense barrier
will bodily uplift a wind current that descends abruptly in the lee, even the
minimum velocity is lower but nearer to the belt, therefore its windbreak
function is not as good as the belt with thin or ventilation structure. Before
wind velocity recovers to its former value within a certain distance in the
lee, next windbreak belt should be set up again, therefore. A network of
windbreak system will be formed for the whole area of the oasis. According to
the research in Xinjiang (Ci, 1978), a narrower forest belt with 4-8 rows of trees
and smaller arid of the network with a distance of 250-300m between two main
belts has a much better function in wind-preventing effects. Its sufficient
protection distance could be 24-38 times the tree height (H) in the leeward.
(2) Obstructing sand-drifts and preventing wind
erosion Because of wind velocity reduction in the lee and increase of the
surface roughness by dense shrubs and grasses, the sand carrying ability of
wind is greatly reduced and, hence, the sand-drift is obstructed and the wind
erosions prevented by the windbreak system. The observation data shows that the
resistance of grass belts to airflow is 17-27 times as many as that of bare
ground. The density and height of grass belts are directly proportional to roughness
and friction resistance of airflow. After being weakened by grass belts, the
drifting sand then enters into forest belts.
(3) Keeping out the horizontal cold current the
windbreak system could protect the winter crops and fruit trees from the cold
wave to a certain degree, but radiation frost is often favored in the leeward side
of the dense belt.
(4) Sheltering
from burning solar radiation windbreak belts and, especially, intercropping
trees and crops can protect crops from the damage by burning solar radiation energy.
2.2 As a biological
filter or conditioner for the oasis ecosystem
Since the wind velocity and turbulence exchange
become weakened near the surface of farmland under the protection of thc forest
belts, a reduction occurs in the horizontal air exchange and in the vertical
exchange of heat energy and water in the air and soil. All these factors
provide good microclimatic conditions for crop growth.
(1)Improving
micro-climate and local climatic of farmlands and residential areas under the protection
of windbreak, evaporation is reduced and a considerable amount of physical evaporated
water is transferred into the form of physiological transpired water, therefore,
damage from hot and dry wind for crops is significantly reduced by the
windbreak system. The accumulation and even spreading of snowdrifts for fields
in the lee increase soil temperature (0.2-0.6℃)
in winter and soil moisture significantly in spring. Although the effect of windbreak
systems on microclimate is varied and complicated, it is usually beneficial to
crop growth and can mal<c the net increase in yield.
(2)The
windbreak belt along irrigation systems can reduce evaporation from water
surface and uprising of ground water table in adjacent land by the powerful
transpiration of trees and, hence, control the secondary soil salinization in a
certain degree, for example, a 6-8 rows forest belt with Populus thevestina and Salix
alba could reduce ground water table for 0.2-0.7m within the range of
75-100m wide in both sides of the belt. The content of salt in the soil was also
decreased accordingly.
(3) Under the forest belt, the contents of organic
matters and nitrogen in soil are prominently higher than those in the desert
soil.
(4) The trees and undergrowth of the forest
belts enriched the biological diversity of the area. Increased the links in the
oasis food chain, and, especially provided a suitable site for nesting, feeding,
and hiding of birds, which was most favored to control the insect pests in the
oasis ecosystem.
The overall effect of the windbreak system in
the oasis is to help control the entropy. Because the windbreak system itself,
as a bio-system in the desert, can convert a portion of the solar energy into
biochemical energy by means of photosynthesis, and as a result of reduction of
evaporation m the whole oasis ecosystem, limited water resource can be fully
used by transpiration. In other words, windbreak system reduces the system
entropy and increases the regulation and stability in the system. Although the
windbreak system needs considerable amount of farmland area and water for irrigation,
but from the view of the productivity. It significantly increases the total yield
m the oasis ecosystem.
3 Direct Economic
Benefits Provided by the Forest Product
(1) Fuel and small-sized woods could be provided
by thinning, selected cutting and pruning from the windbreak belts and timbers
from mature woods.
(2) The economic trees and fruit trees in the
system could produce various kinds of fruits, berries, nuts, oils and other
forest products.
(3) The shrub-grass belt could be used to moderate
grazing or mowing; the scenic and air-purifying values of the windbreak system
in the oasis are also of great importance for the human system. The windbreak
system has become one of the most important means in preventing development of
desertification and protecting agriculture and human living conditions in
China's arid and semiarid zones. The Three North Protection Forest System was
proposed and has been carrying out as one of the important eco-engineering
construction. Which had played and is playing a significant role in improving
and protecting the landscape and environment in Northern China's arid and semiarid
zones.
References
1.
Adli
Bishay and Harold Drcgne. 1991 Desert Development, Part l. New York Harwood Academic
Publishers .
2.
Anundzade,
D..1958. The influence of Forest Belts on the Productivity of the Tea Shnib. Socialisticeskoe
Selskoe hozJajstvo Azcr Daidzana. No. 8.
3.
Anai,
S.. 1960. On the Wind Velocity Profile near the Surface and Accumulation of
Sand Grams at a Coastal Sand Dune. Mcm. Kobc, Mar. Obs. 14, 129-133 pp
4.
Bagnold,
R.A., 1936. The Movement of Desert Sand. Proc. Roy. Soc., 157, A.P. 594
5.
Bagnold,
R.A., 1941. The Physics of Blown Sand and Desert Dunes. W. Morrow, London and
New York- 96
6.
Barrett,
G.W., 1987. A Problem-solving Approach to Resource Management.
7.
C.
J. Barrow, 1991. Land Degradation. Cambridge University Press
8.
Chang,
Hsin-shih. 1978. The plateau zonality of vegetation in Tibet. Act Botanic Sinica
20. (In Chinese)
9.
Chang,
Hsin-shih. 1979. The Vegetation of China. Part 16. Beijing Science Press. (in Chinese)
10.
Ci
Longjun. 1978. Ecological System-Desert, Oasis and Windbreak System. Urumqi Xinjiang
People's Publishing House. (in Chinese)
11.
Ci
Longjun, 1979. Principle and Technology for the Establishment of Forest Belt System
in Oases. Lecture on the Seminar of investigation of desertification of the United
Nations in China.
12.
Ci
Longjun, 1980. The Establishment of the Forest Belt System. Urumqi Xinjiang
People's Publishing House.
13.
Cloudsley-Thompson,
J.L.. 1954. Biology' of Deserts. The institute of Biology Tavistock House
South, Tavistock Square, London, W.C .l
14.
Cloudsley-Thompson,
J.L.. 1977. Man and the Biology of Arid Zone. William Clowes k Sons Limited.
15.
Evans,
D. D. and J.L. Thames.. 1981. Water IV Desert Ecosystems. Dowden, Hutchinson L Ross,
Inc., Stroudsburg, Penn.
16.
F.K.
Hare, 1988. Climatic and Desertification. Beijing Meteorology Publishing House.