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A 10-Year Study on Techniques for Vegetation Restoration in a Desertified Salt Lake Area
Journal of Arid Environments (2002) 52: 483–497 doi:10.1006/jare.2002.1013, available online at http://www.idealibrary.com on
A 10-Year Study on Techniques for Vegetation Restoration in a Desertified Salt Lake Area
Yong Gao%*wz, Guo Yu Qiu%z, Hideyuki Shimizu%, Kazuo Tobe% Baoping Sunz & Ji Wangw %
National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-0053, Japan wInner Mongolia Agricultural University, Hohhot 010019, China zKey Laboratory of Environmental Change and Natural Disaster, The Ministry of Education of China, China Center of Desert Research, Beijing Normal University, Beijing 100875, China zBeijing Forest University, Beijing, 100083, China (Received 18 May 2001, accepted 27 February 2002) Vegetation restoration is one of the most common and effective ways to combat desertification and prevent adjacent areas from sand encroachment in many of the desertified regions of the world. However, vegetation restoration in desertified regions is very difficult because of low rainfall, the mobile ground surface, and cost. An effective, low-cost method of afforestation is urgently required. To determine such a method, a 10-year study was carried out in the Jilantai Salt Lake area. Five different afforestation areas were established: a ‘comparison area,’ a ‘land enclosure area,’ a ‘land enclosure + irrigation area,’ a ‘leveled-afforestation area’ (the dune areas were leveled and then planted with seedlings with added irrigation), and a ‘protected afforestation area’ (the dune areas were planted with seedlings, and the surviving natural vegetation was protected as much as possible). Vegetation-related parameters (survival rate, height, trunk diameter, coverage, canopy size, and density) and environment-related factors (relative humidity, wind velocity, and amount of sand encroachment) were measured by standard methods. Results show that the protected afforestation method had the following advantages: (1) the survival rate was higher for seedlings planted in the protected afforestation area than in the leveled afforestation area; (2) vigor (height, trunk diameter, coverage, and canopy size) was better in seedlings planted in the protected afforestation area than in the leveled afforestation area, especially in the beginning period of revegetation; (3) coverage (of individual species, of all planted vegetation, and of all vegetation) was larger in the protected afforestation area than in the leveled afforestation area; (4) density of naturally germinated plant species was higher in the protected afforestation area than in the other areas, showing that the protected afforestation method provided a suitable growing environment not only for planted species but also for naturally growing species; (5) in the protected vegetation area, relative humidity of air increased and wind velocity was greatly reduced; (6) after the establishment of vegetation by the protected afforestation method, sand encroachment into the salt lake area was significantly reduced. These results suggest that *Corresponding author. Fax: +81-298-50-2586. E-mail: [email protected]. 0140-1963/02/040483 + 15 $35.00/0
# 2002 Elsevier Science Ltd.
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protected afforestation is an effective method of vegetation rehabilitation that has the potential not only to be applied to arid lands in China but also to desertified areas throughout the world; (7) cost-effective calculation shows that the leveled afforestation area costs much more than other areas. # 2002 Elsevier Science Ltd. Keywords: desertification, Jilantai Salt Lake, protected afforestation, vegetation restoration
Introduction Overgrazing and cutting of trees and shrubs for fuel wood are the most important causes of desertification (Mitchell et al., 1998). Vegetation restoration is one of the most common and effective ways to combat desertification and prevent adjacent areas from sand encroachment in many of the desertified regions of the world (Ffolliott et al., 1995). Vegetation-restoration methods vary according to the natural and economic conditions. In desertified areas of China, land enclosure and plantations are commonly applied methods of vegetation restoration. Land enclosure can control overgrazing and woodcutting by enclosing an area with fences, thus enabling the natural reestablishment of vegetation. Planting seedlings is a way of artificially establishing vegetation by using native plants or plant species introduced from outside the area. Because of the low precipitation and sand-dune movement, establishment of vegetation in arid and semi-arid desert regions is one of the most difficult and costly tasks for local people (Gupta, 1995). Jilantai Salt Lake is one of the most important salt resources in China. The area of the salt lake is 120 km2, and the storage volume of salt is up to 110 million tons (Ma, 1990). Because the salt lake is located on the edge of the Ulan Buh Desert, a 9900km2 sandy desert, the quantity and quality of salt production have been seriously affected by the encroachment of sand dunes and wind-blown sand. Population growth in the area surrounding the lake has been accompanied by increased woodcutting and overgrazing, which have further damaged the native vegetation and increased windblown sand. The damage this wind-blown sand inflicts on the salt-producing environment is so serious that the annual production and daily mining are being impacted by the damage, especially in the spring season. The efforts to establish vegetation in this area have been continuing for several decades (Zhang, 1988). The most commonly used procedure has been leveling the dune areas and then planting seedlings of several species, such as Elaeagnus angustifolia, Calligonum arborescens, Hedysarum scoparium, and Tamarix austromongolica. Irrigation is carried out between 1 and 3 times a year. Hereafter, in this paper, we call this traditional method the ‘leveled-afforestation’ method. Over the past several decades, huge amounts of labor and money have been spent on the leveledafforestation method. However, the original vegetation is completely destroyed by the leveling of the sand dunes, and the replanted vegetation cannot control the windblown sand in the initial stage. Moreover, the planted vegetation cannot grow well due to the damage from moving sand. The third disadvantage of the traditional way is its high cost. The main objective of this study was to look for some suitable vegetationrestoration techniques that can overcome these shortcomings and can be applied not only to the local area but also to other arid zones. The native vegetation has survived in the local environment for a very long time and is probably the most suitable for the area. Therefore, we assumed that the best method of rehabilitating the vegetation is by ‘protecting the surviving native vegetation as much as possible’ + ‘planting seedlings.’ For convenience, we hereafter refer to this
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method as the ‘protected-afforestation’ method. To verify this hypothesis, a 10-year study (1983–1992) was conducted in the Jilantai Salt Lake area.
Material and methods Location and natural conditions of the experimental area Jilantai Salt Lake, located on the south-western edge of the Ulan Buh Desert (lat391480 N, long 1051300 E), is an elliptical basin extending from north-east to southwest. It is an evaporating salt lake typical of those in arid areas in China (Fig. 1). The land surface in the experimental area is covered by moving sands with sparse vegetation dominated by Nitraria tangutorum, Haloxylon ammodendron, Ammopiptanthus mongolicus, Reaumuria soongorica, Convolvulus tragacanthoides, Oxytropis aciphylla, Kalidium foliatum, Psammochloa villosa, Sophora alopecuroides, and so on (Fig. 2). These plant species are very resistant to both water stress and salt stress (Tobe et al., 2001). This region has a typical temperate desert climate: dry and hot in summer, cold in winter, plenty of sunshine, very little precipitation, strong winds, and frequent drifting
Figure 1. Location of Jilantai Salt Lake and Ulan Buh Desert.
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Figure 2. Natural landscape of Jilantai Salt Lake.
sands. The characteristics of the local weather, from the local meteorological station’s 30-year record, are summarized in Table 1. The depth of ground-water is 1–3 m and the water quality is poor and brackish. Besides this shallow ground-water, there is a deeply buried layer of relatively good quality ground-water, which is the main source of water for irrigation and domestic use (Mitchell et al., 1998). Establishment of the experimental fields Because the prevailing wind in this area is from the north-west, the north-west area of this salt lake suffers the most serious damage from sand encroachment and dune movement. Therefore, we set up our experimental area to the north-west. Five experimental fields were designed and set up (Fig. 3). Except the Field 1, other four experimental fields were located in one big area enclosed with wire fences. The wire fence was established by Jilantai Salt Development Company after our design. Besides our five fields, there were also several other small experimental fields located in the same enclosed area, which was established by other groups Field 1FComparison area: This area (20 km2) comprised dunes with sparse natural vegetation. Because grazing and woodcutting continued normally, this area is the original landscape without experimental input. Field 2FLand enclosure area: By enclosing the area with fences, grazing and woodcutting were excluded and the vegetation could naturally regenerate. The size of this experimental area was 4 km2. Field 3FLand enclosure+irrigation area: Irrigation was carried out 2–3 times a year by using a movable sprinkler. This measure accelerated the regeneration of natural vegetation. The size of this experimental area was 2 km2. Field 4FLeveled afforestation area: In this experimental area (1 km2), the procedure of irrigation was the same as for Field 3. However, the dunes were first leveled with heavy machinery, and then planted with seedlings of native shrubs (Calligonum arborescens, Hedysarum scoparium, Tamarix austromongolica) and tree (Elaeagnus angustifolia). Dune leveling was carried out by the afforestation sub-division of the Jilantai Salt Development Company in late 1982 and early 1983 after our design.
Table 1. Meteorological characteristics in Jilantai area (average from 1955 to 1985). Data source: Jilantai meteorological station
Month
Jan
Mar
Apr
Jun
Jul
Aug
Sep
6?3 2?0 10?7 18?2 17?9 28?8 32?8 38?6 27?2 21?2 13?9 1?2
23?3 38?4 3?3
25?4 40?9 11?6
23?6 40?5 8?2
17?2 8?8 1?0 8?5 35?8 29?6 22?5 15?6 1?9 13?3 23?2 29?4
5?5 3?3 13?3 22?2 28?6 44?1 56?5 63?8 30?3 24?3 19?3 7?4 236?7 269?6 280?2 319?7 0?9 1?6 5?2 6?6 77?8 181?8 314?2 443?3 39 33 31 29
27?5 66?7 0?9 321?6 12?8 472?7 35
29?0 67?0 8?6 308?8 26?5 458?4 44
26?7 65?5 3?2 296?3 35?5 389?6 50
3?9 SW 18
3?7 NW 19
3?7 NW 14
3?5 SW 18
3?8 NW 20
4?3 NW 24
WNW
NW
W
May
4?2 SW 20
Oct
Nov
Dec
Annual average 8?6 40?9 31?2
19?3 9?5 0?7 8?5 10?5 57?6 46?0 34?5 19?3 67?0 6?1 15?1 26?3 35?2 37?2 276?6 272?5 239?0 232?7 3293?2 12?3 5?6 1?9 0?2 109?9 284?5 187?2 97?1 51?7 3005?2 44 43 45 48 41 3?3 NW 18
3?1 N 16
3?5 SW 15
3?4 SW 20
3?6 SW 24
NW WNWNW NW NENW NW WNW WNW WNW WNW
1?5 7 0
3?0 8 0
6?5 14 0
5?7 15 1
4?5 10 0
4?9 14 0
3?2 13 0
1?8 6 0
1?0 6 0
1?2 4 0
0?8 6 0
34?5 79 7
7?5 13 0
9?3 17 2
9?0 17 0
10?2 19 5
8?0 16 1
6?7 15 1
6?5 16 1
4?0 9 1
4?0 8 0
5?1 15 1
5?8 14 0
82?5 126 37
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Air temperature (1C) Average 10?5 Maximum 11?3 Minimum 31?2 Surface temperature (1C) Average 10?3 Maximum 16?8 Minimum 37?2 Sunshine (h) 239?5 Precipitation (mm) 0?8 Evaporation (mm) 46?9 Relative 46 humidity (%) 3?3 Wind velocity (m s1) Dominant wind direction SW 16 Maximum wind velocity (m/s1) Direction of maximum wind S Days when strong wind occurrs Average 0?5 Maximum 4 Minimum 0 Days when sandy flux occurrs Average 6?2 Maximum 16 Minimum 1
Feb
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a. Comparison
a
b. Land enclosure area c. Land enclosure + irrigation area d. Leveled afforestation area
e
b
e. Protected afforestation area
d
a
c
Wire fence
b Wire fencee d
Salt Lake
e
1: 50000 0
0.5
1.0
1.5 km
Figure 3. Arrangement of five experimental areas.
Field 5FProtected afforestation area: In this experimental area, the procedure of irrigation was also the same as for Field 3. The main characteristic of this method was that the original natural vegetation was not disturbed. The plant species used for revegetation were the same as those used in Field 4. This experimental area is a belt 1 km long and 9 km wide, and located just beside the salt lake; this belt is the salt lake’s main protection against the encroachment of moving sand. We emphasize that there were three common features for the establishment of these experimental fields. Firstly, all of the five fields were established on the same type of sand dune. The dunes in this area are approximately 30 m in height and covered with some vegetation. Secondly, the plant species used to enlarge the revegetation were the same for both the protected afforestation area and the leveled afforestation area. Thirdly, the pattern of placement of the newly planted species was the same in both the protected and leveled vegetation areas. By these efforts, it is guaranteed that the conditions for the ‘protected afforestation area’ and the ‘leveled afforestation area’ are comparable with each other. Measurements Vegetation measurements Permanent sample quadrats of four different sizes were set up in each of the five experimental fields. These quadrats were larger (25 m 40 m, 20 m 25 m, 10 m 20 m), 10 m 10 m, 2 m 2 m, and 1 m 1 m. There were eight larger
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quadrats and, respectively, 9, 20, and 30 of each other quadrat size in each experimental field. Therefore, a total of 335 quadrats were established and investigated. In the larger quadrats (25 m 40 m, 20 m 25 m, 10 m 20 m) and 10 m 10 m-sized quadrats, the coverage and height of each species (including herbs, shrubs, and trees) and other parameters (trunk diameter, canopy size) of each shrub and tree were measured and recorded. In the smaller quadrats (2 m 2 m, 1 m 1 m), number of individuals, height, and coverage of herbs were measured and recorded.
Environmental measurements In each experimental field, the relative humidity of the air was measured by using a DIJ1 hygrometer during July–August, 1986. Humidity was continuously measured for 50 days and all of the data were recorded automatically. Wind velocity was measured in each experimental field by using a DEY–I anemograph at heights of 2?0, 1?0, 0?5, 0?3, and 0?1 m. Wind velocity was measured during April–June 1986 and April–June 1992. In every experimental field, wind velocities at the five different heights were measured and averaged every 1 min. Totally over 60 measurements were made for each field.
Measurement of sand encroachment The amount of sand sediment encroaching into the salt lake area was measured in 1980, 1983, 1986, and 1992. By measuring the sand-covered area and the depth of sand layer, the amount of sand sediment encroaching into the salt lake was calculated.
Results and discussion Survival rate of planted seedlings Survivability is the key to vegetation rehabilitation for arid land forestry (Johnson et al., 1992). As a result of moving sand and low rainfall, the low survival rate of seedlings planted in desertified areas has been a limiting factor for dry land forestry. A method that can improve the survival rate of seedlings is seriously required. Figure 4 shows a comparison between the survival rates of Elaeagnus angustifolia seedlings in the protected afforestation area and those in the leveled afforestation area. In the leveled afforestation area, the numbers of sampling quadrat were 26 and 12 and the observed plant individuals were 1254 and 349, for the observations after 1 year and after 2 years, respectively. In the protected afforestation area, the numbers of sampling quadrat were 12 and 12 and the observed plant individuals were 201 and 195, for the observations after 1 year and after 2 years, respectively. The survival rate after 1 year was equal to the number of living individuals after 1 year divided by the number of planted seedlings. The survival rate after 2 years was equal to the number of living individual after 2 years divided by the number of living individuals after 1 year. In the protected afforestation area, the survival rates after 1 and 2 years were 95%, while in the leveled afforestation area, the survival rates were 60% after 1 year and 75% after 2 years. The survival rate was higher in the protected afforestation area than in the leveled afforestation area. Figure 5 shows a comparison between the survival rates of Tamarix austromongolica after 1 year in the two areas. The numbers of sampling quadrat were 12 and 5 and the observed plant individuals were 843 and 60, for the
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Surviving rate (%)
100
After 1 year After 2 years
80 60 40 20 0
Leveled afforestation
Protected afforestation
Figure 4. Surviving rate of planted E. angustifolia in both the leveled and protected afforestation areas. The bars represent 1 standard deviation. In the leveled afforestation area, the numbers of sampling quadrat were 26 and 12 and the observed plant individuals were 1254 and 349, for the observations after 1 year and after 2 years, respectively. In the protected afforestation area, the numbers of sampling quadrat were 12 and 12 and the observed plant individuals were 201 and 195 for the observations after 1 year and after 2 years, respectively.
leveling and protected afforestation areas, respectively. The survival rate was approximately 90% in the protected afforestation area and 55% in the leveled afforestation area. Similar results were obtained for other species. In the protected afforestation area, the coverage of natural vegetation was approximately 15%, which reduced the harm caused by sand-dune movement and provided a relatively stable ground surface for the growth of seedlings. Hence, a relatively high survival rate was achieved (over 90%). In the leveled afforestation area, the original vegetation was completely destroyed, and the growth of seedlings was affected by wind erosion or by being buried by sand. Consequently, the survival rate was low (between 55% and 75%).
120
Surviving rate (%)
100
Leveled protected
80 60 40 20 0
Figure 5. Surviving rate of planted T. austromongolica in both the leveled and protected afforestation areas. The bars represent 1 standard deviation. The numbers of sampling quadrat were 12 and 5 and the observed plant individuals were 843 and 60, for the leveling and protected afforestation areas, respectively.
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Vigor of planted seedlings Four parameters (height, trunk diameter, coverage, and canopy size) were used to reflect the vigor of seedlings. The growth statuses of several species in both the protected and leveled afforestation areas were investigated in 1984, 1985, 1986, and 1992. The numbers of sampling quadrat were 8 (larger quadrat) and the entire plant individuals in these quadrats were investigated in both the areas. Figure 6 shows the growth status of Tamarix austromongolica in the two areas. In the first 3 years, mean plant height in the protected area was around 70 cm, while the mean height was only 15 cm in the leveled areaFa difference of 55 cm. After 8 years, although plants in the protected area were still taller than those in the leveled area, the difference was reduced to 15 cm (Fig. 6(a)). A comparison of trunk diameters near the ground surface showed that, in the first 3 years, trunk diameter varied from 0?8 to 1?2 cm in the protected area, while it varied from 0?2 to 0?4 cm in the leveled area (Fig. 6(b)). After 8 years, these differences were reduced. Similar results are shown in Figs 6(c and d) for coverage and canopy size, respectively. These results suggest that planted seedlings grow better in the protected-afforestation area than in the leveled afforestation area. Changes in coverage of individual species and total vegetation During the first 3 years, coverage of Tamarix austromongolica in the protected afforestation area increased from 15% to 30%, while it remained at 0?3–0?5% in the leveled afforestation area (Fig. 6(c)). After 8 years, coverage increased to 22% in the
2.5
Trunk diameter (cm)
150
Height (cm)
120 90 60 30
(a)
0 1984
1985
1986
1992
1.0 0.5 0.0 1984
1985
1986
1992
1985
1986
1992
3.0
Canopy size (m2)
30
Coverage (%)
1.5
(b)
40
20 10 0 −10
2.0
1.0
0.0
−1.0 1984
(c)
2.0
1985
1986
1992
Protected afforestation area
1984
(d)
Leveled afforestation area
Figure 6. Vigor of T. austromongolica in different afforestation areas. The number of sampling quadrat was eight (larger quadrat) and the entire plant individuals in these quadrats were measures. The bars represent 1 standard deviation.
Y. GAO ET AL. 70
70
60
60
50
50
Coverage (%)
Coverage (%)
492
40 30
30
20
20
10
10 0
0 1984
(a)
40
1985
1984
1986
(b)
Protected afforestation area
1985
1986
1992
Leveled afforestation area
Figure 7. Coverages of E. angustifolia (a) and H. scopurium (b) in protected afforestation and leveled afforestation areas. The bars represent 1 standard deviation. The number of sampling quadrat was eight (larger quadrat) for each area. Canopy sizes of entire plant individuals in these quadrats were measured and vegetation coverage was calculated by using the canopy size data.
protected afforestation area and 15% in the leveled afforestation area. In the area dominated by Elaeagnus angustifolia (Fig. 7(a)), coverage increased from 20% to 60% during the first 3 years in the protected area, while it increased from 8% to 37% in the leveled area. Similar results were also found for Hedysarum scoparium (Fig. 7(b)). During the experimental period, the total coverage of planted vegetation increased from 20% to 55% in the protected area, while it increased from 10% to 30% in the leveled area (data not shown). Figure 8 shows the changes in mean total vegetation coverage in the comparison area (natural vegetation, Field 1), the protected afforestation area, and the leveled afforestation area. The number of sampling quadrat was eight (larger quadrat) for each area. Canopy-sizes of entire plant individuals in these quadrats were measured and vegetation coverage was calculated by using the canopy size data. During the first 4 years, the coverage of natural vegetation varied from 10% to 15%, which was mainly the result of the variation of annual precipitation. Meanwhile, coverage in the leveled
70
Coverage (%)
60 50 40 30 20 10 0 1983
1984
1985
1986
Year Comparison area
Protected afforestation area
Leveled afforestation area
Figure 8. Variation of mean total vegetation coverage in three different afforestation areas. The bars represent 1 standard deviation. The number of sampling quadrat was eight (larger quadrat) for each area. Canopy sizes of entire plant individuals in these quadrats were measured and vegetation coverage was calculated by using the canopy size data.
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area varied from 0% to 30%. In this area, 2 or 3 years were required to reach a level of coverage equal to that in the natural vegetation area. Coverage in the protected area increased from 15% to 55%, which was larger than the combined coverage of natural vegetation plus the coverage in the leveled area. These results show that the protectedafforestation method can increase coverage faster than the leveled-afforestation method. Coverage is a parameter that not only can reflect the characteristics of the plant community but can also reflect the characteristics of the environment. Qiu et al. (2001) report that canopy coverage is one of the most important factors reflecting the features of the plant community in arid, semi-arid, and dry sub-humid areas, where the ground surface is seldom fully covered by vegetation. Therefore, we conclude that the environment provided by protected afforestation is better for plant growth than that provided by leveled afforestation. Effects on the vigor of natural species After the afforestation, in addition to the planted seedlings, several plant species naturally appeared in the area due to seed germination. Figure 9 shows the densities of naturally germinated seedlings of seven species in different afforestation areas. Although density varies with species, a common characteristic of all seven species is that the density was higher in the protected afforestation area than in the other two areas. For example, the density of Ammopiptanthus mongolicus was 8/(100 m2) in the protected afforestation area, while it was only 3/(100 m2) in the leveled afforestation area and 1/(100 m2) in the comparison area. For some species, the density in the protected afforestation area was as much as 2 times that in the leveled
25
10 8
Oxytropis aciphylla Density /(100 m2)
Density /(100 m2)
Ammopiptanthus mongolicus
6 4 2
20 15 10 5
0
0 Protected Leveled Comparison area afforestation area afforestation area
Protected Leveled Comparison area afforestation area afforestation area
10
6
70 Density / (m2)
Density / (m2)
8
90 Nitraria tangutorum Psammochloa villosa Phragmites australis
4 2
Artemisia ordosica Agriophyllum squarrosum
50 30 10
0 −2
−10 Protected Leveled Comparison area afforestation area afforestation area
Protected Leveled Comparison area afforestation area afforestation area
Figure 9. Density of natural plant species in the three different afforestation areas after 3 years of the start of the experiment. The bars represent 1 standard deviation. The number of sampling quadrat was eight (larger quadrat) for each area and the entire plant individuals in these quadrats were measured.
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afforestation area. The protected-afforestation method provided a suitable growing environment not only for planted species, but also for naturally growing species. Improvement of micrometeorological conditions As discussed in the previous sections, from the point of view of vegetation rehabilitation, protected-afforestation had advantages over the other afforestation methods. In this section, we discuss the effects of the protected afforestation method on air humidity and wind velocity. During July–August 1986, we continuously measured the humidity for 50 days in the 3-year-old protected-afforestation area and in the comparison area. Procedures for data analysis were summarized as follows: step 1, original data were read (one data every 10 min) from recording paper and mean value for each hour was calculated by these six data. By this step, 24 values (corresponding to each hour) were acquired for each day; step 2, the hourly average values for the 50 days were calculated by using the mean value of each hour obtained from step 1. Therefore, every value represented here is the mean value of 300 data and results are shown in Fig. 10. The average relative air humidity in the protected afforestation area varied between 15% and 60%, while it varied between 10% and 45% in the comparison area (Fig. 10). Protected afforestation improved air humidity by 5–15%. This effect was especially significant during the night-time (20:00–08:00), when the difference was as much as 20%. To investigate the effects of vegetation on the reduction of wind velocity, wind velocity was measured at different heights in different experimental areas. Usually, the wind season in this area was in spring season (March to May). Because plant leaves were not fully developed in this season, we selected one day with strong wind velocity after the leaves were fully developed to compare the reduction of wind velocity in four different afforestation areas (7 June 1986). Results were shown in Fig. 11. For convenience, we used relative wind velocity for the comparison (relative wind velocity=[wind velocity]/[wind velocity at a height of 2 m]). In the comparison area, wind velocity was reduced to 90% at a height of 1?0 m, 80% at a height of 0?50 m, 70 Comparison 60
Protected afforestation
Humidity (%)
50 40 30 20 10 0 0
2
4
6
8
10
12
14
16
18
20
22
Time
Figure 10. Daily variation of relative humidity of air (average value from 50 days measurements conducted in July–August 1996) in protected afforestation area (3 years old) and the comparison area. Procedures for data analysis were as follows: step 1, original data were read (one datum every 10 min) from recording paper and mean value for each hour was calculated by these six data. By this step, 24 values (corresponding to each hour) were acquired for each day; step 2, the hourly average values for the 50 days were calculated by using the mean value of each hour obtained from step 1. Therefore, every value represented here is the mean value of 300 data.
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110
Relative wind velocity (%)
90
70
50 Comparison area Land enclosure area 30
Land enclosure + Irrigation area Protecting afforestation area
10 0.4
0.8
1.2
1.6
2.0
Height from ground surface (m)
Figure 11. Reduction of wind velocity at different heights in different afforestation areas (3 years after afforestation).
62% at a height of 0?30 m, and 60% at a height of 0?10 m. In the land enclosure area (Field 2), the corresponding values for these four heights were 85%, 75%, 60%, and 40%, respectively. In the land enclosure + irrigation area (Field 3), the corresponding values for these four heights were 75%, 60%, 40%, and 15%, respectively. In the protected afforestation area, the corresponding values for these four heights were 70%, 40%, 30%, and 10%, respectively. Compared with Field 1, wind velocity was reduced in each of the other three measured fields. In the enclosure area, because there was no grazing or woodcutting, natural vegetation rehabilitated and grew better than in Field 1 and wind velocity reduction was enhanced. In the enclosure + irrigation area, rehabilitation of natural vegetation was further accelerated by watering, and the wind velocity reduction effect was even further improved. However, the natural vegetation + planted vegetation in the protected afforestation area achieved the maximum wind velocity reduction effect. The wind velocity in the protected-afforestation area at a height of 0?10 m was only 10% that at 2 m; therefore, compared with the other fields, wind velocity was significantly reduced.
Effects of dune fixation As mentioned before, the protected afforestation area is the main barrier between the salt lake and the encroaching sand. Therefore, any effect on the fixation of moving sand can be regarded mainly as a result of this belt. The volume of sand that moved into the salt lake area was 1?3 106 m3 year1 before the afforestation (1980–1983), 0?4 106 m3 year1 during the afforestation (1983–1986), and 0?2 106 m3 year1 after the afforestation (1986–1993) (Fig. 12). The amount of sand that moved into the salt lake area decreased to 15% after the establishment of the protected afforestation area. The rate of decrease was approximately 0?11 106 m3 year1. Because the rehabilitated vegetation is still in its early period, there is still some sand moving into the salt lake area; the amount of sand encroaching into the salt lake area is expected to
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After afforestation
In the afforestation
Before afforestation
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Volume (106 m3/year) Figure 12. Volume of sand encroaching into the salt lake area before, during, and after the afforestation. The amount of sand sediment encroaching into the salt lake area was measured once in every 3 years in 1980, 1983, 1986, and 1992. By measuring the sand-covered area and the depth of sand layer, the amount of sand sediment encroaching into the salt lake was calculated with these data.
further decrease as the vegetation grows. Therefore, we conclude that the protectedafforestation method is a suitable way to establish a protective belt.
Conclusion Vegetation restoration is one of the most common and effective ways of combating desertification and protecting adjacent areas from sand encroachment in many of the desertified regions of the world. One commonly used procedure to establish vegetation is to first level the dune areas and then plant seedlings of several species with added irrigation. We call this the ‘leveled-afforestation’ method. In contrast to the leveled-afforestation method, we defined another method comprising ‘protecting the surviving natural vegetation as much as possible’ + ‘planting seedlings’ as the ‘protected-afforestation’ method. Results of a 10-year study in the Jilantai Salt Lake area show that the protected-afforestation method had the following advantages: a. The survival rate of planted seedlings was higher in the protectedafforestation area than in the leveled afforestation area. b. The vigor of planted seedlings (height, trunk diameter, coverage, and canopy size) was better in the protected-afforestation area than in the leveled afforestation area, especially in the beginning period of revegetation. c. The coverage (individual species, all planted vegetation, all vegetation) was larger in the protected-afforestation area than in the leveled afforestation area. d. Density of naturally germinated plant species was higher in the protectedafforestation area than in the other areas. For some species, the density in the protected-afforestation area was as much as twice that in the leveled afforestation area. The protected-afforestation method provided a suitable
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growing environment not only for planted species, but also for naturally growing species. e. In the protected vegetation area, relative humidity of air increased. Compared with other afforestation methods, wind velocity was greatly reduced in the protected afforestation area. f. After the establishment of vegetation by the protected-afforestation method, sand encroachment into the salt lake area decreased to 15%. This amount is expected to further decrease as the vegetation grows. These results suggest that protected afforestation is an effective method for vegetation rehabilitation, and has the potential to be applied not only to the arid lands in China but also to desertified areas throughout the world. The authors thank professor Kuibi Zhang (Inner Mongolia Agricultural University) for his helpful advice and kind support as a project leader. The Jilantai Salt Development Company and Science and Technology Committee of Inner Mongolia financed this work. This work was also partially financed by NKBRSF Project G2000018604 and NSFC 39960064.
References Ffolliott, P.F., Gottfried, G.J. & Rietveld, W.J. (1995). Dryland forestry for sustainable development. Journal of Arid Environment, 30: 143–152. Gupta, G.N. (1995). Rain-water management for tree planting in the Indian Desert. Journal of Arid Environment, 31: 219–235. Johnson, D.E., Borman, M.M. & Ben Ali, M.N. (1992). Evaluation of plant species for land restoration in central Tunisia. Journal of Arid Environment, 22: 305–322. Ma, S. (1990). Aeolian landform development and control methods of sand hazards in the northwestern part of the Jilantai Salt Lake area. Journal of Desert Research, 10(3): 13–24. Mitchell, D.J., Fullen, M.A., Trueman, I.C. & Fearnehough, W. (1998). Sustainability of reclaimed desertified land in Ningxia, China. Journal of Arid Environment, 39: 239–251. Qiu, Guo Yu, Shi, Peijun, Shimizu, Hideyuki, Tobe, Kazuo, Gao, Yong & Omasa, Kenji (2001): Vegetation indicators of desertification in the Mu Us desert and their applicability to remote sensing. Journal of Arid Land Studies, 11: 111–117. Tobe, Kazuo, Zhang Liping, Qiu, Guo Yu, Shimizu, Hideyuki & Omasa, Kenji (2001): Characteristics of seed germination of five non-halophytic Chinese desert shrubs species. Journal of Arid Environment, 47(2): 191–201. Zhang, K. (1988). A study on the techniques of comprehensively treating the sand hazards in Jilantai Salt Lake. Journal of Arid Land Resources and Environment, 2(4): 64–74.
A 10-Year Study on Techniques for Vegetation Restoration in a Desertified Salt Lake Area
Yong Gao%*wz, Guo Yu Qiu%z, Hideyuki Shimizu%, Kazuo Tobe% Baoping Sunz & Ji Wangw %
National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-0053, Japan wInner Mongolia Agricultural University, Hohhot 010019, China zKey Laboratory of Environmental Change and Natural Disaster, The Ministry of Education of China, China Center of Desert Research, Beijing Normal University, Beijing 100875, China zBeijing Forest University, Beijing, 100083, China (Received 18 May 2001, accepted 27 February 2002) Vegetation restoration is one of the most common and effective ways to combat desertification and prevent adjacent areas from sand encroachment in many of the desertified regions of the world. However, vegetation restoration in desertified regions is very difficult because of low rainfall, the mobile ground surface, and cost. An effective, low-cost method of afforestation is urgently required. To determine such a method, a 10-year study was carried out in the Jilantai Salt Lake area. Five different afforestation areas were established: a ‘comparison area,’ a ‘land enclosure area,’ a ‘land enclosure + irrigation area,’ a ‘leveled-afforestation area’ (the dune areas were leveled and then planted with seedlings with added irrigation), and a ‘protected afforestation area’ (the dune areas were planted with seedlings, and the surviving natural vegetation was protected as much as possible). Vegetation-related parameters (survival rate, height, trunk diameter, coverage, canopy size, and density) and environment-related factors (relative humidity, wind velocity, and amount of sand encroachment) were measured by standard methods. Results show that the protected afforestation method had the following advantages: (1) the survival rate was higher for seedlings planted in the protected afforestation area than in the leveled afforestation area; (2) vigor (height, trunk diameter, coverage, and canopy size) was better in seedlings planted in the protected afforestation area than in the leveled afforestation area, especially in the beginning period of revegetation; (3) coverage (of individual species, of all planted vegetation, and of all vegetation) was larger in the protected afforestation area than in the leveled afforestation area; (4) density of naturally germinated plant species was higher in the protected afforestation area than in the other areas, showing that the protected afforestation method provided a suitable growing environment not only for planted species but also for naturally growing species; (5) in the protected vegetation area, relative humidity of air increased and wind velocity was greatly reduced; (6) after the establishment of vegetation by the protected afforestation method, sand encroachment into the salt lake area was significantly reduced. These results suggest that *Corresponding author. Fax: +81-298-50-2586. E-mail: [email protected]. 0140-1963/02/040483 + 15 $35.00/0
# 2002 Elsevier Science Ltd.
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protected afforestation is an effective method of vegetation rehabilitation that has the potential not only to be applied to arid lands in China but also to desertified areas throughout the world; (7) cost-effective calculation shows that the leveled afforestation area costs much more than other areas. # 2002 Elsevier Science Ltd. Keywords: desertification, Jilantai Salt Lake, protected afforestation, vegetation restoration
Introduction Overgrazing and cutting of trees and shrubs for fuel wood are the most important causes of desertification (Mitchell et al., 1998). Vegetation restoration is one of the most common and effective ways to combat desertification and prevent adjacent areas from sand encroachment in many of the desertified regions of the world (Ffolliott et al., 1995). Vegetation-restoration methods vary according to the natural and economic conditions. In desertified areas of China, land enclosure and plantations are commonly applied methods of vegetation restoration. Land enclosure can control overgrazing and woodcutting by enclosing an area with fences, thus enabling the natural reestablishment of vegetation. Planting seedlings is a way of artificially establishing vegetation by using native plants or plant species introduced from outside the area. Because of the low precipitation and sand-dune movement, establishment of vegetation in arid and semi-arid desert regions is one of the most difficult and costly tasks for local people (Gupta, 1995). Jilantai Salt Lake is one of the most important salt resources in China. The area of the salt lake is 120 km2, and the storage volume of salt is up to 110 million tons (Ma, 1990). Because the salt lake is located on the edge of the Ulan Buh Desert, a 9900km2 sandy desert, the quantity and quality of salt production have been seriously affected by the encroachment of sand dunes and wind-blown sand. Population growth in the area surrounding the lake has been accompanied by increased woodcutting and overgrazing, which have further damaged the native vegetation and increased windblown sand. The damage this wind-blown sand inflicts on the salt-producing environment is so serious that the annual production and daily mining are being impacted by the damage, especially in the spring season. The efforts to establish vegetation in this area have been continuing for several decades (Zhang, 1988). The most commonly used procedure has been leveling the dune areas and then planting seedlings of several species, such as Elaeagnus angustifolia, Calligonum arborescens, Hedysarum scoparium, and Tamarix austromongolica. Irrigation is carried out between 1 and 3 times a year. Hereafter, in this paper, we call this traditional method the ‘leveled-afforestation’ method. Over the past several decades, huge amounts of labor and money have been spent on the leveledafforestation method. However, the original vegetation is completely destroyed by the leveling of the sand dunes, and the replanted vegetation cannot control the windblown sand in the initial stage. Moreover, the planted vegetation cannot grow well due to the damage from moving sand. The third disadvantage of the traditional way is its high cost. The main objective of this study was to look for some suitable vegetationrestoration techniques that can overcome these shortcomings and can be applied not only to the local area but also to other arid zones. The native vegetation has survived in the local environment for a very long time and is probably the most suitable for the area. Therefore, we assumed that the best method of rehabilitating the vegetation is by ‘protecting the surviving native vegetation as much as possible’ + ‘planting seedlings.’ For convenience, we hereafter refer to this
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method as the ‘protected-afforestation’ method. To verify this hypothesis, a 10-year study (1983–1992) was conducted in the Jilantai Salt Lake area.
Material and methods Location and natural conditions of the experimental area Jilantai Salt Lake, located on the south-western edge of the Ulan Buh Desert (lat391480 N, long 1051300 E), is an elliptical basin extending from north-east to southwest. It is an evaporating salt lake typical of those in arid areas in China (Fig. 1). The land surface in the experimental area is covered by moving sands with sparse vegetation dominated by Nitraria tangutorum, Haloxylon ammodendron, Ammopiptanthus mongolicus, Reaumuria soongorica, Convolvulus tragacanthoides, Oxytropis aciphylla, Kalidium foliatum, Psammochloa villosa, Sophora alopecuroides, and so on (Fig. 2). These plant species are very resistant to both water stress and salt stress (Tobe et al., 2001). This region has a typical temperate desert climate: dry and hot in summer, cold in winter, plenty of sunshine, very little precipitation, strong winds, and frequent drifting
Figure 1. Location of Jilantai Salt Lake and Ulan Buh Desert.
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Figure 2. Natural landscape of Jilantai Salt Lake.
sands. The characteristics of the local weather, from the local meteorological station’s 30-year record, are summarized in Table 1. The depth of ground-water is 1–3 m and the water quality is poor and brackish. Besides this shallow ground-water, there is a deeply buried layer of relatively good quality ground-water, which is the main source of water for irrigation and domestic use (Mitchell et al., 1998). Establishment of the experimental fields Because the prevailing wind in this area is from the north-west, the north-west area of this salt lake suffers the most serious damage from sand encroachment and dune movement. Therefore, we set up our experimental area to the north-west. Five experimental fields were designed and set up (Fig. 3). Except the Field 1, other four experimental fields were located in one big area enclosed with wire fences. The wire fence was established by Jilantai Salt Development Company after our design. Besides our five fields, there were also several other small experimental fields located in the same enclosed area, which was established by other groups Field 1FComparison area: This area (20 km2) comprised dunes with sparse natural vegetation. Because grazing and woodcutting continued normally, this area is the original landscape without experimental input. Field 2FLand enclosure area: By enclosing the area with fences, grazing and woodcutting were excluded and the vegetation could naturally regenerate. The size of this experimental area was 4 km2. Field 3FLand enclosure+irrigation area: Irrigation was carried out 2–3 times a year by using a movable sprinkler. This measure accelerated the regeneration of natural vegetation. The size of this experimental area was 2 km2. Field 4FLeveled afforestation area: In this experimental area (1 km2), the procedure of irrigation was the same as for Field 3. However, the dunes were first leveled with heavy machinery, and then planted with seedlings of native shrubs (Calligonum arborescens, Hedysarum scoparium, Tamarix austromongolica) and tree (Elaeagnus angustifolia). Dune leveling was carried out by the afforestation sub-division of the Jilantai Salt Development Company in late 1982 and early 1983 after our design.
Table 1. Meteorological characteristics in Jilantai area (average from 1955 to 1985). Data source: Jilantai meteorological station
Month
Jan
Mar
Apr
Jun
Jul
Aug
Sep
6?3 2?0 10?7 18?2 17?9 28?8 32?8 38?6 27?2 21?2 13?9 1?2
23?3 38?4 3?3
25?4 40?9 11?6
23?6 40?5 8?2
17?2 8?8 1?0 8?5 35?8 29?6 22?5 15?6 1?9 13?3 23?2 29?4
5?5 3?3 13?3 22?2 28?6 44?1 56?5 63?8 30?3 24?3 19?3 7?4 236?7 269?6 280?2 319?7 0?9 1?6 5?2 6?6 77?8 181?8 314?2 443?3 39 33 31 29
27?5 66?7 0?9 321?6 12?8 472?7 35
29?0 67?0 8?6 308?8 26?5 458?4 44
26?7 65?5 3?2 296?3 35?5 389?6 50
3?9 SW 18
3?7 NW 19
3?7 NW 14
3?5 SW 18
3?8 NW 20
4?3 NW 24
WNW
NW
W
May
4?2 SW 20
Oct
Nov
Dec
Annual average 8?6 40?9 31?2
19?3 9?5 0?7 8?5 10?5 57?6 46?0 34?5 19?3 67?0 6?1 15?1 26?3 35?2 37?2 276?6 272?5 239?0 232?7 3293?2 12?3 5?6 1?9 0?2 109?9 284?5 187?2 97?1 51?7 3005?2 44 43 45 48 41 3?3 NW 18
3?1 N 16
3?5 SW 15
3?4 SW 20
3?6 SW 24
NW WNWNW NW NENW NW WNW WNW WNW WNW
1?5 7 0
3?0 8 0
6?5 14 0
5?7 15 1
4?5 10 0
4?9 14 0
3?2 13 0
1?8 6 0
1?0 6 0
1?2 4 0
0?8 6 0
34?5 79 7
7?5 13 0
9?3 17 2
9?0 17 0
10?2 19 5
8?0 16 1
6?7 15 1
6?5 16 1
4?0 9 1
4?0 8 0
5?1 15 1
5?8 14 0
82?5 126 37
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Air temperature (1C) Average 10?5 Maximum 11?3 Minimum 31?2 Surface temperature (1C) Average 10?3 Maximum 16?8 Minimum 37?2 Sunshine (h) 239?5 Precipitation (mm) 0?8 Evaporation (mm) 46?9 Relative 46 humidity (%) 3?3 Wind velocity (m s1) Dominant wind direction SW 16 Maximum wind velocity (m/s1) Direction of maximum wind S Days when strong wind occurrs Average 0?5 Maximum 4 Minimum 0 Days when sandy flux occurrs Average 6?2 Maximum 16 Minimum 1
Feb
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a
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e
b
e. Protected afforestation area
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c
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Figure 3. Arrangement of five experimental areas.
Field 5FProtected afforestation area: In this experimental area, the procedure of irrigation was also the same as for Field 3. The main characteristic of this method was that the original natural vegetation was not disturbed. The plant species used for revegetation were the same as those used in Field 4. This experimental area is a belt 1 km long and 9 km wide, and located just beside the salt lake; this belt is the salt lake’s main protection against the encroachment of moving sand. We emphasize that there were three common features for the establishment of these experimental fields. Firstly, all of the five fields were established on the same type of sand dune. The dunes in this area are approximately 30 m in height and covered with some vegetation. Secondly, the plant species used to enlarge the revegetation were the same for both the protected afforestation area and the leveled afforestation area. Thirdly, the pattern of placement of the newly planted species was the same in both the protected and leveled vegetation areas. By these efforts, it is guaranteed that the conditions for the ‘protected afforestation area’ and the ‘leveled afforestation area’ are comparable with each other. Measurements Vegetation measurements Permanent sample quadrats of four different sizes were set up in each of the five experimental fields. These quadrats were larger (25 m 40 m, 20 m 25 m, 10 m 20 m), 10 m 10 m, 2 m 2 m, and 1 m 1 m. There were eight larger
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quadrats and, respectively, 9, 20, and 30 of each other quadrat size in each experimental field. Therefore, a total of 335 quadrats were established and investigated. In the larger quadrats (25 m 40 m, 20 m 25 m, 10 m 20 m) and 10 m 10 m-sized quadrats, the coverage and height of each species (including herbs, shrubs, and trees) and other parameters (trunk diameter, canopy size) of each shrub and tree were measured and recorded. In the smaller quadrats (2 m 2 m, 1 m 1 m), number of individuals, height, and coverage of herbs were measured and recorded.
Environmental measurements In each experimental field, the relative humidity of the air was measured by using a DIJ1 hygrometer during July–August, 1986. Humidity was continuously measured for 50 days and all of the data were recorded automatically. Wind velocity was measured in each experimental field by using a DEY–I anemograph at heights of 2?0, 1?0, 0?5, 0?3, and 0?1 m. Wind velocity was measured during April–June 1986 and April–June 1992. In every experimental field, wind velocities at the five different heights were measured and averaged every 1 min. Totally over 60 measurements were made for each field.
Measurement of sand encroachment The amount of sand sediment encroaching into the salt lake area was measured in 1980, 1983, 1986, and 1992. By measuring the sand-covered area and the depth of sand layer, the amount of sand sediment encroaching into the salt lake was calculated.
Results and discussion Survival rate of planted seedlings Survivability is the key to vegetation rehabilitation for arid land forestry (Johnson et al., 1992). As a result of moving sand and low rainfall, the low survival rate of seedlings planted in desertified areas has been a limiting factor for dry land forestry. A method that can improve the survival rate of seedlings is seriously required. Figure 4 shows a comparison between the survival rates of Elaeagnus angustifolia seedlings in the protected afforestation area and those in the leveled afforestation area. In the leveled afforestation area, the numbers of sampling quadrat were 26 and 12 and the observed plant individuals were 1254 and 349, for the observations after 1 year and after 2 years, respectively. In the protected afforestation area, the numbers of sampling quadrat were 12 and 12 and the observed plant individuals were 201 and 195, for the observations after 1 year and after 2 years, respectively. The survival rate after 1 year was equal to the number of living individuals after 1 year divided by the number of planted seedlings. The survival rate after 2 years was equal to the number of living individual after 2 years divided by the number of living individuals after 1 year. In the protected afforestation area, the survival rates after 1 and 2 years were 95%, while in the leveled afforestation area, the survival rates were 60% after 1 year and 75% after 2 years. The survival rate was higher in the protected afforestation area than in the leveled afforestation area. Figure 5 shows a comparison between the survival rates of Tamarix austromongolica after 1 year in the two areas. The numbers of sampling quadrat were 12 and 5 and the observed plant individuals were 843 and 60, for the
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Surviving rate (%)
100
After 1 year After 2 years
80 60 40 20 0
Leveled afforestation
Protected afforestation
Figure 4. Surviving rate of planted E. angustifolia in both the leveled and protected afforestation areas. The bars represent 1 standard deviation. In the leveled afforestation area, the numbers of sampling quadrat were 26 and 12 and the observed plant individuals were 1254 and 349, for the observations after 1 year and after 2 years, respectively. In the protected afforestation area, the numbers of sampling quadrat were 12 and 12 and the observed plant individuals were 201 and 195 for the observations after 1 year and after 2 years, respectively.
leveling and protected afforestation areas, respectively. The survival rate was approximately 90% in the protected afforestation area and 55% in the leveled afforestation area. Similar results were obtained for other species. In the protected afforestation area, the coverage of natural vegetation was approximately 15%, which reduced the harm caused by sand-dune movement and provided a relatively stable ground surface for the growth of seedlings. Hence, a relatively high survival rate was achieved (over 90%). In the leveled afforestation area, the original vegetation was completely destroyed, and the growth of seedlings was affected by wind erosion or by being buried by sand. Consequently, the survival rate was low (between 55% and 75%).
120
Surviving rate (%)
100
Leveled protected
80 60 40 20 0
Figure 5. Surviving rate of planted T. austromongolica in both the leveled and protected afforestation areas. The bars represent 1 standard deviation. The numbers of sampling quadrat were 12 and 5 and the observed plant individuals were 843 and 60, for the leveling and protected afforestation areas, respectively.
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Vigor of planted seedlings Four parameters (height, trunk diameter, coverage, and canopy size) were used to reflect the vigor of seedlings. The growth statuses of several species in both the protected and leveled afforestation areas were investigated in 1984, 1985, 1986, and 1992. The numbers of sampling quadrat were 8 (larger quadrat) and the entire plant individuals in these quadrats were investigated in both the areas. Figure 6 shows the growth status of Tamarix austromongolica in the two areas. In the first 3 years, mean plant height in the protected area was around 70 cm, while the mean height was only 15 cm in the leveled areaFa difference of 55 cm. After 8 years, although plants in the protected area were still taller than those in the leveled area, the difference was reduced to 15 cm (Fig. 6(a)). A comparison of trunk diameters near the ground surface showed that, in the first 3 years, trunk diameter varied from 0?8 to 1?2 cm in the protected area, while it varied from 0?2 to 0?4 cm in the leveled area (Fig. 6(b)). After 8 years, these differences were reduced. Similar results are shown in Figs 6(c and d) for coverage and canopy size, respectively. These results suggest that planted seedlings grow better in the protected-afforestation area than in the leveled afforestation area. Changes in coverage of individual species and total vegetation During the first 3 years, coverage of Tamarix austromongolica in the protected afforestation area increased from 15% to 30%, while it remained at 0?3–0?5% in the leveled afforestation area (Fig. 6(c)). After 8 years, coverage increased to 22% in the
2.5
Trunk diameter (cm)
150
Height (cm)
120 90 60 30
(a)
0 1984
1985
1986
1992
1.0 0.5 0.0 1984
1985
1986
1992
1985
1986
1992
3.0
Canopy size (m2)
30
Coverage (%)
1.5
(b)
40
20 10 0 −10
2.0
1.0
0.0
−1.0 1984
(c)
2.0
1985
1986
1992
Protected afforestation area
1984
(d)
Leveled afforestation area
Figure 6. Vigor of T. austromongolica in different afforestation areas. The number of sampling quadrat was eight (larger quadrat) and the entire plant individuals in these quadrats were measures. The bars represent 1 standard deviation.
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70
60
60
50
50
Coverage (%)
Coverage (%)
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40 30
30
20
20
10
10 0
0 1984
(a)
40
1985
1984
1986
(b)
Protected afforestation area
1985
1986
1992
Leveled afforestation area
Figure 7. Coverages of E. angustifolia (a) and H. scopurium (b) in protected afforestation and leveled afforestation areas. The bars represent 1 standard deviation. The number of sampling quadrat was eight (larger quadrat) for each area. Canopy sizes of entire plant individuals in these quadrats were measured and vegetation coverage was calculated by using the canopy size data.
protected afforestation area and 15% in the leveled afforestation area. In the area dominated by Elaeagnus angustifolia (Fig. 7(a)), coverage increased from 20% to 60% during the first 3 years in the protected area, while it increased from 8% to 37% in the leveled area. Similar results were also found for Hedysarum scoparium (Fig. 7(b)). During the experimental period, the total coverage of planted vegetation increased from 20% to 55% in the protected area, while it increased from 10% to 30% in the leveled area (data not shown). Figure 8 shows the changes in mean total vegetation coverage in the comparison area (natural vegetation, Field 1), the protected afforestation area, and the leveled afforestation area. The number of sampling quadrat was eight (larger quadrat) for each area. Canopy-sizes of entire plant individuals in these quadrats were measured and vegetation coverage was calculated by using the canopy size data. During the first 4 years, the coverage of natural vegetation varied from 10% to 15%, which was mainly the result of the variation of annual precipitation. Meanwhile, coverage in the leveled
70
Coverage (%)
60 50 40 30 20 10 0 1983
1984
1985
1986
Year Comparison area
Protected afforestation area
Leveled afforestation area
Figure 8. Variation of mean total vegetation coverage in three different afforestation areas. The bars represent 1 standard deviation. The number of sampling quadrat was eight (larger quadrat) for each area. Canopy sizes of entire plant individuals in these quadrats were measured and vegetation coverage was calculated by using the canopy size data.
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area varied from 0% to 30%. In this area, 2 or 3 years were required to reach a level of coverage equal to that in the natural vegetation area. Coverage in the protected area increased from 15% to 55%, which was larger than the combined coverage of natural vegetation plus the coverage in the leveled area. These results show that the protectedafforestation method can increase coverage faster than the leveled-afforestation method. Coverage is a parameter that not only can reflect the characteristics of the plant community but can also reflect the characteristics of the environment. Qiu et al. (2001) report that canopy coverage is one of the most important factors reflecting the features of the plant community in arid, semi-arid, and dry sub-humid areas, where the ground surface is seldom fully covered by vegetation. Therefore, we conclude that the environment provided by protected afforestation is better for plant growth than that provided by leveled afforestation. Effects on the vigor of natural species After the afforestation, in addition to the planted seedlings, several plant species naturally appeared in the area due to seed germination. Figure 9 shows the densities of naturally germinated seedlings of seven species in different afforestation areas. Although density varies with species, a common characteristic of all seven species is that the density was higher in the protected afforestation area than in the other two areas. For example, the density of Ammopiptanthus mongolicus was 8/(100 m2) in the protected afforestation area, while it was only 3/(100 m2) in the leveled afforestation area and 1/(100 m2) in the comparison area. For some species, the density in the protected afforestation area was as much as 2 times that in the leveled
25
10 8
Oxytropis aciphylla Density /(100 m2)
Density /(100 m2)
Ammopiptanthus mongolicus
6 4 2
20 15 10 5
0
0 Protected Leveled Comparison area afforestation area afforestation area
Protected Leveled Comparison area afforestation area afforestation area
10
6
70 Density / (m2)
Density / (m2)
8
90 Nitraria tangutorum Psammochloa villosa Phragmites australis
4 2
Artemisia ordosica Agriophyllum squarrosum
50 30 10
0 −2
−10 Protected Leveled Comparison area afforestation area afforestation area
Protected Leveled Comparison area afforestation area afforestation area
Figure 9. Density of natural plant species in the three different afforestation areas after 3 years of the start of the experiment. The bars represent 1 standard deviation. The number of sampling quadrat was eight (larger quadrat) for each area and the entire plant individuals in these quadrats were measured.
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afforestation area. The protected-afforestation method provided a suitable growing environment not only for planted species, but also for naturally growing species. Improvement of micrometeorological conditions As discussed in the previous sections, from the point of view of vegetation rehabilitation, protected-afforestation had advantages over the other afforestation methods. In this section, we discuss the effects of the protected afforestation method on air humidity and wind velocity. During July–August 1986, we continuously measured the humidity for 50 days in the 3-year-old protected-afforestation area and in the comparison area. Procedures for data analysis were summarized as follows: step 1, original data were read (one data every 10 min) from recording paper and mean value for each hour was calculated by these six data. By this step, 24 values (corresponding to each hour) were acquired for each day; step 2, the hourly average values for the 50 days were calculated by using the mean value of each hour obtained from step 1. Therefore, every value represented here is the mean value of 300 data and results are shown in Fig. 10. The average relative air humidity in the protected afforestation area varied between 15% and 60%, while it varied between 10% and 45% in the comparison area (Fig. 10). Protected afforestation improved air humidity by 5–15%. This effect was especially significant during the night-time (20:00–08:00), when the difference was as much as 20%. To investigate the effects of vegetation on the reduction of wind velocity, wind velocity was measured at different heights in different experimental areas. Usually, the wind season in this area was in spring season (March to May). Because plant leaves were not fully developed in this season, we selected one day with strong wind velocity after the leaves were fully developed to compare the reduction of wind velocity in four different afforestation areas (7 June 1986). Results were shown in Fig. 11. For convenience, we used relative wind velocity for the comparison (relative wind velocity=[wind velocity]/[wind velocity at a height of 2 m]). In the comparison area, wind velocity was reduced to 90% at a height of 1?0 m, 80% at a height of 0?50 m, 70 Comparison 60
Protected afforestation
Humidity (%)
50 40 30 20 10 0 0
2
4
6
8
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Figure 10. Daily variation of relative humidity of air (average value from 50 days measurements conducted in July–August 1996) in protected afforestation area (3 years old) and the comparison area. Procedures for data analysis were as follows: step 1, original data were read (one datum every 10 min) from recording paper and mean value for each hour was calculated by these six data. By this step, 24 values (corresponding to each hour) were acquired for each day; step 2, the hourly average values for the 50 days were calculated by using the mean value of each hour obtained from step 1. Therefore, every value represented here is the mean value of 300 data.
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Figure 11. Reduction of wind velocity at different heights in different afforestation areas (3 years after afforestation).
62% at a height of 0?30 m, and 60% at a height of 0?10 m. In the land enclosure area (Field 2), the corresponding values for these four heights were 85%, 75%, 60%, and 40%, respectively. In the land enclosure + irrigation area (Field 3), the corresponding values for these four heights were 75%, 60%, 40%, and 15%, respectively. In the protected afforestation area, the corresponding values for these four heights were 70%, 40%, 30%, and 10%, respectively. Compared with Field 1, wind velocity was reduced in each of the other three measured fields. In the enclosure area, because there was no grazing or woodcutting, natural vegetation rehabilitated and grew better than in Field 1 and wind velocity reduction was enhanced. In the enclosure + irrigation area, rehabilitation of natural vegetation was further accelerated by watering, and the wind velocity reduction effect was even further improved. However, the natural vegetation + planted vegetation in the protected afforestation area achieved the maximum wind velocity reduction effect. The wind velocity in the protected-afforestation area at a height of 0?10 m was only 10% that at 2 m; therefore, compared with the other fields, wind velocity was significantly reduced.
Effects of dune fixation As mentioned before, the protected afforestation area is the main barrier between the salt lake and the encroaching sand. Therefore, any effect on the fixation of moving sand can be regarded mainly as a result of this belt. The volume of sand that moved into the salt lake area was 1?3 106 m3 year1 before the afforestation (1980–1983), 0?4 106 m3 year1 during the afforestation (1983–1986), and 0?2 106 m3 year1 after the afforestation (1986–1993) (Fig. 12). The amount of sand that moved into the salt lake area decreased to 15% after the establishment of the protected afforestation area. The rate of decrease was approximately 0?11 106 m3 year1. Because the rehabilitated vegetation is still in its early period, there is still some sand moving into the salt lake area; the amount of sand encroaching into the salt lake area is expected to
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Volume (106 m3/year) Figure 12. Volume of sand encroaching into the salt lake area before, during, and after the afforestation. The amount of sand sediment encroaching into the salt lake area was measured once in every 3 years in 1980, 1983, 1986, and 1992. By measuring the sand-covered area and the depth of sand layer, the amount of sand sediment encroaching into the salt lake was calculated with these data.
further decrease as the vegetation grows. Therefore, we conclude that the protectedafforestation method is a suitable way to establish a protective belt.
Conclusion Vegetation restoration is one of the most common and effective ways of combating desertification and protecting adjacent areas from sand encroachment in many of the desertified regions of the world. One commonly used procedure to establish vegetation is to first level the dune areas and then plant seedlings of several species with added irrigation. We call this the ‘leveled-afforestation’ method. In contrast to the leveled-afforestation method, we defined another method comprising ‘protecting the surviving natural vegetation as much as possible’ + ‘planting seedlings’ as the ‘protected-afforestation’ method. Results of a 10-year study in the Jilantai Salt Lake area show that the protected-afforestation method had the following advantages: a. The survival rate of planted seedlings was higher in the protectedafforestation area than in the leveled afforestation area. b. The vigor of planted seedlings (height, trunk diameter, coverage, and canopy size) was better in the protected-afforestation area than in the leveled afforestation area, especially in the beginning period of revegetation. c. The coverage (individual species, all planted vegetation, all vegetation) was larger in the protected-afforestation area than in the leveled afforestation area. d. Density of naturally germinated plant species was higher in the protectedafforestation area than in the other areas. For some species, the density in the protected-afforestation area was as much as twice that in the leveled afforestation area. The protected-afforestation method provided a suitable
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growing environment not only for planted species, but also for naturally growing species. e. In the protected vegetation area, relative humidity of air increased. Compared with other afforestation methods, wind velocity was greatly reduced in the protected afforestation area. f. After the establishment of vegetation by the protected-afforestation method, sand encroachment into the salt lake area decreased to 15%. This amount is expected to further decrease as the vegetation grows. These results suggest that protected afforestation is an effective method for vegetation rehabilitation, and has the potential to be applied not only to the arid lands in China but also to desertified areas throughout the world. The authors thank professor Kuibi Zhang (Inner Mongolia Agricultural University) for his helpful advice and kind support as a project leader. The Jilantai Salt Development Company and Science and Technology Committee of Inner Mongolia financed this work. This work was also partially financed by NKBRSF Project G2000018604 and NSFC 39960064.
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