Influence of variation of soil spatial heterogeneity on vegetation restoration

Transcription

1 2020 Science in China Ser. D Earth Sciences 2005 Vol.48 No Influence of variation of soil spatial heterogeneity on vegetation restoration LI Xinrong Shapotou Desert Experiment and Research Station, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou , China ( Lxinrong@ns.lzb.ac.cn) Received April 12, 2004; revised October 20, 2004 Abstract Numerous hypotheses and conceptional models dealing with the grassland desertification or degradation processes recognize that the invasion of shrubs in grasslands is the most striking feature of the variation of vegetation patterns in the grassland degradation or desertification processes in arid and semiarid regions. This is because the invasion of shrubs in grasslands increases the heterogeneity of the temporal and spatial distribution of primary vegetation and soil resources. As a result, the biological processes of the soil-vegetation system are increasingly concentrated in the fertile islands under shrub canopies, and the soil between shrubs gradually turns into bare area or moving sand under the influences of prolonged wind and water erosion. Most of relative researches support this bio-ecological interpretation for the degraded process of grassland. However, as viewed from the other aspect, the shrub vegetation distributed in patches also serves as the trigger spots for the grassland restoration or desertification reversion, and this has been demonstrated by the practices of combating desertification in China. Nearly 50 years of succession of artificial sand-binding vegetation in the Shapotou area and the regional restoration of eco-environment are the theoretical verification and successful example for the desertification reversion. The establishment of artificial vegetation in the region began with the installation of sand fences and planting xerophytic shrubs relying on less than 200 mm of annual precipitation under the non-irrigation condition, this made the moving sand, an originally uniformly distributed soil resource, occur the variation of spatial heterogeneity. Through the redistribution of precipitation and dustfall by the canopy of xerophytic shrubs, litter accumulation and cryptogamic crust development, soil-forming processes under shrub canopies were accelerated; in the meantime, it also created a favorable condition for the invasion and colonization of annual and perennial plant species. However, with the depletion of soil moisture in the deep layer in the sand stabilization area the coverage of shrubs in the sand-binding vegetation lowered from the highest value of 33% to 6%, the dominant position and leading effect of shrubs in the communities were weakened, furthermore they were gradually taken out from the vegetation composition. This correspondingly weakened the spatial heterogeneity of soil resource distribution. The propagation of numerous cryptogams on fixed sand surface and the colonization of annual and perennial plant species promoted the succession and restoration of the vegetation towards herb-dominated vegetation, which are similar to the primary vegetation types of the adjacent steppified desert and desert steppe. This paper, taking nearly 50 years of succession of sand-binding vegetation in the Shapotou region as an example and using the geostatistical Copyright by Science in China Press 2005

2 Influence of variation of soil spatial heterogeneity on vegetation restoration 2021 method, puts forward and explains the conceptional model of vegetation restoration or desertification reversion of grassland in arid zones. Keywords: Tengger Desert, sand-binding vegetation, soil heterogeneity, succession, eco-restoration. DOI: /04yd0139 Ecological restoration as a new research field of applied ecology can be traced back to the 1950s, it mainly focuses on the studies of ecological restoration of mine fields, tropical forests, wetlands and industry-polluted ecosystems [1-4]. Following the raising of the conception of restoration ecology [5], the holding of a series of international conferences and the founding of the International Association for Restoration Ecology, the studies of ecological restoration has become a quite active highlight research field of international ecological circles. However, most of recent reports on ecological restoration in arid and semiarid zones come from the studies of grassland degradation and restoration in the North American countries and Australia [6-8]. In these countries the mechanism of grassland degradation has been studied in detail and several hypotheses and conceptional models have been put forward, most of these researches recognize that the replacement of grassland primary vegetation (dominated by herbages) by the shrub-dominated communities is an obvious manifestation of grassland degradation or desertification [6,9-12]. Some researchers analyzed and explained the ecological processes using the formation mechanism of the fertile island [6,10,13-16], others from the view of restoration ecology, considered the distribution of shrub vegetation in patches and concluded that shrub vegetation is the trigger spots for the vegetation restoration in arid and semiarid zones, also a basis for the vegetation restoration in the regions [17]. Since the late 1950s, a series of ecological restoration and construction projects have been initiated in arid and semiarid zones of China. However, little information is available on the vegetation restoration of degraded grasslands, especially the study of ecological mechanism for vegetation restoration or desertification reversion under human promotion condition is rarely reported. The establishment of artificial sand-binding vegetation from 1956 onwards at the southeast edge of the Tengger Desert ensured the smooth operation of Baotou-Lanzhou railway for nearly half a century, being a successful example in the moving sand control and ecological restoration along the important traffic lines in the world [18,19], furthermore it also provides long-term fixed site observation data for the study of regional ecological restoration. Nearly half a century of succession since the artificial sand-binding vegetation was established significantly improved the region s eco-environment, the stabilization of sand surface created a condition for the reproduction of numerous animal and plant species. Through nearly 50 years of development and succession, soil cryptogamic crusts in the vegetation region include 9 fungal species, 5 moss species and 24 algal species [20,21]. Fifty years later besides the artificially planted shrub species such as Caragana korshinskii, Hedysarum scoparium, Caragana intermedia, a large number of species naturally colonized in the artificially established vegetation region, including Bassia dasyphylla, Eragrostis poaeoides, Setaria viridis, Sonchus brachyotus, Salsola ruthenica, Artemisia capillaris, Chloris rirgata, Aristida adscensionis, Cleistogenes squarrosa, Allium mongolicum, Scorzonera divaricata, Corispermum declinatum, Euphorbia humifusa, Echinops gmelinii, Agriophyllum squarrosum, Artemisia sphaerocephala, Inula salsoloides, Stellaria dichotoma, Stipa glareosa and Artemisia ordosica. In the sand-binding vegetation region insects increased from 5 species to 50 species [22] ; birds increased to 28 species; other animals increased to 23 species 9 of which are rodents. The colonization and reproduction of large number of species changed the mobile dune-dominated desert landscape into a complex artificial-natural desert ecosystem [23]. Using the fixed site observation data at the Shapotou Desert Experiment and Research Station, Chinese Academy of

3 2022 Science in China Ser. D Earth Sciences Sciences in the nearly 50 years and geostatistical methods, this paper studied the relation between the variations of soil spatial heterogeneity and the succession of vegetation in the sand-binding vegetation districts in the arid desert region aiming at exploring the ecological mechanism for promoting regional vegetation restoration through the establishment of artificial vegetation. 1 Study area description Shapotou area is located in Zhongwei county in the Ningxia Hui Autonomous Region at the southeast edge of the Tengger Desert (37 32 N, E), which is an ecotone between steppified desert and desertified steppe and also a transitional zone between desert and oasis [24]. Natural vegetation in the desert region is dominated by the psammophytes such as Hedysarum scoparium, Agriophyllum squarrosum, etc., with coverage of 1% or so [23]. With an elevation of 1339m, the region is covered by huge, dense and continuous reticulate barchan dunes. Soil substrate is loose, impoverished and mobile blown sand soil, the constant moisture content in sand is only 2%-3% [25], groundwater lies 80m below the surface and therefore cannot be used by plants [26]. Mean annual air temperature in the region is 10.0, minimum temperature 25.1, maximum temperature 38.1, annual sunshine duration 3264 h, mean annual precipitation mm, annual evaporation 3000 mm, mean annual wind velocity 2.9 m s 1, and annual number of dust-storm days is 59d. To ensure the smooth operation of Baotou-Lanzhou railway in the desert section, a sand-binding vegetation protective system, Laying emphasis on stabilization in combination with block, was established by the Chinese Academy of Sciences and other related departments since 1956 [27]. At first, the mechanical sand fences were installed at the right angle to the prevailing wind direction, then 1 m 1 m straw checkerboard sand barriers were erected behind the mechanical sand fences, under non-irrigation condition xerophytic shrubs dominated by Caragana korshinskii, Artemisia ordosica and Hedysarum scoparum, Caragana intermedia, Calligonum arborescens and Atraphaxis bracteata were planted at a spacing of 1 m 2 m or 2 m 3 m with straw checkerboard barriers as protective screen. Finally a 500-m-wide sandbinding vegetation belt was established to the north side of the railway and a 200-m-wide vegetation belt was established to the south side, wide a total length of 16 km. 2 Methods A total of 10 sample plots were selected from the existing fixed observation plots in the sand-binding vegetation districts established in different years (1956, 1964 and 1982) and in the adjacent natural vegetation districts (dominant species in communities are Stipa glareosa, Carex capilliformis, Lespedeza durica, Cleistogenes squarrosa, Aristida adscensionis, Stipa bungeana and Oxytropis psammocharis, with relative coverage 32%, 15%, 15%, 5%, 8%, 5%, and 5% respectively. The total coverage of natural vegetation is higher than 90% due to long-term enclosure.) [28] to obtain the study data, and the previous monitoring items were continued on the basis of long-term comprehensive observations of soil-vegetation system: the survey quadrats for shrub vegetation were 10 m 10 m in size and for herbs were 1 m 1 m in size; the abundance of plant species, community coverage and soil physiochemical properties were observed and determined each year. Soil moisture contents were determined from the samples collected by soil auger using the oven-dried method (0-40 cm) and the neutron moisture probe method ( cm). Soil texture was determined by the pipette method, soil total N and organic matter were determined by the standard methods used by the Chinese Ecosystem Research Network [29]. In the study of the heterogeneity of soil resources, we assumed that the initial soil properties distributed in the sand-binding vegetation districts of different ages are consistent and uniformly because the soil was mobile sand before establishment of sand-binding vegetation in different times, which has the same texture composition, organic matter content and other physicochemical features [19]. In addition, the same method was used for the establishment of the sandbinding vegetation of different ages, i.e. after erecting

4 Influence of variation of soil spatial heterogeneity on vegetation restoration 2023 straw checkerboard sand barriers the xerophytic shrubs were planted in same density, and the fixed site observation plots were fenced to avoid human and animal disturbance [27]. The data obtained from these plots were used to reveal the dynamical changes of the spatial heterogeneity of soil resources with the vegetation succession processes (over time). Soil heterogeneity was analyzed using geostatistical method [30], some 100 sampling points were mechanically arranged in 10 m 10 m vegetation plots of different ages, both the transverse and longitudinal spacing was 1m, the surface of sampling plots was flat, and composite samples were collected form 0-20 cm soil layer. In this study the clay percentage was selected to represent soil texture whereas soil organic matter and total N contents were used to reflect soil nutrient regime. According to the distribution depth of mass root system of herbaceous plants and shrubs, soil moisture contents at 0-40 cm and cm were determined respectively. This is because the soil texture is considered to be an important factor determining the vegetation structure and composition of grasslands under uniform climatic condition [31,32], while soil moisture serves as the driving force to the ecological processes in arid zones [32]. In addition, many studies especially emphasize the importance of soil organic matter and total N contents in the vegetation pattern and succession processes [34,35]. Soil spatial heterogeneity was analyzed and given an ecological explanation through the calculation of semivariance γ (h). This method has been widely used in the analysis of soil heterogeneity [6,30,36,37] and the calculation formula is as follows: γ ( h) = 1 E[ Z( x) Z( x+ h) ] 2, 2 where Z(x) is the value of an attribute Z of the system at x spatial position, Z(x+h) is a regional variable at x+h spatial position. From the semivariance and its curve we can obtain four important parameters. 1) When spacing h increases, the semivariance increases from a non-zero value to a constant, namely sill (sill = C 0 +C); 2) When h = 0, γ (0)= C 0, namely nugget; 3) When γ (h) reaches the sill, the spacing is a (range), and fractal dimension D can be determined by the relation between γ (h) and h, (2γ (h) = h [4-2D] ) [38]. Data were processed using the geostatistical software GS + (version 5, Geostatistics for the Environmental Sciences, Gamma Design Software, Michigan, USA). 3 Results 3.1 Temporal and spatial variations of sand-binding vegetation in nearly 50 years Herbaceous plants were difficult to exist without irrigation in the initial stages when sanddunes have been stabilized due to high evaporation and presence of dry sand layer (10-25 cm) on dune surface. However, the seedling of shrub species has higher tolerance to dry and wind erosion habitat because shrub utilize the moisture of deep soil by relative deep-roots in comparison with herbaceous plants. Therefore, we chose only shrub to plant on stabilized dunes in the initial stage when sand-binding vegetation was established. Agriophyllum squarrosum is unique herbaceous plant which occurred in that time because Agriophyllum squarrosum distributes naturally in mobile dunes before establishment of sand-binding vegetation [18,23,24]. Fifty years after the establishment of sand-binding vegetation the maximum shrub cover reached 33%, with further succession some shrub species such as Caragana microphylla, Atraphaxis bracteata, and Calligonum arborescens etc. gradually went out of the original communities, forty years later the shrub cover decreased to 6% (Fig. 1). Three years after shrubs were planted, herbaceous plants began to invade in the shrub vegetation district, and scattered Agriophyllum squarrosum on moving sand acted as the dominant species, with a coverage of less than 1%. Five years after the establishment of the sand-binding vegetation, some annual species such as Bassia dasyphylla, Eragrostis minor and Scorzonera divaricata, etc. began to colonize in the communities; Thirty years later herbaceous plants increased to fourteen species, besides the dominant species Eragrostis minor and Bassia dasyphylla, other species such as Echinops gmelini, Aristida adscensionis, Setaria vbiridis, Salsola ruthenica, and Corispermum declinatum became

5 2024 Science in China Ser. D Earth Sciences Fig. 1. Changes in plant species richness and coverage during 50 years after sand stabilization and revegetation. common species; in addition, some perennial species such as Stipa glareosa also occurred in the vegetation district years after the establishment of sandbinding vegetation the richness of herbaceous species in the vegetation district ranges between 12 and 15 species. However, the richness of herbaceous species in adjacent natural vegetation reaches 34 species [28]. This finding means that biodiversity restoration is a slow process in arid regions. In the nearly 50 years from herbaceous plants began to invade to their establishment, their coverage tended to increase over time and showed a higher correlation with annual precipitation (Fig. 2), while the shrub coverge in the vegetation district showed a less significant correlation with annual precipitation (Fig. 3). Fig. 2. Relationship between annual rainfall and herbaceous coverage after 15 years since revegetation. Fig. 3. Relationship between annual rainfall and shrub coverage after 15 years since revegetation. 3.2 vegetation succession processes and soil property changes It can be seen from Figs. 4 and 5 that after the establishment of sand-binding vegetation on mobile dunes soil properties changed markedly. The surface soil properties (0-20 cm) in the sand-binding vegetation districts of different ages differed significantly (P < 0.01). With the increase in sand stabilization age the percentage of clay and silt particles in sand significantly in creased, the content of coarse sand particles decreased and gradually tended to become into the soil texture of the natural vegetation district. The total contents of N, P and K in sand also increased with the increase in sand stabilization ages, especially the increase in total N content was more significant (P < 0.01). However the soil nutrient content in the sand-

6 Influence of variation of soil spatial heterogeneity on vegetation restoration 2025 Fig. 4. Changes in soil texture of different vegetative sites and control. Values with different letters are significantly different between two sites at 0.01 level (P < 0.01, n = 10). Fig. 5. Changes in soil nutrients of different vegetative sites and control. The others are the same as in Fig. 4. binding vegetation district was still low as compared with the natural vegetation district (Fig. 5). 3.3 Variations of the spatial heterogeneity of soil properties with the ages of sand-binding vegetation Table 1 lists the statistical characteristics of the determined results of soil parameters. The mean clay percentage in soil mechanical composition increased with increasing sand stabilization ages, the variation in clay content was larger in the later sand stabilization district (1982) than in the earlier sand stabilization district (1956). Mean moisture content in surface soil layer in different sand stabilization districts differed little and was lower than the natural vegetation district. Mean moisture content in cm soil layer decreased with the increase in sand stabilization ages, while the same soil layer in the natural vegetation district had higher moisture content. The total and or-

7 2026 Science in China Ser. D Earth Sciences ganic matter contents in the surface soil layer in the sand stabilization district significantly increased with the increase in sand stabilization ages. As shown in Table 1, the statistical characteristics of soil parameters basically coincide with a normal distribution, also coincide with the calculated results of semivariance and the established model. Table 2 shows that the model of the semivariance curve of the five soil parameters, including soil clay percentage, soil moisture contents in surface soil layer (0-40 cm) and deep soil layer ( cm), soil total N and organic matter contents, in four different sample plots is dominated by spherical model. The moisture and organic matter contents in 0-40 cm soil layer in the 1956 sample plot and the total N content in the 1982 sample plot belong to exponential model, the model of the semivariance curve of clay content in the natural vegetation district is a linear model. Soil spatial heterogeneity in different ages of sand-binding vegetation districts and natural vegetation district differs markedly due to the difference in nugget C 0. Larger C 0 means that the spatial heterogeneity caused by stochastic process constitutes a higher percentage. The ratio between the arch height and sill C+ C 0 reflects the contribution the total spatial heterogeneity. It can be seen from Table 2 that for the clay content in the natural vegetation district the spatial heterogeneity caused by stochastic process constitutes the total spatial heterogeneity, while for the distribution of other determined parameters the autocorrelation heterogeneity reaches a higher percentage (>50%) in the spatial heterogeneity. The determination coefficient R 2 represents the fitting reliability to a certain degree. All these models have a higher determination coefficient. Therefore, it can be thought that all those models can fully reflect the distribution characteristics of spatial heterogeneity of various determined parameters. In addition, the fractal dimension D also can reflect the spatial heterogeneity, and larger D value represents a higher spatial heterogeneity caused by spatial autocorrelation. It can be seen from the calculation of the variation function of soil clay content, 20 years after the establishment of sand-binding vegetation (1982 sample plot) soil texture changed from uniform moving dune sand into the surface soil texture with higher spatial heterogeneity, its range a was 31 m, in other words, the clay content in surface soil layer in the 1-31 m mesoscale Table 1 Statistical characteristics of soil parameters in the sand-binding vegetation sites of different ages and natural vegetation (control) Sample Minimum Maximum Sample Parameter Site Mean S.D. Skewness Kurtosis variance value value number Clay (%) Soil moisture content of 0-40 cm depth (%) Soil moisture content of cm depth (%) Total N/g kg 1 Organic matter/g kg 1 control control control control control

8 Influence of variation of soil spatial heterogeneity on vegetation restoration 2027 Table 2 Semivariance model and relevant parameters in the sand-binding vegetation sites of different ages and natural vegetation (control) Parameter Site Model Co Co+C a C/Co+C R 2 RSS D Clay (%) Moisture content of 0-40 cm depth (%) Moisture content of cm depth (%) Total N/g kg 1 Organic matter/g kg Spherical Spherical Spherical Control Liner Spherical Spherical Exponential (30.42) Control Spherical Spherical Spherical Spherical Control Spherical Exponential (8.19) Spherical Spherical Control Spherical Spherical Spherical Exponential (2.94) Control Spherical Data in bracket are the effective range of exponential model (effective range=a 3). range has a marked distribution feature of sand-binding vegetation (1964 sample plot), its range was also 31 m, but the percentage of the spatial heterogeneity caused by spatial autocorrelation in the total spatial heterogeneity increased (72%). Nearly 50 years after the establishment of sand-binding vegetation (1956 sample plot), the effective range of spatial heterogeneity decreased (25 m), the percentage of spatial heterogeneity caused by spatial autocorrelation also decreased (50%), and for the clay distribution in the surface soil layer in the natural vegetation district the effective range of spatial heterogeneity was only 11 m. As for the distribution of moisture content in surface soil layer in the sand-binding district, it also changed from relatively uniform distribution state before the establishment of vegetation to a distribution state with obvious spatial heterogeneity. However, 40 years after the establishment of sand-binding vegetation the effective range of spatial heterogeneity of soil moisture content started to be decreased, and the percentage of the spatial heterogeneity caused by autocorrelation in the total spatial heterogeneity also decreased. The spatial heterogeneity of moisture content in cm soil layer also intensified after the establishment of sand-binding vegetation. However, after nearly 50 years of vegetation succession the spatial heterogeneity gradually tended to be weakened, its range decreased from 21 m in 1982 vegetation district to 4 m in 1956 vegetation district, and the fractal dimension decreased from 1.91 to From the comparison of soil N and organic matter contents in different years of sand-binding vegetation districts, it can be seen that with the succession of sand-binding vegetation the spatial heterogeneity of soil N and organic matter contents exhibited a processes from intensifying towards gradually weakening. However, as compared to the N content in soil, the spatial heterogeneity of organic matter content changed relatively little over time. It can be seen from Table 2 that as compared with the soils in different years of vegetation districts, the spatial heterogeneity of clay content in surface soil layer, moisture, total N and organic matter contents in surface and deep soil layers in the natural vegetation district has a small effective range, and also has a relatively small heterogeneity, this means that the soil

9 2028 Science in China Ser. D Earth Sciences resources with herb-dominated natural vegetation communities are relatively uniformly distributed. 4 Discussion 4.1 Nearly 50 years of succession of sand-binding vegetation in relation to the habitat Nearly half a century since the sand-binding vegetation has been established in 1956 in the Shapotou region the vegetation has caused a profound change in the region s eco-environment. Meanwhile, it also promoted the restoration of the habitat [22] and ensured the steady and sustainable development of sandbreak system along the Boatou-Lanzhou railway [19]. The feedback effects of the habitat changes drive the succession of original sand-binding vegetation [26]. The first years are after the establishment of sand-binding vegetation one of the periods with the largest species turnover rate or species replacement rate of the plant communities [23], when large number of herbaceous species colonized and the coverage of shrub species in the communities reached its peak (33%). However, annual precipitation in the Tengger Desert is only 180 mm, ground-water table is 80 m below the surface. Hence, it cannot be used by plants. The limited water becomes a leading factor affecting the development of deep-rooted shrub vegetation, and the development of cryptogam crusts on sand surface inhibits the deep infiltration of precipitation into dune sand [20-22]. The reproduction of cryptogam and shallow-rooted plants also alters the temporal and spatial distribution of soil moisture and further deteriorates the effective recharge of water to subsoil, as a result, deep-rooted plants begin to degrade. The amount of rainfall interrupted entirely by cryptogamic crust when rainfall is less than 10 mm [20]. However, the frequency of rainfall event less than 10 mm is more than 50% each year [18]. This led soil moisture in the deep layer to decrease due to no supply by rainfall infiltration, further made shrubs with deep-roots to degrade. Nearly 50 years later, among the originally planted shrubs only Caragana korshinskii, Hedysarum scoparium and Artemisia ordosica remained with coverage of less than 10% (Fig. 1). In addition, following the stabilization of sand dune surface, the deposition of atmos- pheric dustfall and the intensifying of biological processes [39] promote the soil-forming processes on sand surface [19]. Some major factors limiting the biological productivity in arid zones such as soil N, P, K and organic matter contents [40-42] significantly increase in surface sand layer (Fig. 5). The increase in clay content in the surface sand layer (Fig. 4) improves the soil water-holding capacity [43] and increases effective water available for shallower-rooted herbaceous plants [36], and therefore form the pattern of sand- binding vegetation with herbaceous plants as dominant species [23]. 4.2 The variation characteristics of soil spatial heterogeneity in the succession processes of sand-binding vegetation The heterogeneity of habitat greatly contributes to the coexisting of many plant species in the communities [44]. Artificial sand-binding vegetation in the Shapotou region was initially planted on the relatively uniform sand dune stabilized by straw checkerboard barriers, it was established in different years using the same plant arrangement and the same planting techniques, and the fixed observation plots in the vegetation districts established in different years were fenced to avoid human disturbance. In such case, we compared the vegetation districts of different ages and analyzed the dynamical changes of soil resources over time. The geostatistical analytical results show that the relatively uniform sanddune was affected by vegetation, its main properties, for example, the spatial heterogeneity of soil texture, moisture content, organic matter and total N contents was significantly intensified, however due to nearly 50 years of vegetation effects, its spatial heterogeneity began to be weakened and tended to evolve the distribution feature of soil spatial heterogeneity of natural herbaceous vegetation district. Many studies show that the presence of shrub species in the vegetation of arid and semiarid zones is an important cause responsible for the soil heterogeneity [32,45]. Therefore the gradual disappearance of shrub species from the sand-binding vegetation in the Shapotou region after 50 years can be explained as one of main causes resulting in the weakening of soil spatial heterogeneity.

10 Influence of variation of soil spatial heterogeneity on vegetation restoration The conceptional model for the reversion of desertified grassland Mass invasion of shrubs and the vegetation pattern of patchy distribution are a significant mark of grassland degradation in arid and semiarid zones [11,17]. The effects of fertile island caused by shrub-dominated vegetation force the biological, physical and chemical processes of primary vegetation system to be largely concentrated in the range of shrub canopies [14]. Large amounts of nutrients and clay in the wind and water erosion environments were deposited in the range of plant canopy. The interception of shrub canopies alters the distribution of natural precipitation. Under larger precipitation intensity the soil moisture content is markedly higher under plant canopies than the bare land, this is because the bare land has a much higher evaporation rate than the soil under the plant canopies. Even so, some researchers think that the grassland degradation and the formation of fertile islands only represent a new and non-uniform distribution of resources in the primary vegetation-soil system, and the total amount of the resources on the whole remains unchanged [42]. On the other hand, in the arid zones where groundwater is too deep to be used by plants, the mass reproduction of annual plants or the formation of cryptogam crusts under the shrubs limit the infiltration of water into deep sand. As a result, soil moisture regime around the root zone of shrubs worsens due to lack of recharge, the deep-rooted shrubs with dense root system gradually go out of the vegetation composition and the effects of fertile island also gradually weaken, the higher heterogeneity of soil resources become weak or less evident, while the relatively uniformly distributed soil resources tend to evolve the soil properties of primary grassland. The weakening of soil heterogeneity and the relatively uniform distribution of clay, nutrients and precipitation are favourable for the invasion and establishment of herbaceous plants and thereby create a soil substrate habitat for the recovery of vegetation towards primary grassland vegetation. Therefore, the shrub fertile islands distributed in patches are often regarded as the trigger spots for the vegetation restoration of grasslands in arid zones [17]. The vegetation succession and ecological restoration in the Shapotou region in the nearly 50 years showed that only when soil substrate is improved can the biotic communities be restored. The restoration of soil resources and the reproduction of biotic communities interact as precondition and promote each other. Based on the above analysis, Fig. 6 outlines the model of vegetation restoration processes and desertification reversion processes. Soil heterogeneity increases with high heterogeneity H 2 in the degradation processes of primary grassland vegetation (soil resource distribution is relatively uniform and has low spatial heterogeneity H 1 ), or the replacement processes of herbaceous vegetation by shrubs. The establishment of vegetation increases the dune soil spatial heterogeneity (H 1 ), furthermore as the dominance of shrubs in the community increases (with higher coverage), soil has a higher heterogeneity H 2. However, as the sand-binding vegetation evolves from shrub communities towards the herb-dominated communities, the soil heterogeneity begins to be weakened (approximate to H 1 ), the distribution of soil resources gradually becomes uniform and exhibits the distribution feature of soil resources in natural vegetation districts. Therefore, the desertification reversion processes of desertified grassland or the restoration processes of Fig. 6. A conceptional model related to grassland desertification and restoration (desertification reverse) in arid and semiarid zones. Soil heterogeneity increasses in the degradation processes of primary grassland vegetation or the replacement processes of herbaceous vegetation by shrubs. The establishment of vegetation increases the spatial heterogeneity of dune soil, but as the sand-binding vegetation evolves from shrub communities into herb-dominated communities the soil heterogeneity begins to be weakened.

11 2030 Science in China Ser. D Earth Sciences vegetation are a weakening process of the heterogeneity of soil resource distribution, while the desertification or degradation of grassland is an increasing processes of soil heterogeneity form lower level to higher level. 5 Conclusion The ecological restoration in the Shapotou region was realized in an extreme environment, namely using the engineering methods in combination with biological measures, relying on the special ecological functions of xerophytic shrubs in the arid zones or the principle is favourable to forming higher heterogeneity of soil resource distribution, the xerophytic shrub species were selected as pioneer plants to establish sandbinding vegetation on sand dunes. This, on the one hand, stabilized moving sand and created a condition for the colonization and the restoration of biodiversity; and on the other hand promoted soil-forming processes. With the vegetation succession and the reduction of soil heterogeneity, the artificially established sand-binding shrub vegetation evolved towards herb-dominated desert grassland vegetation. The distribution of soil resources also exhibited a change from relatively low heterogeneity to high heterogeneity and then tended to become uniform. Therefore, with exception of climatic factor, we think that the variation of the spatial distribution heterogeneity of soil resources is a factor driving the degradation or restoration of grasslands in arid zones. Soil heterogeneity increased in the degradation processes of primary grassland vegetation, namely the replacement processes of herbaceous vegetation by shrubs. The establishment of vegetation increased the dune soil spatial heterogeneity, but as the sand-binding vegetation evolved from shrub communities into herbdominated communities the soil heterogeneity started to be weakened. This finding benefits to further understand the ecological mechanism of interaction between heterogeneity of soil resources and changes in vegetation patterns. Acknowledgements This study was supported by the Innovation Project of the Chinese Academy of Sciences (Grant No. KZCX3-SW- 324) and the Key Research Plan of the National Natural Science Foundation of China (Grant No ). References 1. Good, J. E. G., Williams, T. G., Survival and growth of selected clones of birch and willow on restored opencast coal sites, Journal of Application Ecology, 1985, 22: Cairns, J. Jr., Restoration of Aquatic Ecosystems, Washington D C: National Academy Press, 1992, Brown, S., Lugo A E. Rehabilitation of tropical lands: a key to sustaining development, Restoration Ecology, 1994, 2: Niering, W. A., Tidal wetlands restoration and creation along the east coast of North America, Restoration ecology and sustainable development (eds. Urbanska, K. M., Webb, N. R., Edwards, P. J.), Cambridge: Cambridge University Press, Aber, J. D., Jordan, W., Restoration ecology: An environmental middle ground, BioScience, 1985, 35: Schlesinger, W. H., Raikes, J. A., Hartley, A. E. et al., On the spatial pattern of soil nutrients in desert ecosystems, Ecology, 1996, 77: Williams, K. S., Terrestrial arthropods as ecological indicators of habitat restoration in southwestern North America, Restoration ecology and sustainable development (eds. Urbanska, K. M., Webb, N. R., Edwards, P. J.), Cambridge: Cambridge University Press, 1997, Majer, J. D., Invertebrates assist the restoration process: an Australian perspective, Restoration Ecology and Sustainable Development (eds. Urbanska, K. M., Webb, N. R., Edwards, P. J.), Cambridge: Cambridge University Press, 1997, Crawford, C. S., Gosz, J. R., Desert ecosystems: their resources in space and time, Environmental Conservation, 1982, 9: Noy-Meir, I., Desert ecosystem structure and function, Hot Deserts and Arid Shrublands (ed. Evenari, M.), Amsterdam: Elsevier Science, 1985, Schlesinger, W. H., Reynolds, J. F., Cunningham, G. L. et al., Biological feedbacks in global desertification, Science, 1990, 247: Schlesinger, W. H., Pilmanis, A. M., Plant-soil interactions in deserts, Biogeochemistry, 1998, 42: [DOI] 13. Charley, J. L., West, N. E., Plant-induced soil chemical patterns in some shrub-dominated semi-desert ecosystems of Utah, Journal of Ecology, 1975, 63: Garner, W., Steinberger, Y., A proposed mechanism for the formation of Fertile Island in the desert ecosystem, Journal of Arid Environments, 1989, 16: Hook, P. B., Burke, I. J., Lauenroth, W. K., Heterogeneity of soil and plant N and C associated with individual plants and openings in North American shortgrass steppe, Plant and Soil, 1991, 138: [DOI] 16. Tongway, D. J., Ludwig, J. A., Small-scale resource heterogeneity in semiarid landscapes, Pacific Conservation Biology, 1994, 1: Lal, R., Carbon Sequestration in Drylands, Annals of Arid Zone, 2000, 39: Zhao, X. L., Approach to the problem of vegetal sand stabilization in the Shapotou region, Research of Shifting Sand Control in the Shapotou Region of Tengger Desert 2 (ed. Shapotou Desert Re-

12 Influence of variation of soil spatial heterogeneity on vegetation restoration 2031 search and Experiment Station, CAS) (in Chinese), Yingchuan: Ningxia People s Publishing House, 1991, Xiao, H. L., Li, X. R., Duan, Z. H, Li, S. Z., Succession of soil-vegetation system in moving sand stabilization processes, Journal of Desert Research (in Chinese), : Li, X. R., Wang, X. P., Li, T. et al., Microbiotic crust and its effect on vegetation and habitat on artificially stabilized desert dunes in Tengger desert, North China, Biology and Fertility of Soils, 2002, 35: [DOI] 21. Li, X. R., Zhou, H. Y., Wang, X. P. et al., The effects of sand stabilization and revegetation on cryptogam species diversity and soil fertility in the Tengger Desert, Northern China, Plant and Soil, 2003, 251: [DOI] 22. Li, X. R., Xiao, H. L., Zhang, J. G. et al., Ecosystem effects of sand-binding vegetation and restoration of biodiversity in arid region of China, Restoration Ecology, 2004, 12: [DOI] 23. Li, X. R., Zhang, J. G., Liu, L. C., Study of plant diversity in the artificial vegetation-environmental succession processes in arid desert region of China, Acta Phytoecologica Sinica (in Chinese), 2000, Li, X. R., Shi, Q. H., Zhang, J. G., Study of plant diversity in the artificial vegetation succession processes in Shapotou region, Journal of Desert Research (in Chinese), 1998, 18: Chen, L. H., Li, F. X., Di, X. M. et al., Blown Sand Soils in China (in Chinese), Beijing: Science Press, 1998, Li, X. R., Ma, F. Y., Xiao, H. L. et al., Long-term effects of revegetation on soil water content of sand dunes in arid region of northern China, Journal of Arid Environments, 2004, 57: [DOI] 27. Shapotou Desert Research Station, Lanzhou Institute of Desert Research, CAS, Sand Stabilization Principle and Measures in the Shapotou Section of Baotou-Lanzhou Railway (in Chinese), Yingchuan: Ningxia People s Publishing House, 1991, Li, X. R., Zhang, Z. S., Zhang, J. G. et al. Association between vegetation patterns and soil properties in the Southeastern Tengger Desert, China, Arid Land Research and Management, 2004, 18: [DOI] 29. Liu, G. S., Soil Physical and Chemical Analysis & Description of Soil Profiles (in Chinese), Beijing: Chinese Standard Press, 1996, Wang, Z. Q., Geostatistics and Its Application in Ecology (in Chinese), Beijing: Science Press, 1999, Sala, O. E., Lauenroth, W. K., Golluscio, R. A., Plant functional types in temperate semiarid regions, Plant Functional Types (eds. Smith, T. M., Shugart, H. H., Woodward, F. I.), Cambridge: Cambridge University Press, 1997, Dodd, M. B., Lauenroth, W. K., Burke, I. C. et al., Association between vegetation patterns and soil texture in the shortgrass steppe, Plant Ecology, 2002, 158: [DOI] 33. Zhao, W. Z., Cheng, G. D., Review on several progresses in dry land eco-hydrological study, Chinese Science Bulletin, 2001, 46: Dunkerley, D., Hydrologic effects of dryland shrubs: defining the spatial extent of modified soil water uptake rates at an Australian desert site, Journal of Arid Environments, 2000, 45: [DOI] 35. Sperry, J. S., Hacke, U. G., Desert shrub water relations with respect to soil characteristics and plant functional type, Functional Ecology, 2002, 16: [DOI] 36. Trangmar, B. B., Yost, R. S., Uehara G., Application of geostatistics to spatial studies of soil properties, Advance Agronomy, 1985, 38: Webster, R., Quantitative spatial analysis of soil in the field, Advance in Soil Science, 1985, 3: Palmer, M. W., Fractal geometry: a toll for describing spatial pattern of plant communities, Vegetatio, 1988, 75: [DOI] 39. Fearnehough, W., Fullen, M. A., Mitchell, D. J. et al., Aeolian deposition and its effect on soil and vegetation changes on stabilized desert dunes in northern China, Geomorphology, 1998, 23: [DOI] 40. Zaady, E., Groffman, P. M., Shachak, M., Litter as a regulator of N and C dynamics in macrophytic patches in the Negev desert soils, Soil Biology and Biochemistry, 1996, 28: [DOI] 41. Connin, S. L., Virginia, R. A., Chamberlain, C. P., Carbon isotopes reveal changes in soil organic matter production and turnover following arid land shrub expansion, Oecologia, 1997, 110: [DOI] 42. Havstad, K. M., Herrick, J. E., Schlesinger, W. H., Desert rangelands, degradation and nutrients. Rangeland desertification (eds. Arnalds, O., Archer, S.), Dorrecht: Kluwer Academic Publishers, 2000, Jury, W. A., Gardner, W. R., Gardner, W. H., Soil Physics, New York: John Wiley, 1991, Tilman, D., Kareiva, P., Spatial ecology: The Role of Space in Population Dynamics and Inter-specific Interactions, Princeton: Princeton University Press, 1997, Ettema, C. H., Wardle, D. A., Spatial soil ecology, Trends in Ecology & Succession, 2002, 17: [DOI]