Changes in soil and vegetation following stabilisation of dunes in the southeastern fringe of the Tengger Desert, China
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- Li, X.R., Kong, D.S., Tan, H.J. et al. Plant Soil (2007) 300: 221. doi:10.1007/s11104-007-9407-1
Properties of the soil and sand-binding vegetation were measured at five sites plus a control on dunes of the Tengger Desert stabilized for periods of up to 50 years. In the topsoil, fine particles, total N, P, K and organic matter increased significantly with increasing site age. However, there were no significant changes in deeper soil profiles (>0.4 m depth). Soil pH, calcium carbonate content, and total salt content tended to increase with age. Soil water in the topsoil changed little with increasing age, but was closely related to rainfall during the 50-year period. For deeper soil layers (0.4–3.0 m) soil water decreased significantly with age. After revegetation, the number of herbaceous species increased up to 30 years and then levelled off to 12–14 species, whereas the number of shrub species decreased from the 10 initial sand-binding species to only 3 species. Shrub cover decreased from a highest average of about 33% to the current 9%, whereas cover and biomass of herbaceous species increased throughout succession from 1956 to 2006. The development of soil and cryptogamic crusts on the surface of stabilized dunes enhanced the colonization and establishment of herbaceous plants due to increasing water availability, clay and silt content and soil nutrients. We propose that changes in properties of the surface soil led to increased interception of water, favoring shallow rooted grasses and forbs over perennial shrubs.
KeywordsLong-term effectsSand-binding vegetationSoil resourcesStabilized dunesSuccessionTengger Desert
In arid–desert ecosystems, soil water and/or nitrogen are the most common abiotic factors that limit plant growth (Smith et al. 1997; Whitford 2002). Accumulation of nutrients and soil organic matter on the soil surface under plant canopies results from complex interactions between biotic processes moderated by plants and soil biota, and abiotic processes driven by atmospheric and biochemical processes (Whitford 1986; Fisher et al. 1988; Moorhead et al. 1988; Schlesinger et al. 1996; Aguiar and Sala 1999). With the exception of climatic factors, changes in soil resources are important determinants of vegetation features at small spatial scales (El-Demerdash et al. 1995; Dodd and Lauenroth 1997; Havstad et al. 2000; Miki and Kondoh 2002; Dale and Adams 2003; Oztas et al. 2003). Most recent studies of vegetation succession in arid regions have examined vegetative dynamics after human disturbance, such as recovery after construction (pipeline corridors, power transmission lines and transportation routes), mining, farming (old fields, abandoned fields and field management), abandonment of population centers (Carpenter et al. 1986; Webb et al. 1988; Harmer et al. 2001; Warren et al. 2002; Ejrnes et al. 2003; EI-Sheikh 2005), and natural events such as landslides (Singh et al. 2001; Sparling et al. 2003). Few studies have monitored the long-term processes of vegetation change associated with stabilization of unstable sand dune systems. Attempting to account for such mechanisms is a challenge for ecologists due to the difficulty in monitoring a number of soil parameters over time. Using a substitution of “space” for “time” is an effective way of studying changes over time (Sparling et al. 2003; Li et al. 2007). Sites that have been stabilized by revegetation at different times offer an ideal opportunity to understand vegetation successional processes in extreme environments, as soil conditions before revegetation are largely driven by geomorphological processes, and vegetation succession commences after the establishment of sand-binding vegetation.
China is one of the most desertified countries in the world, and large areas of arid deserts have been revegetated. To date, revegetation covers more than 2.4 million ha of degraded land in China and includes sand-binding vegetation that has been established in arid desert regions to reduce the impacts of desertification (Li et al. 2004a). The robustness and dynamics of these revegetated areas and how they are best managed for sustainable development are critical issues facing land managers in China (Shapotou Desert Research and Experiment Station, CAS 1980).
In this study, we use data from permanent experimental plots of sand-binding vegetation of different ages at the Shapotou Region of the Tengger Desert, western China, to quantitatively describe vegetation changes associated with changes in soil on stabilized sand dunes. The aim of the work described in this paper is to understand the nature of changes in sand-binding vegetation over the 50-year period since it has been established in order to improve our knowledge of the rehabilitation of desert systems. Our particular objectives are: (1) to examine changes in species composition, cover, and biomass of plant species in sand-binding vegetation over a 50-year period; and (2) to assess the effects of variation in soils on sand-binding vegetation. This study will help contribute to our understanding of ecosystem management of revegetation in extreme desert ecosystems.
Materials and methods
To ensure unimpeded passage of the Baotou–Lanzhou railway through the sand dunes in the Shapotou region, a 16-km-long vegetation protective system was established in the 1950s, with widths of more than 500 m on the northern side and 200 m on the southern side of the railway (Shapotou Desert Research Experimental Station, CAS 1980; Li et al. 2003). Straw-checkerboards, usually 1 m2 in area, were built with wheat or rice straw to stabilize the dune surface (Fig. 1a). The straw remains intact for 4–5 years, which allows time for planted xerophytic shrubs to adapt to an environment with prevalent wind erosion. Straw-checkerboards at a height of 0.15–0.2 m above the ground increase the roughness of the sand surface by 400- to 600-fold, and reduce wind velocity by 20–40% at a height of 0.5 m and by 10% at 2 m above the surface (Zou et al. 1981; Fullen and Mitchell 1994). The quantity of sand transported over a straw-checkerboard is only 1% of that over an uncovered mobile sand dune (Zhu et al. 1992). Xerophytic shrub seedlings such as Artemisia ordosica Krasch, Caragana korshinskii Kom., Caragana microphylla Lam., Calligonum mongolicum Turc’z, Atraphaxis bracteata A. Los, Atraphaxis pungens Jaub. et Spach., Elaeagnus angustifolia L., Salix gordejevii Y. L. Chang and Hedysarum scoparium Fisch were planted within the checkerboard and grown without irrigation (Fig. 1b; Li et al. 2003). Parallel stabilized areas were further expanded one by one along the railway line in 1964, 1982, 1991 and 1999 (Fig. 1c) using the same methods, namely dunes were stabilized by straw-checkerboards with the same design, and 2-year-old seedlings of xerophytic shrubs were planted with the same species allocation and density (16 individuals per 100 m2) (Shapotou Desert Research and Experiment Station, CAS 1980). Thus, the sites were similar to each other in the initial stages of change.
Measurement of soil physicochemical parameters
Soil water content has been monitored continuously for each different-aged site since sand-binding vegetation was established in 1956, 1964, 1982, 1991 and 1999. For comparison, soil water content of mobile dunes was also monitored from 1956 to 2006. To measure soil water content, soil samples were collected manually using a soil core sampler and dried at 105°C for 24 h. Soil samples were taken from different-aged sites and mobile dunes at 16 different depths: 0.1, 0.2, and then every 0.2 m down to 3.0 m, with 10 replicates at each depth. Soil was sampled monthly from 1956 to 1998, except during rain events when samples were taken 1 week after the rainfall (Shapotou Desert Research and Experiment Station, CAS 1991). From 1999 to 2006, we measured soil water content using the neutron method and time domain reflectometry (TDR) (Gardner et al. 2001); the frequency of observations was the same as from 1956 to 1998; volumetric water contents were transformed to gravimetric water contents to keep data consistent.
Monitoring the same site through time is normally considered the most reliable way to measure change (Powlson et al. 1998). As there were no historical data recording changes in most soil properties for the 50 years since the onset of stabilization, we used a “space for time” substitution, sampling sites of differing age. Ten replicate soil samples were collected from each of the six age classes at depths of 0–0.05 m and 0.05–0.2 m from the dune’s hollow areas during the 2006 growing season. The soil sampling sites were the same as the soil water monitoring plots. Arid-dried soil samples were sieved through a 2 mm screen and used for further analysis. Particle size was assessed by the pipette method; soil bulk density was determined by inserting a metallic core (0.05 m in depth and diameter) into the soil. Soil pH was determined using a soil-water ratio of 1:5; soil organic matter (SOM) was measured using the K2Cr2O7 method (Agriculture Chemistry Specialty Council, Soil Science Society of China 1983). Soil water characteristic curves were determined using a pressure plate extractor (CAT 1600, Soil Moisture Equipment Company, Santa Barbara, CA). Soil soluble salts were analyzed using methods described by the Nanjing Institute of Soil Research, Chinese Academy of Sciences (1980). Total N was measured using the Kjeltec system 1026 Distilling Unit (Tecator AB, Höganäs, Sweden). Total soil phosphorus (P) was determined by perchloric acid digestion (Olsen and Sommers 1982), and potassium (K) was measured by hydroflouric acid /perchloric acid digestion (Knudsen et al. 1982).
Vegetation monitoring and observations
To monitor sand-binding vegetation, permanent quadrats were located at the sites corresponding to soil water monitoring plots and soil sampling sites. Monitoring began in the year vegetation was established. The quadrats were located in a 1,600 m × 800 m zone north of the railway line. These study sites are still enclosed and no grazing occurs. A total of 200 quadrats were placed. To investigate herbaceous plants, 20 quadrats of 1 m2 were set up at each of these sites. To investigate shrubs, there were 20 quadrats of 100 m2 each. Species richness (number of species per quadrat) and plant cover were recorded annually from 1956 to 2006. Vegetation measurements were made once at the end of autumn of each year (Shapotou Desert Research and Experiment Station, CAS 1980). However, to reduce wind erosion on the dune surface, the aboveground biomass of herbaceous plants was measured only once in each site, using 20 quadrats of 1 m2 in October 2006. To avoid damage to the vegetation protecting the railway, shrub biomass was not measured.
The correlation between soil water content in different soil layers and annual precipitation was determined using Pearson correlation analysis. Linear stepwise regression was used to determine the correlation between vegetative features and soil parameters. Analyses were performed using the Windows-based SPSS software (10th edn; SPSS, Chicago, IL). To estimate rates of changes in both vegetation and soil characteristics, the related variables were plotted against the time since dune stabilization. The area under the curve for each site was calculated and plotted against time to estimate the increase in soil water-holding capacity (SWHC) with time. The optimal shape of the simulating curve depended on regression analysis and correlation test for different models. This procedure was completed using the Origin 7.0 software package (OriginLab, Northampton, MA).
Changes in soil properties over 50 years
In the topsoil, water content was highly positively correlated with annual rainfall (r = 0.83, P < 0.01), but in deeper soil layers, the correlation with rainfall was poor (r = 0.25, P = 0.11 > 0.01) (Fig. 2). However, at the control site, water content in the mobile dunes was correlated with annual rainfall, even in deeper soil layers (soil water content = 14.76 rainfall −10.02, r = 0.80, P < 0.01).
Changes in plant species richness, cover and herbaceous biomass
The biomass of naturally established species increased from 0.3 kg m−2 in 5-year-old vegetation to 0.6 kg m−2 for 50-year-old vegetation. An apparent increase in the biomass of colonizing species occurs with time (Fig. 6c).
In the Shapotou region of the Tengger Desert, straw-checkerboards have transformed mobile, unstable sand dunes into stable productive ecosystems, resulting in the revegetation of large areas of sandy desert (Shapotou Desert Research and Experimental Station, CAS 1980, 1991). These checkerboards, which remain intact for 4–5 years due to the low rainfall, allow time for the xerophytic plants that are planted within the squares to become established. Mobile sand dunes have a low silt and clay content, and are therefore low in available nutrients to support plant growth (Buckley et al. 1986). However, after revegetation, large amounts of silt- and clay-size particles are deposited on the dune surfaces, altering the composition of the original soil texture (Fig. 4a). The results of this study indicate that a continuous increase in the silt and clay content of surface soils occurs with time since revegetation. Increasing the amount of fine-sized particles in the soil results in an improvement in soil nutrient status (Li et al. 2007) and soil water-holding capacity of the topsoil.
Stepwise regression analysis for soil parameters and vegetative features over time since sand stabilization
Shrub cover = 13.165–26.258 SOM% + 13.008 soil water content (0.4–3.0 m)
Shrub species richness = −12.333 + 0.226 Sand%
Herbaceous cover = −11.02 + 2.189 clay% + 7.206 soil water content (0–0.04 m)%
Herbaceous species richness = −3.201 + 3.906 soil water content (0–0.04 m) % + 3.036 SOM%
Herbaceous biomass = 0.253 + 0.128 SOM% + 0.013 clay%
A number of studies from arid areas worldwide suggest that colonization of herbaceous species is positively correlated with soil silt and clay content (Danin et al. 1989; Dodd et al. 2002). Finer-textured soils not only improve the SWHC, but also enrich the surface with nutrients (Drees 1993). Although the increase of topsoil SOM in our study was slow, it plays an important role in plant colonization and establishment because it acts as a reservoir for essential elements, particularly N and P. SOM is a source of cation exchange capacity and soil pH buffering, and enhances soil porosity, improving the movement of oxygen through the soil (Bohn et al. 2001). Our stepwise regression analyses support this conclusion. SOM had an important effect on herbaceous plant species richness and productivity (Table 1). In addition, forbs and grasses, being shallow-rooted, will benefit from these alterations in the topsoil. Therefore, the improvement of the physico-chemical properties of the soil during the 50-year period created a favorable environment for the germination and establishment of forbs and grasses.
The alteration of soil texture affects the formation and development of cryptogamic crusts (Li et al. 2003), which further trap finer material and accentuate the differences between surface horizons and subsoil (Li et al. 2000). Cryptogamic crusts, which are associated with soils of a higher concentration of finer material at the surface, alter the spatial and temporal distribution of infiltration, intercept rainfall at the surface (Li et al. 2000), and prevent water from infiltrating into deeper soil layers (Li et al. 2000, 2003). This results in a decline in soil water content in deeper soil layers and extraction of water from these layers by deep-rooted shrubs (Li et al. 2004a). Thus, over time, shrub cover is reduced, and shrubs are gradually replaced by annual and perennial herbaceous species such as Poa angustifolia, Cleistogenes songorica, Aristida adscensionis and Stipa glareosa. Thus, as revegetation proceeds, shrubs are gradually replaced by growth forms that are more typical of arid areas, i.e., forbs and grasses. The presence and further development of sand-binding annual and perennial herbaceous plants likely alters the spatial and temporal distribution of soil water in the sand dune system. Long-term monitoring indicates that the soil water content in the vegetated areas was reduced at increasing depths (Fig. 2).
In the absence of further disturbance, perennial grasses typical of desertified steppe will eventually dominate the vegetated dunes. Most changes in soil physicochemical properties occurred in the topsoil, with little impact on the deeper soil layers even after a 50-year period of vegetation growth. This implies that the formation and evolution of soil on sand dunes is slow, and long-term monitoring is therefore necessary to better understand the succession process in dunefields.
Rehabilitation methods using straw-checkerboard and revegetation applied at the Tengger Desert improved the soil environment for colonization and establishment of plant species. Changes in most of the measured soil properties and vegetative features were more rapid during the early stages than at later stages of dune stabilization over the 50-year period. We believe that cryptogamic crusts have altered the physicochemical properties of surface soils, including the spatial-temporal distribution of soil water, altering the distribution of vegetation types by favoring annuals and grasses at the expense of perennial shrubs. Our research reinforces the notion that recovery of soils and vegetation in dunefield will be a slow process.
We gratefully acknowledge Dr. N. Jim Barrow and Dr. David Eldridge for their valuable comments as technical editors for this manuscript. This study was supported by the Chinese National Natural Scientific Foundation (40671011, 90202015).