XiaoAn Zuo， XueYong Zhao， ShaoKun Wang， Xin Zhou， Peng Lv， Jing Zhang

1. Naiman Desertification Research Station， Cold and Arid Regions of Environmental and Engineering Research Institute，Chinese Academy of Sciences， Lanzhou， Gansu 730000， China

2. Laboratory of Stress Ecophysiology and Biotechnology （LSEB）， Cold and Arid Regions of Environmental and Engineering Research Institute， Chinese Academy of Sciences， Lanzhou， Gansu 730000， China

Effects of dune stabilization on vegetation characteristics and soil properties at multiple scales in Horqin Sandy Land， Northern China

XiaoAn Zuo1，2*， XueYong Zhao1，2， ShaoKun Wang1， Xin Zhou1， Peng Lv1， Jing Zhang1

1. Naiman Desertification Research Station， Cold and Arid Regions of Environmental and Engineering Research Institute，Chinese Academy of Sciences， Lanzhou， Gansu 730000， China

2. Laboratory of Stress Ecophysiology and Biotechnology （LSEB）， Cold and Arid Regions of Environmental and Engineering Research Institute， Chinese Academy of Sciences， Lanzhou， Gansu 730000， China

Ecological patterns and processes in dune ecosystems have been a research focus in recent years， however information on how dune stabilization influences vegetation and soil at different spatial scales is still lacking. In this study， we measured vegetation characteristics and soil properties across three spatial scales （10， 100 and 1，000 m2） along gradient dune stabilization stages （mobile dune， semi-fixed dune and fixed dune） in Horqin Sandy Land， Northern China. Vegetation cover over all scales significantly increased with degree of dune stabilization， as well as species richness and C/N ratio at 10 m2scale. Species richness significantly increased with the increase in measured scales at each stage of dune stabilization and was higher in fixed dune than that in mobile dune and semi-fixed dune at 100 and 1，000 m2scales. Over all scales，aboveground biomass was lower in mobile dune than that in semi-fixed dune and fixed dune， and soil organic C， total N，EC， very fine sand and silt + clay contents were higher in fixed dune than those in mobile dune and semi-fixed dune. These results suggest that along the gradient dune stabilization， species richness has strong spatial scale-dependence， but vegetation cover， aboveground biomass and soil properties is generally scale independent （i.e.， the pattern of response is consistent across all scales）. Effect of dune stabilization on vegetation and soil over all spatial scales results in the positive correlation among vegetation cover， species richness， biomass， soil organic C， total N， C/N， EC， very fine sand and silt + clay along the gradient dune stabilization. In addition， species richness at the smallest scale （10 m2） has more sensitive response to dune stabilization. Thus， the monitoring strategies at small scales are essential to detect changes of species diversity in semiarid dune ecosystems.

dune stabilization； sand dune； scale dependence； soil properties； species diversity

1　Introduction

The spatial scale of observation is one of current hot topics in ecology research and may profoundly affect displayed ecological patterns， e.g.， distribution patterns of species diversity at a variety of ecosystems are always scale dependent （Crawley and Harral， 2001；Suarez-Seoane and Baudry， 2002； Chalcraft et al.，2004； Dengler et al.， 2009； Yoshihara et al.， 2010）. Spatial scale greatly affects many aspects of commu-nity distribution， composition and structure， such as species diversity （Harrison et al.， 2006）， productivity（Oline and Grant， 2001）， species turnover （Chalcraft et al.， 2004）， species-area relationship （Turner and Tjorve， 2005） and productivity-diversity relationship（Chase and Ryberg， 2004）. In particular， species richness in arid and semiarid ecosystems is spatial scale dependent （Crawley and Harral， 2001； He et al.， 2006；Palmer， 2006）. Some findings on vegetation changes at different spatial scales in semi-natural ecosystems reflect differences in extent of studies， e.g.， the variation in plot sizes from 1， 10 and 100 m2to field of 1，000 m2（Anderson et al.， 2004； Spiegelberger et al.，2006； Zhou et al.， 2008b）.

Many studies have also shown that the relative importance of the parameters that control ecological patterns has always been strongly affected by spatial scale at which research is conducted， e.g.， vegetation patterns frequently correspond to different environmental conditions at different spatial scales （He et al.，2006； Schooley， 2006； Auestad et al.， 2008） and spatial heterogeneity of soil properties is related to vegetation change and geomorphic position depending on spatial scales （Zuo et al.， 2010； Schwanghart and Jarmer， 2011）. In addition， scale-dependence of vegetation is determined by environment resources （Anderson et al.， 2004；Auestad et al.， 2008）， land use （Spiegelberger et al.，2006） and ecosystem types （Crawley and Harral， 2001）. Especially in natural ecosystems， spatial scale at which a habitat is relatively homogeneous or heterogeneous also strongly depends on the habitat type and classification determined by environmental recourse （Wagner and Edwards， 2001）. Habitat type and quality are also recognized as the important local determinants affecting spatial patterns of species diversity， vegetation composition and soil properties （Li et al.， 2007；Eschen et al.， 2009； Petersen and Drewa， 2009； Zuo et al.， 2009）. Thus， understanding how spatial scales depending on how habitat type influence ecological patterns is very important in ecology research （Guil et al.， 2009； Yan and Liu， 2010； Yoshihara et al.， 2010）.

Many studies have shown that dune stabilization activities in semiarid areas have led to distinct dune habitat types， including mobile dune in which sand pioneer plants are dominant species in barren soils，semi-fixed dunes in which asexual reproduction perennial shrubs are dominant species and fixed dunes in which herbaceous plants are dominant species （Guo et al.， 2008； Zuo et al.， 2009； Yan and Liu， 2010； Li et al.，2011）. Changes of species diversity and vegetation composition （Yan and Liu， 2010）， topographical features （Shirato et al.， 2005）， soil properties （Raji et al.，2004； Zuo et al.， 2010）， soil biology crust （Guo et al.，2008） and soil macrofauna （Liu et al.， 2009） were carried out in the process of dune stabilization. Moreover，different ecological processes also affect patterns of vegetation composition and soil properties at different dune habitats （Su et al.， 2005； Zhao et al.， 2007）. All these effects are beneficial for degraded vegetation restoration and soil development in dune ecosystems. Thus， examining how vegetation and soil respond to dune habitat changes at different spatial scales are very helpful to improve our ability of biodiversity conservation and dune ecosystem management.

Horqin Sandy Land is one of the most severely desertified regions of China （Liu et al.， 1996）. Desertification control has been considered as a principal strategy to maintain the sustainable development of this region. Since the 1970s， many effective measures have been taken to stabilize dunes and to restore degraded vegetation in the area of severe drifting sand，such as building physical sand barriers， building corn straw fence belts and placing wheat straw checkerboards on mobile dunes （Wang et al.， 2004； Guo et al.，2008； Liu et al.， 2009）. In addition， due to the annual precipitation of 350-500 mm， some mobile dunes can also be gradually and naturally restored to semi-fixed or fixed dunes after excluding livestock grazing （Zuo et al.， 2009）. Thus， it is very important to examine how vegetation characteristics and soil properties vary with increasing spatial scales following dune stabilization. This knowledge is necessary for us to better understand changes of ecological pattern during the restoration of degraded dune ecosystems. In this paper， we tested two hypotheses： （1） species richness is spatial scale dependent in the process of dune stabilization；and （2） effect of dune stabilization on vegetation characteristics and soil properties is significant over all spatial scales of observations.

2　Materials and methods

2.1Study area description

The study was conducted in the southwestern part（42°55′N， 120°42′E； 360 m elevation） of Horqin Sandy Land， Inner Mongolia， China. The climate is temperate，semi-arid continental and monsoonal， receiving annual 360 mm in precipitation， with 75% of the precipitation in the growing seasons of June to September. The annual mean open-pan evaporation is about 1，935 mm. The annual mean temperature is around 6.4 °C， with the minimum monthly mean temperature of -13.1 °C in January and the maximum of 23.7 °C in July. The annual mean wind velocity is in the range of 3.2 to 4.1 m/s， and the prevailing wind direction is northwest in winter and spring （Liu et al.， 1996； Zhang et al.， 2005）. The distribution pattern of mainly natural vegetation is characterized by a mosaic of lowland grassland， fixed dune， semi-fixed and mobile dunes. The zonal soils are identified as sandy chestnut soils， which are mostly equivalent to the Orthi-Sandic Entisols of sand originin terms of the FAO-UNESCO system （Su et al.， 2006）. Sand dunes are covered with various native plants，including grasses （e.g.， Cleistogenes squarrosa，Setaria viridis， Phragmites australis， Digitaria ciliaris）， forbs （Mellissitus ruthenicus， Salsola collina，Agriophyllum squrrosum， Artemisia scoparia）， shrubs（e.g.， Lespedeza davurica）， and subshrubs （e.g.， Artemisia halodendron， A. frigida）.

2.2Experiment design

Three major dune types at different stages of dune stabilization were selected in typical areas of vegetation restoration by fencing after the mid-1980s， including mobile dune （MD）， semi-fixed dune （SFD）and fixed dune （FD）， in which vegetation cover is separately less than 10%， 10%-50% and more than 50%. Three replicate 20m×50m plots between 0.6-8.0 km apart from each dune type were chosen with similar topography to establish the modified Whittaker plot （MWP） （Spiegelberger et al.， 2006）. Nested in the MWP were one 100 m2subplot （5m×20m） in the center and two 10 m2subplots （5m×2m） in opposite corners of the plot. Sixteen quadrats （each 0.5m×2.0m）were regularly spaced within the MWP， in which three quadrats is established along the inner border of each 10 m2subplot， four quadrats is established along the outer border of the central 100 m2subplot and six quadrats is established along the inner border of the 1，000 m2plot. In each quadrat， total vegetation cover，species richness and plant cover were measured and plant aboveground biomass （AGB） was also estimated by the harvest method. The AGB were dried at 60 °C in a hot air oven for 48 hours and weighed in the lab. In addition， species richness （total number of species in a sample area） was recorded separately at each scale and each plot.

Concurrently， three random soil samples were collected within each quadrat at 0-20 cm depth using a 3-cm-diameter soil auger， and were pooled to form one composite sample for laboratory analysis. Soil samples were hand-sieved through a 2-mm screen to remove roots and other debris. Soil particle size from international and USDA classification systems was determined by the wet sieving method （Institute of Soil Sciences， 1978）. Soil pH and electrical conductivity（EC） were measured in a 1：1 soil-water slurry and in a 1：5 soil-water aqueous extract， respectively. Soil organic carbon （C） was measured by the dichromate oxidation method of Walkey and Black （Nelson and Sommers， 1982）， and total nitrogen （N） was determined by the Kjeldahl procedure （ISSCAS， 1978）.

2.3Data analysis

Vegetation characteristics and soil properties were calculated separately at different scales （10， 100 and 1，000 m2）. Effects of dune stabilization on vegetation characteristics and soil properties across multiple scales （10-1，000 m2） were assessed using a general linear model with dune stabilization and sampling scale as fixed factors. Differences in vegetation characteristics and soil properties at different scales among different dune stabilization stages were compared using the multiple comparison and one-way analysis of variance （ANOVA） procedures. Results were checked by Tukey's test. The descriptive statistical parameters and significance test were calculated by SPSS （version 19.0）.

3　Results

3.1Effects of dune stabilization and spatial scale on vegetation characteristics

Dune stabilization significantly affected vegetation cover， species richness and aboveground biomass，and spatial scale significantly affected species richness （Table 1）. Vegetation cover over all scales （10，100 and 1，000 m2） increased along the gradient of dune stabilization （P ＜0.05） （Figure 1）. Species richness at 10 m2scale significantly increased along the gradient of dune stabilization （P ＜0.05）， and was higher in FD than that in MD and SFD at 100 and 1，000 m2scales （P ＜0.05） （Figure 1）. Aboveground biomass over all scales was higher in FD than that in MD and SFD （P ＜0.05）， and had no differences between MD and SFD （P ＞0.05） （Figure 1）. In addition，species richness also significantly increased with the increasing spatial scales from 10 to 1，000 m2in MD，SFD and FD （Table1， Figure 2）.

3.2Effects of dune stabilization and spatial scale on soil properties

Dune stabilization also significantly affected soil C，total N， C/N， pH， EC， soil water content， coarse sand，fine sand， very fine sand and silt + clay， but spatial scale had no effect on soil properties （Table 2）. Soil organic C， total N， EC， very fine sand and silt + clay over all scales （10， 100 and 1，000 m2） were higher in FD than those in MD and SFD， and had no differences between MD and SFD （P ＞0.05） （Table 3）. C/N at 10 m2scale increased along the gradient of dune stabilization （P ＜0.05） and was higher in FD than that in MD and SFD at 100 and 1，000 m2scales （P ＜0.05） （Table 3）. Soil water content was higher in FD than that in MD and SFD at 10 m2scale （P ＜0.05）， and had no differences between MD and SFD （P ＞0.05） （Table 3）. In addition， fine sand content over all scales was lower in FD than that in MD and SFD （P ＞0.05）， and had no differences between MD and SFD （P ＞0.05）.

Table 1 General linear model of the effects of dune stabilization degree and spatial scale on vegetation characteristics

Figure 1 Vegetation characteristics at different spatial scales in process of dune stabilization. MD， Mobile dune； SFD， Semi-fixed dune； FD， Fixed dune. Different letters indicate statistical differences between dune stabilization stages at P ＜0.05

3.3Correlation analysis between vegetation characteristics and soil properties

Correlation analysis between vegetation characteristics and soil properties from 10 to 1，000 m2scale along the gradient of dune stabilization showed that there was significantly positive correlation among vegetation cover， species richness， biomass， soil organic C， total N， C/N， EC， very fine sand and silt + clay（P ＜0.01）， and vegetation cover was significantly negative correlated to pH （P ＜0.01） （Table 4）. In addition， fine sand was significantly negative correlated to vegetation cover， species richness and biomass （P＜0.01） （Table 4）. However， there were no significant correlations between vegetation characteristics， coarse sand， and soil water content （P ＞0.05）.

Figure 2 Species-area relationships at different dune stabilization stages. MD， mobile dune； SFD， semi-fixed dune； FD， fixed dune；error bars indicate one standard error for each combination of dune stabilization stage and sampling area

Table 2 General linear model of the effects of dune stabilization degree and spatial scales on soil properties

Table 4 Correlation analysis among vegetation characteristics and soil properties from 10 to 1，000 m2scale along the gradient of dune stabilization （n=36）

4　Discussion

Our results show that dune stabilization increased vegetation cover， species richness， aboveground biomass， soil organic C， total N， C/N， EC， very fine sand and silt + clay contents. In accordance with our hypothesis， our results suggest that dune stabilization has a significantly positive effect on vegetation and soil at all spatial scales， which is similar to previous studies in a variety of dune ecosystems， supporting the idea that dune stabilization plays an important role in restoration of degraded vegetation and soil in dune ecosystems （Su et al.， 2005； Li et al.， 2007； Guo et al.， 2008；Zuo et al.， 2009； Yan and Liu， 2010）. The previous study has shown that when excluding the disturbance of grazing and humans for many years， sand pioneer plant of Agriophyllum squarrosum survives and establishes its population in mobile dunes， resulting in a decrease in sand mobility （Zhang et al.， 2005）. Afterwards， Artemisia halodendron， an asexual reproduction perennial shrub， invades and establishes its population， providing a better environment for establishment of herbaceous seedlings under shrub canopies（Su et al.， 2005）. Thus， with the further development of herbaceous plants， shrub species gradually disappears from semi-fixed dune to fixed dunes （Zhang et al.，2005）. In addition， herbaceous plants also further improve soil properties through the greater accumulation and decomposition of organic litter （Zuo et al.， 2009；Wang et al.， 2011）.

We found that species richness was scale dependent at dune stabilization， which is consistent with other studies in desert or grassland ecosystems （Crawley and Harral， 2001； He et al.， 2006； Palmer， 2006）. Our previous studies have shown that soil properties determine the vegetation composition in dune habitats（Zuo et al.， 2009）， so changes of soil resources depending on increasing spatial scales result in the scale-dependence of species richness in dune ecosystems. Due to the different environmental context， the scale dependence of species richness occurring from 1 to 1，000 m2in dune ecosystems is higher than that in desert ecosystems in arid areas （occurring from 1 to 100 m2） （He et al.， 2006）. In addition， our results also suggest that with increase of dune stabilization degree，semi-fixed and fixed dunes are probably more suitable habitat for the development and establishing of more plant populations with increase in spatial scales， due to relatively better environmental conditions （Guo et al.，2008； Yan and Liu， 2010）.

Our results suggest that vegetation cover and aboveground biomass are generally scale independent（i.e.， the pattern of response is consistent across all scales） following dune stabilization. Although vegetation succession is from sand pioneer plants to shrubs then to herbaceous plants following dune stabilization，the competition effect of dominant plants responding to soil resources at each stage contributes to the relative stabilization of vegetation cover and biomass in plant communities， resulting in that vegetation cover and biomass have no obvious changes with increase in spatial scales. In addition， over each scale， vegetation cover significantly increased with dune stabilization，indicating that vegetation cover is a sensitive index in measurement of dune stabilization or restoration of degraded vegetation.

Overall， our results also suggest that soil properties are generally scale independent （i.e.， the pattern of response is consistent across all scales） following dune stabilization. Over each scale， FD had higher soil organic C， total N， C/N， EC， soil water content，very fine sand and silt + clay as compared to MD and SFD. This suggest that only long term dune stabilization since vegetation restoration produces an increase in these soil properties which determine plant distributions （Zuo et al.， 2009）. Soil organic C and N accumulations have been regarded as indicators of soil fertility which regulates plant growth and plays an important role in the sustainable use of soil. The previous study has shown that particle size distribution also plays an important role in maintaining soil organic matter and soil nutrients （Su et al.， 2004）. Our study is also in agreement with other results that soil organic C and total N has strong positive correlations with very fine sand and silt + clay （Zhou et al.， 2008b；Zuo et al.， 2008）. In addition， at the smallest scale （10 m2）， the increased C/N ratio with dune stabilization suggests that dune stabilization has a stronger effect on soil organic C than on total N at the small scale. This may be partly explained by the heterogeneous spatial distribution of dominant plants during dune stabilization.

Some studies have shown that the display patterns of plant distribution determined by key environment factors at fine scales may affect vegetation patterns at larger scales （Perelman et al.， 2001； Auestad et al.，2008）. Our study shows that species richness has increased sensitivity response to dune stabilization at the smallest scale （10 m2）， thus monitoring strategies at small scales are essential in detecting changes of species diversity in semiarid dune ecosystems. The occurrence of plant species at a site is also determined by the presence of suitable habitats at different scales. Thus， a scaling-up approach would utilize the monitoring data of species composition at small scales in different habitats to predict plant distributions or vegetation patterns at the larger scales. In addition， these results were obtained from multiple spatial scales （10，100 and 1，000 m2）； however whether similar results can be obtained from more spatial scales larger than 1，000 m2remains to be explored.

5　Conclusions

Over all observed spatial scales， dune stabilization has an important role for increasing vegetation cover，species richness， aboveground biomass， soil organic C，total N， C/N， EC， very fine sand and silt + clay. Our study also suggests that following dune stabilization，species richness is spatial scale dependent， but vegetation cover， aboveground biomass and soil properties are generally scale independent （i.e.， the pattern of response is consistent across all scales）. This finding demonstrates the importance of studying effects of dune stabilization or habitat change on vegetation and soil， especially its effect on species richness at different spatial scales. In species diversity conservation practices， maintaining dune stabilization appears to be the most promising approach to conserve the high species diversity in dune ecosystems. Thus， it is necessary to increase our effort in promoting dune stabilization with protective measures and to enhance the natural restoration succession.

Acknowledgments：

The authors would like to thank all the members of Naiman Desertification Research Station， China Academy of Sciences （CAS）， for their help in field work. This paper was financially supported by the Knowledge Innovation Program of the Chinese Academy of Sciences （No. KZCX2-EW-QN313），National Natural Science Foundation of China （No. 41171414）， 'One Hundred Talents' Program of Chinese Academy of Sciences （No. Y451H31001）， and Key Project of Scientific and Technical Supporting Programs （2011BAC07B02-09）.

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Zuo XA， Zhao XY， Zhao HL， et al.， 2009. Spatial heterogeneity of soil properties and vegetation-soil relationships following vegetation restoration of mobile dunes in Horqin Sandy Land，Northern China. Plant and Soil， 318（1）： 153-167. DOI：10.1007/s11104-008-9826-7.

Zuo XA， Zhao XY， Wang SK， et al.， 2015. Effects of dune stabilization on vegetation characteristics and soil properties at multiple scales in Horqin Sand land， Northern China. Sciences in Cold and Arid Regions， 7（1）： 0040-0049. DOI： 10.3724/SP.J.1226.2015.00040.

*Correspondence to： Dr. XiaoAn Zuo， Prof. of Cold and Arid Regions Environmental and Engineering Research Institute， Chinese Academy of Sciences. No. 320， West Donggang Road， Lanzhou， Gansu 730000， China. Tel：+86-931-4967219； E-mail： zuoxa@lzb.ac.cn

August 16， 2014 Accepted： November 20， 2014