AOBPreview originally published online on November 3, 2006
Annals of Botany 2007 99(1):19-28; doi:10.1093/aob/mcl228
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The Role of Reproductive Phenology, Seedling Emergence and Establishment of Perennial Salix gordejevii in Active Sand Dune Fields
1 Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, PR China
2 Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
* For correspondence. E-mail liuzhimin6555{at}yahoo.com.cn
Received: 17 July 2006 Returned for revision: 22 August 2006 Accepted: 4 September 2006 Published electronically: 3 November 2006
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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BACKGROUND AND AIMS: The function of sexual reproduction of perennials in restoration of vegetation of active dune fields frequently has been underestimated. The objective of this study was to evaluate the role of sexual reproduction of the perennial Salix gordejevii in the revegetation of active dunes.
METHODS: Seedling emergence and establishment of S. gordejevii were examined both in controlled experiments (germination at different burial depths with different watering regimes) and in field observations in three dune slacks. The reproductive phenology and soil seed bank of S. gordejevii, the dynamics of soil moisture, the groundwater table and the landform level of three dune slacks were monitored.
KEY RESULTS: Seeds of S. gordejevii began maturation on 1 May, and seed dispersal lasted from 8 May to 20 May. Seeds on the soil surface germinated significantly faster than those buried in soil (P<0·05). Seedling emergence was negatively correlated with landform level. When most seedlings emerged, there was a significantly positive correlation between soil moisture and seedling emergence (P<0·01). Rainfall was negatively correlated with seedling emergence. Seedling establishment was significantly and positively correlated with seedling emergence (P<0·05), and 72·3 % of the emergent seedlings were established at the end of the growing season. These results indicated that (a) seeds matured and dispersed before the rainy season; (b) seeds germinated as soon as they contacted a moist surface and relied more on soil moisture than on rainfall; and (c) more seedlings emerged at lower sampling points in dune slacks.
CONCLUSIONS: In natural conditions, restoration of active sand dune fields generally commences with revegetation of dune slacks where sexual reproduction of perennials contributes greatly to species encroachment and colonization and hence plays an important role in restoration of active dune fields. Furthermore, aeolian erosion in dune slacks, leading to good soil moisture, facilitates seed germination, seedling emergence and establishment of S. gordejevii.
Key words: Aeolian erosion, dune slack, groundwater table, landform level, rainfall, restoration, seedling, sexual reproduction, soil moisture, soil seed bank
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Desertification, as land degradation in arid, semi-arid and dry sub-humid areas, often leads to formation of sand dunes (Zhu and Chen, 1994; Kassas, 1995), and subsequently to sand mobility and aeolian erosion. In the last few decades, much attention has been paid to the establishment and growth of vegetation in desertification regions (e.g. Allen, 1988; Schlesinger et al., 1990; Bowers, 1996; Guo et al., 1998; Lichter, 2000; Yu et al., 2003).
Vegetation processes in the dune slack, as a vegetation island, are involved in the restoration of active sand dune fields (Zhu and Zou, 1987; Zhu and Chen, 1994; Avis and Lubke, 1996; Bakker et al., 2000; Zhang et al., 2005). With the advancement of active sand dunes, dune slacks are changing constantly, with vegetation encroaching upon active dunes (Zhu and Zou, 1987; Cao et al., 2000; Jiang et al., 2003). The study of species establishment in dune slacks is of significant importance to obtain knowledge on how to restore the vegetation of desertified areas. How vegetation develops in dune slacks, however, requires further studies.
Perennial dune plants that endure sand mobility have to rely to a great extent on special reproductive strategies (e.g. Chen and Dong, 2000; Yu et al., 2002; Zhao and Liu, 2002; Yan et al., 2004; Liu et al., 2005). Vegetative propagation is believed to be more important than sexual reproduction with respect to species establishment on active dunes (Liang et al., 2000; Ren et al., 2001; Li et al., 2005). In dune slacks with aeolian erosion, however, vegetative propagation, which prefers sand burial to erosion, might not function effectively.
Salix gordejevii (Salicaceae), a pioneer perennial shrub, is dominant in active sand dune fields (Liu, 1985). Reports have shown that vegetative propagation is responsible for its development on active dunes (Liang et al., 2000; Ren et al., 2001). However, pre-experiment observations showed that many S. gordejevii seedlings could be found in dune slacks, leading to the assumption that sexual reproduction of S. gordejevii also plays an important role in the revegetation of active dune fields.
Seed maturation phase, dispersal time, germination, and seedling emergence and establishment are closely associated with species colonization in active sand dune fields (Maun, 1981; Zhang and Maun, 1990; Huang et al., 2004). It has been observed that seed maturation was very early for S. gordejevii, and large numbers of seedlings existed in dune slacks in early spring. This suggests that next to the seed maturation phase, dispersal and germination are also of importance in seedling establishment. Although there are numerous seeds available after seed maturation in spring, seedling emergence and establishment are not ensured because light seeds of S. gordejevii (Yan et al., 2004) are easily blown away by a strong wind. Therefore, it may be that there are some mechanisms favouring the retention and germination of seeds in dune slacks. Most seedlings are in blowouts of dune slacks, where aeolian erosion is active and soil moisture is good, leading to the hypothesis that aeolian erosion, which leads to good soil moisture, facilitates seed germination, seedling emergence and establishment of S. gordejevii. To test this, seedling emergence and establishment of S. gordejevii were examined both in controlled experiments (different burial depths and watering regimes) and in field observations in three dune slacks with regard to reproductive phenology, soil seed bank, the dynamics of soil moisture, groundwater table and landform level (i.e. relative height).
Salix gordejevii has early seed propagation and a longer growth period compared with other species, which might provide advantageous conditions for the encroachment of other species in dune slacks. It was hypothesized that sexual reproduction of perennials contributes greatly to species encroachment and colonization in dune slacks and, consequently, to restoration of active sand dune fields. To test this, survival of S. gordejevii seedlings was examined at the end of the growing season.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Study site
Investigation was conducted at Wulanaodu (119°39'–120°02'E, 42°29'–43°06'N, 480 m above sea level.) (Fig. 1) in the Horqin Sandy Land, in north-eastern Inner Mongolia, China. The annual average temperature of the study area is 6·3 °C, with January, the coldest month, averaging –14·0 °C, and July, the warmest, averaging 23·0 °C. Mean annual precipitation is 340 mm, of which 70 % falls in June, July and August. Annual mean wind velocity is 4·4 m s–1, and the number of days in which the wind is gale force (>16 m s–1) is 21–80. The windy season lasts from early March to late May; the prevailing wind is north-western.
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Methods for field observations
Three dune slacks (the land between the foot of leeward slope and the foot of windward slope of an active sand dune) in the study site were selected (Fig. 1). The details of the three dune slacks are shown in Table 1 and Fig. 2.
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Five adult plants of S. gordejevii, similar in canopy, were selected, and the date of flowering, fruit maturation and dispersal were recorded every 5 d in 2005 from 19 March onwards.
Five transects, each 2 m apart, were selected at each of the three sites (Fig. 2). Nine sampling points at Site 1 and Site 2, and 13 sampling points at Site 3 were selected at intervals of 4·5 m along each transect. Thus there were 155 sampling points in total. From early March to early July, information on landform level, soil moisture, groundwater table, soil seed bank, seedling emergence and vegetation, and seedling establishment were recorded at each sampling point. At each sampling point, quadrats (1x1 m) were established in pairs. One was used for recording vegetation composition before the growing season and seedling emergence of S. gordejevii at 5 d intervals (emergence quadrat), while the other was used for recording seedling establishment of S. gordejevii at the end of the growing season (establishment quadrat).
Landform
On 15 March 2005, a horizontal level was established in each dune slack. The vertical distance between the horizontal level and the soil surface of each sampling point was considered as the initial level of each dune slack (Fig. 2). Therefore, the value of the landform level is negative. The bigger the absolute value of the landform level, the lower the sampling point. Meanwhile, to monitor the dynamics in landform level, an iron stick was also inserted in each sampling point on 15 March. From then on, the height of the sticks above the sand surface was recorded every 5 d. If the height of the stick was lower than that measured 5 d previously, the sampling point had been buried by sand; if the height was higher than that measured 5 d previously, the sampling point had been eroded by wind. Sticks were reinserted if they fell down or were buried.
Soil moisture
In early May 2005, when seeds of S. gordejevii matured, the soil moisture of 0–5 cm of each sampling point was measured every 5 d. Soil samples (98 cm3 in volume) were taken with soil borers (5 cm in diameter), weighed, and dried at 80 °C for 12 h and weighed again.
Groundwater table
A well at each site was dug, and a permanent pipe was put in each well. The groundwater table was measured every 5 d from early May, 2005.
Soil seed banks of S. gordejevii
Soil samples were collected in late April 2005 at 5 d intervals. Due to the active aeolian erosion in dune slacks, seeds cannot be buried deeply by sand, so the investigation on the soil seed bank was limited to the depth of 0–2 cm. Sampling on the basis of a large number of samples and using small quadrats is more reliable (Bigwood and Inouye, 1988); therefore, soil samples were collected by using a cylindrical trowel of 70 mm diameter (Thompson and Grime, 1979). The length, width and height of seed for S. gordejevii were 1·3, 0·6 and 0·5 mm, respectively (Yan et al., 2004), bigger than a mesh size of 0·5 mm. Therefore, after air-drying, the soil samples were sieved through a 0·5 mm sieve, and only seeds of S. gordejevii were extracted by hand (Meissner and Facelli, 1999). Then, their viability was tested (Paker and Venable, 1996).
Seedling emergence of S. gordejevii
From early May 2005, in each emergence quadrat, seedlings of S. gordejevii were recorded, counted and removed every 5 d.
Vegetation and seedling establishment
On 15 March 2005, the vegetation composition and cover of the emergence quadrat were recorded at each sampling point. In mid-September 2005 (autumn), when the growing season of plants ended, the abundance and cover of newly emerged seedlings of S. gordejevii in the establishment quadrat at each point were recorded.
Methods for germination experiments
In early May 2005, freshly matured seeds of S. gordejevii were collected in the field. On 9 May, the seeds were used for germination experiments in the glasshouse with a 15 h day at 28 °C and a night temperature of 16 °C. The temperatures adopted approximated the mean daily maximum and minimum temperature in 3–5 cm deep soil during rainy days in the local region from May to August. Flowerpots (16 cm high and 15 cm in diameter) were filled with sand collected from the local dune and sieved to remove any seeds of S. gordejevii that may have been present. Five replicates of 50 seeds were sown in flowerpots at different depths and with different watering regimes as shown below. Seedlings were recorded and removed daily until the experiment was terminated on 11 June when no further germination occurred. Ungerminated seeds in each flowerpot were extracted and counted, and their viability was tested following Paker and Venable (1996).
Germination of S. gordejevii at different burial depths
Seeds were sown at five depths, i.e. 0 (soil surface), 0·5, 1, 2 and 4 cm, in flowerpots with pinholes in their bottoms, which were placed in trays (5 cm high), full of water.
Germination of S. gordegevii under different watering regimes
Seeds were buried at a depth of 1 cm, and tap water equal to the rainfall of 2, 4, 6, 8 and 16 mm was added to the flowerpots every 5 d. Soil moisture was also measured every 5 d using a further 175 flowerpots, containing sand but no seeds. Again, water (2, 4, 6, 8 and 16 mm) was added to the pots every 5 d. Five replicates were used for each watering treatment. Every 5 d, 25 flowerpots were taken away after watering and measurement. Soil samples (98 cm3 in volume) of 0–5 cm were taken with a soil borer (5 cm in diameter) 4 h after watering, when free water drainage was nearly completed and soil moisture was almost unchanged. Soil samples were weighed, dried at 80 °C for 12 h and then weighed again.
Data analysis
Tukey's test was applied post hoc (SPSS software, 10th edition, Chicago, IL, USA) to distinguish between seed germination at different burial depths. Correlations between seedling density and other factors (i.e. soil moisture, groundwater table, rainfall, landform level and vegetation cover) were tested using bivariate correlations. Soil moisture is calculated as the percentage of water weight accounting for total soil weight.
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RESULTS |
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Reproductive phenology, germination, and soil seed bank characteristics
Salix gordejevii began flowering on 8 April, seeds started maturating on 1 May, and seed dispersal lasted from 8 May to 20 May 2005.
Seeds on the soil surface germinated significantly quicker than those buried in soil (P<0·05) (Fig. 3). Seeds buried at 1 cm depth germinated only when watering was 16 mm, and soil moisture was not lower than 2·0 %. After the germination experiment, no viable ungerminated seeds were found for each burial depth and for each watering regime.
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During the investigation, few viable seeds (<4) of S. gordejevii were found in the soil profile of 0–2 cm of dune slacks; therefore, the data of the soil seed bank are not presented.
Spatial patterns of seedling emergence
Overall, seedling emergence was negatively correlated with the relative height of the sampling point (only significant for Site 1), i.e. the higher the sampling point, the lower the seedling emergence (Fig. 4A1, A2 and A3). However emergence was positively correlated with soil moisture (P<0·05) along the transect from the foot of the leeward slope to the foot of the windward slope (Fig. 4B1, B2 and B3).
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The dune slacks presented contrasting patterns of seedling emergence (data not shown). At Site 1, there was a seedling peak near the foot of the windward slope, where vegetation cover was low, while at Site 2 seedlings were densest near the foot of the highly vegetated leeward slope. Seedling density peaked at several points in the dune slack at Site 3, being lowest at the foot of each slope.
Temporal patterns of seedling emergence
More than 85 % of seedlings emerged in 1 month, from 17 May to 12 June. Moreover, seedling emergence was low in early May, and reached a maximum in mid-May (Table 2, Fig. 5).
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The relationship between soil moisture and seedling emergence differed depending on the time of emergence. During the period when most seedlings emerged, there was a significantly positive correlation (P<0·01) (Table 2), but at other points in time there were no significant correlations. Rainfall was negatively, but not significantly correlated with seedling emergence at any time of emergence (Fig. 5).
Seedling establishment
Seedling establishment was significantly and positively correlated with seedling emergence (P<0·05) (Fig. 6), and 72·3 % of the emergent seedlings were established at the end of the growing season in dune slacks.
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DISCUSSION |
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Traits of sexual reproduction of S. gordejevii
Reproductive phenology and germination characteristics of S. gordejevii represent the intrinsic traits of its seed propagation. According to the present observations, seed maturation and dispersal of S. gordejevii occurred in the windy season, and wind facilitated seed dispersal. Seeds germinated as soon as they contacted a moist surface (Fig. 7).
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External conditions, i.e. aeolian erosion and hydrological conditions, are crucial for seedling emergence and establishment of S. gordejevii in dune slacks (Fig. 7). With active advancing of sand dunes, a bare belt, i.e. a transitional zone between dune slack and windward slope, would appear behind dunes (Cao et al., 2000). Aeolian erosion is quite active on the belts, bringing the ground surface closer to the groundwater. Capture and germination of seeds are favoured by provision of good moisture, because the seeds of S. gordejevii are both small (0·30±0·02 mg per seed, Yan et al., 2004), and hairy, and likely to adhere to a moist surface, hence aiding the germination process. The present experiment indicated that seeds germinated only when soil moisture was not lower than 2·0 %, but, in the field, seedlings emerged only when soil moisture reached 2·7 %. Furthermore, on average, the soil moisture increased 1·5 % with a decrease of 10 cm in relative height. Aeolian erosion is an important force facilitating seed germination, seedling emergence and establishment of S. gordejevii.
Good hydrological conditions seem to be a guarantee for seed germination, seedling emergence and establishment of S. gordejevii in active sand dune fields. The present study confirmed that seedling emergence relied more on soil moisture than on rainfall (Table 2, Fig. 5). In turn, soil moisture depended more on groundwater supply, and rainfall played a negligible role in improving the groundwater table (Table 3). In dune slacks, groundwater moves up constantly by means of capillarity, thus maintaining soil moisture. Limited amounts of rainfall seem not to be important in improving soil moisture status (i.e. 10 mm rainfall may merely improve 0·1 % soil moisture in dune slacks), which explains why seedling emergence relied more on soil moisture than on rainfall. According to the observations in the field, although many seeds of S. gordejevii occurred under the mother plants, there were few seedlings under the plants. The colonization of adult plants will reduce the soil moisture (Liu, 1985) so that the moisture status is unfavourable for their seed germination. Thus, soil moisture appears to be the main limiting factor for seed germination, seedling emergence and establishment of S. gordejevii.
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It may be concluded that (a) seeds matured and dispersed before the rainy season; (b) seeds germinated as soon as they contacted a moist surface, relying more on soil moisture than on rainfall; and (c) the lower the sampling point in the dune slack, the greater the number of seedlings. In addition, quick germination and active aeolian erosion might explain the small soil seed bank of S. gordejevii in dune slacks, since as soon as the seeds of S. gordejevii fall on the ground surface, they would either germinate rapidly or be blown away by the wind.
Sexual reproduction status of S. gordejevii in active sand dune fields
Salix gordejevii, as a pioneering shrub, is dominant in frequency as well as in density in active sand dune fields (Liu, 1985) (Fig. 8). Vegetative propagation has been said to be responsible for its colonization at the feet of leeward slopes, on leeward slopes and on the crest of active sand dunes (Liang et al., 2000; Ren et al., 2001), but little is known about how sexual reproduction of S. gordejevii functions in revegetation of active dune fields. The present studies support the hypothesis that S. gordejevii successively encroaches and colonizes dune slacks through sexual reproduction as an adaptation to aeolian erosion.
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The encroachment of S. gordejevii by seed propagation in dune slacks is highly favourable for the encroachment of other species in active sand dune fields. The process by which S. gordejevii encroaches in dune slacks may be as follows. Seeds accumulate and germinate in the wind-formed dune slack, and seedlings emerge to develop into adult plants which stabilize the habitat. The formation of S. gordejevii thickets then gives rise to the deposition of large quantities of wind-carried sand. Aeolian erosion occurs in the sites located leeward of S. gordejevii thickets, which lack vegetation, and causes formation of wind-formed holes and wetlands where seeds of S. gordejevii accumulate and germinate, and seedlings emerge. A few years later, new S. gordejevii thickets are formed, and the process begins again. These processes can be used to interpret the formation of the S. gordejevii zonation pattern shown in Fig. 8. With its development in abundance, bare belts of dune slacks are gradually stabilized, and provide advantageous conditions for the encroachment of other species, i.e. Setaria viridis, Artemisia wudanica and xerophytic plants, such as Ulmus pumila and Cleistogenes squarrosa. The occurrence of seed propagation of S. gordejevii is therefore much earlier than that of other plants in dune slacks. Salix gordejevii developed well in active sand dune fields through vegetative propagation on active dunes as well as sexual reproduction in dune slacks.
After dune slacks in active sand dune fields are stabilized and habitats become dry, the development of S. gordejevii is greatly restrained. With the vast encroachment of xerophytes, S. gordejevii gradually withdraws from communities in dune slacks, but this process may take a long time.
Sexual reproduction status of perennials in active sand dune fields
Sexual reproduction of perennials is important in active sand dune fields where strong wind action, including sand burial and aeolian erosion, are particular attributes (Zhu, 1963). Vegetative propagation also functions in colonization of the active dune as an adaptation to sand burial (e.g. S. gordejevii and A. wudanica), due to its stronger capability of spreading risks (Fowler, 1988) and smaller investment in reproduction (Walter, 1992) than is found for sexual reproduction. In dune slacks with aeolian erosion, however, vegetative propagation of perennials cannot work well. Sexual reproduction contributes greatly to species colonization in active sand dune fields. Apart from S. gordejevii, A. wudanica, for example, reproduces by seeds, in combination with its unique reproductive strategies (i.e. canopy seed bank, myxospermy and persistent soil seed bank) (Yan et al., 2004; Liu et al., 2005), which would make it more likely to germinate (Gutterman and Ginott, 1994; Huang and Gutterman, 1998; van Rheede van oudtshoorn and van Rooyen, 1999). It is concluded that perennials establish well in active sand dune fields through either vegetative propagation to adapt to sand burial in active dunes or sexual reproduction as an adaptation to aeolian erosion in dune slacks.
Vegetation restoration of active sand dune fields is realized by two pathways, i.e. restoration of active dunes and revegetation of dune slacks, and a great deal of attention has been paid to the former (Françoise and Servane, 2004). Vegetation development in dune slacks also plays a crucial role in restoration of active sand dune fields, because vegetation restoration of active sand dune fields begins with revegetation of dune slacks in most cases. The impacts of the moving-forward of active sand dunes on vegetation processes of dune slacks are mainly 2-fold: (1) the original vegetation is buried by sand dunes or deposited by wind-carried sand grains; and (2) new vegetation is developing in the sections with aeolian erosion or sand accumulation. Thus, the position of dune slacks, at present located near leeward slopes, was once lying near windward slopes, and was colonized by many perennial seedlings e.g. Phragmites communis, A. wudanica, S. gordejevii, Caragana microphylla and Agrostis clavata, although they are prone to be buried by shifting sand. This suggests that sexual reproduction is effective in the development from bare land to well developed vegetation in dune slacks. Encroachments of dune pioneer perennials by seed propagation in dune slacks could reduce sand movement, and create more suitable and protected habitats which are more favourable for encroachments of other species such as A. wudanica, and gradually different vegetation patterns are formed (Vitousek and Walker, 1989). The vegetation development in dune slacks facilitates restoration of the sparsely vegetated active sand dune fields.
To conclude, sexual reproduction of perennials contributes much to species encroachment and colonization in dune slacks, playing an important role in restoration of active sand dune fields. Furthermore, in dune slacks, aeolian erosion, leading to good soil moisture, facilitates seed germination, seedling emergence and establishment of S. gordejevii.
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ACKNOWLEDGEMENTS |
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We thank Luo Yongming and Wang Hongmei for assistance during the field investigation, and Roy A. Lubke and two anonymous reviewers for valuable comments on the manuscript. The work was financially supported by the project ‘Study on ways to adaptive land utilization and techniques of sustainable grassland management in the arid zone of China’, initiated by the China Science and Technology Ministry.
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