Key Laboratory of Arid and Grassland Ecology of Ministry of Education, Lanzhou University, South Tianshui Road 222, Lanzhou 730000, Gansu, PR China
https://doi.org/10.1016/j.flora.2009.02.006 Abstract We examined the role of the soil seed bank along a grazing disturbance gradient and its relationship with the vegetation of alpine meadows on the Tibet plateau, and discussed the implications for restoration. The seed bank had a high potential for restoration of species-rich vegetation; 62 species were identified in the vegetation and 87 in the seed bank, 39 species being common to both. Mean seed density was 3069–6105 viable seeds m−2. The density of buried seeds increased significantly with increasing disturbance, indicating that restoration of disturbed areas is not seed limited. Seed density and species richness decreased with depth. The proportion of perennial species decreased with decrease in disturbance both in seed bank and in vegetation. A large portion of species with persistent seeds in the disturbed areas indicate that this seed type can be regarded a strategy of adaptation to current disturbances. Detrended correspondence analysis (DCA) showed significant differences of species composition between seed bank and vegetation, except for the seriously disturbed site. Our results suggest that the establishment of new species in severely disturbed areas is more dependent on the seed bank. By contrast, the restoration in less-disturbed and mature meadows does not rely on seed banks, and the establishment of the vegetation in these communities is more likely to rely on seed dispersal from the standing vegetation and on species with vegetative reproduction.
Introduction
Transhumance has been a traditional form for thousands of years and intensive seasonal grazing has a long history in Tibet plateau. With stock number increasing, some areas have been excessively mowed and grazed, resulting in the loss of biodiversity and the degradation of vegetation. However, current management of this region is controversial. Management from a position of ecological ignorance is a potential recipe for disaster, but compensation for disturbances based on peculiar site resources can give a perspective of sustainability (Jalili et al., 2003).
Some authors (e.g., Van Der Valk, 1992) have suggested that the seed bank is the major source of seedling recruitment after disturbance in grasslands. Acting as a propagule reservoir, the seed bank may reduce the probability of population extinctions (Cohen, 1966; Venable and Brown, 1988) and even restore the vegetation after destruction (Grubb, 1997; Wisheu and Keddy, 1991). Viable seed banks may be useful indicators in assessing restoration potential (Bekker et al., 1997). The evaluation of seed banks can therefore give an idea of the recovery potential of degraded rangelands (Solomon, 2003; Tracy and Sanderson, 2000; Tongway et al., 2003) and knowledge of seed bank composition will be a crucial factor in the definition of restoration policy and strategies.
Soil seed banks can be important sources for restoration of species-rich vegetation. High seed density and a large number of species in the seed bank contribute to a potential role of the seed banks in vegetation development during restoration of degraded sites (Bakker et al., 1996; Grime, 2001; Thompson et al., 1997). Understanding the effects of grazing on the seed bank is of crucial importance for conservation and grazing management in these regions. Grazing may have various effects on the density and composition of seed banks in natural grasslands. For instance, grazed sites usually show a decrease in density and species number in the seed banks as compared to ungrazed sites (Bertiller, 1996; Jutila, 1998; Mayor et al., 2003). Under heavy grazing, impoverished seed banks may become a limiting factor for recovery or persistence of the palatable vegetation (Sternberg et al., 2003). However, Kinucan and Smeins (1992) and Meissner and Facelli (1999) reported that grazing had no impact on total seed density. In other studies, even an increase of seed density under a higher grazing intensity was found (e.g., Navie et al., 1996; Russi et al., 1992). It can be argued that these contrasting responses were due to differences in species composition and grazing regimes under different climatic condition and at different altitudes. If seed density increases with disturbance, and rich seed banks exist in degraded habitats, the seed banks will have a potential role in vegetation restoration.
Insight into the mechanisms controlling community composition dynamics in various ecosystems may result from knowledge of seed bank and vegetation relationships, which in turn will allow appropriate land management practices (Hopfensperger, 2007). Community composition can be severely changed by disturbances, succession, and restoration efforts. The relationships between seed banks and standing vegetation have been studied under different aspects, for instance, with respect to restoration and reforestation (Carter and Ungar, 2002; Leck, 2003), to disturbance effects (Amiaud and Touzard, 2004), to successional patterns (Bossuyt and Hermy, 2004; Grandin, 2001), and to management techniques (Kinloch and Friedel, 2005; Lopez-Marino et al., 2000).
However, information about the role of soil seed bank in regeneration of non-successional perennial communities is rather scarce (Kalamees and Zobel, 2002). Here, upon major disturbances such as cultivation, topsoil removal, fire, flooding, and forest felling (Bakker, 1989; Crawley, 1990; Thompson, 1992) the seed bank can be an essential source for re-establishment of the vegetation. The degree of similarity between the species composition of vegetation and the seed bank is predicted to decrease with lower seed production and greater investment in clonal growth in perennial species (Diemer and Prock, 1993).
It is our working hypothesis that the establishment of new species in seriously disturbed areas depends more strongly on the seed bank, whereas the role of seed bank affecting the standing vegetation decreases with decrease in disturbance. This is tested with the seed bank relations of an alpine meadow on the Tibet plateau. We attempt to investigate and compare the composition of seed banks with their vegetation over a range of disturbed biotopes, and to characterize the role of seed banks along such a disturbance gradient. Specifically, we wanted to explore: (1) the size and composition of the soil seed bank along the disturbance gradient; (2) the relationship between the composition of seed bank and standing vegetation along this gradient; (3) whether the seed bank of seriously disturbed communities can be potentially regarded as a source for restoration; so that, (4) conclusions can be drawn for an appropriate restoration management.
Materials and methods
Study site
The study was conducted in an alpine meadow in Maqu (N35°58′, E101°53′), Gansu, China, on the eastern Tibet Plateau, with an elevation of 3500 m above sea level. The average temperature there is 1.2 °C, from −10 °C in January to 11.7 °C in July. The precipitation is 620 mm per year (over the last 35 years), and it is mainly distributed during the short, cool summer. The area has 2580 h of sunshine and more than 270 frost days per year. The soil type is alpine meadow soil.
Seed bank sampling and assessments of vegetation composition were carried out in four different biotopes within the meadow. C, F, L, and S represent a gradient of increasing grazing disturbance and degradation of the biotope. The sites are far apart from each other (500–1000 m between any two biotopes). No fertilizers, neither mineral nor organic, were applied. Under different disturbance intensities, the species composition of vegetation among these four sites is very different, and the mean aboveground biomass per square meter differed significantly among the four sites (F=38.403, P<0.001). The four sites are characterized as follows:
Control meadow (C): A low disturbance, mature meadow functioning as control site for this study. Dominant species are Kobresia humilis, Kobresia capillifolia, Potentilla fragarioides, and Elymus dahuricus. pH: 6.37. The mean aboveground biomass is 834.4±49.6 g m−2(fresh mass).
Fenced site (F): It was fenced in October 1999 and was subsequently only grazed by livestock (e.g. yak and Tibetan sheep) during the winter. This low-intensity grazing removed only the dead biomass, and the live biomass was not affected. It is covered with graminoids, and its height is obviously higher than that in the other communities. Dominant species are E. dahuricus, P. pratensis, and K. capillifolia. pH: 6.62. The mean aboveground biomass is 1299.2±83.5 g m−2.
Largely disturbed site (L): The disturbance intensity in this area was higher than that in site C and less than that in site S. Locally within the site, the vegetation is slightly degraded. The proportion of ruderal species is less and of graminoids higher compared with site S. Dominant species are Stipa aliena, Kobresia graminifolia, and Anemone rivularis. pH: 6.92. The mean aboveground biomass is 642.8±52.7 g m−2.
Seriously disturbed site (S): It was exposed to a long-term overgrazing and trampling by livestock (e.g. yak and Tibetan sheep), resulting in a loss of vegetation. Under this serious disturbance, vegetation is low and sparse in some places where it is covered with ruderal species. Potentilla anserina colonizes these plots. Dominant species are P. anserina, P. pratensis, and E. dahuricus. pH: 7.23. The mean aboveground biomass is 508.0±15.3 g m−2.
Selection of these sites was designed to maximize representation of different types of grassland communities. Both vegetation and environmental factors were homogeneous within each site, as demanded for such studies, e.g. by Acosta et al. (1992). Sv, Lv, Fv, Cv represent vegetation in site S, L, F, C, and Ss, Ls, Fs, Cs represent the soil seed bank, respectively.
Germination tests with seed bank materials were conducted in Hezuo (N34°55′, E102°53′), Gansu, China, also on the eastern Tibet plateau, at an elevation of 2900 m above sea level. The average temperature there is 2.0 °C, and precipitation is 557.8 mm.
Soil collection and processing
Ten randomly selected plots (2 m×2 m) were established in each area, and soil samples were collected from these plots in May 2005. The soil seed bank was sampled by the concentration method (ter Heerdt et al., 1996). Each plot was divided further into 10 subplots (0.4 m×1 m), and 10 cylindrical soil cores (3.6 cm diameter) were taken randomly in each subplot. The soil cores were separated into three fractions: the shallow soil layer (0–2 cm deep), the mid layer (2–7 cm deep), and the deepest layer (7–12 cm deep). Ten cores at each depth were pooled per subplot. Overall, there were 30 samples in each plot (10 samples in every layer), and 300 soil samples at each site. Thus, the area sampled at each site was 1.02 m2, and the total bulk of soil samples was 0.151 m3.
Maintenance of seed trays
The direct germination method of Thompson and Grime (1979) was used to assess the readily germinable seed species (Gross, 1990). The seedling emergence method usually detects more than 90% of species present in soil samples of grassland systems (ter Heerdt et al., 1996).
Each soil sample was put in a cloth bag, and placed on a table in front of a north-facing window for 10 days of direct exposure, then the samples were sieved through mesh sieves (mesh width 0.2 mm), in order to remove plant fragments and stones (Funes et al., 1999; ter Heerdt et al., 1996). Samples were then put in plastic germination trays (width 30 cm) over a layer of sterilized sand (depth 10 cm) that had been kept in an oven maintained at 140 °C for 24 h. The samples were spread evenly in a less than 2 cm layer over the sterilized sand. Control trays with only sterilized sand were set alongside the experiment to detect contamination by wind dispersed seeds. No seedlings germinated in the control trays. The soils were watered regularly three times a day in the first 2 months. In order to favor maximum germination, emerging seedlings were identified and removed or replanted for later identification. Seedlings were removed after identification to maintain a low density in the germination trays in order to encourage maximum germination and allow the germination of other seeds. Germinating seedlings were monitored daily for the first month of the study and weekly for the next three months and every four weeks after that as numbers of germinating seeds diminished. At the end of the first 2 months of the experiment, the soil samples were carefully turned over in order to facilitate the emergence of new seedlings (Roberts, 1981). The soils were stirred after 4–6 weeks to expose the ungerminated seeds following cessation of the initial flush of germinating seeds. After 5 months, the experiment was stopped as no more seedlings occurred for three consecutive weeks, and visual inspection of the samples after germination revealed very few ungerminated seeds. This indicated that an accurate estimation of the number of species in the seed bank samples was reached.
Vegetation sampling
Vegetation sampling was performed during summer (July 2005) when the development of plants was at its optimum. The vegetation was recorded by randomly placing five quadratic frames of 50 cm×50 cm within each of the site where the soil seed bank samples were taken, 20 quadrates at four sites altogether. We recorded the presence and cover of all species within each quadrate.
Data analyses
Differences of the amounts of species and mean number of seeds between different habitats and layers were compared by the one-way analysis of variance and Tukey's range test. The numbers of seeds in the seed bank were log-normally transformed prior to analysis. Analysis of variance was conducted in SPSS 11.5.
To compare the composition and abundance of species in the vegetation (4 habitats) and the seed bank, a multivariate ordination was conducted using Detrended correspondence analysis (DCA) (Hill, 1979; Hill and Gauch, 1980). DCA was used to examine the variation in plant species composition, and was applied to the data of species frequency. Graphical plots of data ordinations were constructed using CANODRAW (Smilauer, 1992). All analyses were performed using the DECORANA program, following the default options for DCA. For both ordinations of the seed bank and vegetation data, the two first ordination axes were retained for interpretation. For seed bank and vegetation data, species frequencies were calculated as the number of seedlings of one species divided by the total number of seedlings in the seed bank for each subplot and quadrate. Our data set finally consisted of one statistical matrix: the “samples–species” matrix Y with species frequency data for p=106 species (columns) in n=60 samples (rows).
Results
Vegetation composition of four habitats
In the vegetation, we recorded a total of 62 species (56.9% of the total species in seed bank and vegetation), belonging to 17 families. Of these 62 species, 9.7% were annuals, 4.8% were biennials, and 85.5% were perennial herbs. The proportion of perennial species decreased with increase in disturbance (Fig. 1). Species richness per quadrate differed significantly among the four sites (F=34.912, P<0.001; Table 1). Site L was the richest with 28.2 species, and site S was the poorest with 10.8 species.
Different superscripts indicate significant differences (p<0.05) (Tukey-test after significant one-way ANOVA). The numbers of seeds in the seed bank were log-normally transformed prior to analysis.
Composition of seed bank and variation in species richness and diversity
Visual inspection of the samples after germination revealed very few ungerminated seeds. This indicated that we had obtained an accurate estimation of the number of species in our samples. No seedlings were recorded in the control trays, indicating that there were no airborne seed contaminants. Five taxa from the seed bank were identified only to family or genus level (Pedicularis sp., Asteraceae sp., Poaceae sp., Galium sp., Carex sp.). After the 5-month germination period, we recorded a total of 17531 seedlings and 87 species. Of these species, 19.5% were annuals, 5.7% were biennials, and 74.7% were perennial herbs. The proportion of perennial species in the seed bank decreased with disturbance gradient (Fig. 1).
The mean number of seeds per plot in 0–12 cm depth differed significantly among the four sites (F=10.613, P<0.001), and the highest was in site S and the lowest in site C (Table 1). The seed density increased with increase in disturbance in each separate layer and in total (Fig. 2). The mean number of seeds in different soil layers differed significantly (Table 1), and seed density showed an obvious decrease with depth in each separate layer and in total (Fig. 2), being significantly higher in the shallow layer than in the mid and deepest layers. The species richness per plot differed significantly in 2–7 cm (F=3.596, P<0.05) and 7–12 cm (F=3.889, P<0.05) among different sites, and increased with increase in disturbance. However, there was no significant difference in 0–2 cm (F=1.738, P>0.05) and 0–12 cm layers (F=2.162, P>0.05). Species richness decreased gradually with depth in each separate layer and in total (Table 1).
Relationship between seed bank and aboveground vegetation
Detrended correspondence analysis (DCA) of the soil seed bank subplots and vegetation quadrates identified groupings of species composition (Fig. 3). The first axis of the DCA separated well group Sv from other vegetation groups, whereas these Cv, Lv, and Fv overlapped with each other along axis 1. The seed bank groups were close to each other; only group Ss had a minor distance to group Fs and Cs. The four seed bank groups and vegetation groups Cv, Lv, and Fv were well separated by axis 1; group Sv was separated by axis 2 from the four seed bank groups. Group Ss was very close to group Sv relative to the other seed bank groups and their corresponding vegetation groups. The major changes concerning the vegetation at site S were not reflected in the seed bank.
The seed density measured (3069–6105 viable seeds m−2) was larger compared with some other alpine meadow, arctic meadows and high-altitude meadows (e.g., 99–1109 m−2 viable seeds m−2: Welling et al., 2004; 0–3367 viable seeds m−2: McGraw and Vavrek, 1989; 2416 viable seeds m−2: McGraw et al., 1991; 0–4080 viable seeds m−2: Molau and Larsson, 2000; 1247–5405 viable seeds m−2: Also et al., 2003), but lower than the numbers given by Klug-PĆ¼mpel and Scharfetter-Lehrl (2008); (6000–34,000 seeds m−2). Several factors could explain high amounts of seeds at such sites. Cold climates (e.g. high mountains or high latitudes) could contribute to the maintenance of many seeds in the soil (Archibold, 1984; Cavieres and Arroyo, 2000). For example, the diversity of both seed predators and fungi tends to be low in high-mountain habitats (McGraw and Vavrek, 1989). Further, the cold climatic conditions may promote low embryonic metabolic rates and slow consumption of seed reserves, favoring seed longevity (Mudoch and Ellis, 2000) and retard germination (Milberg, 1995). In addition, a cold climate also contributes to the maintenance of seed viability and persistence in the soil (Thompson et al., 1997). A high carry-over of seeds from year to year may increase the total seed density in such alpine meadows.
Interestingly, the density of buried seeds increased along the disturbance gradient (Fig. 2); the highest density was found in habitat S and decreased with lower levels of disturbance (i.e., Cs<Fs<Ls<Ss). This is consistent with the scheme presented by Thompson (1978), which proposes that buried seed density increases with disturbance. Matus et al. (2005) also found that overgrazing, similar to other ways of intensified land use (e.g., ploughing), resulted in an increased seed density and higher similarity of vegetation and seed banks (see also Bekker et al., 1997; Halassy, 2001). It has been postulated that due to the consequences of reproductive failure in unfavorable years, there was greater seed dormancy in frequently disturbed environments (Venable and Brown, 1988). In addition, seeds are likely to be dispersed (by wind and livestock) from adjacent vegetation into the disturbed areas. Therefore, loss of vegetation did not significantly affect the seed density in these areas. We can conclude that with increase in disturbance, seed density increases gradually, indicating that restoration of seriously disturbed vegetation is not seed-limited. Rather, the high seed density would contribute to a potential role of the seed bank in vegetation development during restoration.
Seed density depends on the distribution and seed production of dominant species to a large extent. We found that Plantago asihica, P. pratensis, and Artemisia desertorum made up 76.7–86.8% of the seed bank. Soil seed banks that were similarly dominated by a few species have been shown in other studies, including a polar desert in the high arctic (Freedman et al., 1982), and a high subalpine site in the Oregon cascade mountains (Ingersoll and Wilson, 1993). Perennial grasses are abundant in undisturbed areas and are less frequent in areas disturbed by grazing (Bertiller et al., 1995). This is consistent with our results: in vegetation as well as seed bank, the proportion of perennial species decreased with disturbance gradient (Fig. 1).
Seed number and species richness showed a decreasing trend with depth, being significantly higher in the shallow layer than in the mid and deepest layers (Table 1, Fig. 2). Species composition was related to vertical distribution. Thus, some species (e.g., Scirpus pumilus, Lancea tibetica, and Koeleria cristata) were more abundant near the surface, whereas others (e.g., Juncus effusus, Potentilla bifurca, and Chenopodium foetidum) were mainly detected in the mid-layer, and a third group of species (e.g., Stipa capillata, Galium aparine, and Cuscuta europaea) mainly appeared in the deepest layer. Most probably, the vertical structure may lie in the different ability of seeds to penetrate the soil (Thompson et al., 1993). Many other studies have reported similar trends of decreasing seed density with depth (e.g., Kitajima and Tilman, 1996; Klug-PĆ¼mpel and Scharfetter-Lehrl, 2008). The majority of viable seeds are normally concentrated in the first few centimeters of soil surface (Larvorel et al., 1993), and only seeds in the upper centimeters of the soil have a chance to germinate (Fenner, 1985). The proportion of seeds in mid and deepest layers increases along the disturbance gradient (Fig. 2); these seeds can be classified as persistent seeds. A large portion of species with persistent seeds in a dense seed bank can indicate that it is a strategy of adaptation to current disturbance.
The relationship of seed bank and vegetation, and its implications for restoration
In some arctic and alpine habitats such as northern Alaska (Roach, 1983), Mont Jacques-Cartier in Quebec (Morin and Payette, 1988), and the Andes of central Chile (Arroyo et al., 1999), vegetation and seed bank are correlated, but they are only loosely coupled in other subarctic and subalpine communities (e.g., Diemer and Prock, 1993; Ingersoll and Wilson, 1993; Staniforth et al., 1998; Whipple, 1978). We found a relatively low degree of similarity as a whole between the species composition of vegetation and seed bank. These discrepancies have been explained by the minor contribution of the dominant perennial meadow species to the formation of the seed bank. Most studies of grasslands dominated by perennial grasses have found low similarities between the seed bank and the vegetation (Bakker et al., 1996; Edwards and Crawley, 1999; Milberg, 1995; Peco et al., 1998). In addition, in the alpine meadow we studied, the majority of species belong to clonal plants such as Cyperaceae (S. pumilus, K. humilis, K. capillifolia, K. bellardii) and Poaceae (Elymus dahuricus, P. pratensis, P. poophagorum). These species generally have a low seed production because they alternate sexual reproduction with vegetative forms and their seeds have a short-term persistence in soil (Thompson, 1992). Some species were found in the seed bank but were absent in the present-day vegetation (i.e., A. desertorum, Hypecoum leptocarpum, Artemisia hedinii, Elsholtzia densa). This may be due to the interaction of three factors: (1) possibly these species do not have suitable “safe sites” for germination and establishment (van der Valk and Davis, 1976, 1978); (2) they are only able to germinate and establish when disturbance events happen (Thompson and Grime, 1979); (3) these species are provided with long-lived seeds able to form persistent seed banks (Milberg and Hansson, 1993). No seeds of, e.g., S. aliena and Elymus nutans were found in the seed bank, although these species dominated the vegetation. The absence of a seed bank for species relying on vegetative reproduction seems to be a common trait (Milberg, 1992; Rice, 1989).
The result of DCA showed that the species composition of the vegetation at site S was very different from the other three sites; the composition of the seed bank did not show such obvious differentiation among sites. Therefore, changes of vegetation at site S in response to disturbance were not reflected in the seed bank. High grazing pressure probably has affected the species composition in the vegetation more dramatically than in the seed bank (Luzuriaga et al., 2005). The impact of these changes in the vegetation composition is obviously buffered in the seed bank. Whereas the species composition of vegetation was shaped by grazing, the persistent seed bank comprised long-lived dormant seeds, and changes in seed bank species composition lagged behind changes in the vegetation. Milberg and Hansson (1993) similarly found only slight effects of vegetation changes upon seed bank species composition when comparing grazed and non-grazed limestone grasslands.
The relationship between species composition of the seed bank and the vegetation was significantly different at sites L, F, and C. In other words, the seed bank was dissimilar to the vegetation in these areas having the tendency to contain more species with persistent seeds. The vegetation, on the other hand, contains more perennials, more clonal species and fewer selfing species. Low similarity between seed bank and vegetation in these areas indicates that the seed bank plays a minor role in contributing to the regeneration of vegetation, and the managers cannot rely there on soil-stored seed banks for restoration. Establishment of the vegetation in these areas is possibly more likely to rely on actual seed dispersal and on species with vegetative reproduction. The respective contribution of these two ways sustaining the given vegetation should be examined by further studies.
By contrast, the species composition of the seed bank in site S was similar to its vegetation. The degree of similarity between species composition of vegetation and seed bank is predicted to increase with disturbance, due to the greater relative abundance of annuals in the vegetation (Chambers, 1993). Matus et al. (2005) found that the intensity of overgrazing disturbances led to high similarity between seed bank and vegetation composition. Similarities between both community attributes may lie in the opportunistic behavior of species that comprise such a seed bank. As soon as any disturbance occurs, these species become dominant in the vegetation so that the vegetation composition reflects the seed bank composition. Hence, we assume that the establishment of new species in seriously disturbed environments mainly depends on the seed bank. This confirms our hypothesis. For successful restoration and species conservation projects, seed banks or above-ground sources of seeds are necessary (Leck and SchĆ¼tz, 2005). High seed density and sufficient species diversity in the seed bank increase its potential for vegetation restoration (e.g., Grime, 2001). The rich seed bank in the seriously disturbed area, where it contains also more species than the vegetation (Table 1), could play a potential role during restoration management, provided that it does not become too much dominated by ruderals with short life cycles.
Acknowledgements
We would like to thank Yifeng Wang, Xuelin Chen, Wei Qi, Shiting Zhang, and Zhengwei Ren at the Laboratory, and Haihun Liang, Jihong Ma of Hezuo Grassland Workstation for their help with identification of many seedlings and field assistance. We particularly thank Dr. David Gibson, Dr. Peter Kotanen, Dr. Zhigang Zhao, and Dr. Ken Thompson for critically reviewing the initial draft of this manuscript and for their many constructive comments. The study was supported by the Key Project of the National Natural Science Foundation of China granted to Guozhen Du, No. 30470307.
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