Seed Characteristics Determine Species Composition of Degraded Alpine Meadow Communities

doi:10.21203/rs.3.rs-2007896/v1

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Seed Characteristics Determine Species Composition of Degraded Alpine Meadow Communities

https://doi.org/10.21203/rs.3.rs-2007896/v1

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Climate change and human activities are increasing the disturbance to alpine meadows in Qinghai-Tibet Plateau, these changes are reducing the abundance of previously-dominant Gramineae and sedges, with forbs establishing dominance through secondary succession. Forbs replaced native Gramineae and sedges as dominant species. To explain the formation mechanism of this succession, this paper studied the seed characteristics of 14 common plants in alpine meadow communities, including seed number, 1000-grain weight, germination rate, pappus characteristics, fall velocity. It is revealed that seed pappus characteristics are the key factors in determining the dominance of species on the secondary bare land of degraded alpine meadows. The results of the comprehensive evaluation method show that Asteraceae have higher diffuse ability than Gramineae, sedges, Apiaceae and Leguminosae due to lower falling speed of Asteraceae, caused by their pappus. The redundancy analysis results further explain the reasons for the emergence of corresponding dominant species in different degradation stages of alpine meadows. These results are helpful for understanding changes in plant communities in alpine meadows under grazing disturbance. They also provide a theoretical basis for the sustainable utilization and management of the alpine meadow ecosystem and restoration of degraded grassland.

seed characteristics

degraded alpine meadow

Asteraceae

diffuse ability

species composition

The alpine meadows are one of the main grassland types in Qinghai-Tibet Plateau, with the vegetation communities which have many important ecological functions for conserve soil and water, conserve water sources, and maintain biodiversity [1–3]. However, over past decades, overgrown grazing-based human activities, mainly grazing yaks and Tibetan sheep, and climate change have caused to extensive degradation of alpine meadows [4]. Communities dominated by Gramineae and sedges are fundamentally destroyed, forming large areas of secondary bare land [5], disturbing the originally stable alpine meadow ecosystem and significantly reducing ecological function [6]. These losses pose a serious threat to agricultural production, the way of life of local people and regional ecological balance [4].

The formation and development of any vegetation community is determined by these aspects: plant dispersal, plant settlement, competition among plants and the reproductive ability [7–9]. Plants with high diffused ability generally have many key traits, such as a large number of inflorescences and of seeds, rapid reproduction and maturity and strong seed dispersal ability [10]. Seeds with greater dispersal ability often have appendages such as pappus, wings, and hooks, which can help seeds spread to farther places with the help of wind, water, and animals [11–14]. A series of seeds traits such as production, diffusion, germination and competition determine the ability of species to diffuse and establish new populations in new environments [10, 14]. Previous studies have found that the most invasive species in China are Asteraceae [9, 11, 33, 34]. These plants can produce a large number of seeds, have strong diffused ability and the ability to adapt to harsh environments [10]. Once successfully invaded, they threaten the stability of the local ecosystem [15].

Alpine meadows are seriously degraded due to both human and natural factors, resulting in areas of bare land, enabling secondary succession [16]. Forbs grow rapidly on bare land through their propagules, forming new secondary communities [17]. Weeds harmful to grazing livestock, especially some Asteraceae plants quickly replace Gramineae and sedges as dominant species, and last for a long period of time on alpine meadow in Qinghai-Tibet plateau of China [18]. So, the species composition of heavily degraded alpine meadows is also probably related to these characteristics of seeds in the native community. The mechanism is unclear that causes this succession. Research reports related to alpine meadows are extremely rare, and the main mechanism is still unclear [5, 6]. In order to reveal the mechanism influencing secondary succession in heavily degraded alpine meadows, in this paper we compared the seed characteristics of a range of Gramineae, sedges, Asteraceae on the grassland, to explore how seed characteristics affect the diffuse ability of species. This research aims to provide a theoretical basis for understanding the succession of alpine meadow plant communities following disturbance, and provide a basis for the sustainable utilization of alpine meadow ecosystem and the restoration of degraded grassland.

Study site

The study site is located in Amuhu Village, Youganning Town, Henan County, southeastern Qinghai Province. Geographical coordinates of the sample plot are 34°43′27″N, 101°39′18″E (Fig. 1). The altitude of the territory is 3400 ~ 4200m, with an annual average temperature of -1.3 ~ 1.6℃, annual precipitation of 597.1 ~ 615.5mm, and annual average wind speed of 2.3m/s. In areas of the plateau continental climate zone, with typical alpine meadow vegetation types, the frost-free period is 28 days, and the growth period of grassland plants is short, ranging from 120 to 140 days. The plant communities in the study site are mainly composed of Graminoids, sedges, which are good for grazing, and forbs which are bad for grazing (Table 1).

Study methods

Plot setting

Affected by different grazing intensities, alpine meadows in this area show different degrees of degradation, On the basis of the height, coverage, biomass etc. of the vegetation[19], we selected four alpine meadows with different degradation degrees, non-degraded (ND), lightly degraded (LD), moderately degraded (MD) and heavily degraded (HD), as experimental plots, with each plot area not less than 4 ha.

Measurement of species dominance

Species dominance was measured and calculated by following formula: Important value = (relative height + relative density + relative frequency + relative biomass)/4. The greater the important value, the greater the dominance.[20, 21]. Set 9 plots for each degradation degree. Height, the natural height of the plants in the plot; Density, the number of plants in the plot; Frequency, the number of occurrences of species in the plot; Biomass, the dry weight of the plants on the plot.

Test plant seeds

The seeds of 14 most common species, which have higher frequency in the plots, were collected in this study, including 3 species of sedges, 4 species of Gramineae, 5 species of Asteraceae, 1 species of Apiaceae, and 1 species of Leguminosae (Table 1). Figure 2 shows photographs of seeds from the tested plants. Figure 3 shows that the dominant species are changed with the degradation.

Measurement of seed characteristics

For the seeds of each species, the number of seeds per plant, 1,000-grain weight, number and length of pappus, fall velocity (the speed at which seeds fall freely under windless conditions) [22, 23], germination rate were measured.

Comprehensive Evaluation of Plant Seed Invasion Ability

The comprehensive evaluation method of the diffuse ability of different plant species adopts the membership function method. In this study, the comprehensive evaluation method, used by Wang Zan et al [24]., was used to comprehensively evaluate the diffuse ability of each species.

Membership function values of each index of the tested species:

µ(X_ij)=(X_ij-X_min)༏(X_max།X_min) (1)

In the formula: X_ij represents the 1 to ith index value (i represents the14 species respectively. 1 to jth represents the pappus length, pappus number, number of seeds on a single reproductive branch, number of seeds on a single flower, fall speed, germination rate respectively), X_min represents the minimum value of the Jth index, and X_max represents the maximum value of the Jth index. If an index is negatively correlated with intrusion capability, the membership function can be calculated by inverse subordination function. µ(X_ij)=(X_max-X_ij)/(X_max-X_min) [24].

Coefficient of standard deviation: V _j=\(\frac{\sqrt{\frac{\sum _{i=1}^{n}{\left({X}_{ij}-{\stackrel{-}{X}}_{j}\right)}^{2}}{n-1}}}{{\stackrel{-}{X}}_{j}}\). (2)

‾X_j represents the average value of the Jth index of each material. X_ij represents the subordination function value of j character of material i; V_j represents the standard deviation coefficient of the Jth index [24].

The weight of each index: W _j=\(\frac{{V}_{j}}{\sum _{j=1}^{n}{V}_{j}}\). (3)

W_j represents the weight of the Jth indicator [24].

Comprehensive index: D=\(\sum _{i=1}^{n}\left[\mu \left({X}_{ij}\right)\times {W}_{j}\right]\). (4)

D represents the comprehensive evaluation value of the diffuse ability of each species, and the higher the D value is, the stronger the diffuse ability is. D is defined as grade Ⅰ when the value is between 0.7 and 1.0 (including 0.7). The value between 0.5 ~ 0.7 is grade Ⅱ(including 0.5). The value between 0.3 ~ 0.5 is grade Ⅲ (including 0.3). The value between 0.1 ~ 0.3 is grade Ⅳ (including 0.1). Seeds index lower than 0.1 are considered to be grade Ⅴ, and seeds in grade Ⅴ are considered to have no diffuse ability.

Data Processing And Analysis

Microsoft Excel 2019 was used to enter the original data. One-way ANOVA and Spearman correlation analyses were performed on the data with SPSS 25.0 software. Multiple comparisons were used the least significant difference (LSD) with SPSS21.0 Statistics software. The statistical significance level was P = 0.05 (extremely significant at P = 0.01). Graphs were drawn using Microsoft Excel 2019 and SigmaPlot10.0 software. Redundancy analysis used Canoco5.0 software.

Species Dominance

As the degradation intensifies, the dominant species has changed significantly (Fig. 4). In non-degraded grassland, Gramineae is the dominant species. Gramineae plants accounted for 61.48% of the total. Under lightly degraded, grass and sedge plants are dominant in the community. Under moderately degraded, sedge plants become the dominant species, comprising 50.18%. Under heavily degraded, Asteraceae become the dominant taxa, accounting for 72.16% in general. While some Gramineae plants no longer appeared in the heavy degraded plot .

Seed Characteristics

The greatest number of seeds found on a single reproductive branch was Ligularia virgaurea; significantly higher than other plants (P < 0.05) (Fig. 5a). Kobresia humilis and Kobresia pygmaea had the fewest seeds on single reproductive branches, significantly lower than other plants (P < 0.05) (Fig. 5a).

Aster yunnanensis has the largest number of seeds on a single flower, which is significantly higher than other species (P < 0.05) (Fig. 5b). Kobresia humilis and Kobresia pygmaea have the least number of seeds on a single flower (Fig. 5b).

The thousand kernel weight of Elymus nutans is the largest, which is significantly higher than other plants (P < 0.05) (Fig. 5c). The thousand kernel weight of Leontopodium leontopodioides is the lowest (Fig. 5c). Among Asteraceae plants, the thousand kernel weight of Saussurea superba seeds is significantly higher than other Asteraceae seeds (P < 0.05) (Fig. 5c).

Seeds Appendant Organ

Although grass plants have awns, the awn does not alter the flight ability of the seeds [25]. Thus the grass awn is not included in the appendant organs. Asteraceae plants have developed accessory organs, the pappus (Table 2). The pappus number of Ligularia virgaurea is the most, which is significantly higher than other plants (P < 0.01) (Table 2). Gramineae, Cyperaceae, Apiaceae, and Leguimnosae do not have pappus (Table 2). The length of the pappus of Saussurea superba is the longest, which is significantly higher than other plant species (P < 0.05) (Table 2).

Seed falling Speed And Germination Rate

The falling speed of Elymus nutans is the highest, which is significantly higher than other plant seeds (P < 0.05). The seeds fall speed of Asteraceae are significantly lower than other species(Fig. 6a). The fall speed of Leontopodium leontopodioides is the lowest (Fig. 6a). There is no difference in the fall speed between Kobresia humilis and Kobresia pygmaea (P > 0.05) (Fig. 6a).

The seed germination rate of Saussurea superba is the highest, which is significantly higher than other plants (P < 0.05) (Fig. 6b). There is no difference in germination rate among Carex breviculmis, Festuca sinensis and Oxytropis ochrocephala (P > 0.05) (Fig. 6b). The germination rate of Oxytropis ochrocephala is the lowest. Among Gramineae, the germination rate of Festuca sinensis is the lowest, which is significantly lower than other Gramineae seeds (P < 0.05) (Fig. 6b). Thousand-grain weight and fall speed are significantly positively correlated (P < 0.05), and pappus length and fall speed are very significantly negatively correlated (P < 0.01) (Table 3).

Correlation Analysis

NSSRB, NSSF, PL, PN and FS of plant seeds are significantly correlated with the frequency of plants appearing in the sample plot (Figs. 7a-e). The correlation coefficient between seed pappus number and frequency is the largest (Fig. 7d). The correlation coefficient between frequency and seed number of single reproductive branch is the smallest (Fig. 7a). Germination rate of seeds is not related to frequency (Fig. 7f).

Comprehensive Evaluation

The seed invasion ability of Ligularia virgaurea is the strongest, at grade II (Table 4). The seed invasion ability of Kobresia pygmaea and Oxytropis ochrocephala are the least (Table 4). The invasion ability is grade Ⅴ. The diffuse ability of Leontopodium leontopodioides, Saussurea superba, Anaphalis lactea and Aster yunnanensis are grade Ⅲ (Table 4). The diffuse ability of Gramineae and Cyperaceae is in the grade Ⅳ, so have little difffuse ability (Table 4). There is a very significant positive correlation between the frequency of plants on heavily degraded alpine meadow and the integrated assessment value of each plant (P < 0.01) (Fig. 8).

Redundancy Analysis

Redundancy analysis results show that seed characteristics account for 93.57% of the total change of frequency (Fig. 9). RDA also shows that fall speed is the most influential factor upon frequency. PN, PL, NSSF and NSSRB are positively correlated with the species frequency on HD (Fig. 9). While ND, LD and MD are on the contrary. FS and GR is negatively correlated with HD, but it is positively correlated with ND, LD and MD (Fig. 9). TKW is positively correlated with the frequency of plant appearance on ND, but negatively correlated with MD, LD and HD (Fig. 9).

Grazing disturbance can cause significant change in grassland plant communities [17, 18, 26–28]. The results of this study show that, from non-degraded to heavily degraded areas, the dominant species of alpine meadow plant community showed a clear trend. Non-degraded meadow is dominated by Gramineae, with sedges becoming increasingly dominant following degradation, and forbs becoming dominant in the most degraded areas (Fig. 4). These results are consistent with those of other studies [18, 27, 28]. Due to the absence of grazing pressure from livestock, and their significant height advantage, grasses in the non-degraded plots can produce large numbers of seeds and remain the dominant species [18, 27]. During the stage of light degradation, the reduction of tall grasses by livestock provided opportunities for the growth of low sedges, so both Gramineae and sedges were dominant at this stage (Fig. 4) [28]. In the moderately degraded stage, Gramineae are easily eaten due to their highest position in the grass layer, limiting their seed formation [28]. On the contrary, low sedges are not easily eaten and their strong grazing tolerance and vegetative reproduction ability make them the dominant species at this stage [28]. In the first three stages of degradation, dense clump-type Gramineae and sedges formed a dense meadow layer, and it was difficult for forb seeds to reach the soil surface through the meadow layer, and it was even more difficult to germinate and settle down [18]. In the heavily degraded stage, due to the over-grazing of Gramineae and sedges, the competitiveness of forbs, which are less foraged by livestock, is improved, and the low alpine meadows trigger a large number of digging activities by pikas [29]. The meadow plants died and disappeared, and a large area of bare land appeared, creating opportunities for the invasion of some plants through secondary succession [28].

The fecundity of plants plays an important role in the expansion of the population of species[30, 31]. This study shows that, the number of seeds of Asteraceae on the alpine meadow is much larger than that of Gramineae (Fig. 5) [15]. Most of the Asteraceae are annual plants with a very strong fecundity, thus excellent ability to mass reproduce [9]. Sedges in alpine meadows are perennials, and have the ability to reproduce both sexually and vegetatively. Sedges have changed their reproductive strategies under grazing disturbance, and they mainly tend to be asexual [32], allocating significant resources to vegetative rather than sexual reproduction (Fig. 5). This is an adaptation strategy for plants to long-term grazing but, because they are dense clumps, their extremely short rhizomes limit their asexual expansion of the area [32, 33]. The respective reproductive strategies of the plants in these alpine meadows can well explain the results presented in our study. Therefore, sexual reproduction has become a decisive factor for plants to invade bare land [30], and rely on seed reproduction to achieve a sudden population increase.

In addition to seed production capacity, the diffuse capacity of plants is also closely related to the ability of seeds to spread [10, 15, 34, 36, 37]. The most critical characteristic affecting the propagation distance is the speed of seed fall [36]. The slower the fall speed, the farther the seeds will be carried by wind or water [35–37]. Our study found that all of the seeds of Asteraceae on alpine meadows have feathery pappus (Fig. 2), and the seeds with a large number and long pappus have a lower fall speed (Table 2) (Fig. 6a), [37]. Thousand-kernel weight, which represents seed mass, also affects seed dispersal [38]. This study found a positive correlation between thousand-kernel weight and seed fall velocity, which is consistent with the findings of others (Table 3) [38]. Large seeds with large thousand-kernel weights and high fall speed limit their own dispersal distances, which in turn affect their ability to invade new habitats [35, 37, 38]. Our results show that Gramineae and sedges have larger thousand-kernel weights, while Asteraceae have smaller thousand-kernel weights (Fig. 5c). The seed fall speed, which is determined by the number and length of the pappus of the seed, and the thousand-kernel weight, can best explain the results of this study, that is, the heavily degraded grassland is dominated by Asteraceae, while the Gramineae and Cyperaceae are few or disappear. The reproductive ability and dissemination ability of plants determine the initial competition ability of plant invasion, while the establishment ability of plants determines the final competition ability of plants [39]. This study found that the seeds of Asteraceae (Saussurea superba), Gramineae (Elymus nutans, Poa pratensis) and Cyperaceae (Kobresia humilis) had higher germination rates (Fig. 6b), but this could not explain well the composition of the plant community appearing on degraded bare land, which indicates that succession on degraded bare land is determined by the comprehensive effects of various characteristics of plant seeds.

In order to further explain the species composition of degraded bare land succession and the reasons for the formation of dominant species, we analyzed the diffuse ability of species with the above characteristics of seeds using a comprehensive evaluation method. The results show that Asteraceae such as Ligularia virgaurea, Leontopodium leontopodioides, Saussurea superba, Anaphalis lactea, and Aster yunnanensis have strong diffuse ability and become the dominant species following secondary succession on degenerated bare ground (Table 4) [29]. The diffused ability of Gramineae is very low, while the sedges have almost no diffused ability (Table 4).This may be why the bare land formed by the heavily degraded alpine meadow is difficult to recover under natural conditions [32]. The results of the redundancy analysis also support the above phenomenon and mechanism, that is grasses and sedges with high 1,000-kernel weight, no pappus, and fast fall speed, appear in non-degraded and lightly degraded alpine meadow (Fig. 9), while Asteraceae with small seeds, low 1,000-kernel weight, large number and long of seed pappus, and slow fall speed, mostly appeared on heavily degraded bare land (Fig. 9). Thus, both seed characteristics and grazing of livestock determine the species composition in the bare land of degraded alpine meadows.

This study focuses mainly on seed fall speed, which is affected by wind influencing the ability of species to spread and invade [35–38]. However, studies have shown that animal transport and water transport of plant seeds also have important effects on seed dispersal and plant invasion [40, 41]. In addition, the allelopathy of Asteraceae can affect interspecific competition [42–44]. The main disadvantage of this study is that these issues were not considered. A more nuanced assessment of successional processes on the alpine meadow is possible, if these factors are considered comprehensively. To this end, long-term observation and research of the alpine meadow seed bank can be carried out by isotope labeling and other methods.

The results of this study show that the seeds of Asteraceae have the characteristics of large quantities of seeds, long pappus, high number of pappus, high germination rate, low 1000-grain weight, and low fall velocity. Conversely, although the seeds of Gramineae are numerous, with a high germination rate, their seeds do not have accessory organs to reduce their falling speed, thus have a weak dispersal ability overall. These characteristics of seeds determine that Gramineae cannot become dominant species in a short period of secondary succession, only after a long period of succession can it become a dominant species. Sedges also have strongly limiting characteristics, namely seeds and no pappus, with high fall speed thus, sedges are outcompeted by Asteraceae on the bare land of degraded alpine meadows. These limitations are also seen where the sedges are previously dominant species but fail to recover following disturbance. Seed characteristics determine the composition of plant communities on the bare land of degraded alpine meadows. These results provide a good theoretical basis for better understanding the degradation and succession laws of alpine meadow plant communities, it also well guide the sustainable utilization of alpine meadow ecosystems and the restoration of degraded grasslands.

ACKNOWLEDGMENT

This study was supported by the national natural science fund project (31560151), science and technology plan projects in Qinghai province (2020-ZJ-785, 2020-ZJ-904). Thanks to Riki Taylor in Auckland, New Zealand, for his helpful feedback on a draft of this paper.

Ethical Approval

Not applicable.

Competing Interests

The authors have no relevant financial or non-financial interests to disclose. All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.

Authors' Contributions

Weimin Yan, Yuanwu Yang, and Xilai Li wrote the main manuscript text. Weimin Yan prepared all the figures and tables in the manuscript. Yuanwu Yang analyzed and processed the data. All authors reviewed the manuscript.

Funding

Partial financial support was received from the national natural science fund project (31560151), science and technology plan projects in Qinghai province (2020-ZJ-785, 2020-ZJ-904).

Availability Of Data And Materials

All date generated or analysed during this study are included in this published article( and it’s supplementary information files)

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Table 1-4 is available in the Supplemental Files section.

No competing interests reported.

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Seed Characteristics Determine Species Composition of Degraded Alpine Meadow Communities

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Abstract

Figures

Introduction

Materials And Methods

Study site

Study methods

Plot setting

Measurement of species dominance

Test plant seeds

Measurement of seed characteristics

Comprehensive Evaluation of Plant Seed Invasion Ability

Data Processing And Analysis

Results

Species Dominance

Seed Characteristics

Seeds Appendant Organ

Seed falling Speed And Germination Rate

Correlation Analysis

Comprehensive Evaluation

Redundancy Analysis

Discussion

Conclusion

Declarations

References

Tables

Additional Declarations

Supplementary Files

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