Shrub encroachment stabilizes aboveground biomass and enhances community diversity in alpine meadow of the Hengduan Mountain, Southwest China

doi:10.21203/rs.3.rs-971214/v1

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Shrub encroachment stabilizes aboveground biomass and enhances community diversity in alpine meadow of the Hengduan Mountain, Southwest China

https://doi.org/10.21203/rs.3.rs-971214/v1

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Aims

To understand the response of alpine meadow to shrub encroachment, we analyzed alpine meadow plant community structure and function and soil properties at different shrub coverage sites in western Sichuan, P.R. China, further providing a reasonable scientific basis to better control or remove the effects of shrub encroachment on local meadow.

Methods

A site with an area of 200 × 200 m along the shrub line was selected as the study area (4300 m a.s.l.) in Yajiageng Valley, Hengduan Mountain, Southwest China. The study area was divided into four treatments. Control, low-, medium- and high -shrub- coverage plots were investigated for community composition, above- ground biomass and belowground biomass were harvested, and root functional traits and soil properties were measured in laboratory in the summer of 2020.

Results

There was no significant difference in the total aboveground biomass among all treatments, however, forb and legume biomass increased, and grass and sedge biomass decreased with increased shrub coverage; Species richness significantly decreased, but diversity and evenness index increased in the medium -shrub -coverage plots; Root biomass at the depth of both 0-10 cm and 10-20 cm significantly decreased in medium- and high -shrub -cover plots; The field capacity, soil total nitrogen (STN) and available nitrogen (AN) concentration were significantly decreased in the medium -shrub -coverage plots, and the total phosphorus (TP) concentration were significantly decreased among all treatments.

Conclusion

The process of shrub encroachment tends to limit the local dominant species and maintain the growth of nondominant species, which further increases the community diversity and maintains the stability of aboveground biomass. Shrub encroachment makes alpine meadow soil became drier and impoverishment and promoted the mineralization of organophosphorus and the dissolution of insoluble inorganic phosphorus, which strengthened the alpine ecosystem nitrogen (N) limit.

Botany

Agricultural Engineering

shrub encroachment. alpine meadow. different shrub coverage

Over the past half century, a phenomenon in which native shrub plant density, cover and biomass increased, defined as “shrub encroachment”, has been widely observed around the world (Auken 2009). Shrub encroachment appears to result from reduced fire frequency (Rossiter N A 2003), long-term climate change (D'Odorico et al. 2010; Heisler et al. 2008) and predator suppression (Kerley and Whitford 2009). Previous study presented evidence that over more than 320 km² of Arctic landscape shrub abundance increased during the past 50 years (Sturm 2001). Currently, in the United States, approximately 330 million hectares of non-forest-lands are experiencing shrub encroachment. (Heisler et al. 2008; Place et al. 2008). In China, about 5.1×10⁶ hm² of meadow has experienced shrub encroachment in Inner Mongolia (Peng et al. 2013). Brandt also indicated that at least 39% of the alpine meadow had been replaced by shrub meadow from 1990 to 2009 on the south-eastern Qinghai-Tibet Plateau. Shrub-encroached grassland (SEG) has emerged as an important vegetation type in arid and semiarid regions (Houghton et al. 1999).

Shrub encroachment is associated with a general weakening of ecosystem functions and processes (Archer 2009; Auken 2003; 2009), and this weakening is often accompanied by great changes in soil resources and vegetation patterns (Schlesinger and Pilmanis 1998). Several articles have revealed that shrub encroachment tends to reduce vegetation density, coverage and productivity (Gibbens et al. 2005; Zavaleta and Kettley 2006), which also generates a significant reduction in species diversity based on the succession process of different steppe communities in North America (Ratajczak et al. 2012). In addition, Eldridge and Myers-Smith showed that shrub encroachment could change the nutrient cycle of carbon (C), water use efficiency and pH, and could change the spatial distribution of landscape resources significantly, with intense biological activity (Eldridge et al. 2011; Li et al. 2013; Myers-Smith et al. 2011). However, field data are very rare (Archer et al. 2004). Recent meta-analyses have demonstrated that the impact of shrub encroachment on ecosystem function is uncertain on a global scale, with equally positive and negative impacts (Eldridge et al. 2011), and most studies have focused only on comparing the differences between shrub and grass patches (Archer et al. 2004; Liu et al. 2018a; Zhou et al. 2018). Few studies have examined the effect of different shrub coverages on meadow (XingYY et al. 2017); thus, information on the function and processing of SEGs in different shrub coverages is lacking.

Alpine meadow are characterized by high biodiversity and endemic species, most of which are close to their physiological tolerance limit and are particularly sensitive to environmental changes (Euskirchen et al. 2010; Korner 2005; Solomon et al. 2007). A recent study demonstrated that the distribution of alpine vegetation has changed rapidly due to shrub encroachment (Chen et al. 2011). This change in plant species distribution and community composition may greatly change the response of alpine ecosystems to global change (Wookey et al. 2010). However, research on shrub encroachment in China is mainly concentrated in the arid and semiarid steppe areas, particularly in Inner Mongolia meadow (Zhou et al. 2018), and there are relatively few studies on alpine meadow, especially in Qinghai Tibet Plateau meadow ecosystems, which limits our understanding of the adaptation mechanism of alpine meadow to shrub encroachment. Therefore, we selected different degrees of SEG to analyse the community structure and function in the alpine meadow area of Yajiageng Valley, which is along the northeast slope of Mt. Gongga, western Sichuan province. This region is an important area for the protection of mountain ecological environments and the economic development of ethnic minority areas. The objectives of this study aimed to solve two scientific problems: 1) How does shrub encroachment affect the community composition and structure of alpine meadow? 2) What are the responses of the alpine meadow to different shrub coverage?

Site description

The study site is located in Gongga Mountain (29.33°-30.33°N, 101.50°-102.25°E), and it is in the transition zone between the subtropical humid monsoon and the temperate subhumid zone of the eastern Tibetan Plateau. The mean annual temperature (MAT) and mean annual precipitation (MAP) are approximately 4 ℃ and 1100 mm, respectively, with obvious wet and dry seasons. The rainy season (from May to October) accounts for more than 80% of the annual precipitation, and the dry season is from November to April. The species composition of the plant community is mainly perennial herbs, and the soil types of the study sites are typical alpine meadow soil.

Experimental design and field sampling

We investigated the vegetation at an elevation of 4300 m in the Yajiageng Valley area of Gongga Mountain in June 2020. According to Wijngaarden’s shrub coverage classification index (Wijngaarden 1985), the area was divided into non- shrub (control) meadow, low-shrub-coverage meadow (coverage 40% - 60%), medium-shrub-coverage meadow (coverage 60% ~ 80%) and high- shrub shrub-coverage meadow (coverage > 80%). Five blocks (3×3 m) were selected from the shrub meadow with different shrub coverage levels. Each block was separated by at least 3 m, and the habitat, topography and soil type were basically consistent. We selected a pair of grass and shrub plots (50×50 cm) in each block for the vegetation investigation, and the distance between each paired shrub and the grass patch was at least 2 m. The density (the number of shrub patches in the block) and size (long and short crown diameter) of the shrub patch were measured to assess the shrub characteristics. In each 50×50 cm grass plot, we recorded the number and height of each species in the plot, and then estimated the total coverage as well as the coverage of and each species coverage. All plants in the plot were harvested at the soil surface to evaluate the aboveground biomass (AGB), which was classified into grass, forbs, sedges and leguminous groups. The roots of plants were collected using a stainless-steel corer (5-cm diameter) from 3 layers at intervals of 0–10, 10–20 and 20–40 to estimate the belowground biomass and analyse the root functional traits (n=5). In addition, soil samples were collected using an auger boring (5-cm diameter) and cutting ring from the same layers in both shrub and grass patches, and plant and animal residues were removed from the soil surface during sampling.

Laboratory analyses

The cutting ring soil sample was used to measure the soil bulk density, field capacity and total porosity. All air-dried soil samples were sieved through a 0.15-mm sieve, and plant roots were carefully removed. The laboratory soil sample analysis method was based on the operating steps of Soil Physical Analysis. The soil pH was measured with a pH meter (PHS-3B, Shanghai). The soil total nitrogen (STN) was measured by the potassium dichromate- sulfuric acid acidification method. The soil available nitrogen (AN) was measured by the NaOH-H₃BO₃ method. The soil total phosphorus (TP) was measured by acid dissolving molybdenum antimony resistance colorimetry. The soil available phosphorus (AP) was measured by sodium hydroxide sodium oxalate. The soil organic phosphorus (OP) was measured by the burning method. The classification of soil inorganic phosphorus (IP) was measured by the extraction method. The root sample was washed until the surface was soilless. The cleaned root was imaged by an Epson scanner, and the root functional traits were analysed by WinRhizo root analysis software (Pro 2009, Regent Instruments Inc. Canada). The total root volume, root length and root surface area and other parameters were recorded. The contents of carbon (C) and N in roots were measured by an elemental analyser (Elementar, Vario Macro cube, Germany).

Statistical analysis

According to Wijngaarden’s classification index, we calculated the shrub coverage in each block, which is the ratio of the crown area of all shrub plants to the block area. To determine the species composition and community characteristics in different stages of shrub encroachment, we described patch cover, patch height, species cover and species height at the plot level. We then calculated the species richness index, Shannon–Wiener diversity index, Pielou evenness index and importance value of each species in the grassy matrix. The ratio of change was equal to (R_i-C)/C, where C was the biomass or species richness in the control plots, and Ri was the biomass or species richness in the different shrub coverage plots. The calculation of the importance value was based on the relative height, cover and frequency at each 20×20 m site. Combined with the root dry weight to be scanned, the specific root length (SRL), root tissue density (RTD), average root diameter and total root biomass were calculated. The SRL (cm/g) is equal to root length (Archer et al.)/ root dry weight (g), and the RTD (g/cm³) is the ratio of root dry weight (g) to root volume (cm³). Statistical analysis was based on SPSS (13.0). One-way ANOVA was used to test whether there was a significant difference between different shrub meadow (P=0.05).

Aboveground biomass change in community and functional groups

The total biomass of the alpine meadow community in the study area showed no difference (Fig. 1), while the functional group biomass changed significantly in meadow with different shrub coverages (p<0.05). We divided the community plants into four functional groups: forbs, grasses, sedges and leguminous. The biomass of grass and sedges decreased in the medium plots, and among them, the sedge biomass decreased the most (approximately 84%, P < 0.05). The biomass of leguminous (approximately 223%) and forb (approximately 59%) plants increased in the medium and high plots, respectively (P < 0.05).

Plant community composition and structure change

The Shannon-Wiener and Evenness indices showed similar change curves; both increased significantly in the medium plots, and the Evenness increased by approximately 39% (Fig. 2, P < 0.05). In contrast to the diversity index, the community richness index decreased significantly in the medium plots, with the richness of forbs and sedges showing the same downward trend (Fig. 3), while the number of leguminous species increased significantly. (P < 0.05).

Root trait response to shrubland coverage

As shown in Fig. 4, the total root biomass decreased significantly in the medium and high plots, especially in the 0-10 cm and 10-20 cm layer (P < 0.05). The variation trend of the total root length (TRL) was consistent with the root biomass, which decreased by approximately 19.5% in medium plots (Fig. 5). In contrast to the TRL, the SRL in the 10-20 cm layer increased significantly in the high shrub coverage plots (P < 0.05).

Soil physicochemical property

The physical and chemical properties of soil in SEGs were affected by different shrub coverages (Fig. 6). The results were as follows: the field capacity of 0-10 cm layer in the medium plots was significantly lower than that in the control (non- shrub) meadow, but increased significantly in the 20-40 cm layer (approximately 51%), while the variation trend of soil bulk density was completely different. The soil bulk density increased in the surface layer (0-20 cm) in low-, medium- and high- shrub- coverage plots, but decreased in the deep layer, respectively (P < 0.05). and the total porosity in each layer showed a decreasing trend in the medium- and high- shrub- coverage plots (P < 0.05).

As shown in Fig. 7, the STN concentration in the top 10 cm experienced a significant reduction in the medium plots, and the change in the AN concentration was consistent with this result (Fig. 7a, c), P < 0.05). The STN concentration of the 20-40 cm soil layer was significantly lower than that in the control meadow, while the AN concentration showed no difference in the low- and medium -shrub- coverage plots. The TP concentration in each layer decreased significantly in low-, medium- and high -shrub -coverage plots, while the AP concentration increased significantly (Fig. 7b, d), P < 0.05).

Community and functional group biomass

As important indicators of ecosystem productivity and system stability, biomass and diversity have always been the focus of research on SEGs (Zhang et al. 2006). Our study showed that the total community biomass of alpine meadow had no significant change in plot with different shrub coverage levels, while the biomass of grasses and sedges decreased significantly in the medium -shrub-encroached plots, which may be due to the reduction of available light in the lower layer of the herbaceous community caused by shrub encroachment. We observed that the average height of grass species increased significantly at the later stage of shrub development by approximately 1cm (P < 0.01), which may increase their competitiveness for light resources (Cornelissen et al. 2003). A previous study showed that the lowest light accounted for only 0.7% of the light in the upper layer, which reduced the coverage of grass and ANPP (Brantley and Young 2007; Lett and Knapp 2010). Research has shown that compared with temperature, the effect of light on the biomass of grass is more intense (MA 2016). Meanwhile, the utilization rate of N was affected by the reduction of light, which further inhibited the growth of Grass (Knapp and Lett 2003). However, the surface soil AN concentration was significantly reduced in the medium -shrub- coverage plots (Fig. 7), which intensified the restraint of grass. The results showed that the biomass of sedges decreased significantly, which may have been because sedges were greatly affected by drought, as a shallow root plant (Liu et al. 2018a). A previous study showed that there was a strong positive correlation between the sedge group and soil water content in the alpine meadow community (Liu et al. 2018b) (R² = 0.959, P < 0.01). We also found a significant positive correlation between the biomass of sedges and the soil surface field capacity water in plots different shrub coverage (P < 0.05). On the one hand, the high density of shrubs makes it difficult for water and nutrients to be preserved (Guan et al. 2016; Zhang and Zheng 2001). The competition for water and nutrients between the shrub and meadow patches caused meadow soil dryness and impoverishment between the shrub patches (Fig. 6) (Kessel et al. 2000; Smith et al. 2000). On the other hand, the root system was affected by the physical properties of the soil, and the high soil bulk density accompanied by low porosity (Fig. 6), and increased the mechanical resistance of root penetration, which was not conducive to root growth (Bréda et al. 1995). Our study showed that the TRL decreased in the plots with low and high shrub coverage levels (Fig. 5), which further inhibited the growth of sedges.

Our research demonstrated that shrub encroachment had a positive effect on the biomass of forbs. This result is probably because shrub encroachment significantly reduced the biomass and coverage of sedge group (Fig. 1, ESM_1. ), as the dominant species in the alpine meadow community (coverage was approximately 49%,P < 0.05), which reduced the shade density of sedges and the competition for light resources, releasing competition pressure on forbs and promoting the growth of forbs (Fahnestock and Knapp 1994; Ren QJ 2008). However, Some studies have shown that the changes in soil structure have different impacts on plant functional groups, the influence of soil bulk density on legumes and forbs is less than that on sedges (Zhang et al. 2016), Recent research further noted that only when the STN and water content are low (Figs. 6,7) was the effect of soil bulk density on forbs is weaker than that on sedges, which may be determined by the root characteristics of forbs (Liu et al. 2018a; Place et al. 2008). Under high soil bulk density (Fig. 6), forbs may produce a large number of branch roots and root hairs to improve their competitive advantage (RS 1979; Russell and Goss 1974). Because of the changes in soil structure, nutrients and moisture, forbs gradually invade the living space of sedge plants and thus increase their biomass (Helsen et al. 2016).

Our study found that the N:P ratio of soil, an indicator reflecting the nutritional status of plants, was approximately 4~6 in this study area; because this value was, less than 14, it indicates that the local plants suffer from N deficiency (Güsewell 2004, vol. ; Tessier and Raynal 2003),which may be due to the increase in AP (Fig. 7), promoting the absorption of N by plants and reducing the accumulation of nitrate nitrogen in the soil (Crews et al. 2000; Yuan et al. 2000). In contrast, the legumes obtain additional N by symbiosis with nitrogen-fixing bacteria and are therefore less limited by N than are non-legumes (spehn 2002)༌which is in accordance with Tilman's "resource ratio hypothesis". When legumes and grasses grow together, the increased phosphorus content makes the ratio of soil nitrogen to phosphorus decrease, which makes N becomes the limiting nutrient and legumes become the dominant species in N competition; thus, the biomass of legumes increased (Tilman 1982). However, this physiological characteristic may reduce its fitness (Voisin et al. 2002), thus affecting the quantity or quality of its offspring. Because the degrees and directions of biomass variation in functional groups within the community were different, the total biomass of the community showed no significant difference in the plots with different levels of coverage, and the mutual shift between the functional groups was also observed in the Qinghai Tibet Plateau (Liu et al. 2018a).

Community diversity

Species diversity is a comprehensive indicator of species richness and evenness, and also reflects the heterogeneity of community composition, structure and function (Ru WM 2006). The results showed that the community richness declined significantly in the medium -shrub- coverage plots, which was consistent with the conclusions of van Auken (Auken 2009). A study in southern Brazil further indicated that the reduction was mainly related to forbs (Guido et al. 2017). Our results also showed a high correlation between them (R²=0.925, P < 0.001). First, the soil TP content significantly decreased (Fig. 7), and our previous research showed that community richness was mainly controlled by the concentration of TP in 10-20 cm soil layer in this region (Zeng F 2021). This significant effect of phosphorus on species richness was also confirmed in the studies by Arnesen and Anic (Anic V 2010; Arnesen and EngelskjN 2007). Second, shrub encroachment has limited the resources of herbaceous plants, and the physiological characteristics of some miscellaneous grasses, such as low vegetation height and different specific leaf areas from grass plants, making them less competitive (Guido et al. 2017). Taking Geranium wilfordii Maxim as an example (ESM_2.), the importance value decreased significantly in the high -shrub -coverage plots (approximately 0.11, P < 0.05), and this value is generally used to evaluate the position and role of a species in the community. The higher the importance value is, the stronger the adaptability to resource utilization and the environment will be (Qian YF 2012).

The evenness index represents only the uniformity of each species distribution, and it is not related to the number of species (KP Ma 1995). The results showed that the evenness index significantly increased in the medium -shrub- coverage plots (Fig. 2). This result may be due to the coverage of the originally dominant plants (sedges) in the community was significantly decreased under the influence of shrub encroachment (ESM_1), thus creating more living space for other species and promoting the development of the community towards homogenization. Taking Polygonum viviparum and Ranunculus japonicus Thunb as examples (ESM_2), their importance values increased significantly in medium -shrub- coverage plots, with the largest amplitude of approximately 76%. This result may be because Polygonum viviparum has both asexual and sexual reproduction strategies, so it can better adapt to the changes in environmental resources caused by shrub encroachment (Fan and Yang 2009).

The Shannon- Weiner index serves as a combination of richness and evenness indices, based on this, the richness decreased less in the plots with different shrub coverages (approximately 13%), while the species distribution was relatively uniform, Thus, the diversity index increases significantly, which may be the effect of a shrub "Nurse", that is, the expansion of the dominant species of local vegetation is restricted by the shrubs of alpine meadow and has a protective effect on rare and non-dominant species (Holzapfel C 1999; Xu et al. 2010).

Root functional trait

As one of the key controlling factors of aboveground productivity, the morphology, distribution range and size directly determine the absorption and utilization ability of soil resources to plants, which plays an important role in the energy flow and material circulation of ecosystems. (Cheng et al. 2009; Jun et al. 2008; Zhang et al. 2000). Our study showed that the SRL of shallow roots and root N content increased significantly in the medium -and high- shrub- coverage plots (Figs. 5, ESM_3), which indicated that plants adopted resource acquisition strategy to achieve the rapid acquisition of water and nutrients (Laliberte and Etienne 2017). We found that the root biomass in the shallow layer decreased significantly in the medium- and high- shrub -coverage plots (Fig. 4), while 80% of the roots were distributed in the 0-20 cm soil layer (Fahey et al. 1988), which reflected the reduction of the investment in underground materials and energy (Zhang et al. 2000). A previous study showed that plants generally improve their competitive ability for nutrients or reduce nutrient losses by reducing the turnover of biomass to adapt to a malnourished environment (Berendse and Elberse 1990).

Soil physicochemical properties

Shrub encroachment can change the nutrient cycling of C, water use efficiency and pH, as well as greatly change the spatial layout of landscape resources (Eldridge et al. 2011; Myers-Smith et al. 2011). Soil bulk density and field capacity water showed completely opposite trends in the surface and deep layers of the soil, which may have been due to the deeper shrub roots improving the soil structure and making the deep soil more looser, which increases its water infiltration rate index (Howard et al. 2012); thus, the water content in the deep layer of the soil increased, while the bulk density decreased (Fig. 6). We found that the concentration of AP was significantly increased by shrub encroachment, as one of the essential elements for the normal growth of plants. Previous studies have shown that the concentrations of OP and AP vary greatly,and the latter mainly depends on the weathering of inorganic phosphorus and mineralization of organic phosphorus in soil (Denslow et al. 2010; Fitter 1999). On the one hand, the strong underground roots of shrubs could transport phosphorus from the deep soil to the surface (He 2017); thus, the concentration of organic phosphorus in the surface soil increased significantly (ESM_4), while the suitable hydrothermal environment under shrub patches accelerated microbial activities. which promoted the mineralization of organic phosphorus (Li et al. 2013; Turner B L 2003). In addition, the soil in the study area is acidic, containing more ferric phytate and aluminum salts, which easily precipitate and accumulate organic phosphorus (SH Zhao 2004). On the other hand, shrub encroachment significantly increased the concentrations of inorganic phosphorus forms (mainly Al-P), which were significantly related to the AP (RX Chen 1998), while the concentrations of potential phosphorus sources, such as O-P and Ca-P, decreased significantly (Table. 1). This result may have been due to unstable substances such as organic acids secreted by roots triggering soil decomposition (Kuzyakov and Blagodatskaya 2015; Mack et al. 2004; Sokol and Bradford 2019), and Gerke J also showed that organic acids could dissolve other insoluble phosphates, the general trend is to promote the transformation of inorganic phosphorus to available forms, which is difficult for plants to use in soil (Gerke and Jrg 1992).

The aboveground community biomass was generally stable in the plots with different shrub coverage levels. This stability was mainly due to the transformation of functional groups within the community, where forbs and legumes occupied a competitive advantage, with their biomass increasing significantly. The position and role of grass and sedge plants in the community decreased significantly due to the limitation of soil nutrients and their own physiological characteristics. The results showed that the species richness decreased significantly under the influence of soil TP concentration, while the evenness and diversity increased significantly, which indicated that the development of local dominant species was limited by shrub encroachment, and the community was developing in the direction of homogenization, with the forage quality decreasing. Root functional traits showed that the root system tended to adopt a resource acquisition strategy, while the root biomass decreased significantly in plots with different shrub coverages, indicating that the underground investment of plants decreased. The soil filed capacity and AN concentration decreased significantly, which suggested that shrub encroachment made the alpine meadow soil dry and aggravated the N limitation of alpine meadow soil. This limitation was mainly due to the transfer of OP from the deep layer to the surface through shrub roots, and this process caused more insoluble phosphate dissolve, which increased the availability of P in the soil, and then affected the absorption and utilization of N by vegetation.

Acknowledgement

This study was supported by the NSFC China Norway International Cooperation Project (41861134039), and the Gongga Mountain Ecosystem Observation Station of the Chinese Academy of Sciences provided experimental conditions.

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Shrub encroachment stabilizes aboveground biomass and enhances community diversity in alpine meadow of the Hengduan Mountain, Southwest China

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Abstract

Aims

Methods

Results

Conclusion

Figures

Introduction

Materials And Methods

Site description

Experimental design and field sampling

Laboratory analyses

Statistical analysis

Results

Aboveground biomass change in community and functional groups

Plant community composition and structure change

Root trait response to shrubland coverage

Soil physicochemical property

Discussion

Community and functional group biomass

Community diversity

Root functional trait

Soil physicochemical properties

Conclusion

Declarations

Acknowledgement

References

Supplementary Files

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