The Impact of Plant Diversity on Carbon Storage Along the Gradient of Altitude in Alpine Grasslands of the Qinghai–Tibetan Plateau

Abstract

Plant diversity plays an important role in shaping the carbon cycling of grassland ecosystems. However, differences in the diversity effect on aboveground and belowground carbon storage remains unclear at specific spatiotemporal scales. A transplant experiment was carried out along the gradient of altitude (4600–5200 m) in alpine grasslands of the Qinghai–Tibetan Plateau in the year 2017. Vegetation characteristics like species richness, vegetation cover and height were measured in the years 2020 and 2021. The plant diversity was described by species richness. Then, I calculated the vegetation biomass to quantify the aboveground carbon storage. The belowground carbon storage was represented by soil organic carbon. The results showed that the effect of species richness on belowground carbon storage was significantly positive (p < 0.05) at most altitudes for both years. However, the diversity effect on aboveground carbon storage was weak and inconsistent. The relationship between species richness and belowground carbon storage remained relatively stable over a period of two years. In contrast, the relationship altered greatly in terms of aboveground carbon storage in terms of inter-annual changes. Precipitation of growing seasons significantly impacted the species richness rather than aboveground carbon storage. Soil temperature was significantly associated with belowground carbon storage. These findings provide a new insight which will help us to assess the relationship between plant diversity and ecosystem functioning. They also allow us to understand how vegetation responds to climate change in high-altitudes areas.

1. Introduction

The causes and consequences of plant diversity are always important issues, especially in light of global warming and increasing climate extremes [1,2,3]. Carbon storage is an important component of ecosystem functioning and is closely associated with plant diversity [4,5,6,7]. The relationship between plant diversity and ecosystem functioning has long been debated, but no consistent conclusions have been reached [8,9,10,11]. Most studies support the idea that plant diversity has a positive effect on ecosystem functioning [12,13,14]. However, negative or no effects were also reported [8,15,16,17]. Various approaches, including theoretical models, manipulative experiments, and observations are often used to evaluate the biodiversity effect. Species were randomly set based on a local species pool for theoretical models and manipulative experiments. However, complex environmental contexts were not sufficiently considered using these two methods. Thereby, observational data provide a way to settle this debate.

Theoretical studies and manipulative experiments often assume similar environmental conditions and vegetation composition to assess the relationship between plant diversity and aboveground biomass. However, these studies were usually implemented at a small spatial scale (or plot level). Environmental factors altered substantially at spatiotemporal scales and strongly impact natural plant communities. Previous studies have tried to clarify the relationship between plant diversity and ecosystem functioning in different environmental scenarios [18,19,20]. However, community composition and species identities differ significantly among sites. Thus, the community structure may distort the interpretation of biodiversity’s effects on ecosystem functioning. However, the impact of the community composition on ecosystem functioning was often ignored at a large spatial scale [20]. Specific environmental change shapes corresponding community structure [21]. Transplant experiments provide a feasible way to ensure similar vegetation composition and species identity. In addition, the effect of plant diversity on ecosystem functioning was rarely tested in high-altitude areas, where vegetation tends to allocate biomass belowground to resist harsh environments. Soil carbon plays an important role in maintaining plant diversity and modulating carbon pools of terrestrial ecosystems [22]. High-elevation plants respond to climate change more conservatively than low-elevation ones [23]. In addition, plants growing at high altitudes are assumed to benefit rather than compete with each other based on the stress gradient hypothesis [24]. It is reported that the temporal scale, rather than the spatial scale, drives the effect of plant diversity on ecosystem functioning [25]. However, environmental effects induced by altitudinal differences may be stronger than those resulting from intra- or inter-annual scales in high-altitude areas.

The warming rate of the Qinghai–Tibetan Plateau (QTP) is roughly twice the global level. In addition, the warming rate at nighttime is higher than that during the daytime [26]. Thus, the plateau is very sensitive to climate change. In addition, the QTP is characterized by a large area of alpine meadow and a wide altitude span. As we know, air temperature decreases by roughly 0.6 °C with an increasing altitude of 100 m. The QTP experienced an enhanced air temperature of 0.3–0.4 °C per decade [27]. These provide an ideal experimental site to study the impact of environmental conditions on vegetation dynamic. The QTP harbors rich species diversity. Soil organic carbon decomposes very slowly due to low temperatures [28]. This study examined the relationship between species richness and aboveground/belowground carbon storage along the gradient of altitude in two years on the QTP and two hypotheses: (1) The relationship between species richness and aboveground carbon storage is stronger and the relationship with belowground carbon storage is weaker at lower altitudes; (2) the difference in the relationship between species richness and carbon storage along the gradient of altitude is larger than that at the inter-annual level. The following questions are addressed: (1) What is the relationship between species richness and aboveground or belowground carbon storage along the gradient of altitude in alpine meadow ecosystems? (2) Are there differences in the relationship between species richness and carbon storage in two consecutive years? (3) How do environmental factors impact species richness and carbon storage?

2. Materials and Methods

2.1. The Study Site

The research was carried out on the south slope of a mountain in Dangxiong County, Xizang Autonomous Region of China. The location was at 30°30′–30°32′ N, 91°03′ E, where experimental plots and a small meteorological station were established in 2006. The altitude ranges from 4600 m to 5400 m. The position of the grass line is roughly 5200 m. Based on the weather records of the local meteorological office, which were collected from 1963 to 2018, the average annual temperature is 1.9 °C. The mean annual precipitation is 473 mm, of which 90% falls from May to September. The dominant plant species is Kobresia pygmaea. Other common species include Kobresia humilis, Potentilla saundersiana, Thalictrum alpinum, Polygonum macrophyllum, Androsace tapete, and Saussurea spp. The vegetation cover ranges from 50% to 90%. The height of plants is generally lower than 3 cm. The landscape is characterized by highland mountains. The soil texture is mainly composed of loam and sandy loam. Most of the land in this region is used for livestock grazing, specifically for yak [29].

2.2. The Experimental Design

The experiment was carried out along the gradient of altitude ranging from 4600 m to 5200 m with an interval of 100 m in October of 2017. Thus, seven elevations were determined. Seven 20 m × 20 m plots were established with an enclosure. Onset HOBO automatic weather station was set in each elevation [29]. Data were recorded every 30 min and were stored by the ‘loggernet’ software. The measured indices include precipitation and 1.5 m aboveground air temperature (Table S1). Additionally, soil temperature and moisture at the depth of 5 cm were automatically recorded via a micro weather station [30]. To keep a similar community structure across the altitude, in the same year, a reciprocal transplanted experiment was conducted across seven elevations (Figure 1). At each elevation, forty-two soil blocks were excavated (length 0.7 m × width 0.7 m × height 0.4 m). Specifically, 6 soil blocks were transplanted to the original elevation as the control and 36 soil blocks on average were transplanted to the other 6 elevations. Each transplanted altitude obtained 6 soil blocks. Each soil block can be deemed as a repetition. That is, the number of repetitions is 48 (42 transplanted soil blocks + 6 situ soil blocks) for each altitude. The transplanted soil blocks included 36 soil blocks transplanted to the other six altitudes and 6 soil blocks transplanted to the same altitude. Downward transplantation denotes an increase in temperature and upward transplantation corresponds to decreasing temperature. A total of 294 soil blocks (7 elevations × 6 repetition × 7 transplantation) were transplanted across altitudes. The soil blocks were randomly arranged during the process of transplantation. The aim of transplanting the soil blocks to the original elevation was to test whether transplantation treatment itself impacts the plant communities of soil blocks.

A survey of plant communities was conducted in August of 2020 and 2021, when vegetation growth was relatively stable. Transplanted plants are assumed to acclimate to the local environmental conditions. Vegetation indices like the number, height and cover of plant species were measured through a sample box of 0.5 m × 0.5 m evenly divided into 100 grid cells. The sampling was guaranteed to be as central as possible to minimize edge effects, which may have affected species richness. The index species richness was used to represent plant diversity. The non-destructive vegetation volume was calculated based on the height and vegetation cover of plant species. The aboveground biomass was then estimated based on the empirical equation between the aboveground biomass and vegetation volume [29]. The aboveground biomass was used to indicate the aboveground carbon storage. Soil samples with a depth of 5 cm were collected from each soil block and the vegetation roots within the samples were removed. The soil organic carbon was measured using air-dried sieved soils [30]. The belowground carbon storage was mirrored by the soil organic carbon.

2.3. Statistical Analysis

The linear regression model was used to quantify the relationship between plant species richness and the AGB (or SOC). The R-square was used to mirror the explanatory power of the relationship between plant diversity and carbon storage. The statistical significance was presented as a p-value of 0.05. ANOVA was used to assess the effects of environmental factors on species richness and carbon storage. For the AGB, the precipitation and temperature of the growing seasons were considered. Soil moisture and temperature were selected in terms of the SOC. All environmental factors were assumed to affect species richness. All analyses were performed by R software (version 4.3.2) [31].

3. Results

3.1. The Relationship Between Species Richness and Carbon Storage Along the Gradient of Altitude

The species richness ranged from 3 to 15 across seven altitudes in the year 2020. At the altitudes 4800 m, 4900 m and 5200 m, species richness significantly increased the aboveground biomass (p < 0.05) (Figure 2). At an altitude of 4800 m, the absolute value of slope for the relationship between species richness and the aboveground biomass was the largest. Although a positive relationship existed at other altitudes, the relationship between plant diversity and aboveground carbon storage was insignificant (p > 0.05).

In terms of belowground carbon storage, soil organic carbon significantly increased with species richness at the altitudes 4800 m, 5000 m, 5100 m, and 5200 m (p < 0.05) (Figure 3). The explanatory power of species richness on soil organic carbon was higher at the altitudes 4800 m and 5100 m (R² = 0.18). In addition, the response of belowground carbon to plant diversity was strongest at an altitude of 5000 m (slope = 3). At other altitudes, the relationship was insignificantly positive (p > 0.05).

3.2. The Differences in the Diversity–Carbon Relationship over a Two-Year Period

The effect of species richness on aboveground biomass differed significantly between the years 2020 and 2021 (Figure 4a). The diversity effect was strong only at an altitude of 4600 m in the year 2021 (p < 0.05). In addition, the relationship between plant diversity and aboveground carbon storage changed from positive to negative in a comparison of the two years, although all negative relationships were statistically insignificant (p > 0.05). In contrast, the effect of species richness on soil organic carbon was very similar between 2020 and 2021, especially at high altitudes (Figure 4b). The relationship between plant diversity and belowground carbon was, in general, significantly positive in the year 2021, except at an altitude 4900 m.

3.3. The Effect of Environmental Factors on Species Richness and Plant–Soil Carbon

The precipitation and air temperature of growing seasons had little effect on the changes in aboveground biomass (Figure 5). Soil temperature strongly affected soil organic carbon. However, this effect did not exist in terms of soil moisture. Species richness was very responsive to precipitation, but very weakly responsive to other factors.

4. Discussion

4.1. The Effect of Plant Diversity on Aboveground and Belowground Carbon Storage Along the Gradient of Altitude in Two Years

• The results showed that the effect of species richness on aboveground biomass did not weaken as the altitude increased; similarly, it did not obviously improve the strength of soil organic carbon as hypothesized. In other words, the association between species richness and aboveground carbon storage did not become stronger and the effect on belowground carbon storage did not weaken as the altitude decreased. The reason for this might be that the initial altitude of 4600 m is very high. The altitudinal differences did not lead to a regular shift in the diverse effects on carbon storage. I found that the relationship between species richness and carbon storage was generally positive across the altitudes in the year 2020. This is consistent with most studies assessing biodiversity’s effects on ecosystem functioning in grasslands [22,32,33,34,35,36]. Further, the plant functional diversity was found be positively associated with aboveground and belowground biomass in semi-arid grassland [37]. According to the stress gradient hypothesis [38], various plant species tend to help each other to resist harsh environmental conditions, while individual plants are assumed to compete for available resources in more favorable habitats. The Qinghai–Tibetan Plateau (QTP) is characterized by an extreme environment. The complementarity between alpine plants may partly promote a positive diversity effect on carbon storage. In addition, the results showed that the slope of the diversity–carbon relationship was generally larger for the aboveground part than the belowground one. This is consistent with the findings of a meta-analysis [19]. This also suggests plant diversity more strongly shapes aboveground carbon storage. Vegetation composition, rather than plant diversity, was found to be a strong predictor of community productivity in grasslands [39]. Thus, I used the transplant experiment across altitudes to minimize the influences of plant community structure on the results. The transplant experiment can ensure that the vegetation composition of different altitudes is similar at the beginning of the research.
• It was found that the effect of species richness on aboveground biomass was significantly altered at the inter-annual level, while the relationship between species richness and soil organic carbon remained relatively stable (Figures S1 and S2). This indicates that the response of belowground biomass to plant diversity is more conservative than that of aboveground biomass. The role of belowground dimension should be highlighted in biomes where most biomass is allocated belowground. In addition, the results showed that five altitudes exhibited a similar relationship between species richness and soil organic carbon within a period of two years, while the association between species richness and aboveground biomass differed significantly in terms of inter-annual changes. Together with the results of altitudes, these indicate that the difference in the relationship between species richness and carbon storage along the gradient of altitude was no larger than that at the inter-annual level. The aboveground part of alpine ecosystems is often characterized by a low temperature and a large wind [40]. In particular, at high altitudes, the effect of plant diversity on belowground carbon storage was consistent within a two-year period. In addition, several studies assumed or have found that the positive effect of plant diversity on productivity escalates over time [5,41]. The increasing positive effect at the temporal scale is attributed to the accumulation of interspecific complementary and diverse plant ecological strategies. We found similar results in terms of the belowground part, but not the aboveground one. Certainly, I am aware of the fact that the experimental indices were measured for only two years, limiting the amount of data available. More experimental data are needed to clarify the regularity of inter-annual changes.

4.2. The Response of Plant Diversity and Carbon Storage to Environmental Factors

The results indicate that the relationship between species richness and aboveground biomass differs with that between plant diversity and soil organic carbon. Similarly, the response of the carbon storage to environmental factors also differed between aboveground and belowground areas. Specifically, soil temperature was found to strongly impact the changes in soil organic carbon. Warming reduced aboveground biomass but increased belowground biomass in alpine grasslands of the QTP [30]. However, the results showed that the precipitation and air temperature of growing seasons had little influence on the aboveground biomass. This result does not match the expectation that precipitation and temperature significantly influence variations in aboveground biomass in alpine grasslands. A high temperature is beneficial for aboveground vegetation growth under moist environmental conditions, while more biomass is allocated to belowground areas in dry habitats on the QTP [42]. Biomass changes increase with the mean annual precipitation difference between altitudes [43]. Since obtaining aboveground biomass in a destructive way was not allowed for the long-term experimental plots, I used the vegetation cover, height and empirical equations to calculate the aboveground biomass. Though these empirical equations have been rigorously validated by Wang et al. [29] at the same experimental sites, the calculated biomass differs from the reality to some extent. The species richness, rather than aboveground biomass, was found to be closely related with the precipitation of growing seasons. The occurrence of extreme precipitation and mean annual precipitation show an increasing variation in northern QTP [44]. The rate of climate change is reported to exceed the adaptation rate of vegetation [1]. This suggested that alpine plants need more time to modulate their own growth to deal with complex environments.

5. Conclusions

The relationship between species diversity and carbon storage is underexamined in regions characterized by high altitudes, whose ecosystems are very sensitive to global warming. Assessing this relationship is very crucial for understanding how alpine vegetation responds to or adapts to climatic conditions. Transplant experiments conducted across altitudes were used to minimize the influence of community composition on the relationship between species richness and carbon storage. The community structure was rarely considered at a relatively large spatial scale. The results clearly show that the effect of plant species richness on belowground carbon storage is generally positive across altitudes over a period of two years, while the relationship between species richness and aboveground carbon storage is inconsistent. It should be stated that an empirical equation was used to calculate aboveground vegetation biomass. In addition, more indices quantifying carbon storage and underlying mechanisms of the diversity–carbon relationship should be considered in future studies. In particular, the belowground aspects like soil physical–chemical factors and microorganism activities urgently need to be assessed. Accessing differences in diversity effects on aboveground and belowground carbon storage at specific spatiotemporal scales provides a new insight to help us understand the relationship between biodiversity and ecosystem functioning. This further highlights the strategies of alpine vegetation adapting to climate change in high-altitude regions.