Applying Specific Habitat Indicators to Study Asteraceae Species Diversity Patterns in Mountainous Area of Beijing, China

Abstract

The distribution pattern and influencing factors of specific species diversity play a crucial role in decision-making for biodiversity conservation. Identifying suitable regional habitat indicators to assess specific species diversity patterns is a global focus topic. A total of 112 sample plots were surveyed to investigate the relationship between Asteraceae species diversity and topography, soil nutrients, and stand factors, using a Structural Equation Model (SEM). Additionally, the Maxent model was utilized to predict the distribution pattern of Asteraceae species diversity in response to specific habitat factors. The findings revealed that soil nutrients, topography, and canopy closure had different impacts on Asteraceae species diversity, with soil nutrients showing the highest relative coefficient, followed by topography and canopy closure. The elevation and slope gradient were identified as direct and indirect influences on Asteraceae species diversity. The contribution rate of potential environmental variables on the Asteraceae species diversity was ranked as follows: STN (29.7%) > SOM (28.5%) > slope (8.5%) > Ele (8.1%). Asteraceae species diversity was found to be abundant in the locations with SOM (>27 g/kg), STN (>1.8 g/kg), Ele (165–333 m), and slopes (5–12 degrees). Soil nutrient content serves as a key indicator for assessing the abundance of Asteraceae species diversity and should be considered in biodiversity conservation.

1. Introduction

The Asteraceae family, holding great economic and medicinal importance, is the predominant family of angiosperms [1]. These plants are known for their resilience and ability to thrive in harsh conditions, playing a vital role in maintaining ecosystem stability [2]. With the intensification of global climate change and the impact of human activities, Asteraceae community ecosystems have been sharply reduced and their protection is facing unprecedented difficulties [3]. Therefore, analyzing the effects of site and stand factors on Asteraceae plant diversity can provide an important theoretical basis for guiding biodiversity conservation [4,5].

The presence and distribution of Asteraceae species are primarily influenced by various factors such as climate, soil nutrients, topography, stand density, and canopy closure [6,7]. Temperature and precipitation play a fundamental role in determining the growth and distribution of understory species on a large scale [8,9,10,11]. From the aspect of stand scale, the site condition has a decisive bearing on the growth and distribution of plant diversity [12,13,14]. Factors like altitude and slope gradient affect the water and nutrient requirements of these plants [15,16]. Soil nutrients, particularly nitrogen, organic matter, and phosphorus, also significantly affect the growth of plants [17,18]. In the current research on understory species diversity, which mainly concerns the effects of environmental factors and stand structure on species composition and diversity [19,20,21], the studies have seldom considered Asteraceae plant diversity; however, existing studies have indicated that Asteraceae plants are strong indicators of site conditions [22].

The majority of the research utilized statistical analysis techniques to examine the correlation between understory plant diversity and environmental factors; however, these methods hardly determine the suitable distribution range of plant diversity accurately, despite a higher abundance diversity index [23,24,25]. Therefore, there is a crucial need for finding a suitable approach to quantify the range of plant diversity. Maximum entropy models offer significant advantages in forecasting species distribution and have become widely utilized in species prediction [26,27,28].

The main goals of our research are to (1) analyze the primary habitat factors that influence Asteraceae species diversity, (2) determine the direct and indirect effects of habitat factors on Asteraceae species diversity, and (3) predict the distribution range of abundant Asteraceae species diversity.

2. Materials and Methods

2.1. Study Sites

The Beijing–Tianjin Sandstorm Source Phase II Forestry Project in Beijing is mainly distributed in its mountainous areas (39°12′–41°05′ N, 115°25′–117°30′ E) and covers about 10,400 km². The areas belong to a warm temperate semi-humid monsoon climate, with an average annual precipitation from 470 to 660 mm and an average yearly temperature from 11 to 13 °C. Land use types consist of woodland and grassland. Zonal soils are mainly composed of meadow soil, mountainous brown forest soil, and mountainous cinnamon soil.

2.2. Field Investigation and Soil Sampling

Based on the Asteraceae species distribution data provided by existing studies [1], a total of 112 representative Platycladus orientalis plantations’ sample sites (20 m × 20 m) were selected for investigation (as shown in Figure 1). The following variables were measured at the sample plots: stand density, canopy closure, elevation, aspect, slope, and geographic coordinates [15]. Ten soil samples were randomly collected from the 0–10 cm soil layer using a standard soil sampler with a 100 cm³ capacity, with the aim of measuring the basic physical and chemical properties of the soil [29].

We randomly laid out ten 1 m × 1 m samples in each standard sample plot [22], and the names of the plants and the number of plants of each species in each sample were recorded for calculating the understory diversity; the environmental and stand factors are shown in

Table 1. Environmental and stand factors data of standard plots.

Environmental and Stand Factors	Ranges	Mean	Standard Deviation
Elevation (m)	114–770	359.69	116.12
Slope (°)	3–39	18.81	10.02
Soil organic matte content (g/kg)	13.84–116.58	44.68	21.35
Soil total nitrogen content (g/kg)	0.98–6.58	2.97	1.25
Soil total phosphorus content (g/kg)	0.24–1.12	0.57	0.15
Soil total potassium content (g/kg)	9.25–40.72	20.25	6.06
Soil available nitrogen content (mg/kg)	91.57–415.24	135.39	80.62
Soil available phosphorus content (mg/kg)	0.09–4.005	6.91	1.59
Soil available potassium content (mg/kg)	37.533–444.82	1.01	0.34
Stand density (plant·hm−2)	500–1975	1035.18	286.71
Canopy closure	0.5–0.95	0.76	0.11

.

2.3. Data Analysis and Modeling

2.3.1. Asteraceae Diversity Index Calculation

In this study, we calculated a total of four Asteraceae species diversity indices. The formula for calculating diversity is as follows:

$$\text{Shannon–Wiener index }H={\sum }_{i=1}^s\ln P_i\times P_i$$

(1)

$$\mathrm{Simpson}\mathrm{index}SD=1-{\sum }_{i=1}^s{p}_i^2$$

(2)

$$\mathrm{Margalef}\mathrm{index}H=(S-1)/\ln N$$

(3)

$$\mathrm{Gleason}\mathrm{index}D={\displaystyle \frac{S}{\ln A}}$$

(4)

where S is the total number of understory herbaceous species; pi is the ratio of the number of individuals of the i_th species to the total number of individuals of all species; N is the total number of herbaceous individuals in the understory.

2.3.2. Maxent Model

The maximum entropy model is a spatial distribution model of the species at the geographical scale [30]. It is based on the maximum entropy theory, and is applied to measure the maximum likelihood of a sample [31]. The objective of the model is to infer the location information through incomplete information and to identify the conditions under which the probability reaches its maximum [32]. The accuracy of the model was verified by using the receiver operating characteristic curve (ROC), and the accuracy was judged by calculating the area (AUC) enclosed by the curve and the abscissa [33,34]. Two sets of data are required for the operation of the maximum entropy model: one is the geographical distribution data of Asteraceae species, which are expressed by the latitude and longitude of each species; the second is the raster data of environmental factors in the study area. The elevation of Beijing is derived from the 30 m × 30 m digital elevation model (DEM), the aspect and slope gradient are extracted from the DEM (http://westdc.westgis.ac.cn, accessed on 12 March 2024), and the soil nutrients data of the study sites are modified by the measured soil nutrients data by means of Kriging interpolation.

3. Results

3.1. The Relationship between the Asteraceae Species Diversity Indices

The correlation of these diversity indices is shown in Figure 2. The results demonstrate that the Shannon–Wiener index had the highest correlation with the other three indices, with an R² of 0.87, 0.74, and 0.70. After a comprehensive comparison of the correlations of these indices, we chose the Shannon–Wiener index to measure the overall level of Asteraceae species diversity.

3.2. Effects of Environmental Factors on Asteraceae Species Diversity

The direct and indirect effects of elevation, slope, and soil nutrient factors on Asteraceae diversity are shown in Figure 3. The soil nutrients had a significant effect on the Asteraceae species diversity (p < 0.01), and the elevation and slope significantly affected the Asteraceae species diversity (p < 0.05). The standardized effects of these factors on the Asteraceae species diversity are shown in

Table 2. Standardized effects of these factors on the understory Asteraceae diversity.

Impact Factors	Direct Effect	Indirect Effect	Total Effect
Elevation	−0.17 *	−0.06	−0.24 *
Slope	−0.21 *	−0.02	−0.23 *
Soil nutrients	0.54 **		0.54 **

Notes: * p < 0.05, ** p < 0.01.

. The direct impact coefficients were soil nutrients (0.54), slope (−0.21), and elevation (−0.17); the indirect impact coefficients were the elevation (−0.06) and slope (−0.02). Overall, soil nutrients were the dominant factors affecting Asteraceae species diversity, with the direct effects being the primary mechanism that dictated diversity. The elevation and slope also had important impacts on Asteraceae species diversity.

3.3. Maximum Entropy Model Results

In this study, the Shannon–Wiener diversity index ranged from 0.27 to 2.01, the average value was 1.01, a screening value equal to the average Asteraceae species diversity at the sample sites was determined, and those values beyond this screening value were considered in the sample plots with abundant Asteraceae species diversity. The ROC curves as shown in Figure 4.

The contribution rate of potential environmental variables on the Asteraceae species diversity was ranked as follows: soil total nitrogen (29.7%) > soil organic matter (28.5%) > slope (8.5%) > elevation (8.1%), and the cumulative contribution rate was 74.8% (

Table 3. Contribution and cumulative contributions of environmental factors to the Asteraceae diversity.

Variable Factors	Contribution Rate (%)	Cumulative Contribution Rate (%)
Soil total nitrogen	29.7	29.7
Soil organic matter	28.5	58.2
Slope	8.5	66.7
Elevation	8.1	74.8
Soil available nitrogen	4.9	79.7
Aspect	4.7	84.4

). The contribution rate of other environmental variables was between 3.5% and 4.9%, which had little impact on Asteraceae species diversity. The above analysis shows that the main environmental variables affecting the Asteraceae species diversity were soil nutrient factors (soil organic matter and soil total nitrogen), of which the cumulative contribution rate was 58.2%. Taking the probability of existence greater than 0.7 as the threshold value, the contribution rate of STN was the highest among the soil factors, reaching 29.7%; when the STN was greater than 1.8 g/kg, the probability of existence was greater than 0.7. The contribution rate of SOM in the soil variables ranked second, which was 28.5%. It can be seen from Figure 5 that when the SOM was greater than 27 g/kg, the probability of existence was greater than 0.7. Obviously, this was suitable for the distribution of the abundant Asteraceae species diversity level when the total nitrogen content of soil was more than 1.8 g/kg. Moreover, the probability distribution remains the largest when it reaches 2 g/kg. When the elevation ranges from 165 to 333 m, the probability of existence was greater than 0.7 (Figure 5). The probability of existence reaches the peak at about 230 m, indicating that low mountain areas were the most suitable for the Asteraceae species growth; when the elevation was less than 100 m or more than 500 m, the probability of existence decreases, indicating that these two elevations were not conducive to their distribution. The contribution rate of the slope degree to Asteraceae diversity was 8.1%; when the slope range was 5 to 12°, the probability of existence was greater than 0.7 (Figure 5). The probability of existence reached the peak value when the slope degree was about 8 degrees, indicating that the gentle slope area was most suitable for their distribution; when the slope degree was less than 2 degrees or more than 22 degrees, there was a rapid decline in the probability, indicating that the two slopes were not conducive to their distribution.

4. Discussion

This study found that the soil organic matter and soil total nitrogen content were the dominant soil factors affecting the Asteraceae species diversity, with a cumulative contribution rate of 58.4%. The main reason was that soil organic matter contains various nutrients required for plant growth, which are some of the main sources of vegetation nutrition and can promote the growth and development of plants [35,36]. A large amount of nitrogen is necessary for plant growth; its abundance, shortage, and supply directly affect the growth level of plants [37,38]. When the soil organic matter content was less than 27 g/kg and the total nitrogen content was less than 1.8 g/kg, the probability of the existence of abundant Asteraceae species diversity decreased rapidly. An increase in soil nutrients such as soil organic matter and nitrogen will increase the competition to occupy ecological niches, which promotes the growth of Asteraceae plants [39,40,41]. Therefore, for the land with a soil organic matter content less than 27 g/kg and a soil total nitrogen content less than 1.8 g/kg in the Beijing Mountain area, a reasonable application of organic fertilizer and nitrogen fertilizer is conducive to increasing the Asteraceae species diversity.

Topographic factors such as the altitude and slope have a vital influence on species distribution, mainly because they are the fundamental factors affecting energy and material variation at the site scale [42,43]. The present study reveals that topographic factors, specifically elevation and slope, exert an impact on the diversity of Asteraceae species, accounting for a cumulative contribution rate of 16.6%. Soil nutrient conditions are better at medium altitudes and on gentle slopes, which provide favorable conditions for the growth and reproduction of Asteraceae plants [44,45]. Within the forest, canopy closure determines the effective light intensity, being able to redistribute light, heat, water, fertilizer, and other resources, thus directly affecting understory herbaceous diversity [46]. However, the canopy close variable has no significant effect on the Asteraceae species diversity in this study.

5. Conclusions

Asteraceae plants have a rich species diversity and strong environmental adaptability; they play a key role in maintaining ecological balance in the mountainous areas of Beijing. However, the response of Asteraceae species diversity to specific habitat factors has been rarely explored. In this study, we utilized a structural equation model and a maximum entropy model to assess the effects of habitat factors on Asteraceae diversity. The results show that soil organic matter and total nitrogen dominated the Asteraceae species diversity, while slope and elevation factors had a significant effect on Asteraceae plants diversity. The effects of canopy closure and stand density on the diversity of Asteraceae were not significant. Asteraceae species diversity was abundant at the location with SOM (>27 g/kg), STN (>1.8 g/kg), Ele (165–333 m), and slopes (5–12 degrees). Soil organic matter and total nitrogen content serve as key indicators for assessing the abundance of Asteraceae species diversity. Our study demonstrated that it is advisable to apply organic fertilizer and nitrogen fertilizer to improve the Asteraceae species diversity in the sites where soil organic matter content and total nitrogen content are less than 27 g/kg and 1.5 g/kg respectively.