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Article

Effect of Geographical Conditions on Moss–Soil Crust Restoration on Cut Rock Slopes in a Mountainous Area in Western Sichuan, China

by
Wanqiu Pu
1,2,
Maoqiang Zhao
2,3,
Jie Du
4,
Yongyao Liu
5 and
Chengmin Huang
2,*
1
Institute for Disaster Management and Reconstruction, Sichuan University-The Hongkong Polytechnic University, Chengdu 610065, China
2
Department of Environmental Science and Engineering, Sichuan University, Chengdu 610065, China
3
Sichuan Institute of Geological Engineering, Investigation Group Corporation Limited, Chengdu 610072, China
4
Jiuzhaigou Administration Bureau, Jiuzhaigou 623402, China
5
The IT Electronics Eleventh Design and Research Institute, Scientific and Technological Engineering Corporation Limited, Chengdu 610021, China
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(3), 1990; https://doi.org/10.3390/su15031990
Submission received: 15 December 2022 / Revised: 4 January 2023 / Accepted: 18 January 2023 / Published: 20 January 2023
(This article belongs to the Special Issue Advances/Developments in Sustainable Materials Engineering)
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Abstract

:
Ecological restoration has great significance on cut rock slopes, which are considered extremely degraded habitats. The development of moss–soil crusts on cut rock slopes as a critical pathway to ecological restoration in mountain areas has been poorly reported. A total of 335 quadrats were selected on cut rock slopes with formation ages between 0 and 60 years to evaluate the evolution characteristics of moss–soil crusts under various geographical conditions (e.g., aspect, lithology, and altitude) in the mountainous area of western Sichuan, Southwest China. The results suggested that moss growth decoupled from soil accumulation within the crusts and was controlled by multiple factors. Moss growth depended on lithology, altitude, and age, while soil weight was mainly influenced by slope aspect. The development of mosses on limestone was better than on sandstone. Moss biomass varied with altitude, consistent with that of rainfall with respect to moss development dependent on moisture. Furthermore, moss development under a semiarid climate was more distinctly impacted by moisture with altitude relative to a humid region, likely owing to the higher sensitivity of the mosses to moisture in the former than in the latter. Moss biomass increased with recovery time, while the rate of moss biomass development was diverse in different geographical areas. The vegetation developed rapidly in low-altitude areas (~1000 m above sea level), resulting in moss biomass increasing from 0 to 24.08 g·m−2 with formation time increasing from 0.5 to 1.5 years and subsequently being restricted by the evolution of higher plants on cut rock slopes, leading to an insignificant difference in moss biomass between 1.5 and 60 years. In high-altitude areas, when the altitude changed slightly (from 2024 to 2430 m above sea level), the moss biomass on cut rock slopes increased linearly with increasing age from 5 to 27 years. Influenced by the surrounding fertile soils and moss bioaccumulation, there were high levels of soil major nutrient content, especially the organic matter content, which reached 377.42 g·kg−1. More soils accumulated on south-facing slopes than on north-facing slopes. This study provided field data to clearly reveal the influence of geographic factors on moss–soil crust development in natural restoration processes in high-altitude mountainous areas.

1. Introduction

A large number of steep rocky slopes have been formed due to the construction of roads (e.g., railways and highways), dams, and mining facilities [1]. On the high and steep cut rock slopes, the vegetation–soil system was partially or completely destroyed, and as a resultant, abominable habitats were produced [1,2,3].
To rehabilitate areas suffering degradation, technologies such as hydroseeding, external soil spray seeding, and erosion control blankets have been widely used [4]. However, most of these technologies were manually recovered using geotechnical techniques, and less consideration was given to natural recovery [5]. Natural restoration was more important in mountainous areas with inconvenient traffic conditions, given the high cost and technical content, large quantities required, potential for secondary damage, and other problems of manual restoration [6,7]. It has been claimed that natural restoration could be more efficient than human efforts in restoring degraded land and vegetation conditions [8,9].
Recently, the natural restoration of cut rock slopes has received increasing attention. Numerous studies have demonstrated soil nutrient accumulation and vegetation community succession following the implementation of ecological restoration programs on cut rock slopes [2,10,11]. Meanwhile, the influence of environmental factors, such as aspect, gradient, and time, on the natural restoration of slope vegetation was discussed. For example, soil properties, e.g., the moisture and organic matter contents of soils on north-facing slopes, are significantly higher than those on south-facing slopes [10,12]. However, these studies have mainly concentrated on low- and middle-altitude mountain areas, and cold and high-altitude regions—where the germination and growth of vegetation are limited by low temperature, insufficient water, and nutrients—have rarely been reported [13].
In alpine and subalpine areas, the natural succession of cut rock slopes is close to the primary succession [14]. As the pioneer species of primary succession and the dominant species of biological soil crusts, mosses play an extremely important role in degraded habitats [15,16]. Moss–soil crusts are an organic complex resulting from an intimate association between soil particles and cyanobacteria, algae, microfungi, lichens, and mosses, which are the most advanced stage of biological soil crusts [17,18,19]. Due to their extensive adaptability and strong reproductive capacity, mosses can survive in grassland, cultivated land, northern forest, mountain rainforest, alpine environments, tundra, desert, and other habitats [20]. The development of mosses in different habitats is heterogeneous in species composition, growth rate, and community structure (coverage and biomass) [21,22,23,24]. Many factors could be responsible for the difference, such as slope aspect, elevation, and time, which have been shown to significantly affect moss development [24,25]. Previous studies have shown that moss is of great significance in promoting restoration of cut rock slopes, which is consistent with the periodic law of vegetation succession and conducive to long-term sustainable development [26]. Similarly, the dominant species and development degree of moss crusts would be different on slopes with different terrain, climate, and other environmental conditions [27]. Nevertheless, the basic research of moss–soil crusts on cut rock slopes—particularly in alpine areas, where the mosses are the main component of ground cover plants—is still lacking.
Large mountainous areas are distributed in China, accounting for two-thirds of the national territory, particularly in western China [28]. The mountainous area in western Sichuan, Southwest China is an ideal place to study natural ecological restoration, as many exposed cut rock slopes exist due to engineering construction and it is difficult to implement manual restoration on the steep slopes of high mountains. Furthermore, the varied natural geographic features (e.g., climate, lithology, terrain, etc.) in this area might be beneficial to explore the geographical effect on moss–soil crust restoration. In this paper, moss coverage and biomass, soil weight, and major nutrient properties were investigated during the restoration of moss–soil crusts on cut rock slopes in western Sichuan to understand the impact of aspect, elevation, lithology, and time on restoration. The aims of this study are to (1) analyze the features of moss–soil crusts on cut rock slopes under different geographical conditions and (2) address the influence of geographical factors on the development of moss–soil crusts on cut rock slopes in alpine and subalpine areas to provide a scientific basis for the implementation of ecological restoration on cut rock slopes in mountainous regions.

2. Materials and Methods

2.1. Study Sites

This study was carried out in western Sichuan, the hinterland of Southwest China (26°03′–34°19′ N, 97°21′–108°12′ E) (Figure 1). Since different geographic locations may have different environmental conditions, a general research area related to climatic differences was defined according to geographic location within the study area: (1) In the Baoxing sampling area, the altitude ranges from 1942 to 4166 m above sea level (a.s.l.). The subtropical–cold temperate humid monsoon climate prevails in this area, with a mean annual temperature (MAT) of less than 10 °C and a mean annual precipitation (MAP) of less than 1000 mm [29]. (2) In the Pengzhou–Shifang sampling area, the altitude lies within ~900 and 1200 m a.s.l. The subtropical humid monsoon climate controls this area, with MAT of 12–14 °C and MAP of 1000 mm or higher. (3) In the Wenchuan–Lixian sampling area, the altitude is between ~2000 and 2500 m a.s.l. This area is dominated by a warm temperate continental semiarid monsoon climate, with a MAT of 6–10 °C and MAP of 540–610 mm [30]. Due to a wide range of altitude differences and complex terrains, the climate obviously varies with increasing elevation in all sample areas. The altitude, rainfall, and other environmental factors in each sampling area are shown in Table 1. In the study area, a large area of cut rock slopes have been produced by the intensive and extensive construction of transportation and mining projects, characterized by high and steep slopes with instability.

2.2. Field and Laboratory Methodology

2.2.1. Sample Collection

After a pilot investigation of cut rock slopes in western Sichuan between October 2017 and October 2018, a total of 335 quadrats on 35 cut rock slopes were chosen for further detailed study (Figure 1), of which 250, 49, and 36 quadrats were surveyed in the Baoxing, Pengzhou–Shifang and Wenchuan–Lixian areas, respectively (Table 2). On each cut rock slope, the surrounding environment was investigated, and the basic conditions of the cut rock slopes were also recorded, e.g., geographic coordinates, elevation, aspect, and height. The formation age of the cut rock slopes was obtained through data review and field investigation. This duration may also be regarded as the restoration age of those slopes because the natural rebuild process started when the slopes were formed and human disturbance ended.
Within each cut rock slope, quadrats with an area of 20 cm × 20 cm were established, and the number of quadrats was determined by the size of the cut rock slopes. Before sample collection, each quadrat was imaged using a camera to estimate the area of the crusts. The samples of moss–soil crusts were obtained with sterilized tweezers and collecting knives to separate the crusts from the rock surface. The dead branches and fallen leaves on the surface of the crusts were picked out, while soil samples were separated from mosses in the crusts. After collection, these samples were put into polyethylene bags and then immediately transferred to the laboratory for sample processing.

2.2.2. Preparation and Analysis of Samples

ArcGIS10.2 and ENVI5.3 software were used to extract the moss–soil crust coverage from the images for each quadrat, utilizing a supervised fuzzy clustering approach and visual interpretation [31]. Moss biomass in the crusts was determined by the sieve washing method. After cleaning with water, the mosses were dried to a constant weight at 65 °C for 48 h, and the dried mosses were weighed with a balance (model PTX-FA110) to calculate the moss biomass per unit area.
The air-dried soil samples, after removing plant residue, were crushed and ground in an agate mortar. The soils were passed through a 100-mesh sieve and preserved for use. More than 50% of the soil samples in the quadrats in this study were less than 10 g in weight. The contents of soil organic matter (SOM), total nitrogen (TN), and total phosphorus (TP) were measured by the potassium dichromate oxidation method, semimicro-Kjeldahl method, and sodium hydroxide melting–molybdenum antimony colorimetric method [32]. Soil TN and TP in 132 and 75 quadrats could not be determined due to the lack of soils.
The soil nutrient stock (SNS) (g·m−2) is the product of soil weight and nutrient content. Therefore, the SNT can be defined as:
SNSi= (Ci × SWi)/S
where Ci, SWi, S, and i represent the nutrient content in the investigated soils (g·kg−1), the soil weight in each quadrat (kg), the sampling area (0.04 m2), and the measured nutrient, respectively.

2.2.3. Statistical Analysis

Correlation coefficient analysis was used to estimate the relationship between moss and soil characteristics. Multivariate analysis of variance was used to test the significance of variations in moss biomass and soil major nutrients under different lithologies and slope aspects. Principal component analysis (PCA) was used to reduce the dimension of recovery feature indicators of moss–soil crusts. There were several quadrats at each sample point, and other geographical factors were fixed when discussing the influence of factor. The data we tested are normally distributed. One-way analysis of variance (ANOVA) was used to test the significance of variations in moss biomass and soil major nutrients under different elevations and formation ages. The criteria for significance in the procedures were set at p < 0.05 (significance) and p < 0.01 (high significance). IBM SPSS 16.0 was used for statistical analysis, and Microsoft Excel 2010 and Origin 2018 were used for data editing and visualization.

3. Results

3.1. Coverage and Biomass of Moss–Soil Crusts on Cut Rock Slopes

Across the study area, moss coverage of the crusts fluctuated greatly, ranging from 0 to 90% (Table 3). Moss coverage at the sampling sites in the Baoxing area ranged from 0.42% to 83.79%, with a mean value of 24.12%, and more than 60% of quadrats had less than the mean value. Only at two sample sites (HDZ1.1 and HDZ2) with a formation age of 27 years was the average moss–soil crust coverage greater than 50%. Moss coverage at the sampling sites in the Pengzhou–Shifang area varied from 0 to 66.91%, with a mean value of 15.64%. The maximum coverage appeared on the cut rock slope with a formation age higher than 60 years (Site LMSZ1), and the cut rock slope with a restoration age lower than 0.5 years had almost no moss crusts (Site YHSZ2). Moreover, moss crusts in the Wenchuan–Lixian area varied between 5% and 90%.
The biomass of moss crusts in the study areas also changed dramatically, with a standard deviation greater than the average value (Table 3). Especially in the Baoxing area, the moss biomass value varied greatly within a range of 0.25 to 888.50 g·m−2. The mean value of biomass in the Pengzhou–Shifang area was 28.48 g·m−2, with values between 0 and 210.44 g·m−2. Consistent with the coverage, the maximum biomass value appeared on the same cut rock slopes, which were distributed at the sites (HDZ1.1 and HDZ2) with a restoration age of 27 years in the Baoxing area and at the site (LMSZ1) with a restoration age of more than 60 years in the Pengzhou–Shifang area. In the Wenchuan–Lixian area, the biomass value oscillated from 0.48 to 220.00 g·m−2, with an average of 43.62 g·m−2. Moss coverage was highly positively correlated with moss biomass in all quadrats (p < 0.01, r = 0.502, n = 335, Figure 2).

3.2. Soil Weight within Moss–Soil Crusts on Cut Rock Slopes

Due to human disturbance and relatively shorter weathering times (less than 100 years), cut rock slopes commonly lack soils derived from rock weathering [33]. In this study, the soil weight in moss–soil crusts was low, varying within 0 and 4.79 kg·m−2 (Table 4). Six percent of the quadrats were observed without soil in the crusts. The soil weights in the Baoxing, Pengzhou–Shifang, and Wenchuan–Lixian areas ranged from 0 to 4.77 kg·m−2, 0 to 4.51 kg·m−2, and 0.02 to 4.79 kg·m−2, respectively. The standard deviation was greater than or close to the average value, indicating the uneven distribution of soils on cut rock slopes. Soil weight was highly positively correlated with moss coverage (p < 0.01, r = 0.218, Table 5) and moss biomass (p < 0.01, r = 0.188, Table 5). Combined with the correlation between moss coverage and biomass, the results showed interaction between the mosses and soils which jointly promoted the development of the moss–soil crusts.

3.3. Soil Major Nutrient Contents within Moss–Soil Crusts

As shown in Table 4, the content of the soil’s major nutrients differed broadly. The SOM content in the Baoxing area ranged from 5.24 to 377.42 g·kg−1, with an average of 86.32 g·kg−1. The SOM content in the Pengzhou–Shifang area ranged from 10.59 to 206.84 g·kg−1. In addition, the SOM content in the Wenchuan–Lixian area ranged between 7.7 and 129.84 g·kg−1.
The mean TN contents in soils within the crusts sampled from the Baoxing, Pengzhou–Shifang, and Wenchuan–Lixian areas were 1.69, 0.76, and 0.84 g·kg−1, respectively (Table 4). Specifically, the maximum TN content appeared on the cut rock slopes with the longest formation ages, which were 30, 60, and 49 years in the Baoxing, Pengzhou–Shifang, Wenchuan–Lixian regions, respectively. The TN content was highly positively correlated with the SOM content (p < 0.01, r = 0.478, Table 5).
The TP content in the Baoxing, Pengzhou–Shifang, and Wenchuan–Lixian areas ranged from 0.02 to 5.05 g·kg−1, 0.15 to 1.37 g·kg−1, and 0.41 to 1.50 g·kg−1, respectively (Figure 4).

3.4. Major Soil Nutrient Stocks within Moss–Soil Crusts

The SOM stocks within the crusts varied broadly (Table 4). Particularly in the Baoxing area, the stocks of SOM were between 0.04 and 361.17 g·m−2, with an average of 24.87 g·kg−1. The mean values of SOM stocks in the Pengzhou–Shifang and Wenchuan–Lixian areas were 96.45 and 47.49 g·m−2, respectively. The maximum stocks also appeared on the cut rock slopes with the longest formation ages, which were 30 and 60 years in the Baoxing and Pengzhou–Shifang areas, respectively. In particular, the maximum SOM stock in the Wenchuan–Lixian area occurred on the cut rock slope with abnormally high soil weight at the site DSE3.
The soil N stocks in the Baoxing, Pengzhou–Shifang, and Wenchuan–Lixian areas ranged from 0.01 to 5.05 g·m−2, 0.34 to 19.92 g·m−2, and 0.07 to 17.60 g·m−2, respectively (Table 4). There was an insignificant difference in soil N stocks between the sampling sites in different areas, except for the two sites (i.e., QQH4 in Baoxing and DSE3 in Wenchuan–Lixian). The differences at these two sites resulted from the abnormal TN content or soil weight. The N stocks were highly positively correlated with the soil weight (p < 0.01, Table 5).
The soil P stocks in the Baoxing, Pengzhou–Shifang, and Wenchuan–Lixian areas ranged from 0.01 to 6.65 g·m−2, 0.13 to 4.10 g·m−2, and 0.02 to 3.51 g·m−2, respectively (Table 4). There was no significant difference in soil P stocks between sampling sites in different regions, except for one site (DSE3) in the Wenchuan–Lixian area, which was related to the high soil weight.

3.5. Principal Component Analysis (PCA)

The PCA results for the nine characteristics of naturally restored moss–soil crusts are shown in Table 6. The contributions of principal components PC1, PC2, PC3, and PC4 to the variation were 42.75%, 18.25%, 13.85%, and 12.83%, respectively, accounting for 87.7% of the total variation value, indicating that these four principal components could explain the majority of the variation in the ecological restoration characteristics of moss–soil crusts. The variables with the highest loading in PC1 were soil weight, SOM stock, P stock, and N stock. Soil weight had a higher loading score in PC1, and was selected as the single indicator of PC1, as all the other preselected variables were related to it (p < 0.01, Table 5). For PC2 the selected indicator was TP content. In PC3 the variables, e.g., SOM and TN contents, showed the highest loadings on this axis (Table 6). TN content was chosen as the indicator since it was significantly correlated with the other preselected variables in PC3 (p < 0.01, r = 0.478, Table 5) and had the highest loading. Finally, in PC4 moss coverage and moss biomass had the highest loading, and moss biomass was selected as the indicator due to its significant correlation with moss coverage (p < 0.01, r = 0.502, Figure 2, Table 5) and the higher loading in the PCA.

4. Discussion

Geographical variables, such as lithotype, microclimate, rainfall, and time—of which, microclimatic factors, such as temperature and humidity, are more relevant than others—exert substantial effects on the development of mosses [24,34,35]. In particular, humidity is a key factor in moss development since water availability is an important component of the key ecophysiological processes in mosses [36]. Given that the environmental variables in the field are difficult to define, the elevation and slope aspect—which are readily quantitatively characterized—might represent the changes in climate factors, such as humidity and temperature. Due to the strong correlation between many environmental variables, we try to fix other geographical factors, in addition to the factors discussed, to better distinguish the influence of factors. Here, we would discuss the effect of elevation, slope aspect, lithology, and formation age on the evolution of moss–soil crusts.

4.1. Influence of Aspect and Lithology on the Natural Restoration of Moss–Soil Crusts on Cut Rock Slopes

An insignificant influence of slope aspect on moss biomass was observed in this study (Figure 3A), although higher moss biomass appeared on north-facing slopes (91.19 ± 136.90 g·m−2) than on south-facing slopes (64.78 ± 86.34 g·m−2), consistent with a previous report [37]. However, the aspect of cut rock slopes had a strong influence on soil weight. The soil weight within the crusts on south-facing slopes was significantly higher than that on north-facing slopes (Figure 3B), depending on the provenance of the soils. It is well known that the development of soils on steep slopes derived from hard bedrock is often very slow, and the estimated production rate of soils developed on granite was 1 m per 20,000 years [38]. At the initial stage of formation on cut rock slopes, the soils in the crusts were absent due to artificial disturbance. In this study, there was little soil on the cut rock slope by the restoration time of 0.5 years, whereas the soil weight reached 0.76 kg·m−2 with the restoration time ranging to 1.5 years. This implied that the soils in the crusts on the cut rock slopes likely came from trapping the existing soils, especially from hill tops, other than in situ rock-weathering products. Furthermore, driven by the higher moisture content and temperature, the weathering intensity on the south-facing slopes was much stronger, including physical weathering, relative to that on the north-facing slopes, resulting in more weathering products and cracks on the south-facing slopes 40 [39]. More weather products and weathering cracks might facilitate providing more soil mass and trapping more grains, leading to the higher soil weight on the south-facing slopes.
The type of crust substrate might affect the development of mosses [40]. In this study, the difference in lithological substrate (i.e., limestone and sandstone) appeared to be the determining factor in moss growth and soil accumulation (Figure 4A). The mean value of moss biomass on a cut rock slope of limestone (96.31 g·m−2) was 1.64 times higher than that of sandstone (58.71 g·m−2). It was previously shown that more abundant mosses, including the number of species and coverage, were distributed on limestone than non-limestone (sandstone) at each site in Eastern Australia, which was attributed to the rock moisture and surface properties [41]. Deep crevices existed between massive limestone outcrops, allowing runoff and subsequent accumulation of water in hollows and crevices. Moreover, the chemical properties of the rock surface also seemed to be a significant factor in the distribution of mosses on limestones. Calciophilic mosses prefer to grow on limestone but are unable to survive on non-limestone [42,43].
The soil weight on sandstone was higher than that on limestone, especially on the south-facing slopes (Figure 4B). This could be related to the higher weathering intensity on sandstone. The formation rate of the weathered soil layer in the limestone area is 0.1–0.4 cm per thousand years in the humid region [44], in contrast with 0.7 to 1.8 cm per thousand years on the vertical surfaces of sandstone even under the semiarid climate in southern Jordan [45].

4.2. Influence of Elevation on Moss Development on Cut Rock Slopes

In fact, the altitude gradient, to a large extent, reflects the difference in climatic conditions, such as precipitation, temperature, and sunshine [46]. The results showed that there was a significant difference in moss biomass among different altitudes (Figure 5). In this study, we found that the variation in moss biomass with altitude was also consistent with that of rainfall. In the humid area (e.g., Baoxing area), the moss biomass and rainfall increased gradually along with the elevation rise, from 1942 to 2353 m on cut rock slopes with a formation age of 5 years and from ~2400 to ~2800 m on cut rock slopes with a restoration age of 27 years (Figure 5A,B). Furthermore, the variation in moss biomass with altitude was more obvious in semiarid climate regions (e.g., the Wenchuan–Lixian area). For cut rock slopes with a formation age of 7 years, the moss biomass increased significantly when the altitude changed by only ~50 m (increased from 2032 to 2081 m) (Figure 5C). Even if the slope was restored for 49 years, the moss biomass also increased significantly with the rising elevation of only 50 m from ~2350 to ~2400 m (Figure 5D).
Previous studies have confirmed that the growth of moss is greatly affected by humidity [47], which is the most important environmental factor in the distribution of moss species on limestone [42]. Relatively humid conditions help create a better growth environment for vegetation, leading to better growth of mosses at altitudes with high rainfall [48]. A positive correlation between lichen and moss abundance and higher moisture was previously demonstrated [49]. It was also found that the moss coverage and biomass in Mount Gongga in western Sichuan increased with altitude and precipitation, while the ground moss coverage reached 95.64% at an altitude of 3750 m [50]. The moss development in the semiarid climate area was more obvious with altitude relative to that in humid regions, likely owing to the higher sensitivity of the mosses to moisture in the former than in the latter [51].

4.3. Influence of Restoration Time on Moss Development on Cut Rock Slopes

Time is one of the critical factors that affect ecosystem succession. Considering that the growth of moss is dependent on the climate and lithology, the selection of sampling quadrats developed on the lithological substrate (i.e., limestone and sandstone) and in different climatic regions (Baoxing, Pengzhou–Shifang, Wenchuan–Lixian areas) was set when the impact of time on moss development was discussed.
According to the comparison of sample sites, the moss crust increased with restoration time, while the rate of moss biomass was diverse in different geographical areas. In the Pengzhou–Shifang area, the moss reached stability quickly. After significant development of moss between the sandstone cut slope with restoration ages of 0.5 years and 1.5 years, with moss biomass increasing from 0 to 24.08 g·m−2 (Figure 6), there was an insignificant difference in the moss biomass with the restoration age increasing from 1.5, 2.5, and 60 years within the slightly varied elevation (1003 to 1177 m) on the cut rock slope of the same lithotype in the Pengzhou–Shifang area (p > 0.05) (Figure 7A). In the Baoxing area, the moss biomass also increased with the formation age on the cut rock slope when the altitude was similar. However, it would take more time for moss to develop to stability in the Baoxing area. For formation ages of 3 and 8 years (3880 and 3716 m a.s.l., sandstone), 5 and 27 years (2430 and 2353 m a.s.l., limestone), and 15 and 27 years (2182 and 2227 m a.s.l., limestone), the moss biomass on the cut rock slope increased significantly with increasing age (p < 0.05) (Figure 7B,C). Our pilot study showed that, on limestone cut slopes in Baoxing, after 5 to 27 years of natural restoration the linear pattern between time (T, year) and moss biomass (MB, g·m−2) might be established (Figure 7C); the fitting formula was defined as follows:
MB= 6.4091 T + 14.4742 (r = 0.83, n = 7, p < 0.05)
The reason for the different times to reach stability of crust development between these two areas was attributed to the environmental conditions triggered by different altitudes. Due to the rapid vegetation succession in the relatively low-altitude mountainous region (~1000 m), and the growth of higher vascular plants limiting the development of moss, a stable level was quickly reached in 1.5 years (Figure 5B). However, in alpine and subalpine areas (over 2000 m a.s.l.), primitive succession is dominant; thus, the mosses are staple plants on the surface, which develop slowly and accumulate continuously [16]. For example, the biomass of lichen symbionts might reach 1892 g·m−2 after hundreds of years of accumulation on rocks in the polar region [52]. In Alaskan forests, moss abundance also showed an increasing tendency with time following the occurrence of fire, peaking at 30–70 years post-fire [53].

4.4. Irregular Soil Characteristics in Moss–Soil Crusts on Cut Rock Slopes during Natural Recovery

As mentioned above, the development of mosses on cut rock slopes was obviously affected by lithology, altitude, and time. However, the soil properties (e.g., soil weight and major nutrient content) in moss–soil crusts was irregular with these factors of cut rock slopes.

4.4.1. Soil Weight

The soil weight values varied with formation time of cut rock slopes. For example, the average soil weight on the cut rock slopes with the restoration age of 15 and 27 years (2024 and 1942 m a.s.l., limestone) was 0.74 kg·m−2 and 0.61 kg·m−2, respectively (Table 7). Comparison with the values of soil weight on the cut rock slopes with different formation ages, the weight of the soils sampled from the crusts varied insignificantly (p > 0.05) between 2 years and 7 years (2013 and 2032 m a.s.l., limestone), 9 and 15 years (2036 and 2024 m a.s.l., limestone), 13 and 15 years (2166 and 2182 m a.s.l., limestone), 5 and 27 years (2430 and 2353 m a.s.l., limestone), 15 and 27 years (2182 and 2227 m a.s.l., limestone), 15 and 27 years (1942 and 2024 m a.s.l., limestone), and 2.5 and 60 years (1043 and 1177 m a.s.l., sandstone) (Table 7), implying the indistinctive effect of restoration age on soil accumulation. Similarly, the influence of altitude on soil weight also seemed unsystematic. This result indicates that soil weight was mainly controlled by (1) the soils on the cut rock slopes from existing mature surface soil near the crusts, especially from hill tops [33], and the slow and weakened soil development on cut rock slopes [54]; and (2) soil loss caused by multiple factors, such as water and wind erosion, microtopography, and the density and biomass moss crusts, other than restoration time and elevation [33,51,55].

4.4.2. Soil Major Nutrient Content

In this study, the nutrient contents in the soils on cut rock slopes were fairly high. The highest SOC and TP contents reached 377.42 g·kg−1 and 5.05 g·kg−1, respectively (Table 4). However, the nutrient contents were less relevant to the factors such as lithology, aspect, altitude, and restoration time, possibly constrained by the material source of soil on the cut rock slope and features of moss crusts. As mentioned above, the soils within the crusts on the cut rock slopes came dominantly from the soils surrounding the crusts, which are rich in the fertility by nature [33]. Moss coverage and biomass showed significant correlations with soil nutrient contents (Table 5). In the process of natural recovery, moss development further prompts the major nutrient content of the soils within the crusts by fixing carbon and nitrogen and with self-decomposition [49,56,57].

5. Conclusions

Affected by the climate in mountainous areas, especially in alpine and subalpine areas, the natural restoration of moss–soil crusts on cut rock slopes is a long-term process with a slow rate of action. These findings unequivocally show that lithology, altitude and recovery time exerted a great impact on moss coverage and biomass, while the slope aspect had a great impact on soil weight in the crusts.
Specific selection of calciophilic mosses may lead to better adaptation of moss crusts on limestone than sandstone. The development of moss was greatly affected by water; thus, the variation trend of moss biomass with altitude was consistent with that of moss biomass with rainfall. The changes in moss development in semiarid climate areas, such as the Wenchuan–Lixian area, were more prominent with altitude. Moss biomass increased with increasing restoration age, while the restoration rate varied in different geographical areas. Vegetation succession was rapid in low-altitude areas (~1000 m); thus, there was no significant difference in moss biomass at restoration ages between 1.5 and 60 years. However, the moss biomass increased slowly in high-altitude mountain areas (~2000 m) because the areas mainly experienced primary succession with slow and sustainable development of mosses. The soils within the moss–soil crusts were affected predominantly by the surrounding soils and the crust features (moss biomass and coverage); thus, the soil major nutrient contents were high. More soils accumulated on south-facing slopes than on north-facing slopes.
The findings might offer guidance for the restoration of cut rock slopes in the future, especially in alpine and subalpine regions. Different cut rock slope faces have different geographical features, such as lithotype, rainfall, and aspect, thus the impact of these factors should be fully considered to propose appropriate management measures. Given that time is a very important factor in the natural restoration of moss–soil crusts on cut rock slopes in mountainous areas, abandoned cut rock slopes should be protected from further disturbance to shorten the ecological process of restoration. In addition, the proper increase in humidity on the rock surface could be helpful for the development of moss–soil crusts.

Author Contributions

Conceptualization, C.H., W.P. and J.D.; methodology, C.H., W.P. and M.Z.; software, Y.L., W.P. and M.Z.; formal analysis, W.P. and J.D.; investigation, W.P. and M.Z.; resources, Y.L.; data curation, W.P. and Y.L.; writing—original draft preparation, W.P.; writing—review and editing, C.H.; visualization, W.P. and Y.L.; supervision, C.H. and J.D.; project administration, C.H. and J.D.; funding acquisition, C.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by National Key Research and Development Program of China (2017YFC0504902) and Project of Assessment on Post-quake Ecosystem and Environment Recovery in Jiuzhaigou (5132202020000046).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data of this study are available from the authors upon request.

Acknowledgments

The authors sincerely thank the handling editor for coordinating the review of our manuscript. The authors also acknowledge anonymous reviewers for their feedback, which certainly improved the clarity and quality of this paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Study area and the distribution and selected landscape of sampling sites in the west of Sichuan, SW China.
Figure 1. Study area and the distribution and selected landscape of sampling sites in the west of Sichuan, SW China.
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Figure 2. Plots between moss coverage and moss biomass in the natural restoration process on cut rock slopes in west Sichuan’s mountainous region.
Figure 2. Plots between moss coverage and moss biomass in the natural restoration process on cut rock slopes in west Sichuan’s mountainous region.
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Figure 3. Moss biomass (A) and soil weight (B) in moss–soil crusts on north-facing and south-facing cut rock slopes in west Sichuan mountainous region. Different letters represent significant difference at p < 0.05 between two slope aspects.
Figure 3. Moss biomass (A) and soil weight (B) in moss–soil crusts on north-facing and south-facing cut rock slopes in west Sichuan mountainous region. Different letters represent significant difference at p < 0.05 between two slope aspects.
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Figure 4. Moss biomass (A) and soil weight (B) in moss–soil crusts of cut rock slopes developed on limestone and sandstone in west Sichuan mountainous region. Different letters represent significant difference at p < 0.05 between limestone and sandstone.
Figure 4. Moss biomass (A) and soil weight (B) in moss–soil crusts of cut rock slopes developed on limestone and sandstone in west Sichuan mountainous region. Different letters represent significant difference at p < 0.05 between limestone and sandstone.
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Figure 5. Variation of moss biomass with altitude in Baoxing and Wenchuan–Lixian areas. MAP: mean annual precipitation; MAT: mean annual temperature; MB: moss biomass. Different letters represent significant difference at a level of p < 0.05.
Figure 5. Variation of moss biomass with altitude in Baoxing and Wenchuan–Lixian areas. MAP: mean annual precipitation; MAT: mean annual temperature; MB: moss biomass. Different letters represent significant difference at a level of p < 0.05.
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Figure 6. Landscape of natural recovery on cut rock slopes of sandstone in Shifang: (A) the natural recovery age of 0.5 years at an elevation of 1124 m; (B) the natural recovery age of 1.5 years at an elevation of 1003 m.
Figure 6. Landscape of natural recovery on cut rock slopes of sandstone in Shifang: (A) the natural recovery age of 0.5 years at an elevation of 1124 m; (B) the natural recovery age of 1.5 years at an elevation of 1003 m.
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Figure 7. Variation of moss biomass with restoration age on cut rock slopes. (A) The altitude within 1003 and 1177 m, sandstone, Pengzhou–Shifang; (B) the altitude within 3716 and 3880 m, sandstone, Baoxing; (C) the altitude within 2024 and 2430 m, limestone, Baoxing. Different letters represent significant difference at p < 0.05.
Figure 7. Variation of moss biomass with restoration age on cut rock slopes. (A) The altitude within 1003 and 1177 m, sandstone, Pengzhou–Shifang; (B) the altitude within 3716 and 3880 m, sandstone, Baoxing; (C) the altitude within 2024 and 2430 m, limestone, Baoxing. Different letters represent significant difference at p < 0.05.
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Table
Table 1. Climate, elevation, and vegetation of studied areas in west Sichuan’s mountainous region, China.
Table 1. Climate, elevation, and vegetation of studied areas in west Sichuan’s mountainous region, China.
AreaClimateElevation
(m a.s.l.)		Mean Annual Precipitation (mm)Typical Natural VegetationPlantTopography
BaoxingSubtropical–cold temperate humid monsoon climate 1942–4116600–1000Theropencedrymion, subalpine coniferous forest, alpine shrubs and meadowsSedge, purslane, pine, cedar Mountainous terrain
Pengzhou–
Shifang		Subtropical humid monsoon climate 900–12001100–1500Evergreen broad-leaved forestDicranopteris pedata, acorus calamus, brichMountainous terrain
Lixian–
Wenchuan		Warm temperate continental semi-arid monsoon climate2013–2426540–610Bushwood, pinus tabulaeformisCoriaria sinica, artemisia argyi, cedarMountainous terrain
Table
Table 2. General geographical description at sampling sites in western Sichuan’s mountainous region, China.
Table 2. General geographical description at sampling sites in western Sichuan’s mountainous region, China.
AreaSample Site IDNumber of Sample QuadratsLatitude and Longitude of Sampling SitesElevation (m a.s.l.)Restoration Age
(Year)		Slope Angle (°)Slope AspectSlope Length (m)Slope Height (m)
BaoxingBX11525°07′90″ N, 102°46′45″ E2036990N510
BXGL11930°47′50″ N, 102°43′40″ E2430~3085N912
JJSA1830°51′38″ N, 102°40′57″ E4116370S353
JJSB830°51′16″ N, 102°41′32″ E3880375S156
JJSC2530°50′34″ N, 102°42′32″ E3716876S358
JJS11230°50′20″ N, 102°41′54″ E3448890N408
JJS21430°49′48″ N, 102°43′12″ E3187880N20015
JJS3830°50′26″ N, 102°43′50″ E2878885S15030
ZGX11530°44′14″ N, 102°46′27″ E2816578S308
ZGX21830°43′55″ N, 102°46′01″ E2640571S1005
ZGX31330°43′35″ N, 103°46′08″ E2430585N6020
QQH2.11030°44′11″ N, 102°44′56″ E21661385S139
QQH2.2630°44′04″ N, 102°44′51″ E21661380N309
QQH31230°41′22″ N, 102°43′21″ E21821570N6050
QQH4430°40′18″ N, 102°44′54″ E20241575N2530
HDZ1.11330°38′37″ N, 102°52′30″ E23532783N2711
HDZ1.21730°38′37″ N, 102°52′30″ E23532790S3020
HDZ2930°38′14″ N, 102°52′06″ E22272776N815
HDZ3.11230°38′05″ N, 102°51′51″ E19422777N207.5
HDZ3.2630°38′07″ N, 102°51′51″ E19422785S72.5
Pengzhou–
Shifang		XY11131°11′52″ N, 103°46′16″ E10432.588S102
XY2931°15′38″ N, 103°48′45″ E967468N35
LMSZ11231°17′26″ N, 103°50′44″ E1177>6078S85
YHSZ11431°18′13″ N, 103°58′05″ E10031.564N106
YHSZ2331°18′08″ N,
103°55′38″ E		1124<0.585S127
Wenchuan–LixianLBZ1531°30′08″ N, 103°40′36″ E2013290N55
DSE1331°26′42″ N, 103°40′36″ E2032780N320
DSE2531°26′35″ N, 103°08′35″ E2081770N76
DSE3631°26′34″ N, 103°08′33″ E2145750N128
BPG1631°23′48″ N, 102°56′03″ E24264960S816
BPG2531°24′27″ N, 102°57′28″ E23504984S78
BPG3631°24′54″ N, 102°58′54″ E22904990S209
Table
Table 3. Moss coverage and biomass within the crusts in three study areas.
Table 3. Moss coverage and biomass within the crusts in three study areas.
AreasBaoxingPengzhou–ShifangWenchuan–Lixian
VariablesMoss Coverage
(%)		Moss Biomass
(g·m−2)		Moss Coverage
(%)		Moss Biomass
(g·m−2)		Moss Coverage
(%)		Moss Biomass
(g·m−2)
Maximum83.97 888.50 66.91 210.44 90.00 220.00
Minimum0.42 0.25 0 0 5.00 0.48
Mean24.12 97.03 15.64 28.48 43.33 43.62
Standard deviation18.36 131.24 16.07 39.51 21.68 58.74
Sample number250 250 49 49 36 36
Table
Table 4. Soil weight and major nutrient contents within the crusts in different areas of western Sichuan’s mountainous region.
Table 4. Soil weight and major nutrient contents within the crusts in different areas of western Sichuan’s mountainous region.
AreasVariablesSW (kg·m−2)SOM (g·kg−1)TP (g·kg−1)TN (g·kg−1)SOM (g·m−2)SP
(g·m−2)		SN
(g·m−2)
BaoxingMaximum4.77 377.42 5.05 5.24 361.17 6.65 5.05
Minimum0 5.24 0.02 0.07 0.04 0.01 0.01
Mean0.43 86.32 0.72 1.69 24.87 0.33 0.91
Standard deviation0.67 71.96 0.59 0.90 34.01 0.61 1.00
Sample number250 211 182 125 211 182 125
Pengzhou–
Shifang		Maximum4.51 206.84 1.37 5.99 574.99 4.10 19.92
Minimum0 10.59 0.15 0.81 7.31 0.13 0.34
Mean0.98 99.63 0.76 2.23 96.45 0.83 4.11
Standard deviation1.01 57.29 0.28 1.49 111.92 0.84 5.73
Sample number49 46 39 21 46 39 21
Wenchuan–
Lixian		Maximum4.79 129.84 1.50 3.93 333.04 3.51 17.60
Minimum0.02 7.70 0.41 0.13 5.28 0.02 0.07
Mean0.84 62.19 0.84 2.07 47.49 0.70 2.02
Standard deviation1.03 34.61 0.23 1.09 66.45 0.83 3.68
Sample number36 32 28 25 32 28 25
SW: soil weight; SOM: soil organic content; TP: total phosphorus; TN: total nitrogen; SOM: total organic stock; SP: total phosphorus stock; SN: total nitrogen stock.
Table
Table 5. Pearson correlation coefficients for moss coverage, biomass, and major soil nutrients within crusts.
Table 5. Pearson correlation coefficients for moss coverage, biomass, and major soil nutrients within crusts.
CVMBSWSOMTPTNSOMSPSN
CV1☐☐☐☐☐☐☐☐
MB0.502 **1☐☐☐☐☐☐☐
SW0.218 **0.188 **1☐☐☐☐☐☐
SOM0.188 **0.141 *−0.282 **1☐☐☐☐☐
TP0.0760.032−0.0890.0511☐☐☐☐
TN0.0910.1080.0080.478 **0.0741☐☐☐
SOM0.141 *0.143 *0.675 **0.147 *−0.0250.422 **1☐☐
SP0.263 **0.142 *0.668 **−0.160 *0.468 **0.0590.567 **1☐
SN0.272 **0.1280.716 **0.130−0.0120.491 **0.918 **0.604 **1
* p < 0.05; ** p < 0.01. CV: moss coverage; MB: moss biomass; SW: soil weight; SOM: soil organic content; TP: total phosphorus content; TN: total nitrogen content; SOM: total organic stock; SP: total phosphorus stock; SN: total nitrogen stock.
Table
Table 6. Loading coefficients of the variables analyzed for the principal components (PC) that comply with the condition of λ > 1.
Table 6. Loading coefficients of the variables analyzed for the principal components (PC) that comply with the condition of λ > 1.
VariablesPrincipal Component (PC)
PC1PC2PC3PC4
CV0.48 0.13 0.01 0.68
MB0.34 0.02 0.16 0.83
SW0.84 −0.23 −0.40 0.01
SOM0.10 −0.13 0.740.15
TP0.22 0.93 0.18 −0.12
TN0.29 −0.16 0.86 −0.13
SOM0.87−0.30 0.18 −0.18
SP0.840.42 −0.17 −0.11
SN0.89−0.29 0.14 −0.21
Cumulative variance (%)42.75 18.25 13.85 12.83
In bold, for each PC, are the variables that comply with the condition of belonging to the range of loadings between the maximum absolute value and 10%. The variables in italic and the underlined loading values identify the indicators selected taking into account Pearson’s bivariate correlations (p < 0.05).
Table
Table 7. Soil weight variation with formation time under the similar altitude in different areas.
Table 7. Soil weight variation with formation time under the similar altitude in different areas.
AreaElevation (m)Formation Age (Year)Soil Weight (kg·m−2)
Baoxing203690.18 ± 0.16 a
2024150.74 ± 0.60 a
1942270.61 ± 0.96 a
2166130.16 ± 0.08 a
2182150.49 ± 0.67 a
243050.17 ± 0.25 a
2353270.22 ± 0.18 a
2181150.49 ± 0.66 a
2227270.18 ± 0.22 a
Pengzhou–Shifang10432.51.43 ± 1.19 a
1177601.49 ± 1.37 a
Wenchuan–Lixian201321.45 ± 1.03 a
203270.16 ± 0.05 a
Different letters denote significant difference at p < 0.05 at different recovery times. The same letter denotes no significant difference at p < 0.05 at different recovery times. The contrast only occurs between two points with similar altitude.
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MDPI and ACS Style

Pu, W.; Zhao, M.; Du, J.; Liu, Y.; Huang, C. Effect of Geographical Conditions on Moss–Soil Crust Restoration on Cut Rock Slopes in a Mountainous Area in Western Sichuan, China. Sustainability 2023, 15, 1990. https://doi.org/10.3390/su15031990

AMA Style

Pu W, Zhao M, Du J, Liu Y, Huang C. Effect of Geographical Conditions on Moss–Soil Crust Restoration on Cut Rock Slopes in a Mountainous Area in Western Sichuan, China. Sustainability. 2023; 15(3):1990. https://doi.org/10.3390/su15031990

Chicago/Turabian Style

Pu, Wanqiu, Maoqiang Zhao, Jie Du, Yongyao Liu, and Chengmin Huang. 2023. "Effect of Geographical Conditions on Moss–Soil Crust Restoration on Cut Rock Slopes in a Mountainous Area in Western Sichuan, China" Sustainability 15, no. 3: 1990. https://doi.org/10.3390/su15031990

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