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Article

Effects of Erosion Micro-Topographies on Plant Colonization on Weathered Gangue Dumps in Northeast China

by
Dongli Wang
1,*,
Jingting Qiao
1,
Ye Zhang
1,
Tong Wu
1,
Jia Li
1,
Dong Wang
2,*,
Xiaoliang Zhao
1,
Haiou Shen
3 and
Junliang Zou
4
1
College of Environmental Science and Engineering, Liaoning Technical University, Fuxin 123000, China
2
College of Mining Technology, Liaoning Technical University, Fuxin 123000, China
3
College of Resources and Environment, Jilin Agricultural University, Changchun 130118, China
4
Institute of Grassland, Flowers, and Ecology, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100097, China
*
Authors to whom correspondence should be addressed.
Sustainability 2022, 14(14), 8468; https://doi.org/10.3390/su14148468
Submission received: 23 June 2022 / Revised: 5 July 2022 / Accepted: 9 July 2022 / Published: 11 July 2022
(This article belongs to the Section Environmental Sustainability and Applications)
Browse Figures
Review Reports Versions Notes

Abstract

:
Micro-topography has been proved to be beneficial for plant colonization in severe environments. There are numerous micro-topographies caused by erosion of gangue dumps in the Northeast China, which can make plant colonization difficult. To determine how these micro-topographies affect plant colonization, the environment conditions, regeneration characteristics, vegetation characteristics of different erosion micro-topographies, such as bare slope, rill, ephemeral gully and deposit body were studied, and their relationships analyzed. The results showed that the content of particles with a size < 2 mm in the deposit body and bare slope was 33.7% and 7.8% higher than that in the ephemeral gully, respectively (p < 0.05), while the content of particles with a size > 20 mm in the ephemeral gully was 2.24 times higher than that in the deposit body. Except for the substrate water content, the substrate temperature and the surface humidity and temperature of the ephemeral gully were significantly different from those of the deposit body (p < 0.05); the surface temperature was the highest (54.6 °C) while the surface humidity and the substrate water content were the lowest among the erosion micro-topographies. The vegetation coverage, the plant and seedling density of the deposit body were significantly higher than those of the ephemeral gully (p < 0.05), with differences of 5.26, 35.9 and 16.8 times, respectively. The vegetation characteristics (Vdc) were more affected by the regeneration characteristics (Rc) as well as surface humidity and temperature (Sht), while Rc was significantly affected by Sht, which was extremely significantly affected by the soil physical properties and substrate water and temperature (p < 0.01). Different plant species had different responses to the environmental conditions of the erosion micro-topographies. In conclusion, the deposit body and rill are likely to promote plant colonization, which is driven mainly by the seed supply and comfortable growing conditions. The ephemeral gully is not suited to plant colonization because of its unhealthy mechanical composition and strong runoff scouring, and because it is prone to drought, high temperature, and a lack of seeds.

1. Introduction

Micro-topography describes the small-scale (the centimetre to decimetre scale) variation in surface elevation, and features depressions, humps, cracks, rocks, and other rough surfaces [1,2,3,4]. Micro-topography affects soil moisture and substrate temperature not only by influencing surface runoff and infiltration processes [5,6] but also by redistributing meteorological factors due to affecting radiation and reflection [4]. Furthermore, micro-topography can trap some organic matter, e.g., litter, seeds, and plant residue or woody debris, which promote the biogeochemical processes [7,8,9]. Due to the temperature and moisture influencing biochemical processes, micro-topography indirectly leads to heterogeneous micro-habitats. Micro-habitats had been proposed by Harper et al. [10] as the term for ‘safe-sites’ to describe micro-sites more favorable for seed germination and establishment in primary succession. The existence of such habitats in many ecosystems is an important issue in the field of plant establishment, colonization, and development. Micro-habitats play an important role in plant regeneration in harsh conditions during early successions, such as grasslands [11], sand land [12], even the foreland of Coleman Glacier [13].
Abandoned coal wastelands, constituting various rocks, mine gangues, and soil, always lead to landscape damage, vegetation community destruction, serious land degradation, and ecosystem function losses [14,15,16,17], and the ecosystem urgently needs to be reconstructed. Vegetation restoration, as the principal approach to ecological reconstruction in abandoned coal wastelands [18], is an effective way to reduce runoff and soil erosion and plays an important role in improving soil quality, maintaining water, climate and microorganism balance on coal wastelands [19,20]. Thus, more and more attention has been paid to revegetation in coal mine areas, especially artificial planting. Commonly, it is very difficult to carry out vegetation restoration under extremely harsh site conditions. Environmental fluctuations may easily change human-induced community stability compared with spontaneous succession in semiarid environments [18], so it is important to promote natural revegetation.
During a natural revegetation process, vegetation settlement and community formation, the key and fragile stages depend not only on high-quality soil but also on local microclimate and topography [21]. It has been found that micro-sites such as rill, under tussock or shade habitats have higher photochemical efficiency, a slower drying process and more seeding survival [22,23,24]. Hu et al. [25] found that the bare land micro-habitat was conducive to seedling colonization in the arid valley area. There are obvious differences in water content among different locations of gully and rill that are more beneficial for seedling emergence and survival on the Loess Plateau in Northwest China [22,26]. Bentos et al. [27] found that the seed bank density of plateau and lowland was twice that of the slope, and the seedling emergence rate of depressions was significantly higher than that of the slope. Spatial distribution of micro-topography can often determine where plant establishment occurs, strongly influences colonization, successional patterns, plant community structure, and ecosystem function [4,13].
Weathered gangue dump, as an artificial reshaped ecosystem, is characterized by complex substrate, poor structure and porosity, low capacity of water and fertilizer preservation, lack of nutrients, and severe heat, pollution and radiation, which seriously restrict revegetation [28,29,30]. Due to the scarce vegetation and unstable slope, the weathered gangue dump is prone to soil erosion by hydraulic and gravity forces, which results in uneven topography on a small scale [31]. Due to continuous soil erosion, there are some micro-topographies distributed widely on the slope of the weathered gangue dump, such as rill, gully, and deposit [32], which were defined as erosion micro-topography in this study. Like all micro-topographies, these erosion micro-topographies redistribute the water and heat conditions, which may affect plant colonization. There is an urgent need to determine how these micro-topographies influence colonization by vegetation.
Thus, this study measured and surveyed the physicochemical properties, substrate hydrothermal conditions, surface temperature and humidity, seed bank, seedling and vegetation characteristics under different erosion micro-topographies, and then analyzed the relationships between each environment condition and vegetation characteristic. The main objectives were to (i) identify the differences of micro-habitats formed by different erosion micro-topographies, (ii) determine the characteristics of the seed bank, seedling and vegetation under different erosion micro-topographies, (iii) analyze the relationships between the site conditions and vegetation characteristics, and (iv) explore the effects and mechanisms of erosion micro- topopgraphy on the plant colonization at the weathered gangue dumps. This study will provide valuable guidance for natural revegetation at the weathered gangue dumps of the abandoned coal wasteland in the arid region of China, which is of great significance to improve the regional ecological environment and promote sustainable development.

2. Methods

2.1. Study Area Description

This study area was carried out in Fuxin City (192–293 m a.s.l.) in Liaoning Province, P.R. China, one of the earliest important energy production bases in China, where coal mining was developed in 1897. It has a semi-arid continental monsoon climate in the north temperate zone, with mean annual precipitation of 539 mm and mean annual evaporation capacity of 1800 mm. The mean annual temperature is 7–10 °C, with mean relative humidity of 50–60%. The main soil types are cinnamon soil, brown soil, meadow soil, and aeolian sand soil. Local representative species include Pinus tabulaeformis, Quercus mongolica, Ulmus pumila, Vitex negundo, etc. The Fuxin area is abundant in mineral resources, including about 1 billion tons of coal reserves and about 10 million tons of coal production per year. At the same time, the annual emission of coal gangue is about 15–20% of the coal production [33].
The representative weathered gangue dump formed during 2008–2009 with typical and serious erosion was selected as the study area in the Gaode district (121°42′48.23″ E, 42°0′03.51″ N, Figure 1). The gangue dump covers an area of 1.25 km2 with the difference of elevation varying from 10 m to 13 m and the gradient varying from 35.4° to 39.1°. There is still spontaneous combustion in different uncertain locations, which causes air pollution and poor vegetation coverage. After years of weathering, the gangue dump surface has suffered from different degrees of erosion, and has formed different types of micro-geomorphologies caused by erosion, including rill, ephemeral gully and deposit body.

2.2. Experiment Design

Based on the representative weathered gangue dump selected above, three representative slopes of weathered gangue dump with typical and serious erosion micro-topographies were selected as sample sites. They were sunny slopes and the conditions were relatively consistent. For each sample site, three replications of bare slope, rill, ephemeral gully, and deposit body were selected as study objects while the adjacent bare slope was selected as the control. The specific characteristics of each erosion micro-topography site are shown in Table 1 and Figure 1. For each erosion micro-topography, we measured the physical and chemical properties and surveyed the surface and substrate temperature and humidity to identify the micro-habitats of different erosion micro-topographies. Moreover, we investigated the soil seed bank, seedling and plant population characteristics. We analyzed the relationships between each environmental condition and vegetation characteristic to explore the effects of erosion-affected micro-habitats on the vegetation settlement at the weathered gangue dumps.

2.3. Determination of Physic-Chemical Properties

In the study area, representative bare slopes, rills, and ephemeral gullies were selected according to contour lines. Three sample zones were selected for each erosion micro-topography, and samples were set according to the upper, middle, and lower slope positions, respectively. For the deposit body, three sample plots were set with three sample points, which were chosen for each sample plot. The samples of 0–10 cm and 10–20 cm of each spot were brought back to the laboratory and the multi-point mixing method was used to process the same substrate layer. In this study, bulk density, capillary porosity, and mechanical composition were chosen to represent the physical properties.
The substrate bulk density and capillary porosity of 0–10 cm and 10–20 cm substrate layers were measured by the cutting ring method.
According to the actual situation, the substrate was divided into the particle sizes < 2 mm, 2–10 mm, 10–20 mm, and >20 mm, respectively. After the collected samples were fully dried, the samples were used to measure the content percentage of each particle group by the drainage method.
The pH and electrical conductivity (EC) were selected to characterize the chemical properties of the weathered gangue dumps substrate, using a PHS-3C pH meter (Shanghai Weiye Instrument Factory, Shanghai, China) to measure pH while using a DDSJ-318 conductivity meter (Shanghai Weiye Instrument Factory, Shanghai, China) to determine the substrate EC.

2.4. Investigation of Hydrothermal Conditions

To determine the hydrothermal conditions of different soil erosion micro-topographies in the gangue dump, the temperature and humidity on the surface of substrate, the temperature, moisture content, and the capillary water holding capacity at the 0–10 cm depth of the substrate were measured. The monitoring points of bare slope, rill, and ephemeral gully were determined according to the contour lines, which were divided into upper, middle, and lower slope positions as for the monitoring points of the physical and chemical properties of the substrate. Drying was used to measure the moisture content of the substrate, and the water immersion method was used to measure the capillary water holding capacity. The temperature and humidity on the substrate surface were measured by a handheld weather meter (Kestrel 5000). All the investigations were carried out at mid-day (13:00) during 3 consecutive sunny days in the peak season of plant growth.

2.5. Regeneration Survey

The regeneration characteristics were performed by the density of the substrate seed bank and seedling. To determine the substrate seed bank characteristics, substrate samples in different erosion micro-topographies were collected in March 2020. Nine sample plots were selected in each erosion micro-topography on bare slopes while three sample plots were selected in deposit bodies. In each plot, 10 soil cores with a diameter of 4.8 cm were collected from the 0–2 cm, 2–5 cm and 5–10 cm substrate layers. The soil seed bank was identified using the seedling emergence method using a greenhouse; for more details see Wang et al. [34].
The seedling characteristics were surveyed in May 2020. The seedling survey of the four erosion micro-topographies were different. Nine sample plots with an area of 0.5 m × 0.5 m for bare slopes were set while nine sample belts of 0.2 m × 1 m size for rills and 0.5 m × 1 m size for ephemeral gullies, with three plots for upper, middle, and lower slope positions, respectively. For deposit bodies, sample plots were 1 m × 1 m. The names and density of seedlings in each plot were recorded.

2.6. Vegetation Survey

The vegetation survey was conducted in August 2020. The sample sites and study objects of the vegetation survey were the same as for the seedling survey. The names, density, and coverage of plants in each plot were recorded.

2.7. Data Analysis

We first used one-way analysis of variation (ANOVA) to test differences in the physicochemical properties, the hydrothermal conditions, soil seed bank, and seedling characteristics under different soil erosion micro-topographies, and the least significant difference method (LSD) multiple comparisons were used to compare the significance of differences (p < 0.05). Statistical analyses of the data were carried out using SPSS 21 for Windows (SPSS Inc., Chicago, IL, USA). Since the density of the soil seed bank varied significantly, a natural logarithm was performed before analyses to improve normality. These figures were drawn with the Sigmaplot 14.0.
The partial least squares path model (PLS-PM) was performed to infer potential direct and indirect effects of environmental factors and the characteristics of soil seed banks and seedlings on vegetation characteristics. Based on the expected relationship between vegetation density/coverage and key factors, a base model that linked soil physical properties (Spp), soil chemical properties (Scp), substrate water content and temperature (Swt), surface humidity and temperature (Sht) and regeneration characteristics (Rc) to vegetation density and coverage (Vdc) was established. All the above variables were latent variables incorporating the observed variables, and were selected based on the value of the path coefficient between the latent variables and their observed variables being greater than 0.6. The Spp was characterized by the particle size of <2 mm and >20 mm, the Scp was characterized by pH and EC, the Swt was characterized by the substrate water content, the Sht was characterized by the surface humidity and temperature, the Rc was characterized by the seedling density, and the Vdc was characterized by the vegetation density and coverage. The 95% bootstrap confidence interval was used to judge whether estimated path coefficients were significant. Path coefficient represents the direction and strength of the direct effect between two variables. The PLS-PM was performed using the package “plspm” in R 4.0.5 (R Development Core, 2016, Vienna, Austria).
The relationships among each index of the substrate and the correlations between substrate under different soil erosion micro-topographies and species were analyzed using R software 4.0.5. The figures were drawn with the “ggplot2” package in R 4.0.5.

3. Results

3.1. PhysicoChemical Properties of the Substrate under Different Erosion Micro-Topographies

As shown in Figure 2, the substrate physicochemical properties of different erosion micro-topographies performed differently. Overall, the mechanical components of particle size under different erosion micro-topographies showed that the content of particles of size <2 mm occupied a main position and was followed by the particle size of 2–10 mm, with the content of particles of size <2 mm varying from 35% to 65% and the content of particles of size 2–10 mm varying from 13 to 39%. The content of the particles of size 11–20 mm and >20 mm was not consistent; the content of the particles of size 11–20 mm was lower than that of >20 mm in the ephemeral gully while it showed the opposite trends in other erosion micro-topographies. However, each mechanical composition under different erosion micro-topographies performed differently. The content of particles with size <2 mm in the deposit body was 33.7% higher than that in the ephemeral gully significantly (p < 0.05), while the content of particles of size >20 mm in the ephemeral gully was 2.24 times higher than that in the deposit body. There was no significant difference in the content of particles of size 2–10 mm and 11–20 mm between different erosion micro-topographies. The capillary porosity of the deposit body was significantly higher by 9.1% and 7.8% than that of the rill and ephemeral gully (p < 0.05). There was no significant difference in the bulk density, pH, and electrical conductivity (EC) of the substrates between different erosion micro-habitats (p < 0.05), but the pH was the highest while the EC was the lowest in the rill.

3.2. Hydrothermal Conditions under Different Erosion Micro-Topographies

The surface humidity and temperature as well as the substrate water content and temperature in different erosion micro-topographies in summer were shown in Figure 3. Overall, the surface humidity and temperature were higher than that of the substrate, except that the surface temperature was lower than the substrate temperature in the deposit body. The surface humidity of the deposit body was 39.9% higher than that of the ephemeral gully, with a significant difference between the surface humidity of the ephemeral gully and deposit body (p < 0.05). The substrate water contents were not significantly different between each erosion micro-topography, and only varied from 4.1% to 11.4%. The temperature of the weathered gangue dump was higher, with the surface temperature ranging from 41.5 °C to 58.6 °C and the substrate temperature ranging from 37.0 °C to 50.8 °C. The mean surface temperature varied differently significantly between each erosion micro-topography (p < 0.05), with a decrease in the order of ephemeral gully > rill > bare slope > deposit body. The substrate temperature of the deposit body was 19.7% higher than that of the ephemeral gully (p < 0.05).

3.3. Vegetation Characteristics under Different Erosion Micro-Topographies

As shown in Figure 4, there were fewer plant species and single composition of each erosion micro-topography in general, with there being 3–5 families, 3–5 genera and 4–8 species. However, the composition of the species, genera and families under different erosion micro-topographies were different, with the species decreasing in the order of deposit body > rill/bare slope > ephemeral gully. The plant density and the coverage in the deposit body in the study area were 35.9 and 5.3 times higher than those in ephemeral gully (p < 0.05). The plants in the ephemeral gully generally showed the characteristics of low density and high coverage, while those in the rill generally presented the characteristics of medium density and medium coverage.

3.4. Soil Seed Bank and Seedling Characteristics under Different Erosion Micro-Topographies

As shown in Figure 5, there was no significant difference in seed bank density among different erosion micro-topographies in general, except the density of seed bank at 0–2 cm depth of the deposit body was significantly higher than that in other erosion micro-topographies (p < 0.05). The seedling density of the deposit body was significantly higher than that of the other erosion micro-topographies (p < 0.05). Additionally, both the density of the seed bank and seedlings of the ephemeral gully were the lowest while those of the deposit were the highest, with the differences being 2.61 and 16.81 times higher, respectively. There was a large variation in the densities of the seed banks and seedlings.

3.5. Effects of Erosion Micro-Topography Conditions on Vegetation Characteristics

The results of PLS-PM are shown in Figure 6. All the environment conditions influenced vegetation characteristics positively with no significance. However, the soil physical properties had a positive effect on surface humidity and temperature with an extremely significant coefficient (p < 0.001). Meanwhile, substrate water content and temperature also had a positive effect on surface humidity and temperature with an extremely significant coefficient (p < 0.01). Surface humidity and the temperature had a significant positive effect on regeneration characteristics (p < 0.05).
The correlations between each site condition and between the site conditions and plant species are shown in Figure 7. There were four plant species significantly related to the site conditions in the gangue dumps. The density of S. viridis and E. hispidula were extremely significantly correlated to the density of seed banks and seedlings (p < 0.01). The surface temperature affected the densities of S. viridis and T. terrestris extremely significantly (p < 0.01). The density of E. hispidula was also significantly correlated to the particle size of <2 mm, capillary porosity, and surface temperature, while the density of S. viridis was significantly correlated to the particle size of >20 mm. The density of T. terrestris was significantly correlated to substrate temperature and the density of E. hispidula was significantly correlated to capillary porosity (p < 0.05).

4. Discussion

4.1. Erosion Micro-Habitats Formed by Micro-Topographies

Micro-habitat refers to habitat on a small scale, which is formed due to the hydrological and biogeochemical processes caused by micro-topography. As mentioned above, numerous studies have shown that micro-topography can reshape the distribution of soil water content and temperature by redistributing the meteorological factors [4,5,6]. Our results showed that the surface humidity and substrate water content of the deposit body were the highest among all the erosion micro-habitats while the surface humidity of the ephemeral gully and the substrate water content of the rill were the lowest, which also verified the viewpoint above. It was found in our study that the surface temperature in each erosion micro-habitat presented obvious differences, especially regarding the surface temperature of the ephemeral gully, which can reach up to 58.6 °C. Thus, the ephemeral gully formed a micro-habitat with higher temperature and lower water content while the deposit body formed a micro-habitat with relatively higher water content. However, Wang et al. [22] found that the ephemeral gully in the Loess Plateau can offer suitable heat and moisture for plant emergence and survival, which is inconsistent with our result. The differences of substrate physical properties caused by erosion were one of the reasons lead to this phenomenon. In our study areas, the content of particles of size > 20 mm in the ephemeral gully was higher than that in other erosion micro-topographies while the content of particles of size < 2 mm was lower, which was due to the cumulative effect of water erosion and the material composition of the weathered gangue dumps. In addition, coal gangue dump has a spontaneous combustion phenomenon; the ephemeral gully has a higher substrate temperature for its deeper gully, which is closer to the inner part of the gangue dump. Thus, the micro-habitat formed in the ephemeral gully is unsuitable for plant colonization while that in the deposit body is comfortable for plant colonization.

4.2. How Do Erosion Micro-Habitats Drive Plant Colonization

Previous studies have reported that micro-habitats can be beneficial to vegetation establishment and development in harsh environments [13,26]. However, our results showed that some erosion micro-habitats were supportive of plant settlement while others were unfavorable for plant colonization. Seed is one of the main factors controlling seedling establishment [35,36], which is the key stage in plant colonization. The results of this study showed that the vegetation density and coverage of the deposit body as well as rills were higher than those of the bare slope and ephemeral gully, with a positive relationship between vegetation characteristics and the density of soil seed bank and seedlings. Micro-topography can trap seeds and plant residue [22,37]. In the study area, we found that the density of the soil seed bank in the deposit body was more than that in the other erosion micro-habitats, which indicates that the deposit body is beneficial to retention of seeds.
It was found that the seedling density in the deposit body and rill was higher than that in the bare slope and ephemeral gully, where more seeds germinated successfully, due to the temperature and humidity of the deposit body. This study showed that the deposit body had the highest surface humidity and the lowest surface temperature, which was more suitable for the survival of plants. The deposit body had the highest sand content and the largest capillary porosity, because the deposit body was located at the foot of the whole gangue dump slope with good sized particles accumulated there from the scouring effect of runoff. According to the field investigation, the vegetation coverage of the deposit body is higher, which can improve the texture and structure of the substrate [38,39]. Soil bulk density and porosity are the basic physical properties of soil, which can directly affect the water holding and permeability of soil [40]. Soil capillary porosity and soil bulk density showed opposite trends [41]. It was found that the capillary water capacity of gangue dumps was significantly affected by the capillary porosity, and the capillary water capacity of deposits body was significantly higher than that of other erosion micro-habitats. This is because the fine-grained substrate composition and large capillary porosity give the deposit body strong ventilation and water permeability, as well as water and heat preservation properties [33]. On the other hand, the higher vegetation coverage of the deposit body makes a large number of plant roots grow and interpenetrate, which can increase the capillary porosity and reduce the bulk density, and affect the water storage and water holding capacity of the soil [42]. Synthesizing the physical and chemical properties of the erosion micro-habitats, the result shows that the mechanical composition of the deposit body tends to be fine-grained. All these results indicate that erosion micro-topography redistributes the hydrothermal conditions due to the mechanical composition caused by the runoff scouring, which can explain successful establishment of seedlings and plant colonization. Thus, we can deduce that the ability to trap seeds or promote seed germination can be a driving factor contributing to plant colonization in these habitats.
Furthermore, the seed germination ecological niche and the temperature range in which the seeds can germinate played an important role in plant survival in the study area according to our field observation. In the early spring, the temperature varied from 3–20 °C, and there were many seedlings of S collina in the rill. This phenomenon shows the seeds of S. collina can germinate under low temperatures, with the seedling growing strongly enough to resist the drought stress before hot season comes. This may be why the S. collina can dominate on the gangue dump. Therefore, the plant colonization is driven not only by the seed supply and comfortable conditions but also by the seed germination characteristics and their relationship with environmental factors. However, the seed germination ecological niche and its effects on plant colonization have received much attention and need to be deeply studied.

4.3. How Do Erosion Micro-Habitats Limit Plant Colonization

Vegetation restoration on gangue dump is limited by many factors, such as heavy metal content, surface temperature, substrate water content, salt content, and so on [43]. Soil pH value reflects the concentration of hydrogen ions in solution around the soil particles, which is one of the important indexes of soil chemical properties [44], and is an important factor affecting plant growth. EC is an important indicator of substrate water-soluble salt, which can reflect the degree of soil salinization, and is an important factor that cannot be ignored in plant growth [32]. In the study area, there was no significant difference in pH and EC among different erosion micro-habitats, with the gangue dump being slightly acidic and the EC of bare slope and the ephemeral gully being relatively higher. The correlation analysis also shows that these factors are not the limiting factors of vegetation growth. On the gangue dump slope, the vegetation in the ephemeral gully is restricted the most. Firstly, the vegetation is limited by there being fewer seeds in the ephemeral gully, due to strong scouring by runoff. Moreover, the surface water and heat characteristics of the ephemeral gully are not favorable for seed germination and seedling establishment, with the lowest humidity and the highest temperature. This happens because the content of particles of size >20 mm in the ephemeral gully is significant due to the highest erosion intensity carrying most of the small particles while the large grains remain. There are more macropores, which is bad for moisture retention. The same correlation research [45] showed that there was a certain correlation between the substrate composition and the surface temperature of coal gangue dump. Moreover, the gangue dump is prone to slow oxidation and spontaneous combustion at high temperatures [46]. The ephemeral gully is so deep that it is influenced mostly by the inner heat and it is difficult for heat to dissipate. This case is proved in our study which showed that there was a significant positive correlation (p < 0.05) between the particle size of >20 mm and the surface temperature, and a significant negative correlation between the bulk density and the capillary water capacity. Thus, erosion of micro-habitat by strong runoff affects its physical properties and hydrothermal conditions, which limits plant colonization due to loss of seeds and seedling death.
Different plants have different responses to the physicochemical properties as well as surface water and heat characteristics of the waste dump. Among them, S. viridis and T. terrestris are significantly affected by the surface temperature, showing a negative correlation. The higher the surface temperature, the worse is the growth condition. However, the effect of the surface temperature on E. indica is slightly lower than that on S. viridis and T. terrestris. Studies have showed that the seeds of E. indica germinate well when the temperature is over 25 °C [47,48], which also proves E. indica has a better resistance to high temperature stress. In the gangue dump, there is a negative relationship between the particle size of >20 mm and the growth of S. viridis. The content of particles of size > 20 mm in the ephemeral gully is the highest, and the surface substrate temperature still remains the highest. The excessive surface temperature exceeds the suitable germination threshold of plant seeds, which burns the roots of seedlings and makes it difficult for them to survive. Therefore, plant colonization is not only limited by higher surface temperature and larger grains, but also depends on the stress resistance of seedlings. These factors especially shape the vegetation composition and succession.

5. Conclusions

Our study showed that the ephemeral gully formed an uncomfortable micro-habitat for plant colonization with less sand but more giant grains as well as higher surface temperature and lower surface humidity, while the deposit body formed an opposite micro-habitat. The vegetation coverage, plant and seedling density of the deposit body were significantly higher than those of the ephemeral gully. The vegetation characteristics were closely related to the regeneration characteristics and surface hydrology conditions. The soil physical properties as well as substrate temperature and water content affected the surface temperature and humidity extremely significantly. The deposit body and rill are likely to drive plant colonization due to their higher sand content and micro-topography, which is useful for trapping water and seeds and provides a favorable temperature for seed germination and establishment of seedlings. The plant colonization is driven not only by the seed supply and comfortable conditions but also by the seed germination characteristics and their relationship with environmental factors. The ephemeral gully is not beneficial for plant colonization because of its unhealthy mechanical composition and strong runoff scouring, and is prone to drought, high temperature, and few seeds. The erosion micro-habitat formed by strong runoff and soil erosion limits plant colonization according to both seed loss and seedling death caused by the physical properties and hydrothermal conditions. Additionally, plant colonization is dependent upon the stress resistance of seedlings, which needs to be studied in the future.

Author Contributions

Conceptualization, D.W. (Dongli Wang); Formal analysis, Y.Z.; Funding acquisition, D.W. (Dong Wang); Investigation, D.W. (Dongli Wang) and J.L.; Software, Y.Z. and T.W.; Supervision, D.W. (Dong Wang) and X.Z.; Writing original draft, D.W. (Dongli Wang); Writing review and editing, D.W. (Dongli Wang), J.Q., H.S. and J.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Liaoning natural resources science and technology innovation project “Soil substrate improvement and bio-remediation technology and application study in western Liaoning mining area” and the project supported by the discipline innovation team of Liaoning Technical University (LNTU20TD-01).

Data Availability Statement

All original data are in the manuscript.

Acknowledgments

We thank Yang Liu, Yi Liu and Ziqian Zhang for their hard work during the field investigation.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. The location of the study region and respective sample sites with vivid photos of different soil erosion micro-topographies.
Figure 1. The location of the study region and respective sample sites with vivid photos of different soil erosion micro-topographies.
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Figure 2. Physical and chemical properties of substrates under different erosion micro-habitats. B, bare slope; R, rill; E, ephemeral gully; D, deposit body. Different letters (e.g., a, b) above the error bar in each column indicate there are significant differences between erosion micro-habitats at p < 0.05 (LSD). The box plots represent the median (middle solid line), the average (dashed line), 25% and 75% percentiles (the lower and upper boundaries of the boxes, respectively), and the 1.5 interquartile range (whiskers). This is applicable to the following figures and tables as well.
Figure 2. Physical and chemical properties of substrates under different erosion micro-habitats. B, bare slope; R, rill; E, ephemeral gully; D, deposit body. Different letters (e.g., a, b) above the error bar in each column indicate there are significant differences between erosion micro-habitats at p < 0.05 (LSD). The box plots represent the median (middle solid line), the average (dashed line), 25% and 75% percentiles (the lower and upper boundaries of the boxes, respectively), and the 1.5 interquartile range (whiskers). This is applicable to the following figures and tables as well.
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Figure 3. The surface humidity, substrate water content, surface temperature and substrate temperature under different erosion micro-topographies. Different letters (e.g., a, b, c) above the error bar in each column indicate there are significant differences under erosion micro-topographies at p < 0.05 (LSD). The meaning of the abbreviations can be found in Figure 2.
Figure 3. The surface humidity, substrate water content, surface temperature and substrate temperature under different erosion micro-topographies. Different letters (e.g., a, b, c) above the error bar in each column indicate there are significant differences under erosion micro-topographies at p < 0.05 (LSD). The meaning of the abbreviations can be found in Figure 2.
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Figure 4. The vegetation density and coverage of different species and total plants in different erosion micro-topographies. Different symbol colors represent plant species in different erosion micro-topographies, with black symbols for bare slope, red symbols for rill, blue symbols for ephemeral gully, green symbols for deposit body. Different letters (e.g., a, b) above each error bar in each column indicate there are significant differences between erosion micro-topographies at p < 0.05 (LSD). The meaning of the abbreviations can be found in Figure 2.
Figure 4. The vegetation density and coverage of different species and total plants in different erosion micro-topographies. Different symbol colors represent plant species in different erosion micro-topographies, with black symbols for bare slope, red symbols for rill, blue symbols for ephemeral gully, green symbols for deposit body. Different letters (e.g., a, b) above each error bar in each column indicate there are significant differences between erosion micro-topographies at p < 0.05 (LSD). The meaning of the abbreviations can be found in Figure 2.
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Figure 5. The density of soil seed bank and seedlings under different erosion micro-topographies. Different letters (e.g., a, b) on right of each row or above the error bar in each column indicate there are significant differences between erosion micro-topographies at p < 0.05 (LSD). The meaning of the abbreviations can be found in Figure 2.
Figure 5. The density of soil seed bank and seedlings under different erosion micro-topographies. Different letters (e.g., a, b) on right of each row or above the error bar in each column indicate there are significant differences between erosion micro-topographies at p < 0.05 (LSD). The meaning of the abbreviations can be found in Figure 2.
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Figure 6. The partial least squares path modeling (PLS-PM) for the effects of oil physical properties (Spp), soil chemical properties (Scp), substrate water content and temperature (Swt), surface humidity and temperature (Sht) and regeneration characteristics (Rc) on vegetation density and coverage (Vdc). The values on the arrows from the circle to the rectangle or circle represent the correlation between variables. Positive and negative effects are indicated by the red and blue lines, respectively. * indicates 0.01 ≤ p < 0.05, which means significant correlation, ** and *** indicate 0.01 ≤ p < 0.001 and p≤ 0.001 respectively, which mean extremely significant correlation.
Figure 6. The partial least squares path modeling (PLS-PM) for the effects of oil physical properties (Spp), soil chemical properties (Scp), substrate water content and temperature (Swt), surface humidity and temperature (Sht) and regeneration characteristics (Rc) on vegetation density and coverage (Vdc). The values on the arrows from the circle to the rectangle or circle represent the correlation between variables. Positive and negative effects are indicated by the red and blue lines, respectively. * indicates 0.01 ≤ p < 0.05, which means significant correlation, ** and *** indicate 0.01 ≤ p < 0.001 and p≤ 0.001 respectively, which mean extremely significant correlation.
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Figure 7. The relationship between the substrate properties of erosion micro-topographies and plant species. Association of the plant species(density) and the substrate properties was analyzed with Mantel tests. The color gradient indicates Pearson correlation coefficients among the site conditions. The dark color represents negative correlation, the light color represents positive correlation, the number in the circle represents the specific correlation coefficient, the asterisk represents the degree of correlation, * indicates 0.01 ≤ p < 0.05, which means significant correlation, ** and *** indicate 0.01 ≤ p < 0.001 and p ≤ 0.001 respectively, which mean extremely significant correlation.
Figure 7. The relationship between the substrate properties of erosion micro-topographies and plant species. Association of the plant species(density) and the substrate properties was analyzed with Mantel tests. The color gradient indicates Pearson correlation coefficients among the site conditions. The dark color represents negative correlation, the light color represents positive correlation, the number in the circle represents the specific correlation coefficient, the asterisk represents the degree of correlation, * indicates 0.01 ≤ p < 0.05, which means significant correlation, ** and *** indicate 0.01 ≤ p < 0.001 and p ≤ 0.001 respectively, which mean extremely significant correlation.
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Table
Table 1. Soil erosion characteristics of each micro-topography.
Table 1. Soil erosion characteristics of each micro-topography.
TypeLength (m)
Min–Max		Width (cm)
Min–Max		Depth (cm)
Min–Max		Description
Bare slope15.40–17.80----The surface is occupied mainly by sheet erosion.
Rill6.82–7.2311–304–8The erosion section is undercut in a “V” shape.
Ephemeral gully7.86–13.5620–558–35The eroded section is inverter trapezoid.
Deposit body------The ground is flat and loose.
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Wang, D.; Qiao, J.; Zhang, Y.; Wu, T.; Li, J.; Wang, D.; Zhao, X.; Shen, H.; Zou, J. Effects of Erosion Micro-Topographies on Plant Colonization on Weathered Gangue Dumps in Northeast China. Sustainability 2022, 14, 8468. https://doi.org/10.3390/su14148468

AMA Style

Wang D, Qiao J, Zhang Y, Wu T, Li J, Wang D, Zhao X, Shen H, Zou J. Effects of Erosion Micro-Topographies on Plant Colonization on Weathered Gangue Dumps in Northeast China. Sustainability. 2022; 14(14):8468. https://doi.org/10.3390/su14148468

Chicago/Turabian Style

Wang, Dongli, Jingting Qiao, Ye Zhang, Tong Wu, Jia Li, Dong Wang, Xiaoliang Zhao, Haiou Shen, and Junliang Zou. 2022. "Effects of Erosion Micro-Topographies on Plant Colonization on Weathered Gangue Dumps in Northeast China" Sustainability 14, no. 14: 8468. https://doi.org/10.3390/su14148468

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