Authors: Su, DH; Zhou, JW; Feng, HB; Zhu, Y; Li, R; Ye, JC; Han, X

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Su, DH, Zhou, JW, Feng, HB, Zhu, Y, Li, R, Ye, JC & Han, X 2023, 'Revegetation of high and steep rocky slopes: Below-ground habitat reconstruction of plants', in B Abbasi, J Parshley, A Fourie & M Tibbett (eds), Mine Closure 2023: Proceedings of the 16th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth,

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Mining, road construction, and other engineering activities result in a huge number of high and steep rocky slopes, the ecological restoration of which is a global issue. In this study, below-ground habitat reconstruction is presented as a revegetation strategy for high and steep rocky slopes. The vegetation was established by drilling holes in the slope surface and planting appropriate arbores, shrubs, and climbing plants. Plant growth, as well as temperature and humidity dynamics on the slope, were observed over five years through planting experiments and continuous monitoring in the study area. The results indicate that the plants showed good adaptability with an overall survival rate of 77%. Moreover, Cotinus coggygria var. cinereus Engl., Parthenocissus tricuspidata (Siebold & Zucc.) Planch., Platycladus orientalis (L.) Franco, and Punica granatum Linn. had final survival rates greater than 80% and great growth rates, which could quickly establish a stable ecosystem on high and steep slopes to produce the shading effect. Therefore, they could be used as the main plants for the revegetation of high and steep rocky slopes. Simultaneously, the water vapor fields of the rocky slope were constructed for each season. The results show that the depth where the plant roots were found in the water vapor saturation zones in each season, which can generate condensation water to supply the plant growth. This research serves as a resource for revegetation and ecological restoration of high and steep rocky slopes.

Keywords: high and steep rocky slopes, revegetation, below-ground habitat reconstruction, water vapor The below-ground habitat is the most critical plant habitat component, consisting of soil and the water, salinity, air, and organic matter contained therein. It is the source of water and nutrients on which plants depend for survival and growth (Feng et al. 2021). Below-ground habitat reconstruction of plants is the artificial restoration of the below-ground habitat necessary for plant growth and survival. Create below-ground habitat conditions that are suitable for plants on high and steep rocky slopes using reasonable engineering. Suitable plants, including arbors, shrubs, and vines, are selected and planted on the restored rocky slopes so that the plants can take root and grow on the rocky slopes and absorb water and nutrients from slope fissures to achieve long-term revegetation of the high and steep slopes. The following main points can be summarized. The quadrat method was utilized to investigate the dominant plant species around the high and steep slopes. Then, sample pits were excavated within the quadrats for the investigation of the below-ground habitat structure of plants. The specifications of the sample pits are shown in Figure 3. Use nails and nylon cord to divide the investigation surface into 10 × 10 small squares of size 10 cm × 10 cm. Counting the number of coarse (root diameter ≥ 10 mm), medium (10 mm > root diameter > 2 mm), and fine (root diameter ≤ 2 mm) roots in each square, analyzing the frequency of the three kinds of roots at various depths, and determining the position of the plant root mass (it’s part of the root system that has the most amounts of root hairs and it is the functional zone for water and nutrient absorption) in the depth range between 20% and 80% of the cumulative frequency. Soil samples were taken at each layer and tested for available phosphorus, rapidly available potassium, and alkaline hydrolysis nitrogen. The investigation can serve as a foundation for the selection of plants and the construction of the habitat structure. The survival rate and growth indicators including apical height, base diameter, and crown diameter of the 15 plants were investigated at 3 months (August 2015), 2 years (August 2017), 3 years (August 2018), 4 years (August 2019) and 5 years (August 2020) after planting. Growth indicators were investigated using a random sampling method with a 15% probability of sampling. The annual growth rate (AGR) was calculated using the following equation: where: = the value of the indicator for the last measurement, = the value for the next measurement, = the number of year past between the measurements. We conducted monitoring experiments to study the distribution and transformation characteristics of the water vapor field. Three horizontal monitoring holes were arranged on the rocky slope, which were 2 m, 10 m, and 18 m away from the ground respectively (Figure 4). The diameter of the monitoring holes was 10 cm, and the depth was 200 cm. The temperature and humidity monitors (iButton DS1923, Maxim Integrated Ltd., San Jose, CA, USA) with automatic recording and storage functions were fixed on a PVC tube and placed at different depths (0 cm, 10 cm, 20 cm, 40 cm, 60 cm, 100 cm, 150 cm, and 200 cm) of the monitoring holes (Figure 4). The monitoring holes were blocked by sealing plugs to ensure the accuracy of the monitoring results. The experiments were conducted in four seasons of a hydrological year: summer (July 14th to July 21st, 2018), autumn (October 27th to November 4th, 2018), winter (January 10th to January 17th, 2019), and spring (April 23rd to 30th, 2019). Uninterrupted data collection was carried out during each monitoring period. The monitoring frequency was every 10 min. The experiments collected more than 80,000 sets of temperature and humidity data. The fissures serve as a channel for plants to obtain water and nutrients as well as a place for plant roots to develop. Therefore, the fissures played a significant role in determining the location of the planting hole, with its density and size serving as the primary indicators. The planting holes had a diameter of 15 cm, a depth of 50 cm, and a downward sloping angle of 45° (Figure 4). The horizontal and vertical distance between the planting holes was 1 m. The selection of plant species was guided by the following principles: a) Mainly native plants; b) Plants with strong tolerance to drought and infertile properties; c) Plants with Strong sprouting of roots; d) A variety of arbors, shrubs, and vines to ensure ecosystem stability; e) Plants with ornamental properties. 15 plants were selected for this study, and the plant species and numbers are shown in Table 1. Density and size can only describe the apparent characteristics of the fissures, but not the connectivity of the fissures. Excellent connectivity provides plants with adequate oxygen, water, and nutrients. The connectivity of fissures can be obtained by hydraulic conductivity tests. Water was injected into the planting hole and the change in water level over time was observed to determine the good or bad hydraulic conductivity. Depending on the rate of water level drop, the hydraulic conductivity of the planting holes was classified into three levels: a) The water level dropped more than 5cm within 1 minute, the hydraulic conductivity was good; b) Water level dropped more than 1cm less than 5cm within 1 minute, the hydraulic conductivity was medium; c) Water level drops less than 1cm within 1 minute, the hydraulic conductivity is poor, and the planting holes of this level were discarded. After completing the above, the mixture of soil and organic fertilizer was filled into the planting holes, which provided initial nutrition for plants. Then, the plant seedlings were transplanted and each plant was numbered to create its growth profile. In 2015, we planted 949 plants by drilling holes in the rocky slope and carried out monitoring experiments on plant survival rates. As shown in Figure 5, the plant survival rate of the rocky slope gradually declined and stabilized with time. In 2020, the survival rate was 76.92%, indicating that the plants have adapted to the harsh habitat of the rocky slope. Meanwhile, we mulched the summit of the rocky slope with 40 cm of soil and planted dozens of plants of the same species and size, but after three months, every plant perished. Additionally, in 2016, the local government conducted ecological restoration work for the quarry by mulching 80 cm of soil at the foot of the rocky slope and planting 126 plants, including P. orientalis (L.) Franco and Prunus cerasifera 'Pissardii', only nine of which survived a year later. The government realized that the restoration was not effective, so 307 additional plants were replanted at the foot of the rocky slope. The survival rate of plants decreased rapidly to 25.95% in the first year, and then slowly declined and stabilized. The results above demonstrated that the survival rate of plants growing on rock walls was significantly higher than that of plants growing in mulch, contrary to what we typically consider soil is a better habitat for plants than rock. We believe there are two reasons for this: On the one hand, the soil exchanges moisture with the atmosphere more intimately due to its higher porosity than the fissure rate of the rock mass. The water holding capacity is weak when the surface lacks coverage with vegetation; On the other hand, the specific surface area of soil is significantly larger than that of rock mass, leading to greater matrix suction of soil than fissures. It is more difficult for plants to absorb water from the soil under the same circumstances. This further confirms the significance of the water vapor field of the rocky slope in supplying the water required for plant growth. Figure 9 depicts the seasonal distribution of water vapor partial pressure in the rocky slope, enabling the analysis of the seasonal characteristics of water vapor migration. The water vapor partial pressure and water vapor partial pressure gradient were highest in summer and lowest in winter and between summer and winter in autumn and spring. The high temperature and precipitation in the study area during the summer and spring resulted in high water vapor content and water vapor partial pressure in the air, so the atmosphere became a source of water vapor for the rocky slope, and the water vapor partial pressure decreased from the shallow to the deep part of the rocky slope, which was consistent with the direction of water vapor migration. The air was cold and dry resulting in a low temperature and water vapor concentration in winter and autumn. While the groundwater saturated zones had a greater temperature than the unsaturated zones, where the liquid water vaporized quickly and had higher water vapor content, it became a source of water vapor for the unsaturated zones. Water vapor migrated from the deep to the shallow part of the rocky slope as its partial pressure decreased. Figure 10 illustrates the seasonal distribution of water vapor saturation zones in the rocky slope. Summer and spring water vapor saturation zones were similar in extent and more widely distributed. It extended from 100 cm to the deep part of the rocky slope in summer and from 40 cm to the deep in spring. Water vapor saturation zones were equally approximated in autumn and winter but significantly different from summer and spring, with substantial shrinkage occurring. It ranged from 20 cm to 60 cm, with a somewhat greater bottom range of up to 100 cm; and the range was from 10 cm to 100 cm in the winter. Condensate is generated in the water vapor saturation zones when the rate of condensation of vapor exceeds the rate of evaporation of water. The formation of condensate, according to Wang et al. (2022), can be divided into three stages: the first stage involves the generation of droplets on the fissure surface; the second stage involves the coalescence of droplets when the number of droplets is great; and the third stage involves the coalescent droplets connecting to form a water film as the number of droplets continues to rise. The plant roots that grew into the fissures could absorb the condensate there, supplying the growth and development of the rocky slope plants. Water vapor saturation zones were generated in the rocky slope at the positions of plant roots in the autumn, winter, and spring; even though the range of the saturation zone began at 100 cm in the summer, it was also produced at shallow depths at night. Therefore, rocky slope plants can obtain water from the fissured rock slope in different seasons. Furthermore, the fissures returned to the unsaturated state of water vapor when the plants absorbed the condensate in them, and the surrounding water vapor would be transported to the regions under the gradient of water vapor partial pressure, causing the fissures to reach the saturated state of water vapor again and continue to produce condensate. As a result, the fissure network functions as a sustained system of water supply pipelines, continuously supplying water to the plants.

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