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Article

Evaluation of Soil Loss Tolerance and Tree Growth Features Based on Planting Ground Methods in the Alpine Center, Degraded Forestland in the Republic of Korea

by
Kyeongcheol Lee
1,†,
Yeonggeun Song
2,†,
Haeun Koo
1,
Hyeonhwa Kim
1,
Hyeongkeun Kweon
1 and
Namin Koo
3,*
1
Department of Crops and Forestry, Korea National University of Agriculture and Fisheries, Jeonju 54874, Republic of Korea
2
Department of Forest Environment Science, College of Agriculture and Life Science, Jeonbuk National University, Jeonju 54896, Republic of Korea
3
Division of Forest Ecology, National Institute of Forest Science, Seoul 55365, Republic of Korea
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2023, 14(2), 200; https://doi.org/10.3390/f14020200
Submission received: 18 October 2022 / Revised: 8 January 2023 / Accepted: 17 January 2023 / Published: 20 January 2023
(This article belongs to the Section Forest Ecology and Management)
Browse Figures
Review Reports Versions Notes

Abstract

:
The Jeongseon Alpine Center, a degraded forestland, has a very unique soil feature as it was filled with soil cut from the nearby area and has not yet been rehabilitated since the end of the 2018 Winter Olympics. Therefore, this study attempted to identify a way to establish a stable and efficient planting groundwork to successfully restore this distinctive region. Six different planting groundworks, control, biochar, leaf-mold, mulching, tillage, and polyacrylamide treatments were constructed in September 2020. Soil-related indices have been tracked while the growth and physiological responses of planted Pinus densiflora (Pd) and Larix kaempferi (Lk) seedlings were monitored until May 2022. Mulching had 2–4 times the average SWC when compared to the control. Simultaneously, there was the least variation in soil temperature change, and total soil loss was only 0.05 ton/ha. Similarly, the leaf-mold had a relatively high SWC, and soil fertility increased, while total soil loss was 6.37 tons/ha, indicating a high trend in line with biochar. The Pd leaf-mold had the highest in Amax, E, gs, and Jmax. Furthermore, following the leaf-mold, mulching showed the second-highest photosynthetic indices in general, and the height and DRC also demonstrated favorable results in the above two treatments. Mulching had 1.6 to 2.2 times higher than the control group in PIabs and PItotal, which represent photosynthetic vitality, so the impact on environmental stress was thought to be less than that of leaf-mold. The mulching of Lk showed the greatest growth and physiological responses, Nonetheless, the photosynthetic indices were rather low when compared to Pd, with no discernible differences across treatments. As a result, the considerable effect of the planting groundwork method could not be demonstrated. Therefore, in the case of Pd, the leaf-mold showed the best growth and physiological response. Nevertheless, in terms of soil temperature maintenance and soil loss for slope stability, mulching is assessed to be the most ideal planting groundwork method.

1. Introduction

Owing to the recent rapid industrialization and increase in demand for tourism and recreational facilities, the forest land-use conversion including conservation areas is continuously increasing in the Republic of Korea, and the number of cases per type of development for non-agricultural development such as quarries for the extraction of soil and minerals, golf courses, and ski resorts is surging [1,2,3]. Therefore, although complete restriction of the development of mountainous areas due to their high proportion is difficult, devising an ecological and stable restoration plan for forest damage that inevitably results from development is necessary.
The restoration of forest ecosystems has mainly been classified into the concepts of creating ecological structures and functions that closely resemble their original state [2,4], landscape maintenance, and prevention of disasters for the improvement of the structure and function of forests [2]. Methods for the restoration of deforested areas include physical stability structures and biological greening techniques that ensure the stability of slopes [3]. Furthermore, of these, biological greening techniques are known to not only prevent soil erosion of slopes and provide stability but also to create landscapes and maintain healthy ecosystems with the surrounding vegetation [5]. However, these restoration methods have been mainly focused on the restoration of slopes at the boundary between forests and roads, and greening techniques have been studied in the mix-seeding of invasive species and native plants [6], utilization of forest topsoil [7], and ecological restoration using soil seeds found in forest topsoil [8,9].
The restoration through planting seedlings in deforested areas has the advantage of shortening the restoration period and preventing the loss of soil caused as a result of the net effect of the roots [2,5]. However, depending on the type of deforestation site, in case of damaged lands located near valleys rather than on slopes adjacent to roads, soil disturbance because of deforestation and urbanization is extreme, exposure to strong winds and sunlight is high, and severe soil loss may occur. Under these conditions, stabilization of the vegetation base at an early stage to help the survival rate and establishment of vegetation while minimizing soil loss is an important strategy in forest restoration. In particular, the soil is the habitat foundation of plants and animals in a healthy ecosystem, and in the case of topsoil, the formation of soil aggregates maintains a stable structure even during heavy rain, thereby facilitating rainfall penetration, long storage of carbon, and greatly contributes to the reduction in atmospheric carbon dioxide [10]. In the Republic of Korea, there is a tendency to not collect and preserve topsoil, which is an important element of the vegetation base, during construction [11]. In these cases, buildings or facilities are built on top after site suspension, and the issue of topsoil is finally considered during the planting stage, but as good topsoil has already been lost, separate soil has been often transported to create a vegetation base. The regeneration of topsoil and its loss from such reckless measures or damage to its condition by other reasons requires efforts over long periods of time and high expenses [11].
In preparation for the Pyeongchang 2018 Winter Olympics, the Alpine Center was built in Jeongseon, Gangwon-do, Republic of Korea. By the Olympic standards, the valley has been filled with soil excavated from the nearby area, creating a large scale of degraded forestland. This region has very distinctive characteristics that are uncommon in other Korean forests, including frequent land loss due to anthropogenic topographic changes, increased soil loss and gravel content, and a soil pH of 7.5. In order to successfully carry out restoration for this type of deforestation, research on creating a rapid and stable tree planting ground that can reflect the effects of soil loss prevention, plant establishment, and growth promotion is required.
To successfully restore damaged forest areas, this study investigates the effects of soil physicochemical changes, water holding capacity, soil loss, and vegetation growth according to planting groundwork methods such as soil supplementation, mulching, and tillage. In addition, attempts to present basic data on how to create an efficient planting ground in degraded forestlands.

2. Materials and Methods

2.1. Establishment of Study and Treatment Areas

The site of the study is a filling slope located in the Alpine Center in Jeonseon-gun, Gangwon-do, and the direction of the slope is facing northeast, with an altitude of 619 m (Table 1), and the construction of the study area was completed by 16 September 2020, after finishing a general survey of the target site on 17 June 2020. A total of six planting groundwork treatment areas were constructed in the order of biochar, leaf-mold, control, mulching using coir net, tillage, and PAM (Polyacrylamide), respectively, and each was constructed in a width of 1 m and length of 10 m, with a buffer zone with a width of 1 m in between each of the treatment areas (Figure 1).
To prevent soil and cobblestone from entering from the outside, acrylic separators were placed on the upper and both sides of the slope of each treatment area, and a sedimentation basin was installed using a black cloth made of an impermeable material on the lower side to measure the amount of lost soil.
In addition, to minimize the damage due to compaction damage due to heavy equipment during the development of treatment areas, all work was carried out manually using small farming tools, and all planting groundwork was treated to a depth of 15 cm and then 2-year-old seedlings of Pinus densiflora and Larix kaempferi were planted in each treatment area. After completing the treatment areas, a soil surface penetrometer (FS-45, Forestry Suppliers Inc., Jackson, MS, USA) and a clinometer were utilized to investigate the slope characteristics per treatment area, and as a result, the soil hardness was in the range of 19.7–22.7 mm, and the slope in a range of 11°–15° (Table 1).
The leaf-mold used for the treatment area was collected and mixed with the organic material layer of pine trees and the A layer of soil (≤15 cm) around the treatment area, and to minimize the influence of cobblestone and buried seeds, a 2 mm sieve was used for filtering and homogenization, and biochar was directly produced using forestry by-products under anaerobic oxidation conditions through the cooperation of the East Coastal Forest Fire Center of Gangwon Province. The PAM used in the experiment has anionic characteristics, and a domestically manufactured product satisfying the standard of specific gravity of 0.75–1.25 g cm−3 at 20 °C was used. The treatment ratio for each soil conditioner was 3% of the soil weight, and after dissolving 200 g of PAM in water, it was evenly sprayed throughout the treatment area. In addition, the weather environment including temperature and humidity, rainfall, wind direction, and wind speed in the survey site was periodically monitored every 20 min through an Automatic Weather System, and one soil moisture and temperature sensor were buried in each treatment area.

2.2. Soil Physicochemical Investigation

To investigate soil physicochemical changes according to the composition of the vegetation base, soil texture, soil acidity (pH), cation exchange capacity (CEC), total nitrogen content (T–N), effective phosphoric acid content (avail.–P2O5), organic matter content (OM), were investigated, and the survey was conducted twice in March 2021 and May 2022, and the collected soil was air-dried and filtered through a 2 mm sieve and used for analysis. The physicochemical properties of all soils followed the soil and plant analysis method of the Rural Development Administration [12] and Methods of Soil Analysis of Soil Science Society of America [13]

2.3. Soil Loss Investigation

Changes in the amount of soil loss due to the slope fixing effect by treatment area were monitored from 16 September 2020, to 20 July 2022. After taking a sample of lost soil from the sediment basin created in the treatment area was collected, transferred to the laboratory, dried at 80 °C, and measured by weight [9].

2.4. Growth Characteristics

To determine the difference in growth according to the vegetation base treatment of Pinus densiflora and Larix kaempferi, the height and diameter at the root collar (DRC) were investigated around May 2022, and the DRC was measured at 1 cm above the ground surface using a vernier calliper, and height was measured using a tape measure.

2.5. Chlorophyll and Carotenoid Content

After completion of the photosynthesis experiment, photosynthetic pigments for each treatment area were investigated. Five leaves were collected from each treatment zone, and 0.1 g of leaf piece was put into a 20 mL glass bottle containing 10 mL of DMSO (dimethyl sulfoxide) solution and pigments extracted for 6 h [14] in an incubator set to 60 °C. Here the absorbance of the extracted solution was measured at wavelengths of 663, 645, and 470 nm using a UV/ VIS Spectro-photometer (HP 8453, Hewlett-Packard, Palo-alto, CA, USA), and chlorophyll a, b, and a + b, and carotenoid contents were calculated [15,16,17].

2.6. Photosynthesis Response

The photosynthesis response was carried out on 19–22 April 2022, for Pinus densiflora, and on 9–12 May 2022, at a later time for Larix kaempferi, in consideration of the opening period of the leaves. The experiment was carried out using a Portable Photosynthesis System (Li-6800, Li-Cor Inc., Lincoln, NE, USA), and an LED light source attached to the photosynthesis measuring device, PPFD (Photosynthetic Photon Flux Density) was set to 1200µmol·m−2·s−1, and maximum photosynthetic rate (Amax), stomatal transpiration rate (E), and stomatal conductance (gs) were investigated. The leaves were immediately collected from the field after completion of the photosynthesis experiment, and the leaf areas were measured using Winseedle (ver. 2020a) program and used for calculation. Instantaneous transpiration efficiency (ITE), intrinsic water use efficiency (WUEi), and stomatal limitation (Ls) were calculated using the measured results [18,19]. In addition, by examining the CO2 response curve (A-Ci) in mesophyll cells by the concentration of CO2 (0 µmol m−2 s−1 –1400 µmol mol−1), the maximum carboxylation rate (Vcmax) and maximum electron transport rate (Jmax) were calculated [20]. At this time, the common measurement conditions maintained the air inflow to the chamber at 600 μmol·s−1, and a temperature of 25 °C ± 1 °C.

2.7. Chlorophyll a Fluorescence

The chlorophyll a fluorescence response carried out the OKJIP analysis (polyphasic rise of chlorophyll a fluorescence transients), and a chlorophyll fluorescence analyzer (Plant Efficiency Analyzer, Hansatech Instrument Ltd., King’s Lynn, UK) was applied to leaves adapted to the dark for 20 min and irradiated for 1 s with 3500 µmol·m−2·s−1 of light, and the chlorophyll fluorescence density was investigated for 50 µs (O stage), 300 µs (K stage), 2 ms (J stage), 30 ms (I stage), and 500 ms (P stage). Based on the OKJIP analysis results, main environmental stress indicators such as VJ, VI, VK (Relative variable fluorescence at the J, I, and K-step), Fo/Fm (Quantum yield at t = 0 for energy dissipation), Fv/Fm (Maximum quantum yield of primary photochemistry at t = 0), RC/CSo (Amount of active PSII RCs per CS at t = 0), PIabs (Performance index on absorption basis), PItotal (Measuring the performance up to the PSI end electron acceptors), and SFIabs (The structure function index on absorption basis) were analyzed [21,22,23].

2.8. Data Analysis

The variance homogeneity was verified using Levene’s test, and if the assumptions were satisfied, one-way analysis of variance (ANOVA) and DMRT (Duncan’s Multiple Range Test) post hoc analysis was used. Additionally, a principle component analysis (PCA) was used to illustrate the relationship between planting ground, environmental conditions, and physiological indices. The statistical analysis was carried out using the SPSS statistics program 19.0 (SPSS INC., Chicago, IL, USA) and a biplot based on the PCA results was prepared using a CSM analyzer (ver. 7).

3. Results and Discussion

3.1. Meteorological Environment and Growth-Based Characteristics

The Republic of Korea is topographically stretched from north to south, surrounded by the sea on three sides, and forests cover approximately 65% of the country. The southern slopes of the forest, in particular, are characterized by prolonged periods of sunshine, shallow soil depth, dehydrated soil conditions, and frequent soil erosion due to concentrated summer rainfall [24]. The study site is a deforested area on the southern slope where the existing vegetation has already vanished. The slope and soil hardness was adjusted to 11–14°, and 20 mm, respectively. If the soil hardness is high, the penetration of rainwater into the soil may be hindered and erosion and runoff from the soil surface may increase [25]. Regarding root system development, soil hardness of 23 mm or more often impedes root growth, and it has been found that stable growth can be observed between 11 and 20 mm [26]. Especially, this site is a mountainous area located at an altitude of 620 m above sea level, with frequent loss of soil due to snowfall in winter and strong winds blowing from the mountainside.
As a result of examining the daily average temperature during the experiment period, a change in a range of a maximum of 25 °C to a minimum of −14 °C was observed, and high temperatures were observed in the period between July and August, and lowest temperatures were observed around December to January (Figure 2C). The rainfall in both 2021 and 2022 was 236 and 119 mm, respectively, between March and April, indicating relatively higher rainfall compared to other periods, and during the rainy season from the end of June to July, while there were many rainy days, the amount of rainfall was relatively small (Figure 2C).
The study area had the characteristic of a damaged existing vegetation environment due to the development of ski slopes and lack of surrounding vegetation that could block the wind, and during the experiment period, the direction of daily winds was mostly from the southwest (SW) direction (Figure 2A), strong winds of 10 m/s or more were also frequently observed in the southwest (SW). This reflects that cold winds during the winter could have a negative effect on the growth of vegetation planted in the valleys, and in the case of Larix kaempferi, some were found to have withered.
The soil water content (SWC) and soil temperature (Soil Temp.) showed some differences by treatment group. In the case of SWC, the control group showed the lowest with an average of 11.5%, indicating that the basic soil environment of the study site is susceptible to dry stress. However, the mulching treatment group showed an average of 27.4%, which was about 2.4 times higher than that of the control group, and in other groups, biochar showed 19.5%, leaf-mold 18.6%, and the tillage treatment area 18.1%, all similar values to each other, whereas the SWC of PAM was 12.0%, showing almost no difference from the control group (Figure 3A–F). As a result, it was found that mulching treatment using a coir net greatly helped in the retainment of soil moisture, while PAM material did not help to significantly improve the retention of soil moisture.
Soil temperature directly affects root growth and microbial activity and is influenced by the amount of sunlight, atmospheric temperature conditions, and the supply and retainment of soil water, and in particular, the lower the amount of soil water, the greater the influence of sunlight and temperature [27]. According to the treatment type, the average value of soil temperature (Soil Temp.) was in the range of 12.5 °C–15.4 °C, showing no significant difference, but on 27 July 2021, which showed the highest temperature during the experimental period, biochar, leaf-mold, tillage, and PAM treatment areas each showed a range of 37.1 °C–39.7 °C, while the control and mulching treatment areas each showed 33.0 °C and 34.4 °C, respectively, showing a relatively lower level. Comparing the period with the lowest soil temperature, as of 9 January 2021, which showed the lowest temperature in winter, biochar, leaf-mold, and the control area each were in the range of −12.1 °C–−12.7 °C, respectively, but the mulching treatment group was −8.1 °C, showing the highest soil temperature among all treatment groups (Figure 3A–F). These results show that the fluctuation of soil temperature is lowest because the coir net material itself acts as a thermal insulator during mulching treatment and maintains soil water from the dry external environment.

3.2. Amount of Soil Loss

Soil loss occurring from erosion in slopes such as in valleys can be a factor that hinders the stable growth of vegetation due to the loss of inorganic nutrients and organic carbon, reduction in water holding capacity, and the loss of plant growth space due to the formation of gulleys [28]. Factors that directly affect soil loss include soil texture, slope, rainfall intensity, wind, and vegetation cover [29]. In the case of the study target site, the restoration of the damaged site through filling was carried out, and the prevention of soil loss and fixation of the slope can act as major for the successful long-term ecological restoration. According to the planting groundwork composition type, cumulative soil loss of 10.28 ton/ha and 6.37 ton/ha in the biochar and leaf-mold treatment groups occurred, respectively, wherein the control group and the mulching treatment group was only 0.05 ton/ha, reflecting its effectiveness for fixation (Figure 4).
As a result of a simulated rainfall experiment to investigate the soil erosion reduction effect of biochar, it has been reported that due to the improvement in soil properties, soil loss was reduced by 50% when biochar was added [30]. However, the results may vary depending on the various factors such as field conditions, biochar input ratio, and the size of the particle. Li et al. [31] reported that when the biochar was added to the soil at a ratio greater than 3%, the soil loss accelerated than in the soil without biochar, this means that the smaller the particle size of biochar and higher input to the soil, the more likely it is to have a negative impact on soil erosion due to runoff or heavy sedimentation. Since the proportion of biochar used in this field is 3% of the soil weight and the area is heavily wind-weathered due to the slope being filled, it is necessary to reduce the amount of biochar by at least 3% to avoid soil erosion.
In the case of the control group, at the time of the slope construction (2015), the area was created by filling with cobblestone and slope compaction. The slope was left under continuous exposure in open space conditions after being used, and the loss of topsoil due to rainfall and surface runoff had already significantly progressed (>75% of the surface stone content) before the experiment, thus the soil loss is believed to have been analyzed as low. Mulching is known to play a role in reducing the direct impact of raindrops by covering the soil and reducing erosion by buffering rainfall runoff [32], and the mulching treatment group installed in the study site accordingly showed the lowest amount of soil loss while the net attached to the ground surface effectively suppressed erosion during rainfall.
PAM (polyacrylamide), a water-soluble polymer, is a compound with a strong adsorption functional group and is known to inhibit runoff by increasing the binding force between soil particles through it [29,33], and tillage treatment also has the effect of improving the physical properties of soil and reducing erosion while improving the formation of soil aggregates [34]. The accumulated soil loss of tillage treatment and PAM appearing in the restoration of deforested areas was also 0.56 ton/ha, which was extremely low compared to biochar and leaf-mold, but was found to have lost about 10 times more than the amount of the mulching treatment group (Figure 4). Benik et al. [35] reported that the amount of biomass accumulation was higher than that of bare soil during mulching treatment for the prevention of roadside slope erosion, and the amount of soil runoff was also reduced by more than 10 times.
In addition, it was found that abrupt soil loss occurred due to the snow melts and flows along with the soil particles, and heavy rainfall (Figure 2C) in the spring of the following year (March–April 2021) after the creation of the treatment group (Figure 4B), The soil loss rate in the control and mulching groups was high even before May 2021, whereas, the soil loss rate in the other groups rose after May 2021. (Figure 4). These results show that if an effective slope fixation method is presented during the formation of the initial vegetation foundation, overall soil loss can be greatly reduced with the subsequent establishment of vegetation.

3.3. Soil Physicochemical Changes

In the case of soil texture, in 2021, the proportion of sand was relatively high and that of clay was low in the tillage and PAM treatment groups, while the proportion of sand and clay was similar in the leaf-mold and biochar treatment groups. In 2022, after 1 year, while little change was observed in the tillage and mulching treatment groups, the proportion of sand greatly increased and that of clay relatively decreased in leaf-mold, and biochar showed a relatively large increase in the proportion of silt (Table 2). This tendency seems to be highly related to the large increase in soil loss due to rainfall and wind after April 2021. In addition, comparing soil chemistry, the pH of biochar and leaf-mold was slightly lower compared to that of other treatment groups, and over time, the total nitrogen content showed a tendency to significantly increase in every treatment group in 2022, whereas pH exhibited almost no change. Herbaceous plant germination occurred due to the construction of experimental sites, and thus partial decomposition took place after the soil micro-animals, roots, and dead plants accumulated in the soil in the form of organic matter while improving the physical properties such as soil pores. Furthermore, outside inflow due to environmental factors such as wind and precipitation can be considered. In particular, in the case of leaf-mold, the total nitrogen content increased the most among the treatments, showing the highest content with 0.14% of the treatment groups in 2022, and overall soil fertility such as CEC, OM content, and effective phosphoric acid also improved. Organic matter rich soils, such as leaf molds, promote microbial activity and accelerate decomposition, lowering soil pH but assisting plant growth in terms of nutrient availability [36]. Lee et al. [37] also reported that soil pH decreased in leaf-mold treated soil, but OM content, nitrogen, effective phosphoric acid, and CEC increased, which can be seen as a similar tendency. In the case of tillage treatment, the OM content was relatively lowest, which is thought to be due to the large loss of OM during the tillage process. In the case of the mulching treatment group, the overall trend was very similar to that of the control group, but the CEC, which refers to the ability of soil to adsorb and exchange ions, showed a rather high tendency. In comparison to the other treatments, the mulching treatment had a higher minimum soil temperature, a narrower range of soil temperature change, and a higher soil moisture content (Figure 3), soil temperature and moisture conditions are the limiting factors for the activity of soil microorganisms and small animals responsible for decomposing organic matter, so it can be seen that leaf mold treatment can promote long-term soil material circulation [36,38]. Biochar not only improves soil chemicals but also responds well to physical and microbial properties [39]. In particular, the combination of biochar and soil is known to improve soil structure and increases porosity and moisture retention capacity [40]. In addition, it is also reported to increase soil electrical conductivity and decrease soil acidity [41], and no significant difference between the biochar treatment group and the control group in the deforested areas was observed. Contrarily, the effective phosphoric acid and OM content in the control group showed a tendency to increase around 2022, while the biochar treatment group exhibited almost no change (Table 2). According to Jones et al. [42], no significant difference was found in short-term results, whereas 3 years of long-term results showed significant changes in the structure of soil microbial community structure. In this study, the change in soil physicochemical properties for one year was not significant, and long-term monitoring is required to re-evaluate the effect of biochar on the soil. Polymer-based soil supplements, such as PAM, have been shown to improve the physicochemical properties of soil and support plant growth due to their amino nitrogen, ester group, and carboxylic carbon components [43], total nitrogen, organic matter, and phosphoric acid tended to increase in 2022 compared with 2021 in this study, but there was no significant difference compared to the change in the control group, so a significant soil improvement effect was not demonstrated.
Overall, the increase in soil loss appears to be a factor that increases the proportion of sand or silt, and soil acidity is slightly acidic when treated with leaf-mold, and soil fertility is improved by increasing cation substitution capacity, total nitrogen content, OM content, and effective phosphoric acid. In addition, in the case of mulching, CEC was found to have increased, although the physicochemical properties of the soil were not affected significantly. Although Biochar and PAM did not have a significant soil improvement effect, it is necessary to study the effects on the soil by observing long-term changes.

3.4. Growth Characteristics

Figure 5 shows the height and DRC of Pinus densiflora and Larix kaempferi according to the planting groundwork composition method. Overall, in the case of Pinus densiflora, the height and DRC both showed the highest significant growth (p < 0.05) in the leaf-mold and mulching treatment groups, whereas the biochar, tillage, and PAM treatment groups exhibited almost no difference from the control group. In particular, when treated with leaf-mold, the height increased by 34% and DFC by 20% compared to the control group, showing the highest tendency. In the case of Larix kaempferi, there were no treatments that showed significant growth compared to the control group, but rather, the height growth, when treated with tillage, decreased by about 21% compared to the control, exhibiting a relatively low trend. In the case of DRC, the highest growth was observed when treated with leaf-mold, but statistical significance was not recognized (p > 0.05).
As a result, there was no significant difference in growth according to the growth-based composition method in Larix kaempferi, whereas, in the case of Pinus densiflora, the leaf-mold and mulching treatment methods were found to be the vegetation basis that enhanced growth, thus the sensitivity to growth was found to be different by species depending on the planting groundwork composition method.

3.5. Chlorophyll and Carotenoid Content

Comparing the chlorophyll and carotenoid content, both Pinus densiflora and Larix kaempferi showed the highest total chlorophyll a, b, and total chlorophyll content in the mulching treatment group, but statistical significance was not recognized (p > 0.05) (Table 3). However, in the case of carotenoid content, the highest value was observed in the control group of Pinus densiflora, and the lowest tendency was observed in the biochar treatment group. As a result, the proportion of total chlorophyll content/carotenoid content in Pinus densiflora was lowest in the control group, while that of the mulching treatment group was relatively high (Table 3). In contrast, Larix kaempferi showed the highest proportion of total chlorophyll content/carotenoid content in the control group, while that of the mulching treatment group showed the lowest tendency.
As a result, according to the planting groundwork composition method, there was no significant change in chlorophyll content in both Pinus densiflora and Larix kaempferi, in the case of Pinus densiflora, the high carotenoid content in the control group led to a lower Tchl/Car proportion, which is believed to have been affected by the relatively large change in soil temperature in the control group, and low SWC environment (Figure 3A). Carotenoids play photo-protection functions, which are very important for photosynthetic systems [44], as well as auxiliary pigment functions, and they tend to increase with damage caused by intense light [45] and drought stress [17].

3.6. Photosynthesis Response

Observing the photosynthesis response, Pinus densiflora showed the highest maximum photosynthetic rate (Amax), stomatal transpiration rate (E), and stomatal conductivity (gs), which were 2.2 times, 3.0 times, and 2.8 times higher than those of the control group, respectively. Mulching and biochar treatment groups also showed relatively high photosynthetic responses, whereas the control group showed the lowest trend (Figure 6), which is thought to lead to poor growth (height, DRC). In the case of leaf-mold, conditions with relatively high soil fertility such as OM and CEC were formed, which led to good growth along with the photosynthesis results, and in the case of the mulching and biochar treatment groups, the growth of Pinus densiflora was improved due to the maintenance of a high soil water environment (Figure 3B,D) that allows smooth photosynthetic response.
Comparing the water utilization efficiency of Pinus densiflora, there was no significant difference in the ITE, whereas the control and PAM treatment group, which showed relatively low SWC (Figure 3A), intrinsic water use efficiency (WUEi) showed a significantly high tendency (Figure 6). Water use efficiency [46], which can determine the effective water retention ability through the amount of water loss from photosynthetic transpiration, has been reported to increase due to dry stress such as in droughts through many studies [17]. Under dry stress, the supply of CO2 and water is greatly restricted due to the closing of stomata, and the diffusion resistance of intracellular CO2 is also increased, leading to decreased photosynthesis [47,48]. Stomatal limitation (Ls) also showed a similar trend with water use efficiency (Figure 6), and through these results, it can be seen that the control and PAM treatment groups of Pinus densiflora increased their moisture control function through stomata to cope with continuous dry stress from strong winds and low soil moisture content.
Looking at Larix kaempferi, mulching, leaf-mold, and biochar treatment groups were found to be relatively high (p < 0.05). Stomatal conductance and stomatal transpiration rate also showed similar trends with the maximum photosynthetic rate, and the control group showed the lowest maximum photosynthetic rate, stomatal conductance, and stomatal transpiration rate (Figure 6). However, overall, no significant difference in photosynthesis and the stomatal response was observed by the treatment group, and the maximum photosynthetic rate, stomatal conductivity, and stomatal transpiration rate showed a relatively low trend compared to that of Pinus densiflora, indicating that this trend is difficult to be seen to bring significant change in growth.
The water utilization efficiency of Larix kaempferi showed a rather high value compared to the control group, but neither the ITE nor the intrinsic water use efficiency was found to be statistically significant (p > 0.05), and the same trend was observed in the stomatal limitation.
The A-Ci curve, maximum carboxylation rate (Vcmax), and maximum electron transfer fate (Jmax) according to the treatment type are shown in Figure 7. Compared to Pinus densiflora, Larix kaempferi showed a relatively low rate of photosynthesis in response to an increased concentration of CO2 (Ci) in the mesophyll interstitial space (Figure 7A), and Larix kaempferi and the control group showed a relatively high mulching treatment while other treatment groups exhibited a lower trend.
The photosynthetic ability of a plant is determined by the balance between the carboxylation rate according to the activity of Rubisco and the regeneration rate of Ribulose-1,5-bisphosphate (RuBP), and the regeneration rate is limited by the electron transfer efficiency [17,49].
In the case of Pinus densiflora, the maximum carboxylation rate (Vcmax) showed a relatively high value in the other treatment groups compared to the control group, but statistical significance was not recognized due to the large deviation. However, the maximum electron transfer rate (Jmax) was 43% higher in the leaf-mold treatment group compared to the control group, showing the highest tendency among all treatment groups, and mulching, biochar, and tilling treatment groups also showed relatively good values (Figure 7B,C). Through this, the planting groundwork composition such as leaf-mold treatment can be seen to help maintain and improve photosynthetic capacity by increasing the electron efficiency affecting the regeneration of RuBP in Pinus densiflora. In the case of Larix kaempferi, the maximum electron transfer rate (Jmax) was not recognized as significant, but the maximum carboxylation rate showed a relatively high trend in the control group. However, similar to the maximum photosynthetic rate, it showed a relatively low value compared to that of Pinus densiflora, and it was found that the difference between the treatment groups was not large (Figure 7B,C).
Through these results, in the case of Pinus densiflora, it can be expected that the overall photosynthetic rate and photosynthetic ability will be improved when treated with leaf-mold, biochar, and mulching, while the control group was found to show an improved water retention response through stomata due to high sensitivity to environmental stress such as drought. However, in the case of Larix kaempferi, the maximum photosynthetic rate and stomatal conductivity were relatively high during mulching treatment, but overall, the difference between treatment groups was not large, thus it is not expected to have a significant effect on the growth.

3.7. Chlorophyll a Fluorescence Response

Chlorophyll a fluorescence analysis and the complex dynamics of photosynthesis provide a basic understanding of photosynthetic biophysical processes [50], and in particular, OKJIP fluorescence analysis can be used as an indicator of stress tolerance and physiological disorders before the appearance of visible signs of stress [21,49]. The subtle changes in chlorophyll fluorescence, VJ, VI and Vk, mean that the ratio of total fluorescence changed relative to the alterations at each step (J, I and K). Comparing the VJ, VI and Vk, both Pinus densiflora and Larix kaempferi showed the statistical significance was not recognized (p > 0.05) (Figure 8). Fo/Fm, Fv/Fm, RC/CSo, PIabs, PItotal and SFIabs calculated through OKJIP analysis are known to be highly sensitive indicators of environmental stress [21,23,51,52], and in particular, PIabs, PItotal, and SFIabs are used as vitality indexes of photosynthetic apparatus.
Fo/Fm refers to excitation energy dissipation to prevent damage to the photosynthetic apparatus and reduce restriction on energy flow, and Fv/Fm refers to the maximum quantum yield of photosystem II in the initial photochemical reaction [17,23]. In the case of Pinus densiflora, changes in Fo/Fm, Fv/Fm, and RC/CSo, which represent the amount of active reaction center of photosystem 2 per leaf area, were not significant in the treatment groups. However, PIabs, PItotal, and SFIabs showed a tendency to be relatively sensitive in the mulching treatment group compared to the control group. (Figure 8).
PIabs refers to the energy conservation efficiency in the process of reducing electron carriers using absorbed light energy [53], and PItotal refers to the performance up to the terminal electron acceptor of photosystem I [54]. In addition, SFIabs is known as an indicator showing the structural and functional responses of photosystem 2 that induce electron transport during photosynthesis [55,56]. In the case of Pinus densiflora, especially in the mulching treatment group, PIabs and PItotal maintained values 1.6 to 2.2 times higher than the control group, indicating that the effect of environmental stress was relatively low (Figure 8), which was an even higher tendency observed in the leaf-mold treatment group, which had the best results for overall indicators including than maximum photosynthetic rate, height, and DRC. These results show that during leaf-mold treatment, the accumulation and use of nutrients in the soil vegetation base increases, which helps photosynthesis and growth, but the effect of environmental stress is rather less in the mulching treatment, indicating that mulching treatment is a material that helps maintain soil moisture and improve resistance to environmental stress such as drought under conditions prone to dry stress such as wind. In the case of Larix kaempferi, it can be seen that the vitality index of the mulching treatment group is very high, similar to that of Pinus densiflora (Figure 8).

3.8. Principle Component Analysis (PCA)

Figure 9 represents the Principle Component Analysis of the experimented two species. Firstly, Pinus densiflora can be divided into three groups; (a) the control, (b) the mulching and biochar, (c) the leaf mold, and PAM and tillage were applied to both A and B. The PC 1 axis can distinguish between groups A, B, and C and it has an explanatory power of approximately 31.1%. Among the representative factors affecting the PC axis 1, the environmental factors are soil moisture contents (SWC), maximum soil temperature (Max Temp.), maximum photosynthetic rate (A), height, maximum electron transfer rate (Jmax), root diameter, RC/Cso, intrinsic water use efficiency (WUEi). Environmental factors such as soil moisture contents (SWC), maximum soil temperature (Max Temp.), maximum photosynthetic rate (A), height, maximum electron transfer rate (Jmax), root diameter, RC/Cso, intrinsic water use efficiency (WUEi) are among the representative factors influencing the PC axis 1. Furthermore, groups B and A, and C can be divided into PC 2 axis, the representative factor is the minimum soil temperature (Min Temp.).
In the case of Larix kaempferi, mulching and control were differentiated based on the PC 1 axis. However, the factors affecting the PC 1 axis were far from the axis, thus it was difficult to present specific factors to classify the groups.

4. Conclusions

As a result of comparing the planting groundwork composition method for the restoration of deforested areas, the average SWC during the investigation period of the control group and the PAM treatment group was lowest at 11.5 to 12.0%, respectively, while the mulching treatment group had an average SWC of 27.4%, about 2.4 times higher than that of the control group, and biochar, leaf-mold, and tillage treatment groups also showed a relatively high level of 18.1 to 19.5%. In particular, during mulching treatment, the soil temperature also showed the smallest variation among all treatment groups, ranging from a maximum of 34.4 °C to a minimum of −8.1 °C. The cumulative soil loss was 10.28 ton/ha and 6.37 ton/ha in the biochar and leaf-mold treatment groups, respectively, showing a tendency to significantly increase during the period of concentrated rainfall after planting groundwork treatment, and in contrast, the cumulative loss of the mulching treatment group was only 0.05 ton/ha, indicating it was the most effective for slope fixation. These results show that the mulching treatment using a coir net greatly helps to maintain soil water from the dry external environment, reduces the extreme change in soil temperature, and has an excellent effect on preventing soil loss. In the case of leaf-mold treatment, it was found that the cation substitution capacity, total nitrogen content, OM content, and effective phosphoric acid were increased compared to the soil loss due to wind and rain, indicating that it greatly helped improve soil fertility.
Comparing the growth and physiological responses of Pinus densiflora and Larix kaempferi according to the planting groundwork composition method, Pinus densiflora had the highest height and DRC in the leaf-mold treatment group, which had the best nutrient conditions and maintained SWC at a relatively high level, and the maximum photosynthetic rate (Amax), stomatal transpiration rate (E), stomatal conductivity (gs), and maximum electron transfer rate (Jmax) were also highest, which were about 1.4 to 3.0 times higher than that of the control group. In the case of the mulching treatment group, overall growth and photosynthetic response were highest after leaf-mold, and PIabs, PItotal, and SFIabs, which are highly sensitive to environmental stress and indicate vitality indexes of photosynthetic apparatus, were observed to maintain a rather better level than leaf-mold. On the other hand, the control group showed a stress response according to dry soil conditions through the chlorophyll fluorescence index and high carotenoid content and coped with continuous dry stress by increasing water use efficiency through increasing moisture control function by stomata, maintaining a low photosynthesis level and low overall growth.
Larix kaempferi showed relatively high height growth, maximum photosynthetic rate, stomatal conductivity, and chlorophyll fluorescence during mulching treatment, but overall, there was no significant difference between the treatment groups.
As a result, in the case of Pinus densiflora, leaf-mold treatment showed the best growth and physiological response, but mulching treatment was also observed to have a relatively high level, thus it is judged that considering SWC, maintenance of soil temperature, and soil loss for slope stabilization, mulching treatment is believed to be the best method for a planting groundwork treatment method.
Therefore, mulching is a simple and cost-effective way to efficiently restore forests that have been harmed by extensive developments. By minimizing soil loss, this method can postpone destruction and aid in creating a stable environment for the initial growth of vegetation. Long term, it can support the realization of a variety of forest values, including preserving soil in deforested areas, restoring biodiversity, and producing timber. While monitoring the long-term effects of these soil treatments on vegetation growth in the actual field, it will be necessary to explain the interrelations with various forest components. Further, depending on the types and degrees of deforestation, it is also necessary to conduct follow-up studies on the application of new soil improvement methods or the degree of mulch coverage that can help stabilize the vegetation groundwork quickly and economically.

Author Contributions

Conceptualization, K.L. and N.K.; methodology, H.K. (Hyeongkeun Kweon) and N.K.; validation, K.L. and Y.S.; formal analysis, K.L. and Y.S.; investigation, Y.S., H.K. (Haeun Koo) and H.K. (Hyeonhwa Kim); resources, N.K.; data curation, Y.S., H.K. (Haeun Koo) and H.K. (Hyeonhwa Kim); writing—original draft preparation, K.L.; writing—review and editing, H.K. (Hyeongkeun Kweon); visualization, H.K. (Haeun Koo) and H.K. (Hyeonhwa Kim); supervision, H.K. (Hyeongkeun Kweon) and N.K.; project administration, N.K.; funding acquisition, N.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data will not available due to privacy issue.

Acknowledgments

This study was conducted as a research project with the support of the National Institute of Forest Science (Management Number: 22-00-51).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Location of the experimental site (A,B), visual soil condition (C), and study plots ((D); Biochar (1), Leaf-mold (2), Control (3), Mulching (4), Tillage (5), and PAM (6); Sediment basin for each treatment (Yellow circles), and the Automatic Weather System (Green circle)).
Figure 1. Location of the experimental site (A,B), visual soil condition (C), and study plots ((D); Biochar (1), Leaf-mold (2), Control (3), Mulching (4), Tillage (5), and PAM (6); Sediment basin for each treatment (Yellow circles), and the Automatic Weather System (Green circle)).
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Figure 2. The number of wind directions (A), windrose diagram (B), air temperature (C; black line), and daily precipitation (C; blue line) during the experimental period.
Figure 2. The number of wind directions (A), windrose diagram (B), air temperature (C; black line), and daily precipitation (C; blue line) during the experimental period.
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Figure 3. Soil water contents (SWC) and soil temperature (Soil Temp) of study plots (control (A), Biochar (B), Leaf-mold (C), Mulching (D), Tillage (E), and PAM (F)).
Figure 3. Soil water contents (SWC) and soil temperature (Soil Temp) of study plots (control (A), Biochar (B), Leaf-mold (C), Mulching (D), Tillage (E), and PAM (F)).
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Figure 4. Cumulative soil loss (A) and soil loss ratio (B) of study plots during the experimental period.
Figure 4. Cumulative soil loss (A) and soil loss ratio (B) of study plots during the experimental period.
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Figure 5. Growth characteristics of Pinus densiflora (A) and Larix kaempferi (B) under six different planting groundwork treatments. Different letters indicate significant differences (p < 0.05) by DMRT at a 5% level. ns: non-significance.
Figure 5. Growth characteristics of Pinus densiflora (A) and Larix kaempferi (B) under six different planting groundwork treatments. Different letters indicate significant differences (p < 0.05) by DMRT at a 5% level. ns: non-significance.
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Figure 6. Effects of soil treatments on photosynthetic characteristics of P. densiflora and L. kaempferi under six different planting groundwork treatments. Bars represent means ± SD and different letters indicate significant differences (p < 0.05) by DMRT at a 5% level. ns: non-significance.
Figure 6. Effects of soil treatments on photosynthetic characteristics of P. densiflora and L. kaempferi under six different planting groundwork treatments. Bars represent means ± SD and different letters indicate significant differences (p < 0.05) by DMRT at a 5% level. ns: non-significance.
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Figure 7. Effects of soil treatments on A-Ci curves (A), maximum carboxylation rate (B) and maximum electron transport rate (C) of P. densiflora and L. kaempferi under six different planting groundwork treatments. Bars represent means ± SD and different letters indicate significant differences (p < 0.05) by DMRT at a 5% level. ns: non-significance.
Figure 7. Effects of soil treatments on A-Ci curves (A), maximum carboxylation rate (B) and maximum electron transport rate (C) of P. densiflora and L. kaempferi under six different planting groundwork treatments. Bars represent means ± SD and different letters indicate significant differences (p < 0.05) by DMRT at a 5% level. ns: non-significance.
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Figure 8. Radar plots of several chlorophyll fluorescence parameters of P. densiflora (A) and L. kaempferi (B) under six different planting groundwork treatments. The data are shown as a percentage of control.
Figure 8. Radar plots of several chlorophyll fluorescence parameters of P. densiflora (A) and L. kaempferi (B) under six different planting groundwork treatments. The data are shown as a percentage of control.
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Figure 9. Principle Component Analysis (PCA) of growth and physiological parameters of P. densiflora (A) and L. kaempferi (B) under different soil treatments.
Figure 9. Principle Component Analysis (PCA) of growth and physiological parameters of P. densiflora (A) and L. kaempferi (B) under different soil treatments.
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Table
Table 1. Geographical features and soil hardness of study plots.
Table 1. Geographical features and soil hardness of study plots.
ControlBiocharLeaf-MoldMulchingTillagePAM
Elevation (m)619619619619619619
Slope (°)131114131512
Soil hardness (mm)21.722.722.019.721.320.7
Table
Table 2. Soil properties of soil treatments of 2021 (22 March) and 2022 (10 May).
Table 2. Soil properties of soil treatments of 2021 (22 March) and 2022 (10 May).
YearTreatmentSoil TexturepH
(1:5)		CEC
(cmolc/kg)		T–N
(g/kg)		OM
(g/kg)		Avail.−P2O5
(mg/kg)
Sand (%)Silt (%)Clay (%)
2021Control49 25 26 7.46 9.8 0.04 6.70 28.7
Biochar38 34 29 6.21 9.1 0.13 8.30 26.3
Leaf-mold35 33 32 6.08 10.1 0.18 11.30 13.8
Mulching56 28 16 7.35 8.5 0.10 9.00 28.9
Tillage71 19 10 7.64 6.1 0.34 4.40 25.0
PAM70 20 10 7.68 9.9 0.37 5.00 19.8
2022Control58 29 13 7.18 8.9 0.92 10.00 49.8
Biochar30 44 26 6.43 10.4 0.93 7.40 23.3
Leaf-mold52 32 16 6.10 11.2 1.43 20.10 66.6
Mulching63 24 13 7.12 10.6 0.94 10.90 41.6
Tillage69 21 10 7.41 7.2 0.70 3.70 36.0
PAM60 24 16 7.31 7.8 0.82 6.20 38.8
Table
Table 3. Effects of soil treatments on chlorophyll (Chl) and carotenoid (Car) contents of P. densiflora and L. kaempferi under six different planting groundwork treatments.
Table 3. Effects of soil treatments on chlorophyll (Chl) and carotenoid (Car) contents of P. densiflora and L. kaempferi under six different planting groundwork treatments.
SpeciesTreatmentChl (mg·g−1)Car
(mg·g−1)		Chl a/bT Chl/Car
aba + b
P. densifloraControl9.1 ns2.2 ns11.3 ns2.9 c4.1 ns3.9 a
Biochar7.52.09.52.3 a3.74.1 abc
Leaf-mold8.62.310.92.5 ab3.74.5 bc
Mulching9.52.411.92.6 b4.04.6 c
Tillage8.62.210.82.6 b4.04.1 abc
PAM8.52.110.62.7 bc4.14.0 ab
L. kaempferiControl6.0 ns 1.6 ns 7.6 ns 1.6 ns 3.6 a 4.7 b
Biochar4.9 1.3 6.2 1.4 3.8 ab 4.4 ab
Leaf-mold6.0 1.6 7.7 1.8 3.7 a 4.4 ab
Mulching6.9 1.7 8.6 2.1 4.0 bc 4.1 a
Tillage6.3 1.5 7.8 1.8 4.1 c 4.3 ab
PAM6.8 1.7 8.5 2.1 4.0 bc 4.2 ab
Mean with the same letter in each column indicates not statistically different (p < 0.05) by DMRT at 5% level. ns: non-significance.
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Lee, K.; Song, Y.; Koo, H.; Kim, H.; Kweon, H.; Koo, N. Evaluation of Soil Loss Tolerance and Tree Growth Features Based on Planting Ground Methods in the Alpine Center, Degraded Forestland in the Republic of Korea. Forests 2023, 14, 200. https://doi.org/10.3390/f14020200

AMA Style

Lee K, Song Y, Koo H, Kim H, Kweon H, Koo N. Evaluation of Soil Loss Tolerance and Tree Growth Features Based on Planting Ground Methods in the Alpine Center, Degraded Forestland in the Republic of Korea. Forests. 2023; 14(2):200. https://doi.org/10.3390/f14020200

Chicago/Turabian Style

Lee, Kyeongcheol, Yeonggeun Song, Haeun Koo, Hyeonhwa Kim, Hyeongkeun Kweon, and Namin Koo. 2023. "Evaluation of Soil Loss Tolerance and Tree Growth Features Based on Planting Ground Methods in the Alpine Center, Degraded Forestland in the Republic of Korea" Forests 14, no. 2: 200. https://doi.org/10.3390/f14020200

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