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Article

Comparison of Two Site Preparation Treatments for the Growth of Direct-Seeded Fraxinus chinensis subsp. rhynchophylla Seedlings and Their Effects on Soil Temperature and Understory Vegetation

by
Jong Bin Jung
1,
Hyun Jung Kim
2,
Jongwoo Kim
3,
Ji Sun Jung
3 and
Pil Sun Park
3,*
1
Department of Horticulture and Forestry, Mokpo National University, Muan 58554, Republic of Korea
2
Garden Promotion Office, Korean National Garden Culture Center, Damyang 57352, Republic of Korea
3
Department of Agriculture, Forestry and Bioresources, Seoul National University, Seoul 08826, Republic of Korea
*
Author to whom correspondence should be addressed.
Forests 2025, 16(9), 1401; https://doi.org/10.3390/f16091401
Submission received: 13 July 2025 / Revised: 29 August 2025 / Accepted: 30 August 2025 / Published: 1 September 2025
(This article belongs to the Section Forest Ecology and Management)
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Abstract

Direct seeding is considered a versatile and cost-effective approach to forest regeneration; however, its broader application is limited by low seedling survival rates and species-specific regeneration requirements, which often necessitate site preparation. We investigated the emergence, survival, and growth of Korean ash (Fraxinus chinensis subsp. rhynchophylla (Hance) A.E.Murray) seedlings regenerated by direct seeding over six years following two site preparation treatments—scarification and mixing—to determine appropriate site preparation methods for direct seeding and to assess the effects of site preparation treatments on soil, understory vegetation, and seedling growth. Additionally, the seed germination, shoot and root lengths, and biomass of the seedlings were investigated over 50 days in a growth chamber using soils from each site preparation treatment to examine early-stage growth responses. Both scarification and mixing treatments enhanced seed germination and seedling establishment. Seedling emergence rates were similar between the treatments; however, the seedling mortality and the height and coverage of competing understory vegetation were significantly greater at the scarification treatment than at the mixing treatment during the first year (p < 0.05). Both treatments reduced minimum winter soil temperatures during the first two years, with frost heaving identified as a primary cause of early seedling mortality. From the second year onward, seedling growth was significantly greater in the mixing treatment (p < 0.05), which also more effectively suppressed competing vegetation. A shallow depth mixing treatment (<5 cm) is recommended for direct seeding of Korean ash, as it reduces frost heaving damage and facilitates seedling survival and growth by minimizing understory competition.

1. Introduction

Planting seedlings is a widely used regeneration method for plantation and forest restoration [1]. However, planting seedlings often involves a high cost due to labor and transportation, in addition to seedling production [2,3]. Direct seeding is a less labor-intensive and cost-effective alternative to planting for reforestation [4,5]. Direct seeding has been conducted in areas where accessibility is difficult [6], where transplanted seedlings struggle to adapt to the environment, or where large-scale reforestation is being conducted [7]. Direct seeding is highly preferred in large-scale forest restoration due to the high adaptability of direct-seeded seedlings to the restoration sites [5,8]. Despite the advantages of direct seeding, the lower survivorship hinders the wide application of direct seeding [9,10], resulting in limited direct seeding practices [11,12]. Seedling mortality in the early stages of direct seeding occurs due to various factors, including competition, drought, insect pests and diseases, microenvironmental conditions, and seedbed substrates [13,14].
The litter layer and competing understory vegetation have crucial impacts not only on the survival of direct-seeded seedlings but also on their initial growth; therefore, site preparation and management are critical for the success of reforestation via direct seeding [15,16,17]. Understory vegetation hinders seedling growth and increases mortality through competition for nutrients and water or by shading [18]. Fagus sylvatica L. and Quercus robur L. seedlings surrounded by understory competitors showed lower nitrogen content in leaves and decreased shoot dry weight [19]. Selective herbicides or prescribed burning are considered cost-effective site preparation methods; however, they are not always feasible. Mechanical site preparation could be an alternative that can suppress competing vegetation while preventing the negative impacts of chemicals or prescribed burning, which may cause unexpected results on forest ecosystems and non-target organisms such as wildlife and soil fauna [20,21,22,23].
Mechanical site preparation includes scarification, mounding, disc trenching, plowing, mixing, and soil inversion and is practiced to provide a favorable environment for seed germination and seedling establishment and growth [24,25,26]. Mechanical site preparation treatments remove residue from the soil surface and change the soil’s physical properties and nutrient availability [27,28,29]. Mechanical site preparation improved seedling establishment of Pinus sylvestris L. [30] and increased biomass growth of Q. robur sown in Southern Sweden [31]. The growth response of seedlings varied depending on the species, type, and intensity of mechanical site preparation treatments [32,33].
Scarification and mixing are representative mechanical site preparation practices that remove the litter layer and ground vegetation [24,25]. Scarification removes the litter layer and exposes the mineral soil, which restrains the growth of previously existing vegetation and promotes seedling growth [22,34]. However, shallow scarification may stimulate root sprouts or the germination of the soil seed bank, which may intensify competition with directly seeded seedlings [25]. Mixing treatment mingles the organic layer, topsoil, and subsoil and is effective in suppressing the growth of competing vegetation by uprooting existing plants [35,36]. However, the exposure of mineral soil by deep mixing may increase frost heaving, which can sever seedling roots [33]. Each site treatment has its own advantages and impacts on the soil condition; thus, mechanical site preparation treatments should consider the characteristics of the sites and the direct-seeded species.
Korean ash (Fraxinus chinensis subsp. rhynchophylla (Hance) A.E.Murray) is native to Northeast Asia and is distributed throughout the Korean Peninsula. It reaches over 20 m in height and has been traditionally used for making agricultural equipment and furniture [37]. Korean ash is commonly regenerated by seed germination or stump sprouting in natural forests [38]. It is somewhat shade-tolerant, which enables this species to naturally regenerate under the canopy in natural stands and be directly seeded [39,40]. Korean ash seeds show a high seed germination rate of 60%–70%, even in natural forests [41,42], and Genus Fraxinus are largely ignored by rodents and birds with less herbivory damage [43]. Thus, direct seeding was suggested for the establishment of Korean ash plantations to reduce regeneration costs [44]; however, most Korean ash plantations in Korea have been established by planting seedlings [45], and direct seeding for Korean ash is rare. Optimal priming and pelleting techniques to improve seed germination [46] or site preparation treatments [40] were tested for the direct seeding of Korean ash. However, few studies examined the effects of site preparation treatments on soil, understory vegetation, or seedling dynamics during direct seeding in Korea. A lack of experience and limited information on the early stage of seedling growth complicated the initial management and hesitated the direct seeding trial of Korean ash.
This study aimed to identify optimal site preparation strategies for direct seeding and to elucidate treatment effects on soil conditions, understory vegetation, and the establishment and growth of direct-seeded Korean ash seedlings. Two site preparation treatments—scarification and mixing—were evaluated. Assessing the effects of these treatments on seedlings and site environmental conditions through practical application would help promote the wider adoption of direct seeding for Korean ash. We conducted field experiments involving the two site preparation treatments and investigated the emergence, survival, and growth of Korean ash over 6 years. Additionally, we conducted a growth chamber experiment on the seed germination and initial growth of Korean ash on different seedbed substrates that were subjected to site preparation to scrutinize initial seedling growth. We hypothesized that (1) site preparation treatments would enhance seed germination and seedling establishment, and (2) the mixing treatment would be more effective at suppressing competing understory plants and consequently lead to higher seedling survival rates and greater initial height growth of Korean ash than the scarification treatment.

2. Materials and Methods

2.1. Seed Collection and Preparation

Seeds for the field experiment were collected from natural broadleaved forests on Mt. Gariwang (37°28′40″ N 128°29′57″ E, 965 m asl), Pyeongchang-gun, Gangwon-do, in October 2013. The collected seeds were air-dried at room temperature, then sealed in a Ziplock bag and stored in a refrigerator at 2–5 °C. In March 2016, the seeds were submerged in distilled water and maintained at room temperature for 24 h [47]. Subsequently, empty, spotted, or unsound seeds that did not sink were identified and removed through visual inspection. Seeds that sank in water were retrieved, and among them, 300 visually sound seeds were manually selected. The selected seeds were soaked in 35% hydrogen peroxide for 1 min for surface sterilization and rinsed with distilled water prior to seeding [48].
Seeds for the germination and initial growth experiment in the growth chamber were obtained from the National Institute of Forest Science of Korea [seed lot number: AA0308040020]; they were collected in 2000 in Hongcheon-gun, because we could not obtain enough seeds from Mt. Gariwang. The seed collecting location at Hongcheon-gun had the geographic proximity to Mt. Gariwang, and both seed collecting locations had similar climatic and soil conditions. Korean ash exhibited relatively low genetic differentiation among populations in the Korean Peninsula [49], and the seeds retained high germination rates even over 10 years of storage [50]. A total of 180 healthy seeds were selected by visual inspection. The seeds from Hongcheon-gun were treated in the same way as the seeds used for the direct seeding experiment in the field.

2.2. Site Preparation Treatments and Direct Seeding in the Field

Field experiments were conducted at the Taehwasan University Forest of Seoul National University, Gwangju-si, Gyeonggi-do, Korea (37°18′45.43″ N 127°18′43.65″ E, 143 m asl). Meteorological data from the Icheon Weather Station at 80 m asl, which is 16.1 km from the study site [51], indicated that the annual mean air temperature was 11.7 °C, with a maximum daily mean temperature of 30.2 °C in August and a minimum daily mean temperature of −8.5 °C in January at the study site. The mean annual precipitation was ca. 1300 mm from 1991 to 2020. The soils are brown forest soils and are classified as the Songsan Series (coarse loamy, mixed, mesic family of Typic Dystrudepts). The mean depth of the litter layer and A horizon was ca. 2–3 cm and 20 cm, respectively. The soil pH was 5.3, the total organic carbon content was 14.8 g/kg, and the mean soil cation exchange capacity was 11.5 cmol kg−1 in the A horizon of the Songsan Series [52].
The study site was located on the northeastern skirt of Mount Taehwa (Figure 1) amid Japanese larch (Larix kaempferi (Lamb.) Carrière) plantations; however, they were severely affected by Typhoon Bolaven in 2012, resulting in major windthrow disturbances [53]. All remaining trees, snags, and logs were removed from the larch plantation, and a 225 m2 site was established in March 2016.
Two site preparation treatments were applied: scarification and mixing. The site was divided into two sections, and two mechanical site preparation treatments were applied in each section: one section for scarification and the other section for mixing. For scarification, the litter layer and topsoil were removed using steel hand rakes, and the bare soil was exposed. For mixing, the litter layer, surface soil, and subsoil at 30–50 cm depth were mixed using a 5-ton excavator (DX55 ACE, Doosan Infracore, Incheon, Republic of Korea).
Six 5 m × 2 m plots were established for each scarification and mixing treatment, with a minimum distance of 2 m between each plot to minimize spatial autocorrelation. Within each plot, a 1 m × 1 m subplot was randomly established, and 25 seeds were manually sown 20 cm apart from one another in each subplot in March 2016. A total of 300 seeds (25 seeds × 6 replication × 2 treatment) were sown, and 1–2 cm of soil was placed on top of the sown seeds to prevent loss by animal predation and seed desiccation [54,55]. Marking flags were placed for each sown seed for monitoring throughout the study period. After an understory vegetation survey in July, manual weeding was conducted across all plots in July 2016 and 2018 due to the presence of lush understory vegetation.

2.3. Seedling Establishment, Survival and Growth at Two Site Preparation Treatment Sites

Seedling establishment, survival, and growth were monitored for six years from March 2016 to December 2021. The emergence of the sown seeds, seedling mortality, cause of mortality, and seedling height were measured every month from March to August of the first year of the experiment and every two months during the second and third years. The survey was conducted in the second or third week of the relevant month. In March and August in the first year, the survey was conducted in the fourth week. A total of 202 seedlings that emerged out of 300 sown seeds were monitored. In the 4th and 6th years, the survival, cause of mortality, and height of the seedlings were recorded at the beginning and after the growth period (April and November 2019 and May and December 2021).
The causes of mortality were classified based on observed symptoms during each survey, as follows [56]: desiccation if the seedling showed signs of dryness; frost heaving if the seedling was uplifted or its roots were exposed at the time of death; cutting if the aboveground stem was cut; or unknown if the seedling was missing or none of the above symptoms were observed. During the study period, no signs of damage caused by weather or animals (e.g., pecking marks by birds, pest outbreak, or evidence of soil disturbance by wild boars) were observed. Seedling height was measured as the vertical height of the tallest part (terminal bud or growing tip) to the nearest 0.1 cm using a folding ruler.

2.4. Understory Vegetation Survey at Two Site Preparation Treatment Sites

Although site treatments were conducted, herbs and understory plants grew and competed against the germinants and seedlings at the experimental sites. To identify the species composition of understory competitors, all plant species in the 5 m × 2 m treatment plots were recorded from March to August in the first year at the same time as the seedling monitoring.
The height (cm) and coverage (%) of the herbaceous and woody plants in the treatment plots were measured, recorded and compared between treatments at the same time as the seedlings were monitored from the first to the third year (2016–2018) of the experiment and in May 2019. The height of herbaceous and woody plants was measured as the vertical height of the tallest part to the nearest 0.1 cm using a folding ruler from five randomly selected individuals in each plot. In March and April, when there were fewer than five plants present in some subplots, three individuals were measured. Plant coverage was measured by visually estimating the percentage of the 1 m × 1 m subplot covered by plants.

2.5. Soil Survey and Analysis at Two Site Preparation Treatment Sites

Two automatic weather stations, each equipped with a HOBO data logger (H21-002, Onset Computer Corporation, Bourne, MA, USA) and a soil temperature sensor (S-TMB-M002, Onset Computer Corporation, Bourne, MA, USA) were installed at each treatment. Each treatment section was divided into two subsections, and a data logger and soil sensor were placed near the center of each. Soil temperature (°C) was measured at 30-min intervals at a 5 cm soil depth from 5 March 2016 to 12 December 2021. The observed soil temperatures were averaged for each treatment, and daily and monthly mean temperatures were calculated. The mean temperatures for the coldest month (January) and the warmest month (August) of each year were then compared between the two treatments.
Soil samples were collected from three equally spaced locations at each of the scarification and mixing treatment sites. Soil samples were collected from a depth of 0–5 cm using a 100 cc stainless-steel cylinder and a shovel in June 2016, when most seeds were germinated. Competition between newly emerged seedlings and understory plants primarily occurs in the topsoil. Soil samples were analyzed individually without pooling to preserve site-specific variability. Collected soil samples were transported to the laboratory for soil bulk density, soil moisture content, and soil pH measurement.
The soil samples were weighed before and after oven-drying at 105 °C for 24 h, and soil moisture content and bulk density were determined. Soil moisture content (θm, %) was calculated by subtracting the dry soil mass from the wet soil mass and dividing by the dry soil mass. Soil bulk density was determined as the dry soil mass per unit volume (g cm−3). Soil for soil pH measurement was air-dried and sieved using a 2 mm mesh sieve. The soil pH was measured using a Metrohm 827 pH meter with a Primatrode NTC glass electrode (Metrohm, Herisau, Switzerland) after mixing the air-dried soil with distilled water at a 1:5 volume ratio, shaking for 30 min, and allowing the mixture to settle for 30 min [57]. Before the measurement, the electrode was calibrated using Certipur® pH 4, 7, and 10 buffer solutions.

2.6. Seed Germination and Early Seedling Growth at Growth Chamber Experiment

2.6.1. Growth Chamber Experiment Design

The seed germination and initial stage of seedling growth were investigated in the growth chamber to examine early seedling growth in more detail. Buried seeds and vigorous herbs hindered the detection of germinants in the field. The rates of seed germination and early seedling growth on the seedbed substrates of the control, scarification, and mixing treatments were compared.
Soils were collected from the field study site in early June 2016 and used for seedbed substrates in the growth chamber. Soils for the scarification and mixing treatments were taken from their respective treatment plots. For control treatment, the litter layer and soil were collected from the Japanese larch plantation near the study site. A 2-cm-thick layer of Japanese larch litter was placed on top, and 3 cm of A horizon soil was laid at the bottom of the container, mimicking the seedbed substrates (2–3 cm depth of litter layer) in the Japanese larch plantation for the control treatment.
Three 5 × 4 cell (3.4 cm diameter and 5 cm depth) propagation containers were used for each of the two site preparation treatments and the control. Each container was treated as one replicate. A total of nine containers were randomly distributed using a random number table in a growth chamber. Each cell contained one seed. A total of 180 seeds were used for three treatments with three replications (25 seeds × 3 treatments × 3 replications).
The seed germination experiment was conducted for 50 days from 21 June to 10 August 2016. The seeds were watered with 10 mL of distilled water every other day. The temperature and humidity in the growth chamber were maintained at 25 °C and 70%–80%, respectively [58]. The photoperiod was set at 12 h of light and 12 h of dark, with an illumination of 25,000 lux during the light period. The criterion for germination was the protrusion of a radicle greater than 2 mm in length. Seed germination and seedling survival were monitored during watering.

2.6.2. Growth Measurements and Indicators

The seed germination percentage, mean germination time (MGT), days to reach 50% of the final germination rate (T50), and germination value (GV) were calculated and compared among the seedbed substrates of the site preparation treatments [59,60,61].
Twenty healthy seedlings in each treatment were selected, and the height and biomass of the seedlings were measured 50 days after germination. Seedlings in the control treatment were not included in the analysis, because few seedlings survived in the control treatment during the experiment. The shoot and root lengths of the seedlings were measured using Vernier calipers after washing under running water. Leaf area was measured using an LI-3000C instrument (LI-COR Biosciences, Lincoln, NE, USA). The seedlings were divided into stems, roots, and leaves and oven-dried at 70 °C until the weight was constant, after which the dry mass was measured. The root–shoot ratio (R/S) and leaf weight ratio (leaf mass/whole plant mass, LWR) were calculated based on dry mass.

2.7. Data Analysis

The soil bulk density, soil moisture content, soil pH, and the height and coverage of herbaceous plants were compared between the scarification and mixing treatments using the Mann-Whitney U test (significance level α = 0.05). The normality assumptions were not met due to the small sample size, as tested with the Shapiro–Wilk test [62].
Seedling emergence rate was calculated as the percentage of the number of emerged seedlings to the number of seeds sown. The seedling mortality rate was calculated by dividing the number of dead or missing seedlings by the total number of emerged seedlings. Seedling establishment rate was calculated as the percentage of the number of surviving seedlings to the number of seeds sown [13]. The seedling emergence rate, mortality rate, and establishment rate of direct-seeded Korean ash from each survey were compared between the scarification and mixing treatments from March 2016 to December 2021 using the Mann-Whitney U test (α = 0.05).
The height of individual Korean ash seedlings on each survey date was compared between scarification and mixing treatments using a generalized linear mixed model (GLMM) with a Gaussian distribution and an identity link function. In GLMM, fixed effect was the treatment, and each plot was considered as a random effect. The significance levels were determined using Tukey’s post hoc test (adjusted p-values). The relationships between seedling height, seedling survival, and height and coverage of herbaceous plants in June from 2016 to 2018 were analyzed using Spearman’s rank correlation for the effects of competitive understory plants on the survival and growth of Korean ash. The height and coverage of the seedlings and understory plants in June were used for correlation analysis, because plant growth was greatest in June.
Seed germination percentage, MGT, T50, and GV in the growth chamber were compared among treatments using a nonparametric Kruskal-Wallis test, followed by post hoc Dunn’s test (n = 3). p-values were adjusted using the Bonferroni method. The stem height, root length, and biomass of the plants that were germinated and grown in the growth chamber were compared using the Mann-Whitney U test (α = 0.05).
Statistical analyses were conducted by employing the wilcox.test function for the Mann-Whitney U test, lme4 package [63] for GLMM with multiple comparisons by Tukey contrasts using the glht function in the multcomp package [64], and Shapiro–Wilk test and cor.test function for Spearman’s rank correlation in R software version 3.6.1 [65].

3. Results

3.1. Soil Characteristics at the Scarification and Mixing Treatment Sites

The soil temperature in the first and second summers was greater in the mixing treatment than in the scarification treatment. The highest daily mean soil temperatures in the summer of 2016 were 26.2 °C and 27.9 °C in the scarification and mixing treatments, respectively. However, from 2017 onward, the highest daily mean soil temperatures were higher in the scarification treatment than in the mixing treatment. The lowest daily mean soil temperature was lower at the mixing treatment than at the scarification treatment in the first and second winters (Figure 2). The soil temperature decreased to below −6 °C in the second winter in the mixing treatment. However, the lowest daily mean soil temperature remained above −1 °C from the third winter onwards for both treatments: −0.2 ± 0.8 °C for the scarification treatment and 0.1 ± 1.2 °C for the mixing treatment. In both treatments, the temperature ranges narrowed during the study periods. The highest daily mean soil temperature in the summer decreased, and the lowest daily mean temperature in the winter increased with time and tree growth on the forest floor. The soil temperatures at the scarification and mixing treatments became similar after the third year.
No significant differences in soil bulk density, soil moisture content, or soil pH were found between the two treatments (Table 1). The soil bulk density was 0.81 ± 0.12 g cm−3 and 0.94 ± 0.02 g cm−3 in the scarification and mixing treatments, respectively (n = 3, p = 0.20, Mann-Whitney U test). The soil moisture content was greater in the scarification treatment than that in the mixing treatment at 42.16 ± 8.90% and 32.87 ± 4.64%, respectively (n = 3, p = 0.30). The soil pH was 4.67 ± 0.06 and 4.71 ± 0.02 in the scarification and mixing treatments, respectively (n = 3, p = 0.20).

3.2. Understory Plants at the Scarification and Mixing Treatment Sites

Understory vegetation differed between the two treatments. In total, 81 species were recorded, of which 61 were found at the scarification treatment and 58 at the mixing treatment. More tree, subtree, and shrub species were found in the understory at the scarification treatment than at the mixing treatment, including 29 and 22 species, respectively. Seedlings from stump sprouts of previously existing trees and perennial herbaceous plants that occurred in moist/slightly wet areas were major understory competitors in the scarification treatment (Table S1). On the other hand, annual or biennial grasses in Poaceae, sedges in Cyperaceae, and forbs in Polygonaceae newly invaded the mixing treatment. In total, forty-nine herb species were recorded in the first year of the experiment. Among the herb species, 20 (62.5% out of 32 herb species) and 11 (30.5% out of 36 herb species) perennial herb species were found at the scarification and mixing treatments, respectively.
The height and coverage of understory plants diverged most in the spring and early summer, showing significant differences between the two treatments (p < 0.05), according to the Mann–Whitney U test. The height and coverage of understory plants in the scarification treatment were significantly greater than those in the mixing treatment (p < 0.05; Figure 3a). The mean height of the herbs in the summer ranged from 80 to 100 cm in the scarification treatment and from 38 to 67 cm in the mixing treatment. The coverage of the herb layer in the summer reached more than 80% in the scarification treatment, which was significantly greater than that in the mixing treatment (p < 0.05, Mann–Whitney U test; Figure 3b).

3.3. Seedling Emergence, Survival, and Height Growth Between the Scarification and Mixing Treatments

Seedling emergence rate was similar between the two treatments, as 68.0 ± 8.0% in the scarification treatment and 66.7 ± 19.3% in the mixing treatment (Figure 4a). Most seedlings emerged in May 2016, two months after seeding, although seedling emergence continued until August 2016. Seedling mortality occurred mostly in the first year after seeding (Figure 4b). Once the seedlings survived until June in the second year, most of them survived thereafter. With the seedling emergence rate of 67%–68%, the first-year seedling establishment rate was 52.0 ± 10.1% and 62.0 ± 15.8% in the scarification and mixing treatments, respectively (Figure 4c). The seedling establishment rates became similar between the two treatments from the third year, with scarification at 40.0 ± 9.8% and mixing at 37.3 ± 16.1% in August 2018 (p = 0.68, Mann–Whitney U test).
Seedling mortality was greater in the scarification treatment than in the mixing treatment in the first year (p < 0.05, Mann–Whitney U test) and was not significantly different between the two treatments from the second year (Figure 4b). However, the causes of mortality differed. Desiccation was the second most common cause of mortality (18.4%) at the scarification treatment (Figure 5). Seedling mortality caused by desiccation was greater during the first year of the growing season at the scarification treatment. However, seedling mortality rapidly increased in the spring of the second year at the mixing treatment, offsetting the differences in mortality observed during the first year. Frost heaving was the major cause of mortality, accounting for 73.3% of seedling mortality. Repeated freezing and thawing of the soil resulted in the exposure of seedling roots to cold air, which froze and dried the roots.
The seedlings showed fixed growth as the shoot began to grow from April to June, finishing its annual shoot growth before August (Figure 6). The seedling height was significantly greater in the mixing treatment than in the scarification treatment in the second year (p < 0.05). The second-year seedling heights in October 2017 were 9.9 ± 3.3 cm and 13.9 ± 8.1 cm in the scarification and mixing treatments, respectively. The pattern of greater seedling height in the mixing treatment than in the scarification treatment continued throughout the remaining years. The seedling height in December 2021 was 51.8 ± 32.7 cm (median 45.5 cm, n = 53) in the scarification treatment, which was significantly lower than 96.1 ± 81.1 cm (median 74.0 cm, n = 55) in the mixing treatment (p < 0.05, the effect size 0.82 with the 95% CI [0.06, 1.59]).
Seedling mortality in June 2016 showed a positive correlation with the coverage of understory plants (ρ = 0.62, p < 0.05); however, seedling survival and growth showed no significant correlation with understory plants after the first year (Figure 7).

3.4. Seed Germination and Seedling Growth on Scarified and Mixed Substrates in a Growth Chamber

The growth chamber experiment examined the emergence and initial growth of seedlings in response to substrate types. Germinants began to appear on Day 8 on the treated substrates with scarification and mixing and on Day 10 on the control. The final germination rates were 55.0 ± 4.1% on the scarified substrate and 45.0 ± 4.1% on the mixed substrate, respectively (p = 0.23, Kruskal-Wallis test). However, the percentage of germinated seeds on the litter (control) was only 6.7 ± 6.2%, which was significantly lower than that on the treated substrates (p < 0.05; Figure 8). The MGT was 10 ± 0.0 days on the control, 12 ± 1.8 days on the scarified substrate, and 13.4 ± 1.7 days on the mixed substrate (Table 2). The T50 and GV on the scarified and mixed substrates were significantly greater than those in the control condition (p < 0.05, Kruskal-Wallis test).
The seedling growth at each organ significantly differed among the treatment substrates, except for shoot length and dry mass (p < 0.05; Table 3). The root length, root biomass, leaf area, leaf biomass, and R/S on the scarified substrate were significantly greater than those on the mixed substrate (p < 0.05, Mann–Whitney U test). The LWR on the scarified substrate was significantly lower than that on the mixed substrate (p < 0.05, Mann–Whitney U test).

4. Discussion

4.1. Effects of Site Preparation Treatments on Seed Germination and Seedling Emergence

In the growth chamber, the percentage of germinated seeds on the scarification and mixing treatments ranged from 45.0 to 55.0%, whereas the percentage of germinated seeds in the control was 6.7%, as only a few seeds germinated in the control. In addition, all seedlings died except one unhealthy individual during the 50-day period in the control. The litter layer blocked light and hindered roots from reaching the soil, negatively affecting seed germination and seedling establishment [66,67]. A higher seed germination percentage and seedling emergence in the treatments than in the control showed that both the scarification and mixing treatments improved the seed germination and seedling emergence of Korean ash [40]. Mean germination time (MGT) did not differ significantly among the treatments and control. However, germination value (GV) was the highest in the scarification treatment, and T50 was the highest in the mixing treatment. The highest GV in scarification indicated that the germination ceased more rapidly after reaching its peak, implying that scarification created favorable conditions, leading to simultaneous germination of most seeds. The highest T50 value showed slower seed germination, which might be caused by soil compaction and reduced soil aeration in the mixing treatment.
In the field, seedling emergence ranged from 66.7% to 68.0%, which was consistent with previous findings, reporting emergence rates of 66%–72% under mechanical site preparation [40]. Seedlings began to emerge in May 2016 in both treatments, which was two months after seeding, and most seedlings emerged in the same month in the field, which coincided with previous results [68,69]. A certain temperature and growth period are required for the after-ripening of immature seeds. Two to three months of stratification procedures at a temperature near 20 °C are recommended for after-ripening for the germination of Fraxinus seeds in North America [70]. Korean ash seeds also need a certain period of after-ripening in the field where the daily temperature fluctuates; for this reason, seedlings emerge two to three months after seeding [68].
The optimal germination temperature of Korean ash seeds ranged from 15 to 20 °C, which was reached in May at the study site, activating the germination [41,71]. In Gangwon Province, where the temperature is lower than in the study area, Korean ash seedlings emerge later than at the study site [42]. The temperature remained constant at 25 °C in the growth chamber. The after-ripening period was shortened, and germination progressed faster in the growth chamber than in the field. The temperature and timing at which Korean ash seedlings emerge coincide with the time when understory vegetation begins to flourish. Site preparation treatment that suppresses understory vegetation would improve the emergence and growth of Korean ash seedlings by reducing understory competition for space and resources.

4.2. Seedling Mortality and Soil Condition Between the Scarification and Mixing Treatments

In the first year after direct seeding, seedling mortality in August was 23.6% at the scarification treatment, which was significantly greater than that of 5.2% at the mixing treatment (p < 0.05). This difference occurred when the seedling height was lower than that of the competing understory herbs. Seedling mortality in the first year was significantly correlated with high plant coverage (p < 0.05; Figure 7). The belowground growth of direct-seeded F. excelsior L. seedlings was significantly affected by competitive understory plants due to competition for soil moisture and nutrients [72]. The higher height and cover of understory vegetation in the scarification treatment than in the mixing treatment (Figure 3) indicated that the competition for light, soil moisture, and soil nutrients against the surrounding plants was greater at the scarification treatment than at the mixing treatment, which caused greater seedling mortality at the scarification treatment than at the mixing treatment.
The causes of seedling mortality clearly differed between the scarification and mixing treatments. The amount of seedling damage caused by frost heaving in the second spring was greater in the mixing treatment than in the scarification treatment, causing 73% of seedling mortality. The mixing treatment exposed deeper mineral soil than the scarification treatment. The soil temperature in the first and second winters was lower in the mixing treatment than in the scarification treatment (Figure 2). The deep site preparation that exposed the mineral soil to a depth of over 10 cm lowered the soil temperature and induced frost heaving damage in the plants [56]. In addition, the mixing treatment had less herb coverage that buffered the soil temperature from the cold atmosphere than the scarification treatment. Lower winter temperature in the mixing treatment induced the development of freezing zones in the soil, leading to the migration of soil moisture and the formation of ice lenses that expanded and lifted the ground surface, uprooting or damaging seedlings [73]. From the third winter onward, winter temperatures increased alongside the growth of ground cover and the development of the root system, which likely reduced frost heaving damage.
Physical site preparation using heavy machines causes soil compaction and increases the soil bulk density, which hinders the root growth of seedlings and decreases the R/S ratio, increasing seedling susceptibility to frost heaving damage [73,74,75]. As an excavator was used for the mixing treatment, the resulting soil compaction might contribute to greater frost heaving damage in the mixing treatment than in the scarification treatment. The root growth of the seedlings in the growth chamber was lower in the mixing than in the scarification, demonstrating the negative effects of higher bulk density on root development. Considering that both root number and length in Korean ash seedlings propagated from cutting increased with soil porosity [76], it is likely that reduced soil pore space in the mixing treatment constrained root development, contributing to frost heaving damage.
The causes of the majority of seedling mortality were rarely identified at the scarification treatment; however, drought damage was found, along with frost heaving damage, in some seedlings. Frost heaving might cut off roots, reducing root absorption of soil moisture and increasing seedling vulnerability to drought [33]; thus, desiccation might be related to frost heaving.
Site preparation for seedling growth should consider not only aboveground growth but also belowground growth. Shallow site preparation that mixes the organic layer and topsoil to a depth of less than 5 cm may be preferred in light of the stable soil temperature and lower soil bulk density to reduce frost heaving damage to seedlings [56,77].

4.3. Seedling Growth and Understory Vegetation Between the Scarification and Mixing Treatments

The height growth occurred from April to June. Korean ash seedlings complete their annual height growth in the early part of the growth period and form winter buds in August, based on their photosynthetic capacity [69,78]. First-year height growth was similar between the scarification and mixing treatments, however; the seedling height in the mixing treatment exceeded that of the scarification treatment from the second year, and the height difference increased with time. This difference in height growth was explained by the competition with understory plants, as biomass allocation and initial growth of seedlings were primarily influenced by surrounding competitors [79].
The height and coverage of understory vegetation at the scarification treatment were significantly greater than those in the mixing treatment (p < 0.05; Figure 3), indicating that the mixing treatment more effectively suppressed competing understory vegetation. The number of perennial herb species in the scarification treatment was nearly double that at the mixing treatment (Table S1). In the mixing treatment, grasses, sedges, and narrowleaf forbs were newly established, indicating that this treatment created a disturbed microsite environment and removed existing vegetation, allowing species from incoming seeds to occupy the space. Such a newly formed environment might reduce shading and competitive pressure on seedlings. In contrast, more perennial herb species and root sprouts survived in the scarification treatment than in the mixing treatment due to less root damage from scarification than from the mixing treatment. The competition for light, moisture, and nutrients with perennial herbs and seedlings from sprouts was greater at the scarification treatment, which hindered photosynthesis in first-year seedlings and resulted in lower height growth at the scarification treatment than at the mixing treatment from the second year onward [80,81]. Moreover, trees originating from stump sprouts continued to grow and overshade Korean ash seedlings, further suppressing seedling growth at the scarification treatment.
The average understory height at its maximum was 67.4 ± 25.8 cm in the mixing treatment and 112.7 ± 16.9 cm in the scarification treatment. Seedling height was projected to surpass the understory height three and five years after direct seeding in the mixing and scarification treatments, respectively. This suggests that weeding is necessary for the first 3–5 years to reduce competition and shading from surrounding herbs [40,82]. Weeding can decrease initial seedling mortality and promote seedling growth [83]. In our study, the mixing treatment was preferred, because it reduced competition with understory plants, which was more critical than frost heaving and heavy machinery use at the study site. Additionally, in the growth chamber experiment, although seedlings in the mixing treatment had smaller leaf areas and lower leaf dry mass, they exhibited a higher leaf weight ratio (LWR), indicating a greater proportional investment in leaves. Increased investment in leaves may enhance the early-stage photosynthetic capacity and promote rapid height growth, potentially providing a competitive advantage over understory plants. However, once seedlings overtopped the understory plants, it remains uncertain whether the early height advantage observed under the mixing treatment will persist. Based on the growth chamber experiment, the higher root R/S ratio, root length, and root dry mass observed under scarification suggest that enhanced root development may improve water acquisition, nutrient uptake efficiency, and drought resistance in the long term.
Mixing treatment may induce nutrient leaching and decrease soil microbial activity [84]. Since both aboveground and belowground growth of Korean ash respond strongly to nitrogen fertilization [85], applying nitrogen may enhance soil nutrient availability. Additionally, applying a straw mulch layer before winter can mitigate the negative effects of mixing treatment by improving soil conditions favorable for seedling growth and survival [86]. Straw coverage in the use of an excavator was found effective in reducing frost heaving mortality of one-year-old seedlings of broad-leaved species [87]. Therefore, the combined practices of site preparation, weeding, and mulching could positively affect direct-seeded seedlings by providing temperature insulation and retaining moisture.
Our results compared seedling mortality and growth, soil temperature, and understory vegetation between two mechanical site preparation treatments; however, uncertainty remained due to methodological limitations. The lack of a control in the field prevented us from assessing treatment effects relative to the untreated conditions. In addition, an environmental survey including vegetation, soil, and climate prior to treatment could have more effectively verified our treatment effects. Therefore, future studies employing a Before–After–Control–Impact (BACI) design would be valuable for clearly determining treatment effects. Seedling mortality caused by frost heaving and the influence of understory coverage primarily occurred during the first year. However, monitoring that ended in August did not fully capture seedling dynamics and their relationship with the environment during the latter part of that year. Therefore, more intensive and detailed observations, particularly during the cold season, are necessary.
Although our study plots were located in a relatively homogeneous environment over a small area (225 m2) and were not expected to exhibit substantial variation in climatic factors (e.g., temperature and precipitation) or soil conditions, microsite differences could still influence seedling dynamics. However, only soil temperature was monitored in the study, making it difficult to assess the specific effects of other environmental factors. Frost heaving damage may be influenced not only by soil temperature but also by other factors such as soil moisture, soil physicochemical properties, and air temperature. Consequently, monitoring these environmental changes in relation to different mechanical site preparations is important for understanding their effects on direct-seeded seedlings. Further studies are needed to clarify the relationship between the microsite environment and seedling dynamics.

5. Conclusions

Successful direct seeding requires not only viable seeds but also suitable seedbed conditions. Both scarification and mixing treatments facilitated seed germination and seedling establishment by six to eight times compared to the control by removing the litter layer. Compared to scarification, the mixing treatment was more effective at suppressing competing understory vegetation, particularly perennial herb species and root sprouts. Therefore, the mixing treatment is preferred due to higher seedling survival during the first year and improved seedling height growth. However, site preparation treatments that exposed mineral soil lowered winter soil temperatures and increased summer soil temperatures during the first two years, leading to frost heaving in the cold season. Changes in soil temperature resulting from these treatments stabilized from the third year onward as the trees grew. Mixing treatments are primarily recommended on sites where frost heaving damage is less of a concern. The use of heavy machinery for mixing treatments at depths greater than 30 cm may cause soil compaction, potentially impeding root growth. Therefore, shallow mixing treatments less than 5 cm deep, combined with measures to mitigate frost heaving damage, are recommended for the direct seeding of Korean ash. Incorporating a control treatment in the field is expected to enhance the applicability of our study.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/f16091401/s1: Table S1: A list of the plant species and their life cycles that appeared at the scarification and mixing treatment sites in the first year of site preparation.

Author Contributions

Conceptualization, J.B.J. and P.S.P.; methodology, J.B.J. and P.S.P.; formal analysis, J.B.J., H.J.K., and P.S.P.; investigation, J.B.J., H.J.K., J.K., and J.S.J.; data curation, J.B.J., H.J.K., J.K., and J.S.J.; writing—original draft preparation, J.B.J. and P.S.P.; writing—review and editing, J.B.J. and P.S.P.; visualization, J.B.J. and P.S.P.; supervision, P.S.P.; funding acquisition, P.S.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the R&D Program for Forest Science Technology (Project No. 2013069C10-1919-AA03, FTIS 2022461B10-2424-0201, and RS-2024-00404133) of the Korea Forestry Promotion Institute, Korea Forest Service.

Data Availability Statement

The data from the current study are available from the corresponding author upon reasonable request.

Acknowledgments

We acknowledge the Korea National Institute of Forest Science for providing the Korean ash seeds and Taehwasan University Forest, Seoul National University, for providing the study sites. We thank Myeong Pil Kim, Seunghyun Lee, and Yongseob Lee of Seoul National University Forests for their help in the site preparation treatments and Daeho Choi, Joosung Ha, and Eunho Choi for the fieldwork and laboratory experiments. We acknowledge the Research Institute of Agriculture and Life Sciences, Seoul National University, for language assistance and the anonymous reviewers for their valuable comments.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Locations of (c) the scarification and mixing treatment sites in (b) the Taehwasan University Forest (grey area) of Seoul National University, (a) Gwangju-si, Gyeonggi-do, Republic of Korea.
Figure 1. Locations of (c) the scarification and mixing treatment sites in (b) the Taehwasan University Forest (grey area) of Seoul National University, (a) Gwangju-si, Gyeonggi-do, Republic of Korea.
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Figure 2. Daily mean soil temperature at the (a) scarification and (b) mixing treatment sites from March 2016 to December 2021 at the Taehwasan University Forest of Seoul National University, Gwangju, Gyeonggi-do, Republic of Korea.
Figure 2. Daily mean soil temperature at the (a) scarification and (b) mixing treatment sites from March 2016 to December 2021 at the Taehwasan University Forest of Seoul National University, Gwangju, Gyeonggi-do, Republic of Korea.
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Figure 3. (a) Height (cm) and (b) percent coverage (%) of the herb layer in the scarification and mixing treatment sites. The circles and bars indicate the means and standard deviations, respectively (n = 6). Asterisks represent significant differences in the same month between the scarification and mixing treatments at p < 0.05, according to the Mann–Whitney U test.
Figure 3. (a) Height (cm) and (b) percent coverage (%) of the herb layer in the scarification and mixing treatment sites. The circles and bars indicate the means and standard deviations, respectively (n = 6). Asterisks represent significant differences in the same month between the scarification and mixing treatments at p < 0.05, according to the Mann–Whitney U test.
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Figure 4. Cumulative values of (a) seedling emergence (%), (b) seedling mortality (%), and (c) seedling establishment (%) of Fraxinus chinensis subsp. rhynchophylla in the scarification and mixing treatments. The circles and bars indicate the means and standard deviations, respectively (n = 6). Asterisks represent significant differences in the same year and month between the scarification and mixing treatments at p < 0.05, according to the Mann–Whitney U test.
Figure 4. Cumulative values of (a) seedling emergence (%), (b) seedling mortality (%), and (c) seedling establishment (%) of Fraxinus chinensis subsp. rhynchophylla in the scarification and mixing treatments. The circles and bars indicate the means and standard deviations, respectively (n = 6). Asterisks represent significant differences in the same year and month between the scarification and mixing treatments at p < 0.05, according to the Mann–Whitney U test.
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Forests 16 01401 g005
Figure 5. Causes of seedling mortality at the (a) scarification (n = 49) and (b) mixing (n = 45) treatment sites. The numbers in the graph represent the proportion of the total mortality.
Figure 5. Causes of seedling mortality at the (a) scarification (n = 49) and (b) mixing (n = 45) treatment sites. The numbers in the graph represent the proportion of the total mortality.
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Figure 6. Comparison of height of Fraxinus chinensis subsp. rhynchophylla seedlings between scarification and mixing treatments from 2016 to 2021. The circles and bars indicate the means and standard deviations, respectively. Asterisks indicate that the means are significantly different at p < 0.05, according to the generalized linear mixed model (GLMM) with multiple comparisons by Tukey contrasts.
Figure 6. Comparison of height of Fraxinus chinensis subsp. rhynchophylla seedlings between scarification and mixing treatments from 2016 to 2021. The circles and bars indicate the means and standard deviations, respectively. Asterisks indicate that the means are significantly different at p < 0.05, according to the generalized linear mixed model (GLMM) with multiple comparisons by Tukey contrasts.
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Figure 7. Spearman’s rank correlation coefficient matrix between height and mortality of Fraxinus chinensis subsp. rhynchophylla seedlings and height and percent cover of the herb layer in 2016, 2017, and 2018 (n = 12). Spearman’s coefficients (ρ) at the significance level (p < 0.05) are shown in the box.
Figure 7. Spearman’s rank correlation coefficient matrix between height and mortality of Fraxinus chinensis subsp. rhynchophylla seedlings and height and percent cover of the herb layer in 2016, 2017, and 2018 (n = 12). Spearman’s coefficients (ρ) at the significance level (p < 0.05) are shown in the box.
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Figure 8. (a) Cumulative and (b) final germination percentages of Fraxinus chinensis subsp. rhynchophylla seeds in the control, scarification, and mixing treatments in the growth chamber. The error bars are the standard deviations (n = 3). Means followed by the same letter are not significantly different among treatments at p < 0.05, according to Kruskal—Wallis test with post hoc Dunn’s test.
Figure 8. (a) Cumulative and (b) final germination percentages of Fraxinus chinensis subsp. rhynchophylla seeds in the control, scarification, and mixing treatments in the growth chamber. The error bars are the standard deviations (n = 3). Means followed by the same letter are not significantly different among treatments at p < 0.05, according to Kruskal—Wallis test with post hoc Dunn’s test.
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Table 1. Soil bulk density, soil pH, and soil moisture content at the scarification and mixing treatment sites 3 months after treatment (mean ± standard deviation). Different letters indicate that the means are significantly different between the scarification and mixing treatments at p < 0.05 according to the Mann–Whitney U test (n = 3).
Table 1. Soil bulk density, soil pH, and soil moisture content at the scarification and mixing treatment sites 3 months after treatment (mean ± standard deviation). Different letters indicate that the means are significantly different between the scarification and mixing treatments at p < 0.05 according to the Mann–Whitney U test (n = 3).
TreatmentSoil Bulk Density (g cm−3)Soil Moisture Content (θm, %)Soil pH
Scarification0.81 ± 0.12 a42.16 ± 8.90 a4.67 ± 0.06 a
Mixing0.94 ± 0.02 a32.87 ± 4.64 a4.71 ± 0.02 a
Table 2. Germination percentage, mean germination time (MGT), time to 50% of final germination (T50), and germination value (GV) of Fraxinus chinensis subsp. rhynchophylla seeds in the growth chamber (mean ± standard deviation). Means followed by the same letter are not significantly different among the site preparation treatments at p < 0.05, according to the Kruskal-Wallis test with a Dunn comparison test (n = 3).
Table 2. Germination percentage, mean germination time (MGT), time to 50% of final germination (T50), and germination value (GV) of Fraxinus chinensis subsp. rhynchophylla seeds in the growth chamber (mean ± standard deviation). Means followed by the same letter are not significantly different among the site preparation treatments at p < 0.05, according to the Kruskal-Wallis test with a Dunn comparison test (n = 3).
TreatmentGermination (%)MGT (Day)T50 (Day)GV
Control6.7 ± 6.2 b10.0 ± 0.0 a4.0 ± 2.8 b0.1 ± 0.2 b
Scarification55.0 ± 4.1 a12.0 ± 1.8 a9.7 ± 0.3 ab3.8 ± 1.1 a
Mixing45.0 ± 4.1 a13.4 ± 1.7 a11.3 ± 1.2 a2.2 ± 0.2 ab
Table 3. Shoot and root lengths, leaf area and dry mass of each organ, the root–shoot ratio (R/S ratio), and the leaf weight ratio (LWR) of Fraxinus chinensis subsp. rhynchophylla seedlings at 50 days after sowing (mean ± standard deviation) in a growth chamber. Different letters on the same organ indicate that the means are significantly different between the scarification and mixing treatments at p < 0.05, according to the Mann–Whitney U test.
Table 3. Shoot and root lengths, leaf area and dry mass of each organ, the root–shoot ratio (R/S ratio), and the leaf weight ratio (LWR) of Fraxinus chinensis subsp. rhynchophylla seedlings at 50 days after sowing (mean ± standard deviation) in a growth chamber. Different letters on the same organ indicate that the means are significantly different between the scarification and mixing treatments at p < 0.05, according to the Mann–Whitney U test.
TreatmentShoot Length (cm)Root Length (cm)Leaf Area
(cm2)		Shoot
Dry Mass (mg)		Root
Dry Mass (mg)		Leaf
Dry Mass (mg)		R/S Ratio (mg/mg)LWR
(mg/mg)
Scarification (n = 20)3.6 ± 0.8 a10.2 ± 2.6 a9.0 ± 2.5 a20.0 ± 8.4 a43.2 ± 18.1 a42.5 ± 13.0 a0.67 ± 0.85 a0.42 ± 0.07 b
Mixing
(n = 20)		3.6 ± 0.8 a7.1 ± 4.0 b6.5 ± 2.8 b16.6 ± 9.8 a23.8 ± 17.5 b32.4 ± 14.2 b0.44 ± 0.73 b0.48 ± 0.08 a
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MDPI and ACS Style

Jung, J.B.; Kim, H.J.; Kim, J.; Jung, J.S.; Park, P.S. Comparison of Two Site Preparation Treatments for the Growth of Direct-Seeded Fraxinus chinensis subsp. rhynchophylla Seedlings and Their Effects on Soil Temperature and Understory Vegetation. Forests 2025, 16, 1401. https://doi.org/10.3390/f16091401

AMA Style

Jung JB, Kim HJ, Kim J, Jung JS, Park PS. Comparison of Two Site Preparation Treatments for the Growth of Direct-Seeded Fraxinus chinensis subsp. rhynchophylla Seedlings and Their Effects on Soil Temperature and Understory Vegetation. Forests. 2025; 16(9):1401. https://doi.org/10.3390/f16091401

Chicago/Turabian Style

Jung, Jong Bin, Hyun Jung Kim, Jongwoo Kim, Ji Sun Jung, and Pil Sun Park. 2025. "Comparison of Two Site Preparation Treatments for the Growth of Direct-Seeded Fraxinus chinensis subsp. rhynchophylla Seedlings and Their Effects on Soil Temperature and Understory Vegetation" Forests 16, no. 9: 1401. https://doi.org/10.3390/f16091401

APA Style

Jung, J. B., Kim, H. J., Kim, J., Jung, J. S., & Park, P. S. (2025). Comparison of Two Site Preparation Treatments for the Growth of Direct-Seeded Fraxinus chinensis subsp. rhynchophylla Seedlings and Their Effects on Soil Temperature and Understory Vegetation. Forests, 16(9), 1401. https://doi.org/10.3390/f16091401

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