Introduction
Vegetation restoration is a critical element of successful ecological restoration. If the ultimate goal of ecological restoration is to enhance biodiversity, then fully restoring vegetation provides habitats for a variety of species (Cho, 2024). To this end, a range of planting techniques have been developed in this field. Vegetation restoration techniques in ecological restoration are generally categorized as active or passive. Active restoration involves directly planting vegetation, whereas passive restoration relies on natural plant regeneration without human intervention. The choice between these techniques should be based on the environmental characteristics and restoration potential of the target site. Recent studies have reported that forest restoration can be more effective in areas where no planting has been conducted, compared to areas where active planting has been implemented (Jung et al., 2022; Jung, 2024). Furthermore, one study found that in riparian zones, 20 years after restoration efforts began, the resulting vegetation community differed significantly from the originally planted flora (Ji, 2024). Conversely, areas where planting was conducted had a higher richness of woody species compared to areas undergoing natural succession. However, naturally regenerated areas exhibited a higher proportion of native, bird-dispersed, and insect-pollinated species (Pedersen et al., 2025). These studies suggest that the ultimate development of vegetation during restoration is influenced by various factors, including environmental conditions and site management practices. In cases where vegetation restoration is required, passive restoration techniques—those that do not involve planting—are sometimes employed. However, active restoration techniques that involve planting are more commonly used. Consequently, various planting methods suited to specific target areas have been implemented. The National Institute of Ecology (2015) provides guidelines for restoration planting techniques, which state that woody species in restoration sites can be established either through direct seeding or by planting seedlings/saplings. Nevertheless, the use of seedlings/saplings remains relatively uncommon in ecological restoration projects (Cho, 2022).
Meanwhile, tree mortality (i.e., dead trees) is often unavoidable during vegetation restoration, particularly when various planting techniques—including the use of seedlings/saplings—are applied. This mortality is primarily attributed to environmental stress resulting from inter-individual or intraspecific competition in the early stages after planting, planting density, soil conditions, and competition from weeds. Tree mortality, including both standing and fallen individuals, is a natural phenomenon frequently observed during restoration efforts, and it can influence both the overall success of the restoration process and the long-term stability of the ecosystem. In other words, tree mortality is not merely a negative phenomenon; it can play a crucial role in the development of resprouts. Resprouts emerging from dead trees grow rapidly, facilitate forest regeneration, and serve as valuable resources that contribute to the maintenance of biodiversity. Through vegetative propagation, these resprouts can help maintain or restore forest structure without the need for artificial planting, thereby supporting low-cost and highly efficient forest management (Choung and Choung, 2019). Additionally, resprouts grow quickly using the existing root system and stored assimilates of the parent tree and have high resistance to various environmental stresses during the early stages of development (Lee et al., 2000). These traits enable self-reproduction even after the parent tree’s death, which benefits forest continuity and ecosystem resilience. In particular, resprouts can effectively contribute to sustainable forest systems—namely, the long-term management and maintenance of forests without artificial planting—since they enable self-renewal while minimizing external resource input. Due to these characteristics, resprouts are a low-cost and advantageous option for the sustainable maintenance of forests. In particular, they have positive ecological effects by promoting the restoration of native species and suppressing the growth of non-native species in the understory (Matula et al., 2020).
Resprouts from dead trees have long been utilized as a regenerative mechanism in tropical dry forests. Recent research indicates that resprouts exhibit higher carbon absorption capacity than seedlings/saplings and can play a significant role in enhancing the resilience of ecosystems (Mcdonald, 2008; Mölder, 2019; Matoušvá et al., 2022). In South Korea, resprouts are also used to restore post-wild-fire sites, which contributes to the long-term recovery of the hierarchical structure of natural forests (Choung et al., 2000; Lee et al., 2004). However, in ecological restoration projects, research on tree mortality caused by early high-density planting, and subsequent resprouting remains limited (Yu and Cho, 2023). In South Korea, most previous studies on such resprouting have focused on timber production following artificial disturbances, primarily from a forest management perspective. Studies linking resprouting to ecological restoration have largely addressed the natural recovery of bare land, including wildfire-affected areas, from a long-term perspective (Choung et al., 2000; Lee et al., 2004; Choung and Choung, 2019). Consequently, there is a lack of research on resprouting dynamics specifically within areas where ecological restoration projects are implemented. The survival and growth of resprouts from dead trees are influenced by various environmental and biological factors. Among these factors, planting period (or post-planting period) is a major factor that determines the climatic conditions and environmental stress experienced during the early growth stages, directly affecting the establishment and growth of resprouts. Tree size serves as an indicator of the physiological condition of the parent tree, the amount of stored energy, and the extent of root development, all of which can lead to variation in resprouting rates and resprout growth performance. Planting density relates to the level of inter-individual or intraspecific competition; at higher densities, competition for essential resources, such as water, light, and nutrients, intensifies, potentially affecting the survival rate and growth of resprouts. Lastly, the extent of mulch thickness is a critical factor in enhancing the early growth environment of resprouts by promoting soil moisture retention, suppressing weed growth, and regulating soil temperature. These factors serve as key independent variables in understanding the occurrence and growth of resprouts from dead trees and in developing effective forest restoration and management strategies. Therefore, this study aimed to investigate and analyze tree mortality and subsequent resprouting over a 29-month period following the establishment of experimental plots with varying tree sizes, planting densities, and levels of mulch thickness, as previously described. The findings are expected to serve as foundational data for developing appropriate planting techniques and management strategies for future forest restoration efforts.
Research Methods
Research scope
The experimental site for this study was located at the Dong-A University Toerae Farm (975–19, Gimhae-daero, Hallim-myeon, Gimhae-si, Gyeongsangnam-do), covering an area of 1,181.7 m2. Prior to the establishment of the experimental plots, the land was used for cultivating naked barley (Hordeum vulgare var. nudum). Gimhae-si, where the experimental site is located, has shown a gradual increase in average annual temperature over recent years, recording 14.9°C in 2022, 15.5°C in 2023, and 16.1°C in 2024. The city’s annual precipitation also fluctuated, with a relatively low level of 874.0 mm in 2022—when the experimental site was established—followed by 1,879.6 mm in 2023 and 1,720.0 mm in 2024. Average annual relative humidity increased over the same period, reaching 62.1% in 2022, 67.8% in 2023, and 69.6% in 2024.
The subsoil at the experimental site is classified as loamy, with an effective soil depth ranging from 50 to 100 cm and exhibiting good drainage characteristics. The tree species selected for planting in this study was sawtooth oak (Quercus acutissima Carruth.), a representative native species known for its strong drought tolerance and high resprouting capacity (Lee and Cho, 2007). This species is also among those recommended in the Urban Ecological Axis Restoration Project Guidelines issued by the Ministry of Environment (MOE, August 2024).
To determine the effects of tree size, planting density, and mulch thickness on the post-planting mortality and subsequent resprouting of sawtooth oaks, the experimental site was designed as follows. First of all, in line with ecological restoration techniques that recommend using younger trees whenever possible (Clewell et al., 2005; National Institute of Ecology, 2015; Cho, 2021), four categories of tree size were selected based on diameter at root collar (DRC): 2 cm, 4 cm, 6 cm, and 8 cm. In this study, DRCs of 2 cm (R2) and 4 cm (R4), classified as sapling sizes, were selected based on a literature review (Lee, 2020), which suggests that trees with a diameter at breast height (DBH) of less than 4 cm are suitable for planting. A DRC of 6 cm (R6) was adopted based on the Landscaping Standards issued by the Ministry of Land, Infrastructure, and Transport (MOLIT, 2021), which stipulate a minimum DRC of 6 cm for planted trees. Lastly a DRC of 8 cm (R8) was selected in consideration of the common use of 8–12 cm DRC trees in public landscaping projects (Kang and Kim, 2009) and the typical specifications for tree layers in ecological restoration initiatives (Cho, 2022). Planting density is an important factor in determining canopy spread and minimizing disturbance to the ground cover layer after planting. The experimental plots were designed with three levels of planting density. High density was defined as 2 to 7 plants per square meter, based on the ecological planting technique developed by Miyawaki (1999). Medium density was set at 1 to 3 plants per square meter—approximately half of the high-density level. Low density was defined as 0.2 plants per square meter, as proposed by the Landscape Design Standard (Ministry of Land, Infrastructure and Transport, 2019). Moreover, to determine the effects of mulch thickness on tree mortality rate and subsequent resprouting, this experiment was conducted with three treatment groups: a non-mulched area (NM; 0 cm), a 5-cm mulch treatment, and a 10-cm mulch treatment. Wood chips were used uniformly as the mulching material. The experimental plots were square-shaped, each measuring 2 m by 2 m, and a total of 132 sawtooth oak saplings were planted across 36 quadrats, arranged according to tree size, planting density, and mulching treatment level (Fig. 1). The experimental site was established in April 2022. All planted individuals were surveyed for mortality, and data were collected on mortality rates, resprouting occurrence, and resprout growth. Monitoring was carried out for a total of 29 months, from May 2022 to September 2024.
Research method
In this study, dead trees were defined as standing or fallen dead trees within forests, in accordance with the Management Indicators and Measurement/Evaluation Standards for Urban Forests (Korea Forest Service, 2024). For the purposes of this study, operational criteria were further established to distinguish between dead trees with resprouting and those without (Table 1). The criteria for identifying dead trees were categorized into three stages, based on the Guidelines for Monitoring and Maintaining Ecological Restoration Projects (MOE, Jan. 2017) and the Standards for Investigating Defects in Apartment Housing, Estimating Repair Costs, and Determining Defects (MOLIT, 2021). According to these guidelines, a tree is considered dead if at least two-thirds of its branches wither or if its branches and leaves are in such poor condition that recovery is unlikely.
Resprouts are generally defined as bud burst or shoots that emerge from dormant or adventitious buds, rather than from buds formed during normal branch development. However, since water sprouts that mainly originate from branches are difficult to identify as individual plants, this study focused on individuals developing from roots and the root collar to improve the accuracy of the investigation and the identifiability of individuals during their development. The resprouting timing and number of resprouts from dead trees were visually monitored on a monthly basis. The field survey results were recorded on site and subsequently organized into a time series using Microsoft Excel 2021. Mortality rates were calculated for each survey session, rather than as cumulative values, to analyze temporal patterns in tree mortality, recovery of tree vigor, and remortality. Accordingly, individuals initially classified as dead trees but later exhibiting signs of recovery were reclassified as healthy and excluded from mortality rate calculations. To assess the growth of resprouts from dead trees, their length and diameter were measured twice, in September 2023 and September 2024. The length of each resprout was measured in millimeters using a 50-meter fiberglass tape (Komelon), from the point where the resprout emerged from the xylem. The diameter of each resprout was measured twice at the point of emergence from the xylem, in millimeters, using 24-cm digital vernier calipers (O-ON). When multiple resprouts were present on a single tree, the diameter of each was measured and the average value was used for analysis. The average mortality rate for each tree size over a 10-month period was then converted into a survival rate, calculated using the formula: (1 - mortality rate) × 100.
To examine the effects of multiple independent variables on a single dependent variable, a multiple linear regression analysis was conducted. The data were analyzed to determine how factors such as tree size, planting density, and mulch thickness affect tree mortality by period and its resprouts, and whether any of these independent variables have a statistically significant effect. All statistical analyses—including the standardized coefficient (β), ANOVA, coefficient of determination (R2), and collinearity diagnostics—were performed using SPSS software (Version 29.0).
Results and Discussion
Analysis of tree mortality
Analysis of tree mortality by post-planting period
Among the 132 sawtooth oak saplings planted in the test bed, 52 were found dead one month after planting. However, by the third month, the number of dead trees had decreased to 44, indicating that 8 saplings had regained their vigor. By the fifth month, the number of dead trees had increased to 55, suggesting that some of the previously recovered saplings had died again. From 13 months after planting—one year post-planting—through to the final 29-month observation period, the mortality rate remained stable at approximately 2 ± 0%, with no significant fluctuations (Fig. 2). This pattern suggests that some saplings initially succumbed to transplant stress but recovered their vigor (Lee and Bae, 2022). A significant number of dead trees were observed one month after planting, with some fluctuations continuing into the second year. Additionally, individuals exhibiting weak vigor in the spring appeared to have the potential to recover during the summer stem growth period (Son et al., 2024). However, of the 18 trees initially presumed dead in the spring but showing signs of recovery during the summer, eight died again by the end of the fall growth period. These eight individuals did not recover vigor by the fall of the third year and were ultimately classified as dead trees. In the year of planting, the mortality rate was high immediately after transplantation, likely due to transplant shock and the severe drought that occurred that year. In the second year, the mortality rate increased slightly as summer progressed. From the third year onward, however, no new dead tree formation was observed.
Analysis of mortality rate by tree size
An analysis of the mortality rate by tree size (Fig. 3) revealed that R2 trees had a high mortality rate—attributed to insufficient leaf growth and reduced vigor—within the first month after planting. However, this rate declined to 13% by the third month, suggesting that the trees had begun to recover from transplant shock and regain vigor. Between 5 and 13 months post-planting, the mortality rate was at 18%. A slight increase of 3±0% was observed in the summer of the 15th month, but the final mortality rate remained at 18% after overwintering. The mortality rate of R4 trees consistently exceeded that of R2 trees. It was 28% one month after planting and then slightly declined to 26% by the third month. However, it gradually increased between 5 and 17 months after planting, eventually returning to 28%. The mortality rate of R6 trees was 40% one month after planting. Unlike R2 and R4 trees, however, the rate did not decrease at the third month but instead continued to rise. It reached 60% by the seventh month, indicating a rapid increase during the early post-planting period. After two years, the mortality rate began to decline gradually, ultimately reaching 53%—a 10±0% decrease compared to the rate at the 17th month. The mortality rate of R8 trees was 50% one month after planting and continued to rise, reaching 75% by the fifth month. Although partial recovery was observed between the 7th and 13th months, the rate increased again, returning to 75% by the 15th month. Finally, the mortality rate after overwintering decreased by 12±0%, maintaining 63%. Notably, the rate tended to increase in the fall following the first summer after planting, likely due to drought stress, as the average annual precipitation in 2022 was only 874.0 mm. In contrast, during 2023 and 2024, when average annual precipitation returned to normal levels, the seasonal effect on mortality was not significant.
Three months after planting, the mortality rate of smaller saplings (R2 and R4 trees) decreased, suggesting that most trees capable of recovering from initial withering tend to be small. Furthermore, mortality was lower in smaller saplings at the time of planting. Therefore, as smaller saplings are more adaptable than mature trees to environmental conditions (Kwon, 1997; Clewell et al., 2005; Cho, 2021), they appear to have a higher potential for recovery after experiencing transplant shock within the first month post-planting. In contrast, R6 and R8 trees exhibited the lowest mortality rates one month after planting, but these rates tended to increase over time. This suggests that for ecological restoration purposes, planting R4 or smaller trees may result in higher survival rates.
An analysis of average survival rates by tree size revealed values of 80.4% for R2, 69.1% for R4, 44.5% for R6, and 33.1% for R8. It was analyzed that higher survival rates indicate healthier tree survival.
Analysis of tree mortality by planting density
An analysis of tree mortality by planting density revealed that the mortality rate in the low-density plots remained consistently at 42% from one month after planting through to the final 29-month observation (Fig. 4). The mortality rate in the high-density plots decreased to approximately 13 ± 0% by the third month after planting. However, a renewed increase in plant mortality was observed between 5 and 7 months after planting, with the final mortality rate reaching 36%. In the medium-density plots, the lowest mortality rate was observed one month after planting. However, the rate increased steadily from 1 to 5 months after planting. Subsequently, it decreased by approximately 2 ± 0% at 7 months, but rose again to 42% following the overwintering period. Thereafter, the highest mortality rate was recorded at 38%. As demonstrated in previous studies, high-density planting resulted in a significant umber of dead trees during the early stages of planting due to intense intraspecific competition (Han and Park, 2022), leading to a relatively high mortality rate. In contrast, the low-density plots, which had only one tree was planted per plot, exhibited a similarly high mortality rate despite minimal competition with neighboring trees. This may be attributed to the relatively smaller canopy spread and greater sunlight penetration observed in low-density plots compared to medium- or high-density plots. Increased light penetration likely accelerated soil moisture evaporation and promoted weed growth on the forest floor. The proliferation of weeds, in turn, intensified competition for soil nutrients. Therefore, it is suggested that planting techniques—such as promoting tree height growth and thereby facilitating early canopy closure through planting at densities of 1–2 trees/m2, or employing mulching techniques to reduce soil moisture evaporation and suppress weed emergence—are necessary (Park et al., 2022).
The average survival rates by planting density were 0.600% for high-density plots, 0.617% for medium-density plots, and 0.580% for low-density plots. While medium-density plots exhibited the highest survival rate, the rates observed in high- and low-density plots were relatively similar.
Analysis of tree mortality by mulch thickness/depth
An analysis of tree mortality by mulch thickness showed that the mortality rate was highest in plots without mulching (0 cm) one month after planting, followed by 61% in 10-cm mulched plots and 27% in 5-cm mulched plots (Fig. 5). After 17 months after planting, the mortality rates were 73% in the non-mulched plots, 36% in the 5-cm mulched plots, and 23% in the 10-cm mulched plots. At 29 months post-planting, these rates had declined to 61%, 32%, and 18%, respectively. Notably, the experimental site experienced relatively low rainfall during the summer period (May to July 2022), three months after planting. Consequently, the trees in the non-mulched plots likely experienced more severe moisture stress due to the lack of protective ground cover. The 10-cm mulched plots exhibited a mortality rate nearly half that of the 5-cm and non-mulched plots, suggesting that mulching contributed to improved tree survival by reducing soil moisture evaporation and blocking invasive species (Park et al., 2022). At the experimental site, it was also observed that greater mulch depth corresponded with reduced weed coverage. Lee (2021) suggested an optimal mulch thickness of 5 to 7 cm. This study found that a 10 cm mulch depth resulted in a higher tree survival rate. However, excessive mulching with organic materials may generate decomposition heat or hinder root respiration and growth due to excess moisture (Zhu, 2024; Cho, 2024). Therefore, further studies are needed to determine the optimal mulch depth that balances these benefits and potential drawbacks.
The average survival rates by mulch thickness were 0.335% in the non-mulched plots, 0.676% in the 5-cm mulched plots, and 0.800% in the 10-cm mulched plots. The lowest survival rate was observed in the non-mulched plots, and there was a clear trend of increasing survival rate with greater mulch thickness.
A multiple linear regression analysis using the stepwise method was conducted to evaluate the effects of post-planting period, tree size, planting density, and mulch thickness on tree mortality. The results indicated that the regression model was statistically significant (F = 13.119, p <. 001), suggesting its appropriateness. The model explained 51.0% of the variance in tree mortality, with an adjusted coefficient of determination (adjusted R2) of 0.510 (Table 2). The null hypothesis that tree size, planting density, and mulch thickness have no effect on tree mortality were rejected (t = 2.147, p < .039; t = 4.461, p < .001; t = −3.853, p < .001, respectively), and the alternative hypothesis was accepted. These variables were found to significantly affect tree mortality. However, the independent variable post-planting period did not show a significant effect (t = −0.339, p = .744). To determine the relative effect of the variables—post-planting period, tree size, planting density, and mulch thickness—standardized regression coefficients (β) were compared. Among these, planting density (β = 0.528) had the greatest relative effect on tree mortality, followed by mulch thickness (β = −0.456), tree size (β = 0.254), and post-planting period (β = −0.119).
Analysis of resprouting from dead trees
Analysis of resprouting from dead trees by post-planting period
Resprouting from dead trees was analyzed based on integrated data, regardless of the original tree size. In the year of planting, 10 resprouts emerged from 9 dead trees in the fall. In the second year after planting, 33 to 34 resprouts were observed from 10 dead trees during the summer and fall. In the third year, 24 to 27 resprouts were recorded from 9 dead trees (Figs. 6 and 7). The first resprouts were observed four months after planting, at which time the proportion of dead trees with resprouts was only 2%. However, this increased to 16% by 14 months and eventually reached 18%. The ratio of the number of resprouts per dead tree with resprouting showed a continuous increase from four months after the initial observation. By the fall of the 17th month, this value peaked at 340%, and later stabilized at 267%, indicating that, on average, three or more resprouts were produced per dead tree. As reported in previous studies (Kwon et al., 1998; Min et al., 2017), this finding suggests that management practices such as regulating the number of resprouts may be necessary, depending on their growth status. On the other hand, if such resprouts from dead trees grow successfully, additional planting to repair defects may not be necessary. Therefore, continuous monitoring of their growth is necessary. In forest fire areas, trees damaged by fire are often naturally restored through resprouting (Choung and Choung, 2019). In the same context, long-term monitoring of trees presumed dead in ecological restoration sites is needed. If resprouting occurs, it may be necessary to consider a plan to support the continued growth of the resprouts rather than implementing defect repair measures.
Analysis of resprouting from dead trees by tree size
At 29 months after planting, the number of resprouting individuals among dead trees and the total number of resprouts by tree size were as follows: R8 trees produced 9 resprouts from 3 individuals, R6 trees produced 8 resprouts from 3 individuals, R4 trees produced 7 resprouts from 3 individuals, and R2 trees exhibited no resprouting (0 resprouts from 0 individuals; Fig. 8). This pattern was consistent with the order in which tree mortality occurred most frequently by tree size (Fig. 3). Among the dead trees, the highest ratio of resprouting individuals was observed in R4 trees. Additionally, while resprouting in trees of other sizes began as early as 4 months after planting, R2 trees did not exhibit resprouting until 15 months after planting. Notably, two or more resprouts emerged from a single parent tree, and the average number of resprouts appeared to vary significantly depending on the number of dead trees by size class. Therefore, when considering the potential for resprouting, it is recommended to plant trees of at least R4 size or larger.
Analysis of resprouting from dead trees by planting density
The number of resprouts from dead trees varied by planting density, with 15 observed in high-density plots, 9 in medium-density plots, and none in low-density plots (Fig. 9). The ratio of resprouting trees to total dead trees was 22% in high-density plots, 18% in medium-density plots, and 0% in low-density plots. In low-density plots, although some resprouts initially emerged, they died by the following spring. This outcome is likely due to multiple contributing factors associated with tree mortality, particularly the severe drought conditions in 2022. That year, winter precipitation in Gimhae City that year was only 12.4 mm—substantially lower than the annual average of 88 mm—suggesting damage caused by desiccating winds and drought as a possible cause. In high- and medium-density plots, between 1 and 5 resprouts per dead tree were recorded, whereas in low-density plots, no resprouts ultimately survived. Therefore, when considering resprouting from dead trees, securing a high- or medium-density planting scheme is recommended.
Analysis of resprouting from dead trees by mulch thickness/depth
The number of resprouts from dead trees varied by mulch thickness: 15 resprouts were observed in the non-mulched plots (0 cm), 8 in the 5-cm mulched plots, and 1 in the 10-cm mulched plots (Fig. 10). The corresponding ratios of resprouting trees to total dead trees were 22% in the non-mulched plots, 14% in the 5-cm mulched plots, and 13% in the 10-cm mulched plots. Accordingly, the highest ratio of resprouting trees was observed in the non-mulched plots, aligning with the trend in mortality rates by mulch thickness. This result is likely due not to enhanced growth or development of resprouts in the non-mulched plots, but to the increased mortality rate, which provided more opportunities for resprouting. However, the number of resprouts per individual dead tree did not differ significantly among the mulching treatments.
A multiple linear regression analysis was conducted to determine the effects of post-planting period, tree size, planting density, and mulch thickness on resprouting. The stepwise selection method was employed as the analysis approach. The analysis indicated that the regression model was statistically significant (F = 4.020, p < .016; Table 3), suggesting its appropriateness. The adjusted R2 value was 0.206, indicating that the model explained 20.6% of the variance in resprouting. The null hypothesis—that post-planting period (t = 4.220, p < .001), planting density (t = 2.272, p < .030), and mulch thickness (t = −2.121, p < .042) have no effect on resprouting—was rejected. Accordingly, the alternative hypothesis was accepted, indicating that these variables significantly affected resprouting. Howevert, tree size (t = 1.549, p = .131) was found to have no significant effect. To determine the relative effect of each variable—post-planting period, tree size, planting density, and mulch thickness—on resprouting, standardized regression coefficients (β) were compared. It was found that among these variables, post-planting period had the greatest effect on resprouting (β = 0.760), followed by planting density (β = 0.342), tree size (β = 0.233), and mulch thickness (β = −0.320).
Analysis of resprout growth from dead trees
Analysis of resprout growth from dead trees by post-planting period
Resprouting was first observed five months after planting. By 29 months after planting, a total of 24 resprouts had emerged from 9 out of 49 dead trees (Table 4). At five months after planting, six resprouts were recorded, with a mean length of 1,038.3 ± 487.3 mm and a mean diameter of 11.6 ± 5.5 mm. At six months after planting, five resprouts appeared, with a mean length of 869.4 ± 493.0 mm and a mean diameter of 10.6 ± 7.5 mm. At 12 months after planting, the largest number of resprouts was observed, with a total of 20 resprouts showing a mean length of 809.2 ± 468.6 mm and a mean diameter of 10.9 ± 7.3 mm. Compared to the values at six months after planting, the mean length was 6 cm shorter, while the mean diameter was 0.3 mm greater. At 13, 14, and 17 months after planting, one resprout was recorded in each case. Notably, the resprout that emerged at 14 months after planting exhibited greater diameter growth than the one observed at 13 months after planting. By 29 months after planting, a total of 24 resprouts had emerged, with a mean length of 1,401.0 ± 866.8 mm and a mean diameter of 16.60 ± 10.34 mm. The diameters of the resprouts were generally small, with the exception of one resprout that emerged at 14 months after planting and exhibited substantial growth. In addition, all six resprouts that developed at five months after planting survived through 17 months after planting and remained alive until the final observation at 29 months after planting. These results suggest that resprouts emerging relatively early tend to exhibit superior growth and greater adaptability compared to those that appear later.
Meanwhile, an analysis of resprouting period based on the time of mortality showed that the first resprout (n = 1) appeared at least one month post-mortem, with the latest resprouts occurring 11 months (n = 8) and 13 months (n = 1) post-mortem. On average, resprouting occurred 7.25 ± 3.7 months after tree mortality. Resprouts were most frequently observed in the fall of the year of death, particularly from September to October (n = 11), and in the spring of the following year, most notably in April (n = 12). Additionally, one individual was observed to resprout in June of the following year after death.
Taken together, seasonal resprout growth was most prominent in the fall, followed by the spring after overwintering. As shown in Fig. 6, resprouts from dead trees were observed in the summer of the first year after planting. However, these resprouts died in the fall, preventing any measurement of growth, and no resprouting was observed even in the summer of the second year. In contrast, a substantial number of resprouts emerged in the fall following the summer and again in the spring after overwintering, with these individuals continuing to grow up to the 29th month.
Analysis of resprout growth from dead trees by tree size
The mean resprout length by tree size was highest in R8 trees (142.0 ± 84.6 cm), followed by R4 (140 .4 ± 66.2 cm) and R6 trees (137.6 ± 103.2 cm; Table 5). Similarly, the mean resprout diameter by tree size was greatest in R8 trees (18.57 ± 10.44 mm), followed by R4 (17.08 ± 9.16 mm) and R6 trees (13.90 ± 10.62 mm). No surviving individuals were recorded for R2 trees; therefore, average resprout length and diameter could not be measured for this group. Notably, the ranking of tree size based on both resprout length and diameter was consistent, suggesting that both the original tree size and resprouting period may have affected resprout growth. Among the resprouts from R4 trees, those that emerged in the spring of the 14th month, after overwintering, exhibited the highest growth, with an average length of 90.5 cm and diameter of 16.7 mm. This was followed by resprouts in the summer of the 14th month with an average length of 87.0 cm and diameter of 14.5 mm, two resprouts in the 5th month with an average length of 78.0 cm and diameter of 14.5 mm, and another two resprouts in the same month with an average length of 78.0 cm and diameter of 7.3 mm. The lowest growth was observed in resprouts that appeared in the 6th month, with an average length of 59.5 cm and diameter of 5.9 mm. Among the R6 trees, resprouts that emerged five months after planting exhibited the greatest growth, with an average length of 168.5 cm. This was followed by resprouts that developed in the spring of the 12th month (72.8 cm) and those in the spring of the 13th month (50.0 cm). The lowest growth was observed in resprouts that appeared six months after planting, with an average length of 45.1 cm. Unlike R4 and R6 trees, R8 trees showed the best growth in resprouts that developed six months after planting, with two individuals averaging 142.5 cm in length and 19.3 mm in diameter. Conversely, the resprouts that emerged five months after planting exhibited the least growth among R8 trees, with two individuals averaging only 65.0 cm in length and 8.7 mm in diameter.
These results suggest that when resprouting occurs at similar times—such as in fifth and sixth month from the same parent plant—competition for nutrients within the root system may arise. Consequently, one of the two resprouts may have exhibited reduced growth compared to resprouts that developed either individually from other parent plants or at more staggered intervals.
Analysis of resprout growth from dead trees by planting density
The average resprout length by planting density was greatest in the high-density plots (142.3 ± 79.1 cm), followed by the medium-density plots (136.4 ± 97.9 cm; Table 6). The average resprout diameter was also largest in the high-density plots (18.27 ± 10.07 mm), followed by the medium-density plots (13.76 ± 10.15 mm). In the low-density plots, no individuals survived by the final observation at 29 months. As shown in Fig. 9, a single resprout emerged in the low-density plots five months after planting, but it died in the fall of the same year.
Based on planting density and resprouting period, eleven tree deaths occurred in the year of planting, and 23 occurred two years after planting. However, resprouts that emerged in the high-density plots and survived from the fall of the first year exhibited lower growth compared to those that resprouted later. Furthermore, the average length and diameter of resprouts that emerged in the spring of the 14th month, after overwintering, were greater than those of the three resprouts that appeared five months after planting.
Analysis of resprout growth from dead trees by mulch thickness
The average resprout length by mulch thickness was greatest in the non-mulched plots (164.6 ± 85.5 cm), followed by the 5-cm mulched plots (100.3 ± 76.2 cm) and the 10 -cm mulched plots (90 .0 ± 0 cm; Table 7 ). Similarly, the average resprout diameter was highest in the non-mulched plots (19.60 ± 11.27 mm), followed by the 5-cm (11.73 ± 6.04 mm) and 10-cm mulched plots (10.10 ± 0 mm). Both resprout length and diameter were greatest in the non-mulched plots, followed by the 5-cm and 10-cm mulched plots. Accordingly, the non-mulched plots exhibited both the greatest resprout growth and the highest ratio of resprouting trees to dead trees, indicating that increasing mulch thickness did not positively affect resprout development or growth.
Conclusion
In this study, after planting 132 trees of sawtooth oak (Quercus acutissima Carruth) in an experimental site, we surveyed and analyzed their mortality rate and resprouting from dead trees among them over a 29-month period. These trees are commonly used in ecological restoration projects. Our survey and analysis, conducted at the experimental site, revealed an overall mortality rate of 42% in the year of planting, which increased slightly to 44% at 18 months post-planting before decreasing to 37% at 29 months post-planting. Notably, the mortality rates at 29 months after planting, when analyzed by tree size, planting density, and mulch thickness, were consistently lower than those observed one month after planting. This finding underscores the critical importance of post-planting management for 2 to 3 years, aligning with previous research and indicating that the experimental site is entering a stabilization phase. Furthermore, these results provide logical support for the common practice of setting a two-year defect liability period in landscaping projects. A statistical analysis of the survey data revealed that tree size, planting density and mulch thickness all affected tree mortality. Of these factors, higher planting density was found to lead to a greater incidence of dead trees.
Regarding individual planting techniques, it was observed that smaller planted trees exhibited a lower mortality rate. This suggests that saplings possess superior environmental adaptability to restoration target areas compared to more mature trees, a finding consistent with related studies. Conversely, although an increase in tree size generally correlated with a higher mortality rate, the R4 trees displayed the highest resprouting rate. This likely stems from the active state of dormant buds within the saplings, which leads to a high resprouting rate. These results also indicate that planting smaller trees within the R4 size class, as opposed to mature trees of R6 or greater, can not only reduce mortality but also actively leverage the resprouting pattern from dead trees. Furthermore, should these resprouts continue to survive and grow, they could potentially be excluded from the categorization of defects.
Meanwhile, the average mortality rate over 29 months post-planting was lowest in medium-density plots at 38.4%, followed by high-density plots at 40% and low-density plots at 42%. These findings suggest that low-density planting might have been relatively more affected by soil moisture retention issues likely due to limited canopy spread formation, as well as competition from weeds. Conversely, high-density planting appears to have resulted in a higher mortality rate primarily due to intensified inter-individual competition. Considering planting techniques for future forest restoration, medium-density planting emerges as the most appropriate strategy. However, if the cost of trees is relatively low, thereby precluding the necessity for defect repair in high-density planting scenarios, then high-density planting could also be practically applied in ecological restoration projects.
Regarding mulch thickness, it was found that higher mulch levels were associated with lower mortality rates. At 29 months after planting, the mortality rate was 61% in the non-mulched plots, 32% in the 5-cm mulched plots, and 18% in the 10-cm mulched plots. This aligns with previous findings suggesting that an optimal mulch thickness of 5 to 7 cm is recommended to avoid restricting soil aeration (Lee, 2021). However, excessive mulch thickness—particularly beyond 10 cm—can impede air exchange in the soil, and the heat generated during the decomposition of organic materials such as wood chips (one of the most commonly used mulches) may negatively affect tree root systems. This may be related to the observed trend that greater mulch thickness corresponds to a decrease in the average diameter and length of resprouts.
In this study, the average number of resprouts per tree was 2.5 in the non-mulched plots, 4 in the 5-cm mulched plots, and 1 in the 10-cm mulched plots, with the highest number observed in the 5-cm mulched plots. However, the average length of resprouts was greatest in the non-mulched plots, at 164.40 cm, followed by 100.30 cm in the 5-cm mulched plots and 90.00 cm in the 10-cm mulched plots. Similarly, the average diameter of resprouts was 19.60 mm in the non-mulched plots, 11.73 mm in the 5-cm mulched plots, and 10.10 mm in the 10-cm mulched plots, again showing the highest values in the non-mulched condition. This may be attributed to the reduced competition between existing trees and resprouts in the non-mulched plots, which could have been advantageous for resprout growth. Of the planting technique-related factors, such as tree size, planting density, and mulch thickness, only tree size did not appear to affect resprouting. In contrast, planting density and mulch thickness were found to have an effect on resprouting, with the time elapsed since planting emerging as the most influential factor.
In summary, when sapling-sized trees were planted at a medium density (1 to 3 trees per m2) and mulch was applied at a greater depth, the mortality rate tended to be lower. However, as previously noted, when mulch thickness remained within the optimal range of less than 10 cm, it also had a positive effect on both resprouting and resprout growth. In contrast, low-density planting using mature trees resulted in high mortality rates regardless of mulch depth, along with significantly lower resprouting rates. Therefore, for effective ecological restoration projects, it is advisable to prioritize the use of saplings classified as R4 or smaller in size and to maintain a medium planting density. This approach helps to minimize mortality due to inter-individual competition and allows for quicker canopy expansion, thereby enhancing stability against external factors such as weed encroachment. However, since saplings have a relatively limited canopy spread during the early stages compared to mature trees, covering the ground with mulching materials—such as wood chips—can aid in weed suppression and contribute to growth stabilization. Additionally, in ecological restoration projects, planting R4-sized saplings at medium or higher density is expected to facilitate the establishment of a continuous forest structure, minimizing the need for subsequent replacement of dead individuals.
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