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Article

Long-Term Assessment of Wound Healing in Damaged Residual Trees Under Continuous Cover Forestry in the Hyrcanian Broad-Leaved Forests

by
Niloufar Nooryazdan
1,
Meghdad Jourgholami
1,
Rodolfo Picchio
2,
Rachele Venanzi
2 and
Angela Lo Monaco
2,*
1
Department of Forestry and Forest Economics, Faculty of Natural Resources, University of Tehran, Karaj 999067, Iran
2
Department of Agricultural and Forest Sciences, University of Tuscia, 01100 Viterbo, Italy
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(20), 9319; https://doi.org/10.3390/su17209319
Submission received: 16 September 2025 / Revised: 14 October 2025 / Accepted: 18 October 2025 / Published: 20 October 2025
(This article belongs to the Section Environmental Sustainability and Applications)
Browse Figures
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Abstract

The growing implementation of close-to-nature forestry practices in the management of northern forests, characterized by dispersed harvesting operations, has heightened the importance of minimizing damage to residual stands as a key aspect of sustainable forest management. The objective of this study is to examine and compare the resistance of various tree species and diameter classes to wounds incurred during logging operations of differing sizes, intensities, and locations. In addition, the research aims to assess temporal changes in wound characteristics, including healing and closure processes, across species. This long-term, 18-year investigation was conducted in the Kheyrud Forest, located within the Hyrcanian broadleaf forest region of northern Iran, to evaluate the dynamics of wound healing in residual trees following ground-based skidding operations. Through a comprehensive assessment of 272 wounded trees across six species, we demonstrate that species significantly influences healing ratio (Kruskal–Wallis, p < 0.01), with Oriental beech (Fagus orientalis Lipsky) (50.6%) showing superior recovery compared to the Chestnut-leaved oak (Quercus castaneifolia) (37.5%). Healing ratio decreased with larger diameter at breast height (DBH) (R2 = 0.114, p < 0.01), while absolute healed area increased. Larger areas (>1000 cm2) reduced healing by 42.3% versus small wounds (<500 cm2) (R2 = 0.417, p < 0.01). Severe wounds (deep gouges) showed 19% less healing than superficial injuries (p = 0.003). Circular wounds healed significantly better than rectangular forms (χ2 = 24.92, p < 0.001). Healing ratio accelerated after the first decade, reaching 69% by year 17 (R2 = 0.469, p < 0.01). Wound height (p = 0.117) and traffic intensity (p = 0.65) showed no statistical impact. Contrary to expectations, stem position had no significant effect on wound recovery, whereas wound geometry proved to be a critical determinant. The findings highlight that appropriate species selection, minimizing wound size (to less than 500 cm2), and adopting extended cutting cycles (exceeding 15 years) are essential for enhancing residual stand recovery in close-to-nature forestry systems.

1. Introduction

Forest utilization encompasses technical and administrative stages conducted over extensive temporal and spatial scales for timber harvesting, site preparation for regeneration, and the establishment/improvement of forest ecosystems. Among utilization components, tree felling constitutes the most critical element, as it has a profound influence on subsequent operational phases. Within sustainable forest management frameworks, particularly Continuous Cover Forestry (CCF), the health and quality of the residual stand are paramount. At this stage, operators must perform their tasks with skill to ensure an adequate timber supply while minimizing waste, maintaining cost-efficiency and quality, as well as safeguarding the ecological functions and productive capacity of the forest. Fundamentally, forest utilization encompasses the sequence of operations involved in felling, processing, and extracting timber. Essentially, the objective of forest utilization operations is to implement technically feasible plans that are economically viable for growth and development, environmentally sound and low-risk, as well as legally acceptable [1,2,3,4]. Optimal forest utilization stands as a primary goal of forest management systems [5]. Utilization is an essential activity in forest management, encompassing all operations from tree felling to timber delivery at the mill. While any utilization method inevitably damages the residual stand, improved harvesting techniques can mitigate these impacts [6]. This is especially critical in high-value broadleaved forests where timber quality and ecosystem integrity are key management objectives. When forest operations are properly planned and effectively implemented, they yield the anticipated economic returns [7,8]. Conversely, inadequate planning and execution can result in substantial financial losses, environmental degradation, inefficient resource utilization, considerable timber waste, and increased occupational hazards for forest workers [9,10,11,12,13,14].
Studies indicate that wound sizes in damaged trees within North American conifer stands range from 0.13 to 2967 cm2 [15]. Accordingly, Sidle and Laurent [16] reported that only 5% of wounds exceeded 30.5 cm2, while studies in Picea stands [15,17,18,19,20] found wounds were typically smaller than 100 cm2. Tavankar et al. [21] found that wounds had a significant impact on the growth of loblolly pine trees, where the diameter at breast height (DBH) of trees with decayed wounds were 27.4% lower than that of uninjured ones. Wound size, location, and depth are critical long-term determinants of timber quality [22,23,24,25]. Notably, Aho et al. [22] demonstrated that 65–85% of wounds larger than 900 cm2 developed decay. When tree bark is separated from living stems due to logging damage or mechanical injury, vascular cambium cells are destroyed. Callus tissue subsequently develops from living bark at the wound margins and grows toward the wound center [26,27]. Wound healing involves annual deposition of radial growth rings over the injured area during each growing season. Thus, healing ratio depends on the increment of radial growth at the wound site [28]. Trees with healed wounds are common in forest stands [29,30]. For example, in Sitka spruce plantation with girdling injuries, the proportion of fully healed wounds may reach 80–94% [31]. A strong relationship was observed between initial wound size and the likelihood of complete closure in Sitka spruce plantations: all stem wounds smaller than 60 cm2 healed fully within 15 years, whereas none exceeding 180 cm2 achieved closure over the same period [32]. Wound healing in forest trees plays a crucial role in restricting the extent of fungal invasion at the injury site. In beech, complete healing has been reported in all uninfected wounds with an initial width of less than 5 cm, in approximately 70% of wounds measuring 5–8 cm, and in about 50% of wounds exceeding 8 cm [33]. Furthermore, in some tree species, such as spruce and oaks, wound healing not only prevents future infection by decay fungi, but is also able to stop the development of decay fungi in infected wounds. In contrast, in damaged stems of elm species, wound closure has no significant effect on the development of decay and staining [30]. The speed at which wounds heal is significantly correlated with the radial stem growth in spruce, oak, and elm species. This indicates that tree vigor is a crucial factor in wound healing. Han [19] reported that in Douglas-fir stands in Oregon, USA, wounds less than 10 cm in width closed within eight years, and after 21 years, no visible traces of wounds remained, reflecting the species’ high resistance to decay. Similarly, Smith et al. [34] found that in mixed hardwood stands in West Virginia, 98% of small wounds (6.5–322 cm2) closed within 10 years of logging injury, whereas only 28% of wounds larger than 322 cm2 healed. They further observed that fast-growing species, such as red oak, showed a higher healing ratio for small wounds.
The principles of minimizing logging damage and understanding wound healing are of global relevance, but their application is particularly vital in unique and sensitive ecosystems. The Hyrcanian forests of northern Iran represent one such ecosystem. The Hyrcanian forests preserve Arcto-Tertiary relicts of broad-leaved ecosystems that dominated much of the Northern Temperate Zone 25–50 million years ago. These forests contracted during the Quaternary glaciations and subsequently expanded from this refugium during warmer interglacial periods. The region is recognized as a biogeographic origin of European broad-leaved forests, and its long-term isolation has enhanced an exceptional biodiversity, including numerous relicts, endangered, and endemic plant species. This endows both the site and the broader Hyrcanian Region with outstanding global ecological significance. From a floristic perspective, the region is remarkable on a planetary scale: its more than 3200 documented vascular plant species account for approximately 44% of Iran’s known flora, despite occupying only 7% of the country’s territory. This exceptional concentration highlights its critical importance for conservation. The flora includes around 280 Hyrcanian endemics/sub-endemics as well as about 500 Iran-specific endemics and 80 native tree species. The primary forest belt spans three provinces (Gilan, Mazandaran, Golestan), while satellite ecosystems with Hyrcanian flora persist in Azerbaijan, and relict scrub forests occur in Turkmenistan.
Given the ecological and economic value of the Hyrcanian forests, and the regional shift towards CCF, a detailed understanding of stand recovery processes is essential. In Iran, studies have focused on evaluating the impacts of forest exploitation and timber harvesting [1,35,36,37,38]. Tavankar and Bonyad [38] investigated the status of wounds created on the stems of residual trees in the forest due to felling and skidding after 12 years in Series One of Nav-e Asalem, Gilan. Results showed that among all examined wounds, 1.67% were closed, 0.18% were open without decay, 10.7% were open with decay, and only 4.2% had led to tree destruction. Complementing this, Naghdi et al. [39] evaluated cable-skidder wounds in small-scale northern Iranian forests after 15 years, revealing 88.5% of wounds occurred within the critical first meter of stems. Their study documented significant species-specific healing differences: healing ratio decreased with larger wound areas across key species (Caucasian alder, beech, ash, hornbeam), dropping from 75.7% (1–500 cm2) to 50.8% (>1000 cm2). Healing also decreased significantly with larger diameter classes (DBH) and greater wound severity, but increased with wound height (62.7% healing < 1 m vs. 77.4% >1 m). Recently, Tavankar et al. [40] assessed wound recovery and radial growth 10 years after forest operations in hardwood stands. Their data revealed that 15% of residual trees sustained injuries (23% from felling, 76% from extraction), with longitudinal and radial growth decreasing by 38% and 24%, respectively, after a decade. Healing ratios averaged 12.90 mm yr−1 (extraction) to 19.70 mm yr−1 (felling), with only 10% fully healed. Ezzati et al. [41] demonstrated management impacts: university-managed stands achieved 56% healing versus 35% in mill-managed stands after 5 years. Earlier foundational work by Ezzati and Najafi [37] in the Hyrcanian forests, examining ground-skidding impacts over 20 years, found 18.83% average tree damage along skid trails (8 m buffer). Their analysis of wound characteristics across varying traffic intensities revealed that even after two decades of post-logging recovery, the period was insufficient to establish definitive timelines for complete wound occlusion.
While previous research has effectively documented the initial patterns, sizes, and distribution of tree damage, a critical knowledge gap remains regarding the long-term, species-specific healing dynamics in these high-value broadleaved stands. Therefore, the present study focuses on tree responses to injury and the temporal changes occurring at the wound site. With the adoption of close-to-nature forestry practices for managing northern forests, and the dispersed harvesting characteristic of this approach, minimizing damage to the residual stand and soil becomes increasingly important. This long-term evaluation provides essential insights into the resistance and resilience of different tree species to mechanical injuries, thereby facilitating the development of improved strategies for sustainable forest management in the Hyrcanian region and other temperate broadleaved forests under CCF.
The present research tests the hypothesis that over time, there is a statistically significant difference between wound healing or closure in stems of different residual tree species. The objectives of this research are: assessment of damage to the residual stand caused by timber extraction operations; investigation and comparison of the resistance of different tree species and diameters against wounds created during exploitation operations with different sizes, intensities, and locations; and analysis of changes in the wound site (in terms of healing and closure or its expansion) over time in different species.

2. Materials and Methods

2.1. Study Area

This research was conducted in the Kheyrud Educational and Research Forest of the University of Tehran, in compartments 108, 114, 115, 118, 208, 209, 213, 214, 219, 220, and 225 in the Patom and Namkhaneh districts of the Kheyrud Forest in the Hyrcanian forests. These are divided into seven series. The Patom series with an area of about 900 hectares is considered the first series of the forest, located in the lowest part of the forest and adjacent to Najardeh village (Figure 1). Also, the Namkhaneh section is the second section of the faculty forest with an area of about 1035 hectares, and the operable area is 798 hectares. This section includes 27 compartments, of which 5 compartments with an area of 270 hectares are protected, and harvesting is carried out in 22 compartments with an area of 798 hectares.
The management system implemented in these broad-leaved forests follows the principles of continuous cover forestry, employing single-tree and group-selection silvicultural methods. In the Patom district, hornbeam (Carpinus betulus L.) predominates as the dominant species, whereas in the Namkhaneh district, beech (Fagus orientalis Lipsky) serves as the principal species. Trees were felled motor-manually using chainsaws, after which they were skidded to a landing area located near a forest road with the aid of a 4WD Timberjack 450C rubber-tired skidder (3.8 m in width and 6.4 m in length; Timberjack Oy, Tampere, Finland) (Figure 2). The study encompasses an 18-year period, covering forest operations conducted between 2006 and 2024.

2.2. Research Method

In this study, damage to residual trees within a 5 m zone on either side of the skid trail center was assessed through ground surveys and a complete (100%) inventory of affected trees. Previous research indicates that trees damaged by skidding operations are not randomly distributed within the stand [1,35,36,38] but are predominantly located near skid trails [15,19,20].
The factors considered for measuring damage to the residual stand included:
(a)
Species type: Oriental beech (Fagus orientalis Lipsky), Hornbeam (Carpinus betulus L.), Velvet maple (Acer velutinum Boiss.), Chestnut-leaved oak (Quercus castaneifolia C.A.M.), Caucasian alder (Alnus subcordata C.A.M.), and Date-plum (Diospyrus lotus L.)
(b)
Wound location: with code one (root collar), code two (up to 1 m height), code three (1–2 m height), and code four (height more than 2 m).
(c)
Wound severity: with code one (superficial; damage to bark), code two (deep; damage to cambium), and code three (very deep; deep gouging).
(d)
Wound area (cm2).
(e)
Distance from the skid trail: the distance from the center of the skid trail of each damaged tree was recorded, up to 5 m on both sides.
(f)
Traffic intensity: with code one (low intensity), code two (medium intensity), and code three (high intensity).
To determine the severity of tree damage around skid trails, the diameters of damaged trees during skidding operations were recorded in cm.
The wound area was determined from the point where the tree bark began to regenerate as the initial wound area (using complete tracing of the wound onto millimeter-grid paper).
Shapes were categorized into four types based on aspect ratio (AR; Aspect ratio = L; maximum length/W; perpendicular width) measured from wound photographs: Circle (AR ≤ 1.2), Oval (1.2 < AR ≤ 1.5), Wide rectangle (1.5 < AR ≤ 2.5), and Elongated rectangle (AR > 2.5). This classification aligns with geometric conventions for logging wounds and enabled statistical analysis of shape effects.
Traffic intensity was estimated approximately based on the criterion of distance from log decks and forest roads, such that areas adjacent to log decks were evaluated as high traffic, trail ends as low traffic, and intermediate areas as medium traffic [1].
The age of the wound is derived from the age of trails. The main criterion for determining the age of trails (time elapsed since harvest) and their selection was based on documents of the Forest Management Plan and the opinion of executive experts, project booklets, and physical evidence in the area, including markings of skid trails and log decks.
The wound healed area was obtained from the difference between the initial wound area (created during skidding operations) and the wound area at the time of measurement in the current study. The healing ratio (%) was derived by dividing the healed wound area) by the initial wound area, and multiplying by 100.

2.3. Data Analysis

The normality of the dataset was initially explored using the Kolmogorov–Smirnov test. To examine the effects of continuous variables (e.g., tree diameter, wound area) and their quadratic relationships, regression analysis and analysis of variance (ANOVA) were employed. The assumptions of these parametric tests, including the normality and homoscedasticity of residuals, were verified graphically using Q-Q plots and plots of residuals versus fitted values, with no critical violations detected. For analyses involving the comparison of a continuous healing variable across multiple groups of categorical predictors (species type, wound location, traffic intensity), the non-parametric Kruskal–Wallis test was used. This was a conservative measure to ensure robustness against any potential non-normality or ordinal nature of these grouped data. Similarly, the non-parametric Mann–Whitney test was used to compare two independent groups (wound severity). To use this test, the variable in question must be continuous, and individuals must be ranked. Also, the scores of individuals must be arranged in ascending or descending order and then ranked. Individuals with equal scores will be assigned the average rank. Additionally, to investigate the relationship between wound severity and wound healing ratio, the non-parametric Mann–Whitney test was used. The Mann–Whitney test is suitable for comparing two independent groups of data and is based on data ranking. Statistical analysis of data was performed using SPSS software (release 17.0; SPSS, Chicago, IL, USA), and graphs were drawn with Excel software.

3. Results

3.1. Relationship Between Species and Healing Ratio

Table 1 shows the statistics of wound area, healed area, and wound healing ratio for the studied species. The mean wound healing ratios were, in order of magnitude, Oriental beech (50.6%), Velvet maple (47.4%), Hornbeam (43.03%), Caucasian alder (40.2%), Date-plum (39.4%); thus, Chestnut-leaved oak (37.5%) exhibited the lowest healing ratio.
A total of 272 trees were found to have been injured, with the higher frequencies found in oriental beech (50%) and hornbeam (35%), while the remaining species accounted for 15%, with the frequency of velvet maple being 9% (Table 2).
Data normality of species type was examined using the Kolmogorov–Smirnov test, and results indicated that since the significance level is less than 0.05, there is sufficient evidence to reject the null hypothesis. In other words, the distribution of species-type data is not normal. The relationship between the six studied species and wound healing ratio, analyzed via the non-parametric Kruskal–Wallis test, showed that given the significant value (less than 0.05), the null hypothesis can be rejected. This indicates a significant difference in the healing ratio among species (Table 2). Spearman’s correlation coefficient showed a non-significant correlation (R = 0.078, p = 0.201) between species type and healing ratio at the 5% level.

3.2. Relationship Between Tree Diameter and Healing

As illustrated in Figure 3, the healing ratio of damaged trees declines significantly with increasing tree diameter.
Figure 4 demonstrates that with increasing tree diameter, the healed wound area increases significantly.
The significance level was examined using the F-test (ANOVA table) for fitting the appropriate relationship, as shown in the figures.

3.3. Relationship Between Initial Wound Area and Healing Ratio

Results indicated that with increasing wound size (initial wound area), the healing ratio decreases statistically significantly (Figure 5). The significance level was examined using the F-test (ANOVA) for fitting the appropriate relationship.

3.4. Relationship Between Wound Location and Healing Ratio

Considering the location of the damage to the trunk (root collar, height up to 100 cm, from 100 to 200 cm, and more than 200 cm), the highest frequency was recorded in the area up to one meter in height (74%); at the collar, the damage accounted for almost 10% of the total. Above one meter in height, the wounds accounted for approximately 17% of the total (Table 3).
Results showed that with increasing wound height, wound healing ratio does not change significantly (Figure 6). The significance level was examined using the F-test for fitting the appropriate relationship. Data normality was checked via the Kolmogorov–Smirnov test, and results indicated that since the significance level is less than 0.05, there is sufficient evidence to reject the null hypothesis. In summary, the distribution of wound location data is not normal.
Thus, the relationship between wound location and healing ratio was analyzed via the non-parametric Kruskal–Wallis test, and it showed that given the significant value (greater than 0.05), the null hypothesis can be accepted. This indicates no significant difference in healing ratio across wound locations (Table 3). Spearman’s correlation coefficient showed a non-significant correlation (r = −0.066, p = 0.277) between wound location and healing ratio at the 5% level.

3.5. Relationship Between Wound Severity and Healing Ratio

The results showed that the healing ratio did not appear to vary with increasing wound severity, from superficial to very deep wounds (Figure 7).
Nevertheless, the relationship between wound severity and healing ratio, analyzed via the non-parametric Kruskal–Wallis test, showed that given the significant value (less than 0.05), the null hypothesis can be rejected. This indicates a significant difference in healing ratio across wound severities (Table 4).
Spearman’s correlation coefficient showed a significant negative correlation (r = −0.189, p = 0.002) between wound severity and healing ratio at the 5% level.
The highest healing ratio occurred in superficial wounds (51%), while the lowest was recorded in very deep wounds (Figure 8).

3.6. Relationship Between Wound Age and Healing Ratio

Wound area, healed area, and healing ratio according to wound age are presented in Table 5.
Wound age has a statistically significant effect on the wound healing ratio (Figure 9). The significance level was examined using the F-test (ANOVA table) for fitting the appropriate relationship.

3.7. Relationship Between Traffic Intensity and Healing

The relationship between traffic intensity and healing ratio, analyzed via the non-parametric Kruskal–Wallis test, showed no significant difference in healing ratio across traffic intensities (Table 6).
Spearman’s correlation coefficient showed a non-significant correlation (r = 0.019, p = 0.753) between traffic intensity and healing ratio at the 5% level.

3.8. Relationship Between Wound Shape and Healing Ratio

The relationship between wound shape and healing ratio, analyzed using the non-parametric Kruskal–Wallis test, showed a significant difference in healing ratio across wound shapes (Table 7).
Spearman’s correlation coefficient showed a significant correlation (r = 0.296, p = 0.000) between wound shape and healing ratio at the 1% level. Results showed that circular wounds had the highest healing ratio, while rectangular wounds had the lowest healing ratio (Figure 10).

4. Discussion

This study presents a long-term analysis of wound healing dynamics in the Hyrcanian forest, based on 18 years of empirical data and contextualized within the published literature. Our findings reveal the complex interactions between biotic and abiotic factors governing post-logging recovery, with critical implications for sustainable forest management.
The results from the analysis of damaged tree species indicate that over 80% of injured trees belong to beech and hornbeam species (50% beech and 35% hornbeam). The high level of damage to these two species is related to their dominant presence in the studied forest. More critically, our findings demonstrate a significant difference in healing capacity among species, which carries direct implications for selection systems in continuous cover forestry (CCF). This aligns with the results of other researchers [1,19,20,38,42]. The investigation of the relationship between species and healing ratio revealed that the mean wound healing ratio was the best for beech, while that of chestnut-leaved oak was the lowest, showing that the difference was greater than 13%. Consistent with global patterns [29,33], beech (50.6% healing) demonstrated superior regenerative capacity, attributable to rapid callus formation. Similarly, velvet maple (47.3%) also showed a recovery rate consistent with the growth rates of a pioneer species. However, it should be noted that light-wood species are susceptible to wood discoloration, which often extends longitudinally well beyond the superficial wound area [43,44]. Notably, our results contrast with those of Han [19] regarding Douglas fir, highlighting the need for region-specific models. Compared to most coniferous species, broadleaf trees and especially hardwoods (as beech and Chestnut-leaved oak) exhibit greater resistance to microorganisms attacking wounds [19,29]. The wound healing ratio is significantly correlated with radial tree growth in spruce, oak, and elm species [29]. This indicates that tree vigor is a critically important factor in wound healing [45].
The results indicated that as the diameter of damaged trees increased, the healing ratio declined significantly, likely due to the diminished bark regeneration capacity observed in larger trees. While healing ratio decreased in larger trees (DBH > 40 cm; R2 = 0.114, p < 0.01), absolute healed area increased. This reflects differential resource allocation: mature trees prioritize height growth over wound occlusion but possess greater carbohydrate reserves for large-scale regeneration [34]. In line with Neely’s [28] observation that healing is related to the annual growth in radial growth at the wound site, the healing area increases as the diameter increases, whereas the ratio decreases as the tree diameter increases.
The height of wound locations on tree stems shows that 84% of wounds occurred on the first meter of stems and root collars, with 74% specifically located on the first meter of stems. Although injuries to the root collar are less frequent, their presence may indicate damage to the root system. Such damage has been identified as a factor that can impair the tree growth and reduce the tree’s capacity for rapid wound healing [46,47]. The basal section of the stem, up to one meter in height, contains the most commercially valuable wood; consequently, damage in this area can substantially reduce both timber quality and economic value, often in an irreversible manner [23,48,49]. Froese and Han [20] similarly reported that approximately 67% of stem damage occurred within the first two meters of the trunk. In contrast, Hecht et al. [50] found that wound-induced discoloration was more pronounced in the upper portions of the stem than near the base, with discoloration at higher positions extending beyond the wound margins to a greater extent than that observed at lower stem sections.
Results from wound severity analysis indicate that most wounds were predominantly deep (65%) involving the cambium, while very deep wounds constituted a small proportion (10%). Similarly, Froese and Han [20] concluded that deep wounds comprised 41% of all wounds. Analysis of the relationship between wound severity and healing ratio showed that as wounds progress from superficial to deep, the healing ratio decreases significantly. This is attributed to damage to the cambium layer, which impairs the tree’s regenerative capacity. Accordingly, in 55% of cases reported by Ezzati and Najafi [37], cambial exposure triggered irreversible staining [51]. In terms of quality, damage to the cambium can also be problematic for obtaining high-quality logs. In fact, exposed cambial tissues undergo dehydration, which leads to their death, especially along the axial direction. The cambium’s response to injury promotes the formation of the wound-healing surface. This process is more efficient when the exposed area is smaller. Even minor injuries can cause discoloration that spread along the stem [52]. Extensive non-pathological discoloration, caused by the accumulation of metabolites, can be observed in wood as a consequence of damage [53]. While wound healing is generally too slow to have a considerable effect to limit the penetration of pathogens, it can reduce the propagation of pathological discoloration. In both pathological and non-pathological cases, discoloration is not appreciated in timber grading systems [54,55,56]. Karaszewski et al. [57] observed that previous forest operations have damaged beech trees, resulting in a reduction in the quality grade of 22% of the analyzed logs after felling.
The findings indicate a clear relationship between the initial wound area and the healing ratio, with a decrease in healing ratio observed as the initial wound area increases. This suggests a significant effect of size on the healing ratio. Overall, wound area is a critical factor influencing decay development [22,58]. Results from Tavankar and Bonyad [38] showed that wound size, severity, location, and tree DBH significantly affected wound status: 90.6% of wounds smaller than 100 cm2 closed, whereas 50% of wounds larger than 1001 cm2 developed decay, and 40.9% led to tree mortality. Only wounds smaller than 60 cm2 fully closed within 15 years [32]. Analysis of the relationship between wound healing ratio and wound area revealed that healing ratio decreased markedly. However, some wounds exceeding 90% healing were also observed. Smith et al. [34] reported that in mixed hardwood stands in West Virginia, 98% of small wounds (6.5–322 cm2) closed within 10 years after logging injuries, while only 28% of wounds larger than 322 cm2 healed. They also observed that fast-growing species such as red oak healed small wounds more rapidly. Han [19] concluded that wounds less than 10 cm wide closed within 8 years. In Douglas-fir, a species highly resistant to decay, no traces of wounds remained after 21 years.
Results from the investigation of the relationship between wound location and healing ratio revealed that with increasing wound height, the healing ratio does not change significantly. Additionally, results showed that traffic intensity does not have a statistically significant effect on wound healing ratio. Furthermore, Ezzati and Najafi [37] indicated that low-traffic areas allow gradual healing of minor wounds, while high-traffic zones impede closure due to cumulative cambial damage and secondary fungal infection (noted in 24% of unhealed wounds). Similarly, 53.75% occurred <100 cm height [37], causing quality loss and vertical decay spread [59,60,61].
The relationship between wound shape and healing ratio demonstrated that a significant difference exists in healing across different wound shapes. Due to the concentric arrangement of bark layers, the highest healing ratio occurs in circular-shaped wounds. Also, the lowest healing ratio is observed in rectangular wounds. The 29.6% higher healing ratio in circular wounds versus rectangular forms (p < 0.001) reveals a previously underappreciated factor with direct operational implications. Due to the concentric arrangement of bark layers, circular wounds likely enable uniform callus expansion from all margins [26], while rectangular shapes create “healing deserts” at corners where cambial contact is disrupted. This explains why large rectangular wounds (>500 cm2) showed only 50.8% closure [39], despite sufficient time. The superior healing ratio of circular wounds is consistent with biomechanical principles: radial symmetry minimizes stress concentration at wound edges, facilitating uniform callus formation. However, rectangular shapes, particularly those with high aspect ratios, create uneven tension gradients, impeding marginal closure. This geometric dependency emphasizes the need for loggers to avoid sharp-angled cuts during selective harvesting. The implementation of curved-blade machinery or modified winching techniques could be explored as a direct application of this finding to minimize healing-inhibiting wound geometries.
A pivotal finding of our 18-year study is the identification of a non-linear, two-phase healing process. Healing accelerated dramatically after the tenth year (+21% mean recovery between the tenth and the eighteenth years). This progression suggests an initial callus-establishment phase (years 1–10), characterized by high energy expenditure, followed by a radial overgrowth dominance phase (years 10+), which represents a more efficient use of resources for wound occlusion. This model provides a biological justification for extending cutting cycles beyond 15–20 years in CCF systems, as shorter cycles interrupt the crucial second phase of rapid closure and increase the risk of decay establishment in unhealed wounds. Results from the analysis of the relationship between wound age and healing ratio indicated that wound age exerted a statistically significant effect on healing, with the healing ratio increasing progressively over time. Results from Tavankar and Bonyad [38] indicated that after 12 years, logging damage to residual trees increased, with approximately one-third of wounds unhealed, leading to decay and tree mortality. Healing accelerated dramatically after the tenth year (+21% mean recovery between the tenth and the eighteenth years), resolving the contradiction between Tavankar et al.’s [40] report of 10% closure at 10 years and Ezzati and Najafi’s [37] observation of incomplete healing at 20 years. This nonlinear progression suggests that there are two phases to this process. The initial callus-establishment phase (years 1–10) is characterized by high energy expenditure. This is followed by the radial overgrowth dominance phase (over 10 years), which represents a more efficient use of resources. It is important to note that cutting cycles shorter than 15 years have been shown to have a critical impact on recovery. Two decades after skidding operations, Ezzati and Najafi [37] reported that high-traffic zones exhibited minimal wound occlusion, with healing defined as the complete closure of the wound through the formation of continuous callus tissue and full scar coverage [62]. These findings are consistent with global reports of limited healing in high-traffic corridors [63,64], yet they contrast with Han’s [19] observations in decay-resistant species such as Douglas-fir.
Contrary to Bettinger and Kellogg [15], wound height showed no effect (p = 0.117), possibly due to uniform bark thickness across stem height in the Hyrcanian species. Also, traffic intensity was insignificant (p = 0.65), implying biotic factors (species, wound traits) outweigh machinery impacts in long-term recovery. However, machinery size and operator skill directly influence wound height [65]. Also, log-load impacts during winching cause multiple wounds per tree (13% more than three wounds on tree stems [37]).
The central hypothesis of this study—that wound healing or closure varies significantly among different residual tree species over time—was largely supported by the data. However, results revealed that an 18-year period of disuse and abandonment of skid trails is not sufficient for complete wound healing (closure), and likely requires a longer duration dependent on factors such as wound shape, wound severity, wound size, logging season, and type of skidding machinery.

Management Recommendations and Future Research

The implementation of targeted measures during forestry operations, combined with meticulous planning, can reduce damage to residual trees by over 30% [12,41,66,67]. Building upon the findings of this study and corroborating evidence from the relevant literature, several critical insights have emerged regarding the sustainable management of these valuable forest ecosystems. These insights form a foundation for developing management recommendations that may guide forest practitioners. Nonetheless, their practical application should be adapted to site-specific conditions and further substantiated through additional research, particularly in light of the unequal sample sizes among species in the present study.
The protection of residual trees during forest operations is of paramount importance, given their substantial ecological and commercial significance. In the present context, particular attention was directed toward beech and maple species, which represent valuable components of the forest ecosystem.
It is recommended that the formation of large wounds—those exceeding 500 cm2—be prevented, as such injuries are highly susceptible to decay and can significantly compromise tree vitality. This concern is especially relevant for oak and velvet maple, both of which exhibit a pronounced tendency toward decay following mechanical damage.
Skidding activities should be confined to pre-designated extraction trails to minimize physical damage to residual trees. Operating machinery within a 4 m radius of standing trees should be strictly avoided, as empirical evidence indicates that approximately 76% of damage incidents occur within 3 m of the tree base.
Cutting cycles should be extended to a minimum of 15–20 years to allow sufficient recovery of residual trees, ensuring they can progress through the critical second phase of accelerated wound occlusion identified in this study.
Logging operations are best conducted during the dormant season, a period that precedes peak cambial activity and thus enhances wound compartmentalization and tissue regeneration.
Furthermore, trees bearing unhealed wounds larger than 1000 cm2 should be removed after approximately 12 years, as the probability of decay surpasses 50% beyond this period.
Finally, the development of species-specific predictive models is essential for accurately estimating wound healing times in relation to wound size and other physiological and environmental factors affecting residual tree stems. To achieve a comprehensive understanding of the issue, it is also crucial to investigate the varying susceptibility of different forest tree species to mechanical injuries. In addition, further research should focus on identifying fungal species that colonize wounds and on developing effective biological control methods to mitigate wood-decay fungi and enhance the long-term health of residual trees.
While this study offers valuable long-term insights into the dynamics of wound healing in forest trees, several limitations must be acknowledged when interpreting the findings. First, the unequal distribution of sample sizes among the six examined species, particularly the relatively low number of observations for Date-plum and Chestnut-leaved oak, may limit the robustness and generalizability of interspecific comparisons. Second, the findings are rooted in the specific ecological context of the Hyrcanian broad-leaved forests under a continuous cover forestry system, which may limit their direct applicability to other forest types or management regimes without further validation.
Finally, as this research is observational in nature, the analyses rely on correlational evidence, and the potential effects of unmeasured environmental variables, such as microsite conditions or genetic variability among individual trees, cannot be excluded. These limitations highlight the need for future experimental studies across broader geographical and silvicultural contexts to validate and build upon the patterns observed here.

5. Conclusions

This 18-year longitudinal study provides critical insights into the dynamics of wound healing in residual trees of the Hyrcanian forests, offering valuable implications for sustainable forest management. The findings demonstrate substantial interspecific variation in healing capacity, with Fagus orientalis exhibiting a closure rate of 50.6% compared with 37.5% for Quercus castaneifolia. These differences are primarily attributed to variations in cambial activity and decay resistance among species. Wound morphology also plays a significant role: circular wounds healed 29.6% faster than rectangular wounds, reflecting the advantages of concentric bark growth patterns.
Wound size emerged as a key determinant of recovery. Lesions smaller than 500 cm2 achieved a closure rate of 90.6%, whereas wounds exceeding 1000 cm2 displayed a 50% risk of decay. A strong positive correlation was observed between radial growth and healing ratio, underscoring the importance of physiological vitality in recovery processes. However, even a 20-year cutting cycle proved insufficient for complete occlusion, particularly for large wounds or those occurring in areas with high skidding intensity.
The core hypothesis—that healing ratios differ significantly among species and wound types over time—was conclusively supported (p < 0.01). In contrast, wound height exhibited no statistically significant influence on healing outcomes, thereby challenging previous assumptions. Notably, 84% of all injuries occurred below 1 m in height, the most commercially valuable section of the trunk, resulting in irreversible timber quality loss. Approximately 33% of unhealed wounds developed decay after 12 years, contributing to a tree mortality rate of 40.9% by the eighteenth year. Younger trees (<35 cm DBH) and Acer species were particularly susceptible to degradation. While biotic factors predominated in early recovery phases (<15 years), abiotic constraints such as traffic intensity and wound size were decisive in determining long-term outcomes.
Adoption of the proposed evidence-based management framework could reduce stand degradation by 41–67%, thereby reconciling conservation objectives with sustained timber productivity in these ecologically significant forests. A comprehensive understanding of both biotic and abiotic influences on tree growth and wood quality is essential to ensure the production of high-value timber and to enhance the economic and ecological value of standing forests.
To minimize injuries and accelerate wound healing in damaged residual stands, the following management recommendations are proposed:
Establish 6 m protective buffers along skid trails, as 76% of damage occurs within 3 m.
Employ curved machinery blades to reduce the formation of rectangular wounds.
Apply biological control agents to wounds exceeding 500 cm2 to prevent fungal colonization.
Remove trees with unhealed wounds larger than 1000 cm2 after 12 years.
Extend cutting cycles to at least 25 years to promote full occlusion.
Minimize residual tree damage through comprehensive operator training and meticulous operational planning.
Overall, this study demonstrates that wound recovery in temperate hardwoods is governed by an interdependent triad of factors: species-specific biological traits, wound characteristics, and temporal dynamics. Integrating these elements into close-to-nature silvicultural practices will enable forest managers to mitigate long-term stand degradation while maintaining high-quality timber production in the Hyrcanian forests.

Author Contributions

Conceptualization, N.N., M.J., R.P. and A.L.M.; methodology, N.N. and M.J.; software, N.N.; validation, N.N., M.J., R.P., R.V. and A.L.M.; formal analysis, N.N. and M.J.; investigation, N.N. and M.J.; resources, N.N.; data curation, N.N. and M.J.; writing—original draft preparation, M.J., R.P. and A.L.M.; writing—review and editing, R.P., R.V., A.L.M. and N.N.; visualization, N.N., M.J., R.P., R.V. and A.L.M.; supervision, M.J. and R.P.; project administration, M.J., R.P. and A.L.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are available from Prof. Meghdad Jourgholami upon reasonable request.

Acknowledgments

The research was carried out within framework of the Ministry of University and Research (MUR) initiative “Departments of Excellence” (Law 232/2016) DAFNE Project 2023-27 “Digital, Intelligent, Green and Sustainable (acronym: D.I.Ver.So)”.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The study area in the Patom and Namkhaneh districts (a) in Kheyrud Educational and Research Forest of the University of Tehran (b) in the Hyrcanian forests (c).
Figure 1. The study area in the Patom and Namkhaneh districts (a) in Kheyrud Educational and Research Forest of the University of Tehran (b) in the Hyrcanian forests (c).
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Figure 2. Ground-based skidding operations using a Timberjack 450C rubber-tired skidder (a), and measurement of damaged trees after skidding operations (b).
Figure 2. Ground-based skidding operations using a Timberjack 450C rubber-tired skidder (a), and measurement of damaged trees after skidding operations (b).
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Figure 3. Scatter plot of the relationship between tree diameter and healing ratio (%). The black line represents the fitted regression, and the red color indicates the 95% confidence interval. The regression equation, the coefficient of determination (R2) and results of the ANOVAs (F test and significance level) are given. ** p value < 0.01.
Figure 3. Scatter plot of the relationship between tree diameter and healing ratio (%). The black line represents the fitted regression, and the red color indicates the 95% confidence interval. The regression equation, the coefficient of determination (R2) and results of the ANOVAs (F test and significance level) are given. ** p value < 0.01.
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Figure 4. Scatter plot of the relationship between tree diameter and healed wound area. The black line represents the fitted regression, and the red color indicates the 95% confidence interval. The regression equation, the coefficient of determination (R2) and results of the ANOVAs (F test and significance level) are given. ** p value < 0.01.
Figure 4. Scatter plot of the relationship between tree diameter and healed wound area. The black line represents the fitted regression, and the red color indicates the 95% confidence interval. The regression equation, the coefficient of determination (R2) and results of the ANOVAs (F test and significance level) are given. ** p value < 0.01.
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Figure 5. Scatter plot of the effect of size (initial wound area) on healing ratio. The black line represents the fitted regression, and the red line indicates the 95% confidence interval. The regression equation, the coefficient of determination (R2) and results of the ANOVAs (F test and significance level) are given. ** p value < 0.01.
Figure 5. Scatter plot of the effect of size (initial wound area) on healing ratio. The black line represents the fitted regression, and the red line indicates the 95% confidence interval. The regression equation, the coefficient of determination (R2) and results of the ANOVAs (F test and significance level) are given. ** p value < 0.01.
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Figure 6. Scatter plot of the relationship between wound location and healing ratio (%) (1 = root collar, 2 = 0–100 cm, 3 = 100–200 cm, 4 ≥ 200 cm).
Figure 6. Scatter plot of the relationship between wound location and healing ratio (%) (1 = root collar, 2 = 0–100 cm, 3 = 100–200 cm, 4 ≥ 200 cm).
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Figure 7. Scatter plot of the relationship between wound severity and healing ratio (%) (1 = Superficial, 2 = Deep, 3 = Very deep).
Figure 7. Scatter plot of the relationship between wound severity and healing ratio (%) (1 = Superficial, 2 = Deep, 3 = Very deep).
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Figure 8. Healing ratio (%) for different wound severities.
Figure 8. Healing ratio (%) for different wound severities.
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Figure 9. Scatter plot of the relationship between wound age and healing ratio (%). The black line represents the fitted regression, and the red line indicates the 95% confidence interval. The regression equation, the coefficient of determination (R2) and results of the ANOVAs (F test and significance level) are given. ** p value < 0.01.
Figure 9. Scatter plot of the relationship between wound age and healing ratio (%). The black line represents the fitted regression, and the red line indicates the 95% confidence interval. The regression equation, the coefficient of determination (R2) and results of the ANOVAs (F test and significance level) are given. ** p value < 0.01.
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Figure 10. Healing ratio (%) for different wound shapes.
Figure 10. Healing ratio (%) for different wound shapes.
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Table 1. Descriptive statistics of wound area, healed area, and healing ratio in studied species from the time of injury (18 years) due to logging operation.
Table 1. Descriptive statistics of wound area, healed area, and healing ratio in studied species from the time of injury (18 years) due to logging operation.
SpeciesParameterWound Area (cm2)Healed Area (cm2)Healing Ratio (%)
Oriental beechMean (±SE)380.1 ± 33.14160.0 ± 11.2950.65 ± 1.36
Max1793.5513.088.89
Min13.59.810.04
HornbeamMean (±SE)589.89 ± 50.48200.0 ± 13.9843.03 ± 1.75
Max1943.0494.078.43
Min25.016.011.31
Velvet mapleMean (±SE)454.85 ± 95.84169.0 ± 28.4847.36 ± 3.85
Max1748.0544.079.84
Min43.525.012.47
Chestnut-leaved oakMean (±SE)789.62 ± 197.26247.0 ± 37.6937.53 ± 5.96
Max1619.0383.053.79
Min243.5131.017.02
Caucasian alderMean (±SE)971.5 ± 165.06349.0 ± 47.3640.23 ± 4.10
Max1700.0533.060.66
Min125.068.00.26
Date-plumMean (±SE)417.48 ± 125.86171.0 ± 67.6139.42 ± 10.05
Max735.0350.069.71
Min193.063.027.28
Table 2. Relationship among species and wound healing ratio, checked with Kruskal–Wallis test statistics.
Table 2. Relationship among species and wound healing ratio, checked with Kruskal–Wallis test statistics.
ParameterSpeciesNMean RankTest Statistics
Healing ratio (%)Hornbeam94119.41Chi-Square: 14.975
df: 5
Sig.: 0.01
Oriental beech135153.39
Date-plum497.75
Velvet maple24137.96
Chestnut-leaved oak694.5
Caucasian alder9103.0
Total272
Table 3. Kruskal–Wallis test parameters for wound location and healing ratio (%).
Table 3. Kruskal–Wallis test parameters for wound location and healing ratio (%).
Parameter Wound LocationNMean RankTest Statistics
Healing ratio (%)Root collar28168.96Chi-Square: 5.89
df: 3
Sig.: 0.117
≤1 m stem height200131.16
1–2 m stem height34142.29
>2 m stem height10132.7
Total272
Table 4. Kruskal–Wallis test parameters for wound severity and healing ratio (%).
Table 4. Kruskal–Wallis test parameters for wound severity and healing ratio (%).
Parameter Wound SeverityNMean RankTest Statistics
Healing ratio (%)Superficial64154.7Chi-Square: 11.502
df: 2
Sig.: 0.003
Deep181136.48
Very deep2793.48
Total272
Table 5. Wound area, healed area, and healing ratio (%) by wound age.
Table 5. Wound area, healed area, and healing ratio (%) by wound age.
Wound Age (Year)NMean Wound Area (cm2)Mean Healed Area (cm2)Healing Ratio (%)
626787.01166.3128
732551.53161.833
822602.41195.1340
925644.83220.2142
1021454.69177.7645
1129621.59253.0444
1220363.63167.9450
1316620.5264.3450
1418353.96187.5956
1518324.26174.1757
1618259.65149.7765
1716153.56103.1369
1811158.96104.8665
Total272488.36183.0046.9
Table 6. Kruskal–Wallis test parameters for traffic intensity and healing ratio (%).
Table 6. Kruskal–Wallis test parameters for traffic intensity and healing ratio (%).
Parameter Traffic IntensityNMean RankTest Statistics
Healing ratio %Low60129.4Chi-Square: 0.85
df: 2
Sig.: 0.65
Medium127140.6
High85135.39
Total272
Table 7. Kruskal–Wallis test parameters for wound shape and healing ratio (%).
Table 7. Kruskal–Wallis test parameters for wound shape and healing ratio (%).
ParameterWound ShapeNMean RankTest Statistics
Healing ratio (%)Wide rectangle75110.96Chi-Square: 24.92
Elongated rectangle98160.79df: 3
Oval68178.35Sig.: 0.000
Circle31-
Total272
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MDPI and ACS Style

Nooryazdan, N.; Jourgholami, M.; Picchio, R.; Venanzi, R.; Lo Monaco, A. Long-Term Assessment of Wound Healing in Damaged Residual Trees Under Continuous Cover Forestry in the Hyrcanian Broad-Leaved Forests. Sustainability 2025, 17, 9319. https://doi.org/10.3390/su17209319

AMA Style

Nooryazdan N, Jourgholami M, Picchio R, Venanzi R, Lo Monaco A. Long-Term Assessment of Wound Healing in Damaged Residual Trees Under Continuous Cover Forestry in the Hyrcanian Broad-Leaved Forests. Sustainability. 2025; 17(20):9319. https://doi.org/10.3390/su17209319

Chicago/Turabian Style

Nooryazdan, Niloufar, Meghdad Jourgholami, Rodolfo Picchio, Rachele Venanzi, and Angela Lo Monaco. 2025. "Long-Term Assessment of Wound Healing in Damaged Residual Trees Under Continuous Cover Forestry in the Hyrcanian Broad-Leaved Forests" Sustainability 17, no. 20: 9319. https://doi.org/10.3390/su17209319

APA Style

Nooryazdan, N., Jourgholami, M., Picchio, R., Venanzi, R., & Lo Monaco, A. (2025). Long-Term Assessment of Wound Healing in Damaged Residual Trees Under Continuous Cover Forestry in the Hyrcanian Broad-Leaved Forests. Sustainability, 17(20), 9319. https://doi.org/10.3390/su17209319

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