Impact of Increment Coring on Growth and Mortality across Various Size Classes of Khasi Pine (Pinus kesiya) in Northern Thailand

Abstract

In response to concerns that increment coring with an increment borer might contribute to the dieback of pine trees in Thailand, this research aimed to evaluate the effects of increment coring on the growth of Khasi pine (Pinus kesiya Royle ex Gordon) at Doi Khuntan National Park in northern Thailand. Increment coring is commonly used in dendrochronology, but its impact on tree growth needs to be better understood. This study involved the selection of pine trees of varying diameters, categorizing them into control (uncored) and experimental (cored) groups. Subsequently, data were collected bimonthly from September 2018 to April 2023, except for interruptions from February 2020 to December 2021 due to the COVID-19 pandemic. Tree diameters at breast height were measured, and image analysis was used to monitor the wound healing every two months. A repeated-measures ANOVA was used to compare the growth of cored and uncored groups and the wound healing rates among small-, medium-, and large-tree groups. The growth of cored and uncored Khasi pines within the same and different diameter classes showed no significant differences nor did the wound healing rates. The findings indicated that increment coring had an insignificant impact on the tree growth across all diameter classes, with wounds healing effectively within 14 months. These results support the continued and safe use of increment coring with an increment borer as a non-destructive method for collecting tree-ring and wood samples for climate research and for providing valuable insights into forest management practices.

1. Introduction

Sustainable forest management involves the planning and implementation of practices that maintain and enhance the economic, social, and environmental values of forests, ensuring optimal benefits for current and future generations, based on current scientific and traditional knowledge [1,2]. Achieving this goal requires detailed information on the tree growth rates, harvested yields, rotation periods, and regeneration [3]. Continuous measurements of tree growth are thus essential for supporting effective forest management plans and long-term forest monitoring [4].

In the absence of forest inventories containing continuous collection of growth data, dendrochronology, or tree-ring analysis serves as a reliable method for determining the annual growth rates of trees [5]. This involves the extraction of wood samples using an increment borer, a widely used instrument with a 0.5 cm diameter, which cores a tree from bark to the pith [6]. This technique results in only minor wounds and eliminates the need to fell trees. The wood samples not only support forest management plans by providing monitoring and assessment data [7,8,9] but also enable the study of climate change and reconstruction of past climate by correlating tree-ring data with instrumental climate records [10,11,12,13,14].

At Phu Toei National Park located in the Suphan Buri province, central Thailand, an increasing number of dead pine trees have raised concerns and have been speculated to be caused by increment coring for past dendrochronological studies, despite the lack of evidence to support this claim. Although Temchai et al. [15] suggested that the death of lowland natural pine trees in the park was related to climate change, a lack of sufficient academic documentation on the impact of increment coring on tree growth in Thailand has led to difficulties in obtaining research permissions. This diminished confidence in the use of increment borer for wood collection highlights the challenges in securing approval for research involving wood sample collection in Thailand’s natural forests.

Similar to findings in other temperate countries, research on the effects of increment coring on various tree species has shown no significant differences in decay and mortality between cored and uncored Norway spruce (Picea abies (L.) H.Karst.) and European beech (Fagus sylvatica L.) trees [16,17,18]. Similarly, no significant differences in mortality rates were found between cored and uncored samples of white fir (Abies concolor (Gordon & Glend.) Lindl. ex Hildebr.) and red fir (A. magnifica A.Murray bis). In the tropical zone, increment coring did not have any effect on trunk diameter and survival within the first year after wood sample collection in 11 tree species collected in Singapore [19].

To address concerns about increment coring in Thailand and the hypothesis that increment coring with an increment borer does not significantly affect growth and mortality across various size classes of pine trees, this research aimed to clarify its effects on pine tree growth at Doi Khuntan National Park. This study investigated the tree growth and wound recovery every 2 months over a 56-month period from September 2018 to April 2023, excluding February 2020 to December 2021 due to the COVID-19 pandemic. Natural Khasi pine (Pinus kesiya) trees of three size classes (small-, medium-, and large-diameter) were selected and divided into control (uncored) and experimental (cored) groups. The findings of this research are expected to alleviate concerns among authorities about the impact of increment borers on tree growth. The results will also provide a solid basis for other researchers to request for permissions to collect wood samples using increment borers. By comparing growth among small, medium, and large trees in both the control and cored groups, this study will help the forest managers and officers in selecting appropriate methods for measuring annual tree growth without causing perceptible disturbances in the tree ecosystem.

2. Materials and Methods

2.1. Study Site

The study site for Pinus kesiya, growing naturally in the northwestern part of Thailand, is Doi Khuntan National Park, located in Lamphun province between latitudes of 18°22′ and 18°37′ N and longitudes of 99°12′ and 99°25′ E (Figure 1). The park features mountain pines growing in mixed evergreen and deciduous forests at elevations ranging from 850 to 1000 m above mean sea level. The climate is characterized by three distinct seasons: the rainy season from May to October, winter season from November to February, and summer season from March to April. Temperatures in the park range from 38 °C in summer to 5 °C in winter, with an average annual rainfall of approximately 1034 mm [20].

The Kanchanaburi Formation from the Tanaosi Group of the Silurian–Devonian geological periods underlies the national park, with shale in the northwest and talc in the east. Granite from the Triassic period forms the bedrock in all the other areas. At lower elevations, which were degraded for nearly a century, deciduous forests dominated by teak (Tectona grandis L.f.) have made a significant recovery, while heavily degraded lands are now richly mixed with bamboo. Mixed evergreen and deciduous forests cover elevations between 850 and 1000 m above sea level, while evergreen hardwood and pine forests are found at higher elevations, which are prone to frequent wildfires throughout the park. The national park boasts a diverse flora, with over 165 families and 1285 species of vascular plants documented [21].

2.2. Sample Selection and Periodic Growth Measurement

Khasi pines naturally growing in Doi Khuntan National Park, located 3.5 km from the park office, were measured for diameter at breast height. Based on diameter measurements ranging from 25.4 to 72.2 cm, Khasi pines were classified into 3 diameter classes: small (25.4–41 cm), medium (41.1–56.6 cm), and large (56.7–72.2 cm). In each size class, at least 6 trees were selected based on the criteria of having straight trunks, no injuries from fire, chipping, or bark peeling, and no suppression from other trees. These trees were further divided into 2 groups: cored and uncored. For the cored group, two wood cores were collected from each tree using a 0.5 cm increment borer (Haglöf Sweden AB, Långsele, Sweden) at the opposite sides at a height of 1.0 m, or 0.3 m below the diameter measurement point of 1.3 m. The uncored group was left untouched to serve as control for growth comparison. According to the criteria for tree selection, the total number of cored and uncored trees across all diameter classes was 26 and 14, respectively. Within the small-diameter class, the number of cored and uncored trees was equal at 5 trees each. In the medium-diameter class, there were 3 cored and 11 uncored trees, while in the large-diameter class, there were 6 cored and 10 uncored trees. Since the Khasi pines were cored in September 2018, tree diameters at breast height were measured bimonthly over a period of 56 months, from September 2018 to April 2023, except for February 2020 to December 2021 due to the COVID-19 pandemic. Additionally, photographs of the cored holes were taken to monitor wound healing until they were completely closed and filled with wood cells or other substances.

2.3. Periodic Growth Comparison

The bimonthly growth data of all the selected Khasi pine trees were analyzed and compared within each diameter class and among the diameter classes. Growth trends were visualized using line graphs and bar charts to elucidate any differences within and among the diameter classes over time.

In each size class, the periodic growth data of the cored and uncored sample trees were compared using repeated-measures ANOVA [22]. To address the assumption of sphericity in the repeated-measures ANOVA, Mauchly’s test was performed, revealing violations due to the unequal sample sizes. The Greenhouse–Geisser correction was applied to adjust the degrees of freedom, resulting in more reliable p-values and confidence intervals. The F-values and p-values were also calculated to assess the significance of the differences between cored and uncored groups. This inferential statistical test determines any statistically significant difference between the means of two groups periodically remeasured from September 2018 to January 2020 and January 2022 to April 2023. Additionally, repeated-measures ANOVA was used to determine whether there were any statistically significant differences (at p < 0.05) between the means of two or more independent (unrelated) groups. In this study, six independent groups of periodic growth data were defined, comprising three diameter classes each of the cored and uncored sample trees. Prior to the analysis, the data were checked for normality and the homogeneity of variances in sphericity was determined, and any violations were corrected using the Greenhouse–Geisser adjustment [23]. The results were interpreted in terms of the mean growth differences, with post hoc tests applied to identify specific group differences where applicable.

2.4. Analysis of Increment Coring Effects on Tree Growths

The statistical interpretation of periodic growth data was primarily used to describe the effects of increment coring on tree growth. Additionally, wound healing data, measured from the images of cored holes taken from the date of coring until the holes were fully healed, were analyzed using the ImageJ software (https://imagej.net/ij/) [24,25]. The wound area was measured at regular intervals to assess the percentage of closure over time. These data helped assess whether the trees were able to recover from injuries caused by increment coring. Repeated-measures ANOVA was used to compare the different percentages of wound healing among small-, medium-, and large-tree groups at p < 0.05. The results were interpreted to identify significant differences in wound healing rates among the different tree size classes, with post hoc tests applied to indicate any group specific differences.

3. Results

3.1. Periodic Growth Data

3.1.1. Small-Size Class

In September 2018, the diameters of five Khasi pines in both the uncored and cored small-diameter groups were initially measured, resulting in average values of 29.00 (±4.65) cm and 30.68 (±5.15) cm, respectively. Between September 2018 and March 2019, the average diameters of both the groups slightly declined to 28.80 (±4.63) cm and 30.42 (±5.24) cm. By July 2019, the average diameters increased slightly to 28.95 (±4.58) cm and 30.58 (±5.19) cm, followed by a decline to 28.75 (±4.61) cm and 30.40 (±5.17) cm in January 2020. The gradual increase in growth was observed for both the groups in February 2022, with average diameters reaching maximum values in August 2022 at 29.38 (±3.22) cm and 31.28 (±5.31) cm, respectively. By April 2023, average diameters had slightly decreased to 29.21 (±3.46) cm and 31.00 (±5.43) cm (Figure 2a). Figure 2b illustrates the relative change in diameter relative to September 2018. These variations in diameter aligned with regional rainfall patterns, characterized by a sharp increase from April to May (exceeding 150 mm), a mild decline in June and July (approximately 120 mm), another sharp increase in August and September (200 mm), and a rapid decline to about 100 mm in November, with negligible rainfall from December to February and a slight increase in March (Figure 3).

3.1.2. Medium-Size Class

Since September 2018, both the uncored and cored groups experienced a decrease in average tree diameter from 46.73 (±4.12) cm and 51.29 (±4.11) cm to 46.37 (±4.20) cm and 50.93 (±4.08) cm, respectively, until March 2019, marking the end of the dry season. The average diameters of these Khasi pines then increased steadily until the end of wet season in September 2019, reaching 46.73 (±4.50) cm and 51.39 (±4.20) cm, followed by a decrease during the dry period to 46.53 (±4.45) cm and 51.17 (±4.22) cm in January 2020. Remeasurements in February 2022 indicated an increase to 47.62 (±5.18) cm and 51.93 (±3.61) cm. The average diameters peaked during the wet season, reaching 47.79 (±5.20) cm and 52.91 (±3.49) cm in August 2022. Growth declined again during the dry season at the end of the year through to the beginning of the next year. By April 2023, a transition period between the dry and wet seasons, the average diameters of both uncored and cored groups slightly decreased, as indicated by values of 47.88 (±4.49) cm and 52.57 (±3.45) cm, respectively (Figure 4).

3.1.3. Large-Size Class

The uncored and cored groups of Khasi pines in the large-size class were measured during the same periods as the small- and medium-size classes (Figure 5). The initial average diameters measured in September 2018 were 59.76 (±4.13) cm and 63.07 (±5.56) cm for the uncored and cored groups, respectively. During the dry season, the average diameters of both groups gradually declined, reaching 59.29 (±4.26) cm and 62.74 (±5.60) cm in March 2019. Throughout the wet season from April to October 2019, as described in Figure 3, the average tree diameters of both groups increased slightly to 59.69 (±4.31) cm and 63.32 (±5.64) cm. By January 2020, during the dry period, the average diameters had decreased to 59.58 (±4.24) cm and 63.01 (±5.58) cm. Remeasurements in February 2022 showed an increase to 61.11 (±4.31) cm and 64.67 (±5.70) cm. The wet season during the year 2022 resulted in the highest average diameters recorded in August, with 61.12 (±2.78) cm for the uncored group and 65.11 (±5.90) cm for the cored group. This growth declined steadily from October 2022, during the transition from the wet to the dry season, reaching 60.73 (±2.84) cm and 64.81 (±6.19) cm in April 2023, which marked the beginning of the wet season.

The periodic growth data for the small-, medium-, and large-size classes of Khasi pines indicated both similarities and differences in diameter fluctuations over time. All the three size classes exhibited an overall decreasing trend in diameter during dry seasons and increase during wet seasons, reflecting the impact of seasonal rainfall patterns on tree growth. However, the magnitude of these fluctuations varied among the size classes. The small-size class showed the least variation in diameter, with initial measurements at 29.00 cm (uncored) and 30.68 cm (cored) and maximum diameters of 29.38 cm and 31.28 cm, respectively, in August 2022. The medium-size class experienced moderate fluctuations, from 46.73 cm (uncored) and 51.29 cm (cored) and peaking at 47.79 cm and 52.91 cm in August 2022. The large-size class had the most significant changes, with initial measurements of 59.76 cm (uncored) and 63.07 cm (cored) and maximum diameters of 61.12 cm and 65.11 cm in August 2022. While all groups followed a similar seasonal growth pattern, the extent of growth and shrinkage was more pronounced in larger trees, suggesting that tree size influences the responsiveness to seasonal climatic variations.

3.2. Wound Healing Rates from Increment Coring

From September 2018 to January 2020, the wound healing characteristics of increment-cored Khasi pines were monitored. By November 2019, all cored holes were filled with woody cells, with no tree mortality. The average wound healing rates for each tree size class, measured as the ratio of the filled hole to the total hole size, are presented in Figure 6.

The medium-size class Khasi pines demonstrated the fastest initial recovery, with a filling rate of 10.68% (±12.00) by November 2018, two months after coring. This was followed by the large- and small-size classes, with filling rates of 3.91% (±5.40) and 2.50% (±5.58), respectively. During this period, only one, six, and four trees in the small-, medium-, and large-size classes, respectively, showed any visible healing, while the remaining trees had bare holes. By January 2019, healing rates increased rapidly across all size classes, with the medium-size class again showing the fastest rate at 58.01% (±21.07), followed by the large-size class at 52.67% (±10.34) and the small-size class at 45.13% (±19.60).

During March 2019, more than 50% of the cored holes were filled in all trees, with the medium-, large-, and small-size classes achieving filling rates of 83.24% (±13.29), 80.55% (±13.23), and 78.91% (±15.18), respectively. Healing continued at a slower pace in May 2019, with the medium-, large-, and small-size classes reaching cumulative rates of 91.72% (±7.59), 86.85% (±13.39), and 82.11% (±6.28), respectively. The healing rates further decelerated in July and August, with the medium-size class accumulating rates of 95.85% (±4.16) and 97.43% (±4.85), the large-size class accumulating rates of 90.67% (±10.94) and 94.04% (±7.52), and the small-size class accumulating rates of 89.01% (±3.46) and 89.14% (±5.81). By November 2019, all bored holes were completely filled, indicating a 14-month recovery period for all size classes.

The wound healing rates from increment coring showed notable differences and similarities among tree size classes. Medium-sized Khasi pines consistently exhibited the fastest healing rates, followed by large-sized pines and small-sized pines. However, statistical analysis was used to justify the similarities and differences between these healing rates. Despite these differences, all size classes achieved complete wound closure within the 14-month period, demonstrating a robust healing potential across the diameter classes.

3.3. Comparison of Periodic Growth and Wound Healing Rate

3.3.1. Periodic Growth

• Growth comparison of cored and uncored trees in the small-size class:

A repeated-measures ANOVA was used to analyze the variations in diameter in both the cored and uncored trees of the three diameter classes, to determine any the effects of time and group interactions. The results indicated a significant main effect of time, F(7, 56) = 7.248, p = 0.00, suggesting significant growth changes in the small-size class over the 16 months from September 2018 to January 2020. However, the time and group interaction was insignificant, F(7, 56) = 1.129, p = 0.359, indicating similar growth patterns between cored and uncored trees of this size class. Additionally, Mauchly’s test of sphericity was not violated, as indicated by a χ²(27) = 35.123, p = 0.228.

Following the decline of the COVID-19 pandemic, tree growth was remeasured every two months from February 2022 to April 2023. The results showed an insignificant main effect of time, F(7, 35) = 2.249, p = 0.053, and insignificant interaction between time and group, F(7, 35) = 0.581, p = 0.766. This suggests that the growth patterns of cored and uncored trees in the small-size class from September 2018 to April 2023 did not differ significantly.

• Growth comparison of cored and uncored trees in the medium-size class:

The repeated-measures ANOVA for the medium-size class indicated that Mauchly’s test of sphericity was violated, χ²(27) = 43.104, p = 0.036. Applying the Greenhouse–Geisser correction, significant main effect of time was observed, F(3.011, 36.129) = 9.121, p = 0.00, indicating significant growth changes over the 16 months from September 2018 to January 2020. However, the interaction between time and the group was insignificant, F(3.011, 36.129) = 0.743, p = 0.534, suggesting similar growth patterns between cored and uncored trees during this period.

Post-COVID-19 measurements from February 2022 to April 2023 also indicated that the assumption of sphericity was violated, χ²(27) = 79.236, p = 0.00. After applying the Greenhouse–Geisser correction, the analysis found an insignificant main effect of time, F(1.474, 14.736) = 1.420, p = 0.266, and an insignificant time and group interaction, F(1.474, 14.736) = 1.381, p = 0.274. Thus, the growth patterns of cored and uncored trees in the medium-size class from September 2018 to April 2023 did not differ significantly.

• Growth comparison of cored and uncored trees in the large-size class:

For the large-size class, the repeated-measures ANOVA indicated a violation of Mauchly’s test of sphericity, χ²(27) = 43.194, p = 0.049. Using the Greenhouse–Geisser correction, the analysis indicated a significant main effect of time, F(3.938, 55.129) = 13.557, p = 0.00, suggesting significant growth variation over the 16 months from September 2018 to January 2020. The time and group interaction was not significant, F(3.938, 55.129) = 0.444, p = 0.774, indicating similar growth patterns between cored and uncored trees during this period.

Measurements from February 2022 to April 2023, following the pandemic, also showed a violation of sphericity, χ²(27) = 95.401, p = 0.00. Applying the Greenhouse–Geisser correction, the analysis found a significant main effect of time, F(1.582, 18.981) = 5.952, p = 0.014, but an insignificant time and group interaction, F(1.582, 18.981) = 1.811, p = 0.194. Therefore, the growth patterns of cored and uncored trees in the large-size class from September 2018 to April 2023 did not differ significantly.

• Growth comparison of cored and uncored trees across all size classes:

To analyze the effect of time and the interaction of time with group on tree growth, all trees were divided into cored and uncored groups. The results indicated a violation of Mauchly’s test of sphericity, χ²(27) = 106.302, p = 0.00. Using the Greenhouse–Geisser correction, the analysis revealed a significant main effect of time, F(3.577, 135.916) = 19.920, p = 0.00, indicating significant growth variations over the 16 months from September 2018 to January 2020. However, the time and group interaction was not significant, F(3.577, 135.916) = 0.830, p = 0.497, suggesting similar growth patterns for cored and uncored trees during this period.

Post-COVID-19 measurements from February 2022 to April 2023 also indicated a violation of sphericity, χ²(27) = 222.259, p = 0.00. Applying the Greenhouse–Geisser correction, the analysis found a significant main effect of time, F(1.673, 51.862) = 5.044, p = 0.014, but an insignificant time and group interaction, F(1.673, 51.862) = 0.366, p = 0.657. Therefore, the growth patterns in cored and uncored trees from September 2018 to April 2023 did not differ significantly.

Therefore, repeated-measures ANOVA on the variations in diameter of small-, medium-, and large-size groups for both the cored and uncored trees was used to examine the effects of time and group interaction on tree growth as summarized in

Table 1. Statistical analysis of Khasi pine growth using repeated-measures ANOVA.

Size Class	Period	Greenhouse–Geisser Correction	Time Effect	Time × Group Interaction
Small	September 2018– January 2020	Not Violated	F(7, 56) = 7.248, p = 0.00 (Significant)	F(7, 56) = 1.129, p = 0.359 (Insignificant)
	February 2022– April 2023	Not Violated	F(7, 35) = 2.249, p = 0.053 (Insignificant)	F(7, 35) = 0.581, p = 0.766 (Insignificant)
Medium	September 2018– January 2020	Violated, Correction Applied	F(3.011, 36.129) = 9.121, p = 0.00 (Significant)	F(3.011, 36.129) = 0.743, p = 0.534 (Insignificant)
	February 2022– April 2023	Violated, Correction Applied	F(1.474, 14.736) = 1.420, p = 0.266 (Insignificant)	F(1.474, 14.736) = 1.381, p = 0.274 (Insignificant)
Large	September 2018– January 2020	Violated, Correction Applied	F(3.938, 55.129) = 13.557, p = 0.00 (Significant)	F(3.938, 55.129) = 0.444, p = 0.774 (Insignificant)
	February 2022– April 2023	Violated, Correction Applied	F(1.582, 18.981) = 5.952, p = 0.014 (Significant)	F(1.582, 18.981) = 1.811, p = 0.194 (Insignificant)
All Sizes	September 2018– January 2020	Violated, Correction Applied	F(3.577, 135.916) = 19.920, p = 0.00 (Significant)	F(3.577, 135.916) = 0.830, p = 0.497 (Insignificant)
	February 2022– April 2023	Violated, Correction Applied	F(1.673, 51.862) = 5.044, p = 0.014 (Significant)	F(1.673, 51.862) = 0.366, p = 0.657 (Insignificant)

. For the small-size class, significant growth changes were observed over the 16 months from September 2018 to January 2020, but no differences were found between the growth patterns of cored and uncored trees. Similarly, measurements taken from February 2022 to April 2023 showed no significant growth changes or difference between the groups. In the medium-size class, significant growth changes were noted from September 2018 to January 2020, with no significant differences in growth patterns between cored and uncored trees. Measurements from February 2022 to April 2023 also showed no significant changes or differences in growth. For the large-size class, significant growth changes were observed from September 2018 to January 2020, but the growth patterns of cored and uncored trees were similar, which was also the case from February 2022 to April 2023. Overall, the growth patterns of cored and uncored trees did not differ significantly from September 2018 to April 2023.

3.3.2. Wound Healing Rates

The wound healing rates of the small- and large-diameter trees were slightly lower than those of medium-sized trees, as shown in Figure 6. A repeated-measures ANOVA was conducted to determine any interactions between time and group on the healing rates. As Mauchly’s test of sphericity was violated, χ²(20) = 100.703, p = 0.00, the Greenhouse–Geisser correction was applied to the within-subject effects. The analysis indicated a significant main effect of time, F(2.695, 61.996) = 290.557, p = 0.00, indicating that the wound healing rate varied significantly across the seven measurement points taken every 2 months over the 14 month period from September 2018 to November 2019, when the holes were observed to have completely filled. Conversely, the time and group interaction was not found to be significant, F(5.391, 61.996) = 0.393, p = 0.964, suggesting that the healing rates were consistent across all class sizes in the small, medium, and large trees.

3.4. Data Interpretation for Increment Coring Effects on Tree Growth

Based on the analysis of periodic growth and wound healing rates of Khasi pines in Doi Khuntan National Park, it was observed that increment coring did not have a significant adverse effect on tree growth. Throughout the investigation period from September 2018 to April 2023, there were no significant differences in the growth patterns of cored and uncored trees across the small-, medium-, and large-diameter classes. In the small-size class, the cored group grew 0.11 cm faster than the uncored group, while in the medium-size class, the uncored group grew 0.04 cm faster than the cored group. In the large-size class, the cored group grew 0.78 cm faster than the uncored group. Repeated-measures ANOVA confirmed these observations, indicating no significant time and group interaction on growth for any size class (small-size class: F(7, 56) = 1.129, p = 0.359; medium-size class: F(3.011, 36.129) = 0.743, p = 0.534; large-size class: F(3.938, 55.129) = 0.444, p = 0.774). Although the wound healing rates of the small and large trees were slightly lower than those of the medium-diameter trees, the differences were not significant. The repeated-measures ANOVA indicated a significant main effect of time on wound healing (F(2.695, 61.996) = 290.557, p = 0.00), but no significant time and group interaction effect (F(5.391, 61.996) = 0.393, p = 0.964) was observed.

4. Discussion

4.1. Periodic Growth Patterns and Seasonal Influence

Across all size classes for both the cored and uncored groups, fluctuations in periodic growth data were in correspondence with the seasonal rainfall patterns. This indicates a strong influence of seasonal climatic conditions on Khasi pine growth in this region. Pumijumnong and Wanyaphet [26] observed a positive correlation between growth and rainfall during the wet season for Pinus kesiya in northern Thailand, with pine trees remaining dormant in March and April and reactivating in May with the onset of rain.

Using dendrochronological techniques, Inthawong et al. [27] analyzed the climate–tree growth relationship for Pinus latteri in northern Thailand and found that annual rainfall had the greatest influence on pine growth, with anomalous growth patterns linked to extreme rainfall events. In northeastern Thailand, D’Arrigo et al. [28] revised the pine tree-ring chronologies developed by Buckley et al. [29] and found that suppressed growth was associated with annual rainfall. Pumijumnong and Eckstein [30] reported a significant relationship between the declining temperatures and increasing rainfall during the pre-monsoon period from March to May, which stimulated pine growth in northern Thailand. Similarly, decreased temperatures and increased rainfall during the pre-monsoon period have also promoted the growth of other tree species, such as teak (Tectona grandis), in Thailand and India [14,31].

Pinus kesiya, naturally distributed in southeastern Asia, also exhibited growth patterns influenced by rainfall when introduced to Zambia, where dry season rainfall was a limiting factor for pine growth [32]. Conversely, in northeastern part of India, the cambial activity of Khasi pines was initiated in March, mainly induced by increased temperatures [33], while Chaudhary and Bhattacharyya [34] also found a positive correlation between precipitation during the month of December of the previous year and March in the growing year and the radial growth of P. kesiya in Shillong, Meghalaya in northeastern India.

Additionally, small trees exhibited the least variation in diameter changes, whereas large trees showed the highest fluctuations. Pumijumnong and Wanyaphet [26] found that the middle-aged P. kesiya trees had a shorter dormant season than the young-aged trees, allowing for a longer growing period in each season. However, findings reported by Anderson-Teixeira et al. [35] and Crespo-Antia et al. [36] suggested that larger trees were more sensitive to climate variations, with warmer temperatures and drought impeding growth and increasing the risk of mortality in large trees.

4.2. Wound Healing Rate Following Increment Coring

Wound healing rates following the increment coring were consistent across all size classes, with complete wound closure achieved within 14 months. The medium-sized trees exhibited the fastest initial recovery, but similar eventual healing was observed for all classes. Eckstein and Dujesiefken [37] suggested that wound healing largely depends on the vigor of a tree. Although there is qualitative similarity among different tree species regarding wounds caused by an increment borer, the quantitative response is different. Compartmentalization is defined as the active reaction of a tree to wounding [38,39]. Trees respond to injury or infection by compartmentalizing the affected area, effectively walling it off from the rest of the tree. However, this process does not kill or halt the activity of microorganisms within the compartmentalized regions. Additionally, trees do not have a specific response to different microorganisms, with the compartmentalization being a general response to the injury itself [40].

Eckstein and Dujesiefken [37] also recommended that boreholes created by an increment borer in deciduous trees should be made during the vegetation period, as trees are more effective at sealing wounds during this time compared to their dormant period. During the vegetation period, numerous vessels are sealed, and parenchyma cells and fibers are filled with accessory content such as tyloses or by the closure of bordered pits [37]. Additionally, untreated boreholes could lead to a greater cambial dieback around the boreholes. However, Dujesiefken et al. [41] suggested that treatment with LacBalsam or polyurethane had little to no influence on the wound reactions. Boreholes sealed with impregnated wood dowels had extensive discoloration and cambial dieback due to the toxic effect of creosote [41]. Therefore, leaving the wound holes untreated is the best approach, as trees can naturally recover and compartmentalize the damage effectively on their own.

In conifers, increment borers have been reported to cause minimal injury due to the trees’ ability to rapidly fill the borehole with resin and compartmentalize the damage [37,42]. Neo et al. [19] found that the median time for borehole closure in some tropical trees was 10 months, which is faster than the 14 months observed in this study.

4.3. Increment Coring Effects on Tree Growths

Based on the studies of periodic growth patterns comparing cored and uncored Khasi pines and their wound healing rates, it can be confirmed that tree-ring measurement using an increment borer does not affect tree growth or cause tree mortality. This finding is consistent with several other studies. Wunder et al. [16] compared the mortality rates of 551 trees cored in 1965 with those of similar trees in an uncored control group over 40 years later. They found no influence of increment coring on the mortality rate of Norway spruce within the study region. In addition to mortality rates, Wunder et al. [17] investigated whether coring induced stem-wood decay using high-resolution imaging techniques such as sonic and electric resistivity tomography. This study assessed the decay status of stem-wood in 22 pairs of cored and uncored Norway spruce from 1965–1966 up to 2011 and found no significant difference in decay status between the two groups.

Similarly, van Mantgem and Stephenson [43] found no differences in mortality rates between cored and uncored conifers, specifically Abies concolor (white fir) and A. magnifica (red fir), twelve years post-coring. Portier et al. [18] also confirmed that three European tree species in Switzerland and Ukraine—Picea abies, A. alba, and Fagus sylvatica—remained alive approximately 10 years after coring, with no statistical evidence of negative effects on growth or mortality. Additionally, Neo et al. [19] monitored the effects of increment coring on eleven tropical tree species over the course of a year, finding no significant differences in trunk diameter changes between cored and uncored trees. They concluded that increment coring does not negatively affect the survival and growth of certain tropical forest tree species within the first year after coring. Therefore, based on the findings of this research and previous studies mentioned earlier, mature trees of all sizes could be safely cored using an increment borer with similar wound healing rates. Importantly, no cored trees died during the investigation period, further supporting the conclusion that increment coring with an increment borer for dendrochronological analysis and other forest management purposes does not negatively impact tree vitality and growth.

5. Conclusions

This study conducted at Doi Khuntan National Park provides comprehensive insights into the effects of increment coring on the growth of pine trees. It was concluded that increment coring for dendrochronological studies and other forest management purposes does not significantly impact the growth patterns, health, or mortality of Khasi pine trees naturally distributed in Thailand across different size classes. Throughout the study periods from September 2018 to January 2020 and January 2022 to April 2023, no notable differences were observed between cored and uncored trees. Moreover, wound healing rates were consistent across the different size classes, with all wounds healing completely within 14 months, indicating minimal adverse effects on tree growth from increment coring. Future research should prioritize examining the long-term effects of increment coring on tree health and growth across diverse tree species and ecological contexts. Additionally, it is essential to investigate the optimal timing and frequency of coring operations to minimize any disturbance to tree physiology and overall forest health in various regions. These efforts are crucial in advancing our understanding and sustainable management of forest resources globally.