Neighborhood Tree Position and Size Had Hierarchical Effects on Korean Pine Growth at Different Opening Degree Levels

Abstract

Certain fluctuations were found when using the opening degree index to evaluate the effects of the adjacent broadleaved trees on the under-canopy target Korean pine (Pinus koraiensis Sieb. et Zucc.) trees in a mixed forest of planted Korean trees and natural broadleaved trees. The fluctuations were probably caused by the different positions and sizes of the adjacent broadleaved trees over the target Korean pine trees. For the purpose of clarifying this issue, the effect of the position and size (marked by height and diameter) of the neighborhood broadleaved trees on the height and diameter growth of Korean pine trees under the canopy in a secondary forest at different opening degree levels was studied. Generally, the mean DBH (diameter at breast height) and mean height of P. koraiensis trees under the canopy were higher when the adjacent broadleaved trees were located on the north or east sides of the target P. koraiensis trees than when they were located on the south or west sides. This was the case at all opening degree levels except the 0.5 opening degree level, where no significant difference was observed among the growth indexes of Korean pine trees based on the different positions of neighborhood trees. The mean DBH and height of P. koraiensis trees under the canopy decreased with the increase in the DBH and height of the adjacent broadleaved trees at all orientations and at all opening degree levels, but only those of the Korean pine trees with adjacent broadleaved trees on the south side in the 12.6–13.5 m height range and those with adjacent broadleaved trees on most sides in the 18.6–21.0 cm and 13.6–14.5 m height ranges at opening degree levels of K = 2.0, 1.5, and 1.0 reached a significant level of difference; whereas those of most Korean pine trees with adjacent broadleaved trees on the south and west sides in the height range of 11.6–12.5 m and DBH ranges of 13.6–16 cm and 16.1–18.5 cm, as well as the ones with adjacent broadleaved trees on most sides in the DBH range of 18.6–21.0 cm and the height range of 13.6–14.5 m at the opening degree level of K = 0.5, reached a significant level of difference, with a few exceptions. In conclusion, the broadleaved trees with a large diameter, great height, and a south or west orientation of the target P. koraiensis trees had a negative impact on their growth, but influence intensities changed at different opening degree levels.

1. Introduction

Most secondary forests, which occupy 59% of the world’s forests and 70% of the world’s natural forests [1], are the consequence of primary forest destruction and constitute a major concern in forest management around the world, especially in terms of the restoration and rehabilitation of secondary forests to near the level primary ones [2,3,4,5,6]. Typically, secondary forests in the humid temperate forest zone in the Changbai Mountains and the Lesser Xing’an Mountains in Northeast China are derived mainly from the long-term destruction of the zonal climax, with the primary mixed forest being of Korean pine (Pinus koraiensis Sieb. Et Zucc.) accompanied by broadleaved deciduous tree species like Fraxinus mandshurica Rupr. And Acer mono Maxim., etc. [3,6,7]. This secomdary forest occupies more than 99% of the natural forest area (which occupies 82% of total forest area) in this forest zone. Therefore, the restoration and rehabilitation of the close-climax mixed forest of Korean pine and broadleaved species from the secondary forests in this area has caused serious concern since the 1950s [3,6,7,8,9,10,11]. Stimulated by this concern, the approach of “planting conifer and preserving broadleaved trees”, i.e., artificially planting Korean trees on the sites with natural, regenerated broadleaved trees or being able to regenerate broadleaved trees, was developed, and a great establishment of mixed forests composed of the planted Korean pine trees and natural broadleaved trees has been achieved with this approach [8,9,10,11,12,13,14,15,16].

Considering that previous studies on such mixed forests have mainly been focused on planting (regeneration) issues during the establishment period and based on stand density or canopy density (at the stand level), with a lower precision level [13,14,15,16], our team conducted research on the growth promotion of the planted Korean pine trees during the post-planting period based on opening degree (at the individual tree level), with a higher precision level [8,9,17]. The opening degree is a quantitative index calculated as the sum of the ratio of the distances to heights, which the distance are the ones between the target tree and its nearest four upper-neighborhood trees, and the heights are the ones of the nearest four upper-neighborhood trees [8,18]. Our previous studies have certified that the opening degree is an effective index for quantitatively evaluating and regulating the growth of Korean pine trees under the secondary forest canopy [8,9]; the diameter and height growth of the Korean pine trees increased along with the enlargement of the opening degree, and K = 1.0 or K = 2.0 are the better treatments if the regulation rotation is about 5 or 10 years [8,9]. Meanwhile, we found that the quantitative regulation effect has some fluctuations that may be caused by the uneven size and different positions of the neighborhood trees around the target Korean pine trees as the index was developed in a relatively uniform forest type of Picea wilsonii Mast. with good application effect [18], but our study forest is composed of several tree species with different sizes and crown properties. These fluctuations may impact the precision of the quantitative regulation activities; therefore, how the type of influencing pattern of the adjacent broadleaved trees’ position and size impacts the growth of the target Korean pine trees at different opening degree levels should be elucidated for more precision regulation and to promote the growth of the target Korean pine trees.

This study was designed for the above issue, taking P. koraiensis individuals as the objective test trees and investigating the effect of the position and size (marked by height and diameter) of their neighborhood broadleaved trees on their height and diameter growth based on the effect primarily evaluated using the opening degree index. The results could provide reasonable reference for the precise application of the opening degree index in regulating the relationship between P. koraiensis trees, either planted or naturally regenerated, with their adjacent broadleaved trees in a secondary forest. It could also provide a method of simplifying the opening degree index, allowing for the wide application of the research results to similar forest scenarios around the world.

2. Materials and Methods

2.1. Study Sites and Its Basic Habitat Conditions

The investigation sites were in Shanhetun Forest Enterprise located in Wuchang City (44°03′34″–44°38′34″ N, 127°28′18″–128°14′36″ E), Heilongjiang Province, China; Maoershan University Forests of Northeast Forestry University located in Shangzhi City (45°21′–45°25′ N, 127°30′–127°34′ E), Heilongjiang Province, China; and Fangzheng Forest Enterprise located in Fangzheng County (45°33′16″–46°9′48″ N, 128°13′41″–129°33′20″ E), Heilongjiang Province, China. They are, respectively, located on the northwest ridge, west extended ridge, and southwest ridge of the Zhangguangcai Range in the Changbai Mountains, with a 2.5–2.9 °C mean annual temperature, 2100–2500 °C ≥ 10 °C accumulated temperature, 550–750 mm mean annual rainfall, and zonal dark-brown soil. The vegetation in this area belongs to the Changbai Mountains flora, and the natural secondary forest in these areas is the typical type derived from the zonal climax primary forest mixed with Korean pine and broadleaved tree species. The main secondary forest types are poplar–birch mixed forest (mainly Populus davidiana Dode or/and Betula platyphylla Suk.), precious hardwood mixed forest (mainly F. mandshurica, Juglans mandshurica Maxim., Phellodendron amurense Rupr., and Tilia amurensis Rupr.), Mongolian oak forest (mainly Quercus mongolica Fisch. Ex Turcz. mixed with some birch and poplar species), and hard shaw forest (F. mandshurica, J. mandshurica, Acer spp., Ulmus spp., Tilia spp., etc.); the main plantation types are P. koraiensis, P. sylvestris L. var. mongolica Litv., and Larix gmelinii (Rupr.) Rupr. (or L. olgensis Henry).

The plots in Shanhetun are located at the secondary forest at the Little Mopanshan area in Shahezi Forest Farm; the plots in Maoershan are located at secondary forest in the Taiping Working Circle in the Maoershan University Forests of Northeast Forestry University; and the plots in Fangzheng are located at the secondary forest at the Fangzheng side of junction between Fangzheng and Linkou County along the S206 road, which belongs to Fangzheng Forest Enterprise. The Korean pine trees were planted under the canopy with a planting density of 2500 trees/ha (spacing 2 m × 2 m), basically with 2–2-type seedlings. The plot sites selected were relatively flat, with a gentle incline of less than 10 degrees, but in order to reduce the influence of the slope, sampling plots were set on the four slope aspects with east, south, west, and north orientations. The soil porosity, organic matter, available nitrogen, available potassium, available phosphorus, and total nitrogen in the 0–10 cm soil layer in Maoershan area are higher than those in Shanhetun and Fangzheng, but there is no evident difference with other soil properties in the 0–10 cm layer and with all soil properties in 11–20 cm layer. The basic stand status and species compositions are shown in Table 1 and Table 2 based on the plot investigation data.

Table 1. The basic stand status in the three investigation sites.

Sampling Sites	Upper Layer Broadleaved Trees				Under-Canopy Korean Pines Trees
	Age (Year)	Density (Trees/ha)	Mean Height (m)	Mean DBH (cm)	Age (Year)	Density (Trees/ha)	Mean Height (m)	Mean DBH (cm)
Shanhetun	30–60	763	12.55	15.67	18	1450	2.35	2.35
Maoershan	30–60	1250	12.17	15.57	18	1250	2.63	2.82
Fangzheng	30–60	1025	12.70	15.80	18	1180	2.58	2.47

Table 2. Tree species composition of broadleaved species in different sampling plots.

Sampling Sites	Slope Aspects	Tree Species Composition
Shanhetun	Eastern slope	3(Ulmus pumila) 2(Acer mono) 2(Betula platyphylla) 1(Fraxinus mandshurica) 1(Phellodendron amurense) + (Populus davidiana) + (Rhamnus davurica) + (Maackia amurensis)
	Southern slope	3(Betula platyphylla) 2(Fraxinus mandshurica) 2(Juglans mandshurica) 2(Tilia amurensis) + (Quercus mongolica) + (Phellodendron amurense) + (Maackia amurensis) + (Ulmus pumila)
	Western slope	2(Populus davidiana) 2(Tilia amurensis) 2(Ulmus pumila) 1(Juglans mandshurica) 1(Fraxinus mandshurica) 1Quercus mongolica) + (Maackia amurensis) + (Rhamnus davurica) + (Phellodendron amurense)
	Northern slope	3(Ulmus pumila) 2(Tilia amurensis) 2(Quercus mongolica) 1(Acer mono) 1(Fraxinus mandshurica) + (Phellodendron amurense) + (Maackia amurensis) + (Populus davidiana)
Maoershan	Eastern slope	5(Tilia amurensis) 2(Fraxinus mandshurica) 1(Acer mono) 1(Quercus mongolica) + (Ulmus pumila) + (Betula platyphylla) + (Juglans mandshurica) + (Populus davidiana)
	Southern slope	4(Fraxinus mandshurica) 2(Phellodendron amurense) 2(Betula platyphylla) 1(Ulmus pumila) + (Quercus mongolica) + (Juglans mandshurica) + (Tilia amurensis) + (Populus davidiana)
	Western slope	4(Betula platyphylla) 2(Tilia amurensis) 1(Ulmus pumila) 1(Fraxinus mandshurica) 1(Juglans mandshurica) + (Quercus mongolica) + (Acer mono) + (Phellodendron amurense)
	Northern slope	4Tilia amurensis) 2(Quercus mongolica) 2(Acer mono) 1(Maackia amurensis) + (Ulmus pumila) + (Phellodendron amurense) + (Populus davidiana) + (Juglans mandshurica)
Fangzheng	Eastern slope	4(Fraxinus mandshurica) 2(Ulmus pumila) 1(Maackia amurensis) 1(Phellodendron amurense) 1(Larix gmelinii) + (Juglans mandshurica) + (Rhamnus davurica) + (Tilia amurensis)
	Southern slope	4(Betula platyphylla) 2(Fraxinus mandshurica) 1(Phellodendron amurense) 1(Populus davidiana) 1(Larix gmelinii) + (Acer mono) + (Quercus mongolica) + (Tilia amurensis)
	Western slope	5(Tilia amurensis) 1(Fraxinus mandshurica) 1(Juglans mandshurica) 1(Betula platyphylla) 1(Maackia amurensis) + (Phellodendron amurense) + (Ulmus pumila) + (Pinus koraiensis)
	Northern slope	4(Tilia amurensis) 2(Ulmus pumila) 1(Quercus mongolica) 1(Acer mono) 1(Maackia amurensis) + (Larix gmelinii) + (Juglans mandshurica) + (Fraxinus mandshurica)

Note: Counting the species composition according to the number of plants, 34.9%–25.0% is referred to as 3; 24.9%–15.0% is referred to as 2; 14.9%–5.0% is referred to as 1; 4.9%–2.0% is referred to as +; and <1.9% is referred to as −.

2.2. Study Methods

2.2.1. Plot and Line Investigation Methods

The basic stand situations were investigated by sampling plots at the four slope directions of east, west, south, and north, selected in each investigation area. The size of each sampling plot was 20 m × 20 m = 400 m², repeated 3 times in each slope aspect, with at least 20 m distance between the plots; 12 plots in each area and, in total, 36 plots in the tree sites were investigated. The species, diameter, and height of the broadleaved trees, whose DBH (diameter at breast height) are ≥4 cm, and the DBH and height of all Korean pine trees were measured and recorded. The stand density, mean DBH, mean height of broadleaved trees and Korean pine trees, and the broadleaved species compositions for each plot were calculated with these data (Table 1 and Table 2).

For evaluating the effects of opening degree, (1) the DBH and height of 30 selected P. koraiensis trees, along with a snakelike line in each plot, were measured and recorded; the species, DBH, and height of the nearest broadleaved trees in the four orientations of east, west, south, and north around the selected target Korean pine trees, as well as the distance between them, were also measured and recorded; (2) in addition, a straight line was set in each slope aspect, and 100 Korean pine trees along with the straight line were selected, along with the same items as above for Korean pine trees, and their surrounding broadleaved trees were also measured and recorded. In total, 130, 520, and 1560 Korean pine trees were measured at each slope aspect, each site, and the three investigation sites as the target trees for opening degree evaluation, as well as for position and size effect certification of the neighborhood broadleaved trees.

2.2.2. Calculation and Statistics of the Data

The opening degree (K) of P. koraiensis trees was calculated based the above related data according to the method described by Cong et al., Shen et al., and Luo et al. [8,9,18] as calculated by Equation (1). K was defined as the sum of the ratio of the distance from the reference tree (target tree) to the nearest neighbor trees in each quadrat to the height of these neighbor trees; the longer the distance and the lower the height for the nearest neighbor trees, the larger the K and the more favorable the growth of the reference tree.

$$K={\displaystyle \sum _{\iota =1}^4(ctg{\alpha }_{\iota })}={\displaystyle \sum _{\iota =1}^4{(d/h)}_{\iota }}$$

(1)

where i is the neighbor tree number, and d and h is the distance and height of one of the four trees nearest to a reference tree, respectively.

Four opening degree levels were divided: the range of K = 0.5 was 0.3–0.7; K = 1.0 was 0.8–1.2; K = 1.5 was 1.3–1.7; and K = 2.0 was 1.8–2.2.

Four ranks of DBH and height range of the adjacent broadleaved trees of the target Korean pine trees were divided as 11.0–13.5 cm, 13.6–16.0 cm, 16.1–18.5 cm, 18.6–21.0 cm and 10.5–11.5 m, 11.6–12.5 m, 12.6–13.5 m, and 13.6–14.5 m, respectively. Within each opening degree level, the DBH and height of the target Korean pine trees were listed, respectively, according to the position of their neighborhood broadleaved trees, i.e., listed for their adjacent broadleaved trees located at east, south, west, or north side (orientation), respectively (as shown in Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11, Table 12, Table 13 and Table 14). The DBH and height of the target Korean pine trees were calculated as the mean and standard error, and their significant differences were tested.

Table 3. Diameter at breast height (DBH) of Korean pine trees with adjacent broadleaved trees at different orientations along with the DBH range of the adjacent broadleaved trees around the target Korean pine trees in the Shanhetun investigation site.

Opening Degree (K)	Adjacent Broadleaved Tree Positions Relative to the Target Korean Pine Trees	DBH Range of Adjacent Broadleaved Trees Arround the Target Korean Pine Trees
		11.0–13.5 (cm)	13.6–16.0 (cm)	16.1–18.5 (cm)	18.6–21.0 (cm)
K = 2.0	East side	2.76 ± 0.023 aA	2.59 ± 0.026 aA	2.48 ± 0.025 aA	2.37 ± 0.023 aA
	South side	2.63 ± 0.012 bA	2.52 ± 0.015 bA	2.40 ± 0.026 bA	2.21 ± 0.008 bB
	West side	2.67 ± 0.012 bA	2.48 ± 0.028 bA	2.39 ± 0.011 bA	2.22 ± 0.028 bB
	North side	2.75 ± 0.017 aA	2.62 ± 0.021 aA	2.51 ± 0.016 aA	2.39 ± 0.022 aA
K = 1.5	East side	2.66 ± 0.021 aA	2.58 ± 0.014 aA	2.51 ± 0.021 aA	2.39 ± 0.031 aA
	South side	2.48 ± 0.016 bA	2.46 ± 0.023 bA	2.44 ± 0.019 bA	2.13 ± 0.016 bB
	West side	2.57 ± 0.019 aA	2.49 ± 0.026 bA	2.43 ± 0.026 bA	2.23 ± 0.024 bB
	North side	2.61 ± 0.015 aA	2.65 ± 0.024 aA	2.58 ± 0.033 aA	2.40 ± 0.018 aB
K = 1.0	East side	2.52 ± 0.014 aA	2.47 ± 0.025 aA	2.36 ± 0.025 aA	2.12 ± 0.056 aB
	South side	2.32 ± 0.022 bA	2.28 ± 0.028 bA	2.22 ± 0.029 bA	2.03 ± 0.034 bB
	West side	2.36 ± 0.024 bA	2.28 ± 0.033 bA	2.24 ± 0.018 bA	2.06 ± 0.025 bB
	North side	2.48 ± 0.019 aA	2.39 ± 0.017 aA	2.31 ± 0.014 aA	2.26 ± 0.037 cA
K = 0.5	East side	2.32 ± 0.035 aA	2.24 ± 0.034 aA	2.14 ± 0.028 aB	2.08 ± 0.043 aB
	South side	2.21 ± 0.025 bA	2.11 ± 0.024 bB	2.07 ± 0.023 aB	2.04 ± 0.045 aB
	West side	2.27 ± 0.026 bA	2.17 ± 0.035 bB	2.10 ± 0.035 aB	2.06 ± 0.032 aB
	North side	2.35 ± 0.019 aA	2.26 ± 0.017 aA	2.16 ± 0.042 aA	2.09 ± 0.016 aB

Note: Different lowercase letters in the same column indicate a significant difference at the 0.05 level among the DBHs of Korean pine trees with adjacent broadleaved trees at different orientations (positions) of east, south, west, and north within the same opening degree level. Different uppercase letters in the same row indicate a significant difference at the 0.05 level among the DBHs of Korean pine trees along with the DBH ranges of the adjacent broadleaved trees at the same orientations (positions) of the adjacent broadleaved trees relative to the target Korean pine trees.

Table 4. DBH of Korean pine trees with adjacent broadleaved trees at different orientations along with the height range of the adjacent broadleaved trees around the target Korean pine trees in the Shanhetun investigation site.

Opening Degree (K)	Adjacent Broadleaved Tree Positions Relative to the Target Korean Pine Trees	Height Range of Adjacent Broadleaved Trees Arround the Target Korean Pine Trees
		10.5–11.5 (m)	11.6–12.5 (m)	12.6–13.5 (m)	13.6–14.5 (m)
K = 2.0	East side	2.76 ± 0.011 aA	2.62 ± 0.018 aA	2.45 ± 0.016 aA	2.33 ± 0.013 aA
	South side	2.66 ± 0.006 bA	2.55 ± 0.023 bA	2.39 ± 0.014 bA	2.24 ± 0.009 bA
	West side	2.66 ± 0.009 bA	2.52 ± 0.013 bA	2.37 ± 0.009 bA	2.25 ± 0.015 bA
	North side	2.73 ± 0.009 aA	2.63 ± 0.015 aA	2.47 ± 0.006 aA	2.31 ± 0.019 aA
K = 1.5	East side	2.63 ± 0.011 aA	2.53 ± 0.028 aA	2.46 ± 0.022 aA	2.31 ± 0.026 aA
	South side	2.52 ± 0.023 bA	2.45 ± 0.016 bA	2.40 ± 0.025 aA	2.18 ± 0.012 bB
	West side	2.47 ± 0.008 bA	2.39 ± 0.017 bA	2.28 ± 0.009 bA	2.15 ± 0.012 bB
	North side	2.61 ± 0.032 aA	2.52 ± 0.033 aA	2.42 ± 0.019 aA	2.29 ± 0.028 aA
K = 1.0	East side	2.49 ± 0.023 aA	2.48 ± 0.032 aA	2.38 ± 0.033 aA	2.13 ± 0.029 aB
	South side	2.33 ± 0.053 bA	2.29 ± 0.027 bA	2.21 ± 0.024 bA	2.04 ± 0.035 bB
	West side	2.38 ± 0.024 bA	2.31 ± 0.016 bA	2.22 ± 0.027 bA	2.06 ± 0.026 bB
	North side	2.51 ± 0.035 aA	2.42 ± 0.023 aA	2.29 ± 0.016 aA	2.20 ± 0.024 aA
K = 0.5	East side	2.35 ± 0.024 aA	2.26 ± 0.024 aA	2.13 ± 0.028 aB	2.10 ± 0.018 aB
	South side	2.24 ± 0.027 bA	2.10 ± 0.025 bB	2.09 ± 0.035 aB	2.06 ± 0.015 aB
	West side	2.29 ± 0.036 bA	2.11 ± 0.013 bB	2.10 ± 0.024 aB	2.06 ± 0.032 aB
	North side	2.37 ± 0.041 aA	2.25 ± 0.024 aA	2.16 ± 0.015 aA	2.11 ± 0.024 aA

Note: Different lowercase letters in the same column indicate a significant difference at the 0.05 level among the DBHs of Korean pine trees with adjacent broadleaved trees at different orientations (positions) of east, south, west, and north within the same opening degree level. Different uppercase letters in the same row indicate a significant difference at the 0.05 level among the DBHs of Korean pine trees along with the height ranges of the adjacent broadleaved trees at the same orientations (positions) of the adjacent broadleaved trees relative to the target Korean pine trees.

Table 5. DBH of Korean pine trees with adjacent broadleaved trees at different orientations along with the DBH range of the adjacent broadleaved trees around the target Korean pine trees in the Maoershan site.

Opening Degree (K)	Adjacent Broadleaved Tree Positions Relative to the Target Korean Pine Trees	DBH Range of Adjacent Broadleaved Trees Arround the Target Korean Pine Trees
		11.0–13.5 (cm)	13.6–16.0 (cm)	16.1–18.5 (cm)	18.6–21.0 (cm)
K = 2.0	East side	3.56 ± 0.021 aA	3.45 ± 0.024 aA	3.32 ± 0.032 aA	3.11 ± 0.025 aA
	South side	3.11 ± 0.014 bA	3.01 ± 0.025 bA	2.76 ± 0.022 bB	2.66 ± 0.023 bB
	West side	3.03 ± 0.009 bA	2.87 ± 0.011 bA	2.64 ± 0.019 bA	2.43 ± 0.011 bA
	North side	3.54 ± 0.018 aA	3.42 ± 0.018 aA	3.21 ± 0.016 aA	2.87 ± 0.014 bB
K = 1.5	East side	3.36 ± 0.055 aA	3.15 ± 0.045 aA	2.87 ± 0.039 aA	2.43 ± 0.051 aB
	South side	3.05 ± 0.048 bA	2.81 ± 0.029 bA	2.76 ± 0.044 aA	2.37 ± 0.035 bB
	West side	3.18 ± 0.036 bA	3.01 ± 0.033 aA	2.80 ± 0.048 aA	2.39 ± 0.035 bB
	North side	3.45 ± 0.043 aA	3.24 ± 0.058 aA	3.02 ± 0.028 bA	2.76 ± 0.047 aA
K = 1.0	East side	3.31 ± 0.026 aA	3.23 ± 0.025 aA	3.11 ± 0.035 aA	2.98 ± 0.026 aA
	South side	3.15 ± 0.016 bA	3.01 ± 0.027 bA	2.54 ± 0.032 bB	2.38 ± 0.035 bB
	West side	3.06 ± 0.019 bA	2.91 ± 0.037 bA	2.66 ± 0.022 bB	2.43 ± 0.025 bB
	North side	3.28 ± 0.032 aA	3.19 ± 0.029 aA	2.98 ± 0.027 aA	2.87 ± 0.019 aA
K = 0.5	East side	2.94 ± 0.036 aA	2.72 ± 0.009 aA	2.43 ± 0.036 aB	2.28 ± 0.034 aB
	South side	2.72 ± 0.027 bA	2.54 ± 0.012 bB	2.37 ± 0.024 aB	2.18 ± 0.045 aB
	West side	2.76 ± 0.024 bA	2.56 ± 0.015 bB	2.34 ± 0.035 aB	2.11 ± 0.029 aB
	North side	2.89 ± 0.034 aA	2.69 ± 0.021 aA	2.44 ± 0.019 aB	2.24 ± 0.017 aB

Note: Different lowercase letters in the same column indicate a significant difference at the 0.05 level among the DBHs of Korean pine trees with adjacent broadleaved trees at different orientations (positions) of east, south, west, and north within the same opening degree level. Different uppercase letters in the same row indicate a significant difference at the 0.05 level among the DBHs of Korean pine trees along with the DBH ranges of the adjacent broadleaved trees at the same orientations (positions) of the adjacent broadleaved trees relative to the target Korean pine trees.

Table 6. DBH of Korean pine trees with adjacent broadleaved trees at different orientations along with the height range of the adjacent broadleaved trees around the target Korean pine trees in the Maoershan site.

Opening Degree (K)	Adjacent Broadleaved tree Positions Relative to the Target Korean Pine Trees	Height Range of Adjacent Broadleaved Trees Arround the Target Korean Pine Trees
		10.5–11.5 (m)	11.6–12.5 (m)	12.6–13.5 (m)	13.6–14.5 (m)
K = 2.0	East side	3.52 ± 0.017 aA	3.43 ± 0.032 aA	3.30 ± 0.023 aA	3.09 ± 0.019 aA
	South side	3.26 ± 0.026 bA	3.13 ± 0.012 bA	2.91 ± 0.008 bB	2.77 ± 0.017 bB
	West side	3.14 ± 0.012 bA	2.97 ± 0.019 bA	2.78 ± 0.022 bA	2.61 ± 0.027 bA
	North side	3.51 ± 0.019 aA	3.39 ± 0.026 aA	3.17 ± 0.013 aA	2.96 ± 0.015 aA
K = 1.5	East side	3.42 ± 0.038 aA	3.19 ± 0.037 aA	2.96 ± 0.023 aA	2.51 ± 0.022 aB
	South side	3.16 ± 0.032 bA	3.01 ± 0.031 bA	2.86 ± 0.033 aA	2.49 ± 0.026 aB
	West side	3.12 ± 0.029 bA	3.05 ± 0.045 bA	2.86 ± 0.036 aA	2.31 ± 0.038 aB
	North side	3.38 ± 0.051 aA	3.21 ± 0.023 aA	3.19 ± 0.018 bA	2.87 ± 0.033 bA
K = 1.0	East side	3.35 ± 0.025 aA	3.25 ± 0.034 aA	3.15 ± 0.026 aA	3.01 ± 0.024 aA
	South side	3.13 ± 0.035 bA	3.09 ± 0.054 bA	2.58 ± 0.036 bA	2.36 ± 0.036 bB
	West side	3.11 ± 0.023 bA	2.99 ± 0.026 bA	2.66 ± 0.015 bB	2.39 ± 0.045 bB
	North side	3.29 ± 0.018 aA	3.23 ± 0.027 aA	2.99 ± 0.024 aA	2.88 ± 0.025 aA
K = 0.5	East side	2.91 ± 0.032 aA	2.75 ± 0.013 aA	2.45 ± 0.023 aB	2.25 ± 0.035 aB
	South side	2.77 ± 0.033 bA	2.55 ± 0.012 bB	2.40 ± 0.019 aB	2.19 ± 0.026 aB
	West side	2.74 ± 0.043 bA	2.57 ± 0.027 bB	2.38 ± 0.026 aB	2.15 ± 0.035 aB
	North side	2.92 ± 0.018 aA	2.68 ± 0.038 aA	2.41 ± 0.037 aB	2.22 ± 0.024 aB

Note: Different lowercase letters in the same column indicate a significant difference at the 0.05 level among the DBHs of Korean pine trees with adjacent broadleaved trees at different orientations (positions) of east, south, west, and north within the same opening degree level. Different uppercase letters in the same row indicate a significant difference at the 0.05 level among the DBHs of Korean pine trees along with the height ranges of the adjacent broadleaved trees at the same orientations (positions) of the adjacent broadleaved trees relative to the target Korean pine trees.

Table 7. DBH of Korean pine trees with adjacent broadleaved trees at different orientations along with the DBH range of the adjacent broadleaved trees around the target Korean pine trees in the Fangzheng site.

Opening Degree (K)	Adjacent Broadleaved tree Positions Relative to the Target Korean Pine Trees	DBH Range of Adjacent Broadleaved Trees Arround the Target Korean Pine Trees
		11.0–13.5 (cm)	13.6–16.0 (cm)	16.1–18.5 (cm)	18.6–21.0 (cm)
K = 2.0	East side	2.83 ± 0.018 aA	2.71 ± 0.023 aA	2.62 ± 0.024 aA	2.43 ± 0.043 aB
	South side	2.71 ± 0.027 bA	2.67 ± 0.031 aA	2.36 ± 0.031 bB	2.24 ± 0.022 bB
	West side	2.68 ± 0.025 bA	2.54 ± 0.038 bA	2.44 ± 0.019 bA	2.19 ± 0.029 bB
	North side	2.76 ± 0.012 bA	2.68 ± 0.041 aA	2.59 ± 0.033a A	2.48 ± 0.023 aA
K = 1.5	East side	2.75 ± 0.008 aA	2.66 ± 0.015 aA	2.56 ± 0.025a A	2.48 ± 0.023 aA
	South side	2.71 ± 0.014 aA	2.63 ± 0.021 aA	2.50 ± 0.028 aA	2.24 ± 0.016 bB
	West side	2.69 ± 0.022 aA	2.58 ± 0.009 aA	2.49 ± 0.013 aA	2.28 ± 0.018 bB
	North side	2.86 ± 0.016 bA	2.78 ± 0.018 bA	2.69 ± 0.006 bA	2.60 ± 0.026 aA
K = 1.0	East side	2.76 ± 0.035 aA	2.66 ± 0.036 aA	2.53 ± 0.025 aA	2.41 ± 0.025 aA
	South side	2.66 ± 0.019 bA	2.57 ± 0.027 bA	2.38 ± 0.018 bB	2.27 ± 0.034 bB
	West side	2.61 ± 0.028 bA	2.52 ± 0.022 bA	2.39 ± 0.009 bA	2.19 ± 0.013 bB
	North side	2.72 ± 0.031 aA	2.63 ± 0.017 aA	2.54 ± 0.032 aA	2.44 ± 0.022 aA
K = 0.5	East side	2.44 ± 0.035 aA	2.36 ± 0.033 aA	2.27 ± 0.017 aA	2.18 ± 0.034 aA
	South side	2.31 ± 0.026 bA	2.28 ± 0.024 bA	2.19 ± 0.023 bB	2.15 ± 0.024 aB
	West side	2.39 ± 0.022 aA	2.31 ± 0.028 bA	2.27 ± 0.026 aA	2.16 ± 0.015 aB
	North side	2.41 ± 0.016 aA	2.37 ± 0.019 aA	2.31 ± 0.035 aA	2.28 ± 0.019 aA

Note: Different lowercase letters in the same column indicate a significant difference at the 0.05 level among the DBHs of Korean pine trees with adjacent broadleaved trees at different orientations (positions) of east, south, west, and north within the same opening degree level. Different uppercase letters in the same row indicate a significant difference at the 0.05 level among the DBHs of Korean pine trees along with the DBH ranges of the adjacent broadleaved trees at the same orientations (positions) of the adjacent broadleaved trees relative to the target Korean pine trees.

Table 8. DBH of Korean pine trees with adjacent broadleaved trees at different orientations along with the height range of the adjacent broadleaved trees around the target Korean pine trees in the Fangzheng site.

Opening Degree (K)	Adjacent Broadleaved Tree Positions Relative to the Target Korean Pine Trees	Height Range of Adjacent Broadleaved Trees Arround the Target Korean Pine Trees
		10.5–11.5 (m)	11.6–12.5 (m)	12.6–13.5 (m)	13.6–14.5 (m)
K = 2.0	East side	2.80 ± 0.016 aA	2.69 ± 0.013 aA	2.59 ± 0.023 aA	2.48 ± 0.016 aA
	South side	2.68 ± 0.012 bA	2.61 ± 0.011 bA	2.53 ± 0.027 aA	2.26 ± 0.011 bB
	West side	2.69 ± 0.033 bA	2.58 ± 0.031 bA	2.46 ± 0.012 bA	2.23 ± 0.018 bB
	North side	2.79 ± 0.025 aA	2.71 ± 0.026 aA	2.62 ± 0.029 aA	2.50 ± 0.026 aA
K = 1.5	East side	2.74 ± 0.016 aA	2.69 ± 0.027 aA	2.58 ± 0.033 aA	2.49 ± 0.019 aA
	South side	2.72 ± 0.026 aA	2.68 ± 0.036 aA	2.44 ± 0.019 bB	2.32 ± 0.018 bB
	West side	2.66 ± 0.019 bA	2.59 ± 0.012 bA	2.51 ± 0.027 bA	2.28 ± 0.024 bB
	North side	2.79 ± 0.023 aA	2.71 ± 0.019 aA	2.62 ± 0.025 aA	2.55 ± 0.035 aA
K = 1.0	East side	2.74 ± 0.035 aA	2.68 ± 0.036 aA	2.56 ± 0.025 aA	2.44 ± 0.025 aA
	South side	2.65 ± 0.019 bA	2.56 ± 0.027 bA	2.41 ± 0.018 bB	2.30 ± 0.034 bB
	West side	2.59 ± 0.028 bA	2.55 ± 0.022 bA	2.42 ± 0.009 bA	2.21 ± 0.013 bB
	North side	2.68 ± 0.031 aA	2.66 ± 0.017 aA	2.57 ± 0.032 aA	2.47 ± 0.022 aA
K = 0.5	East side	2.48 ± 0.035 aA	2.38 ± 0.033 aA	2.33 ± 0.017 aA	2.20 ± 0.034 aA
	South side	2.30 ± 0.026 bA	2.31 ± 0.024 aA	2.23 ± 0.023 bB	2.19 ± 0.024 aB
	West side	2.36 ± 0.022 bA	2.29 ± 0.028 aA	2.28 ± 0.026 aA	2.18 ± 0.015 aB
	North side	2.39 ± 0.016b A	2.35 ± 0.019 aA	2.29 ± 0.035 aA	2.24 ± 0.019 aA

Note: Different lowercase letters in the same column indicate a significant difference at the 0.05 level among the DBHs of Korean pine trees with adjacent broadleaved trees at different orientations (positions) of east, south, west, and north within the same opening degree level. Different uppercase letters in the same row indicate a significant difference at the 0.05 level among the DBHs of Korean pine trees along with the height ranges of the adjacent broadleaved trees at the same orientations (positions) of the adjacent broadleaved trees relative to the target Korean pine trees.

Table 9. Height of Korean pine trees with adjacent broadleaved trees at different orientations along with the DBH range of the adjacent broadleaved trees around the target Korean pine trees in the Shanhetun site.

Opening Degree (K)	Adjacent Broadleaved Tree Positions Relative to the Target Korean Pine Trees	DBH Range of Adjacent Broadleaved Trees Arround the Target Korean Pine Trees
		11.0–13.5 (cm)	13.6–16.0 (cm)	16.1–18.5 (cm)	18.6–21.0 (cm)
K = 2.0	East side	2.74 ± 0.005 aA	2.58 ± 0.027 aA	2.60 ± 0.033 aB	2.36 ± 0.051 aB
	South side	2.69 ± 0.023 bA	2.58 ± 0.027 aA	2.56 ± 0.036 bA	2.44 ± 0.010 bA
	West side	2.73 ± 0.029 aA	2.58 ± 0.027 aA	2.46 ± 0.037 cA	2.32 ± 0.018 aA
	North side	2.74 ± 0.006 aA	2.62 ± 0.038 bA	2.54 ± 0.006 bA	2.29 ± 0.030 cB
K = 1.5	East side	2.48 ± 0.032 aA	2.41 ± 0.034 aA	2.37 ± 0.032 aA	2.31 ± 0.035a A
	South side	2.46 ± 0.026 aA	2.39 ± 0.028 aA	2.28 ± 0.019 bB	2.24 ± 0.051 bB
	West side	2.32 ± 0.041 bA	2.29 ± 0.036 bA	2.26 ± 0.033 bA	2.19 ± 0.029 bB
	North side	2.56 ± 0.029 aA	2.47 ± 0.042 aA	2.41 ± 0.044 aA	2.39 ± 0.033 aA
K = 1.0	East side	2.71 ± 0.024 aA	2.65 ± 0.026 aA	2.53 ± 0.026 aA	2.43 ± 0.015 aA
	South side	2.55 ± 0.031 bA	2.48 ± 0.013 bA	2.26 ± 0.023 bB	2.17 ± 0.019 bB
	West side	2.66 ± 0.014 aA	2.54 ± 0.009 bA	2.46 ± 0.021 aA	2.37 ± 0.042 aA
	North side	2.69 ± 0.016 aA	2.61 ± 0.023 aA	2.57 ± 0.034 aA	2.42 ± 0.044 aA
K = 0.5	East side	2.31 ± 0.023 aA	2.27 ± 0.016 aA	2.21 ± 0.019 aA	2.14 ± 0.023 aA
	South side	2.26 ± 0.024 bA	2.22 ± 0.023 bA	2.18 ± 0.021 aA	2.13 ± 0.028 aA
	West side	2.24 ± 0.018 bA	2.18 ± 0.043 bA	2.15 ± 0.025 aA	2.11 ± 0.015 aA
	North side	2.34 ± 0.032 aA	2.28 ± 0.027 aA	2.21 ± 0.032 aA	2.16 ± 0.029 aA

Note: Different lowercase letters in the same column indicate a significant difference at the 0.05 level among the heights of Korean pine trees with adjacent broadleaved trees at different orientations (positions) of east, south, west, and north within the same opening degree level. Different uppercase letters in the same row indicate a significant difference at the 0.05 level among the heights of Korean pine trees along with the DBH ranges of the adjacent broadleaved trees at the same orientations (positions) of the adjacent broadleaved trees relative to the target Korean pine trees.

Table 10. Height of Korean pine trees with adjacent broadleaved trees at different orientations (positions) along with the height range of the adjacent broadleaved trees around the target Korean pine trees in the Shanhetun site.

Opening Degree (K)	Adjacent Broadleaved Tree Positions Relative to the Target Korean Pine Trees	Height Range of Adjacent Broadleaved Trees Arround the Target Korean Pine Trees
		10.5–11.5 (m)	11.6–12.5 (m)	12.6–13.5 (m)	13.6–14.5 (m)
K = 2.0	East side	2.69 ± 0.018 aA	2.56 ± 0.019 aA	2.48 ± 0.035 aA	2.41 ± 0.037 aA
	South side	2.61 ± 0.032 bA	2.54 ± 0.015 aA	2.49 ± 0.024 aA	2.42 ± 0.019 aA
	West side	2.67 ± 0.012 aA	2.58 ± 0.033 aA	2.49 ± 0.009 aA	2.36 ± 0.023 bA
	North side	2.73 ± 0.023 aA	2.68 ± 0.026 bA	2.54 ± 0.017 bA	2.31 ± 0.045 bB
K = 1.5	East side	2.52 ± 0.032 aA	2.46 ± 0.034 aA	2.41 ± 0.032 aA	2.36 ± 0.035 aA
	South side	2.50 ± 0.026 aA	2.43 ± 0.028 aA	2.38 ± 0.019 aA	2.24 ± 0.051 bB
	West side	2.41 ± 0.041 bA	2.37 ± 0.036 bA	2.31 ± 0.033 bA	2.26 ± 0.029 bA
	North side	2.54 ± 0.029 aA	2.45 ± 0.042 aA	2.39 ± 0.044 aA	2.35 ± 0.033 aA
K = 1.0	East side	2.73 ± 0.028 aA	2.61 ± 0.025 aA	2.56 ± 0.024 aA	2.44 ± 0.015 aA
	South side	2.54 ± 0.029 bA	2.47 ± 0.026 bA	2.28 ± 0.016 bB	2.29 ± 0.024 bB
	West side	2.68 ± 0.031 aA	2.52 ± 0.017 bA	2.49 ± 0.018 aA	2.33 ± 0.025 aA
	North side	2.70 ± 0.018 aA	2.59 ± 0.032 aA	2.56 ± 0.016 aA	2.42 ± 0.032 aA
K = 0.5	East side	2.33 ± 0.017 aA	2.28 ± 0.028 aA	2.25 ± 0.013 aA	2.15 ± 0.035 aB
	South side	2.24 ± 0.041 bA	2.23 ± 0.035 bA	2.21 ± 0.014 aA	2.13 ± 0.025 aA
	West side	2.25 ± 0.025 bA	2.20 ± 0.019 bA	2.18 ± 0.027 aA	2.12 ± 0.012 aA
	North side	2.36 ± 0.028 aA	2.30 ± 0.021 aA	2.23 ± 0.018 aA	2.18 ± 0.019 aA

Note: Different lowercase letters in the same column indicate a significant difference at the 0.05 level among the heights of Korean pine trees with adjacent broadleaved trees at different orientations (positions) of east, south, west, and north within the same opening degree level. Different uppercase letters in the same row indicate a significant difference at the 0.05 level among the heights of Korean pine trees along with the height ranges of the adjacent broadleaved trees at the same orientations (positions) of the adjacent broadleaved trees relative to the target Korean pine trees.

Table 11. Height of Korean pine trees with adjacent broadleaved trees at different orientations along with the DBH range of the adjacent broadleaved trees around the target Korean pine trees in the Maoershan site.

Opening Degree (K)	Adjacent Broadleaved Tree Positions Relative to the Target Korean Pine Trees	DBH Range of Adjacent Broadleaved Trees Arround the Target Korean Pine Trees
		11.0–13.5 (cm)	13.6–16.0 (cm)	16.1–18.5 (cm)	18.6–21.0 (cm)
K = 2.0	East side	2.88 ± 0.042 aA	2.76 ± 0.041 aA	2.66 ± 0.026 aA	2.48 ± 0.028 aB
	South side	2.65 ± 0.034 bA	2.54 ± 0.035 bB	2.48 ± 0.019 bB	2.42 ± 0.018 aB
	West side	2.61 ± 0.031 bA	2.57 ± 0.052 bA	2.54 ± 0.033 bA	2.41 ± 0.022 aB
	North side	2.73 ± 0.029 bA	2.69 ± 0.039 aA	2.62 ± 0.024 aA	2.58 ± 0.033 bA
K = 1.5	East side	2.79 ± 0.014 aA	2.70 ± 0.008 aA	2.64 ± 0.021 aA	2.58 ± 0.033 aA
	South side	2.76 ± 0.028 aA	2.65 ± 0.023 aA	2.45 ± 0.021 bB	2.36 ± 0.023 bB
	West side	2.65 ± 0.013 bA	2.60 ± 0.017 aA	2.51 ± 0.011 bB	2.43 ± 0.018 bB
	North side	2.81 ± 0.009 aA	2.76 ± 0.016 bA	2.68 ± 0.034 aA	2.46 ± 0.016 bB
K = 1.0	East side	2.76 ± 0.025 aA	2.68 ± 0.019 aA	2.62 ± 0.014 aA	2.48 ± 0.023 aB
	South side	2.54 ± 0.029 bA	2.49 ± 0.017 bA	2.31 ± 0.024 bB	2.26 ± 0.034 bB
	West side	2.69 ± 0.014 bA	2.58 ± 0.023 bA	2.33 ± 0.027 bB	2.31 ± 0.032 bB
	North side	2.72 ± 0.034 aA	2.66 ± 0.043 aA	2.54 ± 0.016 aA	2.46 ± 0.021 aA
K = 0.5	East side	2.65 ± 0.019 aA	2.56 ± 0.024 aA	2.45 ± 0.023 aA	2.37 ± 0.024 aA
	South side	2.62 ± 0.016 aA	2.57 ± 0.029 aA	2.43 ± 0.015 aB	2.35 ± 0.025 aB
	West side	2.55 ± 0.023 bA	2.44 ± 0.013 bB	2.39 ± 0.027 aB	2.33 ± 0.026 aB
	North side	2.64 ± 0.032 aA	2.61 ± 0.037 aA	2.46 ± 0.035 aB	2.40 ± 0.035 aB

Note: Different lowercase letters in the same column indicate a significant difference at the 0.05 level among the heights of Korean pine trees with adjacent broadleaved trees at different orientations (positions) of east, south, west, and north within the same opening degree level. Different uppercase letters in the same row indicate a significant difference at the 0.05 level among the heights of Korean pine trees along with the DBH ranges of the adjacent broadleaved trees at the same orientations (positions) of the adjacent broadleaved trees relative to the target Korean pine trees.

Table 12. Height of Korean pine trees with adjacent broadleaved trees at different orientations along with the height range of the adjacent broadleaved trees around the target Korean pine trees in the Maoershan site.

Opening Degree (K)	Adjacent Broadleaved Tree Positions Relative to the Target Korean Pine Trees	Height Range of Adjacent Broadleaved Trees Arround the Target Korean Pine Trees
		10.5–11.5 (m)	11.6–12.5 (m)	12.6–13.5 (m)	13.6–14.5 (m)
K = 2.0	East side	2.79 ± 0.033 aA	2.68 ± 0.027 aA	2.61 ± 0.032 aA	2.53 ± 0.034 aA
	South side	2.63 ± 0.028 bA	2.59 ± 0.025 bA	2.52 ± 0.046 aA	2.40 ± 0.026 bB
	West side	2.61 ± 0.012 bA	2.58 ± 0.033 bA	2.56 ± 0.038 aA	2.44 ± 0.026 bB
	North side	2.86 ± 0.017 aA	2.79 ± 0.029 aA	2.69 ± 0.019 bA	2.59 ± 0.017 aA
K = 1.5	East side	2.82 ± 0.025 aA	2.72 ± 0.025 aA	2.62 ± 0.016 aA	2.56 ± 0.022 aA
	South side	2.77 ± 0.013 aA	2.68 ± 0.018 aA	2.58 ± 0.027 bA	2.41 ± 0.025 bB
	West side	2.64 ± 0.012 bA	2.61 ± 0.013 aA	2.57 ± 0.034 bA	2.45 ± 0.033 bB
	North side	2.81 ± 0.017 aA	2.77 ± 0.028 aA	2.65 ± 0.018 aA	2.58 ± 0.028 aA
K = 1.0	East side	2.74 ± 0.024 aA	2.66 ± 0.028 aA	2.63 ± 0.028 aA	2.45 ± 0.017 aB
	South side	2.56 ± 0.033 bA	2.51 ± 0.015 bA	2.35 ± 0.033 bB	2.29 ± 0.024 bB
	West side	2.70 ± 0.016 aA	2.55 ± 0.024 bA	2.37 ± 0.036 bB	2.35 ± 0.017 bB
	North side	2.75 ± 0.014 aA	2.61 ± 0.026 aA	2.58 ± 0.018 aA	2.51 ± 0.019 aA
K = 0.5	East side	2.66 ± 0.025 aA	2.58 ± 0.035 aA	2.47 ± 0.034 aA	2.35 ± 0.034 aA
	South side	2.59 ± 0.009 bA	2.60 ± 0.026 aA	2.44 ± 0.013 aB	2.36 ± 0.014 aB
	West side	2.58 ± 0.024 bA	2.48 ± 0.027 bB	2.43 ± 0.024 aB	2.32 ± 0.017 aB
	North side	2.61 ± 0.015 bA	2.65 ± 0.016 aA	2.42 ± 0.017 aB	2.39 ± 0.025 aB

Note: Different lowercase letters in the same column indicate a significant difference at the 0.05 level among the heights of Korean pine trees with adjacent broadleaved trees at different orientations (positions) of east, south, west, and north within the same opening degree level. Different uppercase letters in the same row indicate a significant difference at the 0.05 level among the heights of Korean pine trees along with the height ranges of the adjacent broadleaved trees at the same orientations (positions) of the adjacent broadleaved trees relative to the target Korean pine trees.

Table 13. Height of Korean pine trees with adjacent broadleaved trees at different orientations along with the DBH range of the adjacent broadleaved trees around the target Korean pine trees in the Fangzheng site.

Opening Degree (K)	Adjacent Broadleaved Tree Positions Relative to the Target Korean Pine Trees	DBH Range of Adjacent Broadleaved Trees Arround the Target Korean Pine Trees
		11.0–13.5 (cm)	13.6–16.0 (cm)	16.1–18.5 (cm)	18.6–21.0 (cm)
K = 2.0	East side	2.80 ± 0.041 aA	2.77 ± 0.034 aA	2.69 ± 0.018 aA	2.56 ± 0.029 aB
	South side	2.79 ± 0.036 aA	2.60 ± 0.026 bB	2.52 ± 0.035 bB	2.48 ± 0.035 bB
	West side	2.66 ± 0.022 bA	2.58 ± 0.024 bA	2.48 ± 0.047 bB	2.41 ± 0.034 bB
	North side	2.84 ± 0.025 aA	2.76 ± 0.018 aA	2.66 ± 0.027 aA	2.57 ± 0.012 aA
K = 1.5	East side	2.81 ± 0.023 aA	2.75 ± 0.044 aA	2.70 ± 0.037 aA	2.62 ± 0.028 aB
	South side	2.76 ± 0.028 aA	2.71 ± 0.024 aA	2.67 ± 0.046 aA	2.46 ± 0.019 bB
	West side	2.66 ± 0.029 bA	2.61 ± 0.035 bA	2.50 ± 0.027 bB	2.47 ± 0.038 bB
	North side	2.79 ± 0.033 aA	2.71 ± 0.025 aA	2.65 ± 0.025 aA	2.61 ± 0.024 aA
K = 1.0	East side	2.81 ± 0.026 aA	2.77 ± 0.013 aA	2.62 ± 0.024 aB	2.52 ± 0.052 aB
	South side	2.75 ± 0.021 aA	2.69 ± 0.024 bA	2.51 ± 0.022 bB	2.44 ± 0.046 aB
	West side	2.64 ± 0.014 bA	2.58 ± 0.022 bA	2.41 ± 0.012 bB	2.33 ± 0.026 bB
	North side	2.78 ± 0.011 aA	2.71 ± 0.019 aA	2.64 ± 0.026 aA	2.49 ± 0.022 aB
K = 0.5	East side	2.66 ± 0.026 aA	2.54 ± 0.023 aA	2.44 ± 0.019 aA	2.36 ± 0.015 aA
	South side	2.51 ± 0.031 bA	2.42 ± 0.016 bB	2.37 ± 0.011 aB	2.31 ± 0.025 aB
	West side	2.53 ± 0.022 bA	2.42 ± 0.027 bB	2.40 ± 0.015 aB	2.33 ± 0.022 aB
	North side	2.62 ± 0.025 aA	2.54 ± 0.031 aA	2.43 ± 0.027 aA	2.39 ± 0.037 aA

Note: Different lowercase letters in the same column indicate a significant difference at the 0.05 level among the heights of Korean pine trees with adjacent broadleaved trees at different orientations (positions) of east, south, west, and north within the same opening degree level. Different uppercase letters in the same row indicate a significant differences at the 0.05 level among the heights of Korean pine trees along with the DBH ranges of the adjacent broadleaved trees at the same orientations (positions) of the adjacent broadleaved trees relative to the target Korean pine trees.

Table 14. Height of Korean pine trees with adjacent broadleaved trees at different orientations along with the height range of the adjacent broadleaved trees around the target Korean pine trees in the Fangzheng site.

Opening Degree (K)	Adjacent Broadleaved Tree Positions Relative to the Target Korean Pine Trees	Height Range of Adjacent Broadleaved Trees Arround the Target Korean Pine Trees
		10.5–11.5 (m)	11.6–12.5 (m)	12.6–13.5 (m)	13.6–14.5 (m)
K = 2.0	East side	2.81 ± 0.033 aA	2.76 ± 0.024 aA	2.66 ± 0.026 aA	2.54 ± 0.037 aA
	South side	2.80 ± 0.028 aA	2.71 ± 0.018 aA	2.58 ± 0.016 bB	2.44 ± 0.027 bB
	West side	2.69 ± 0.036 bA	2.61 ± 0.022 bA	2.46 ± 0.034 cB	2.39 ± 0.015 bB
	North side	2.85 ± 0.019 aA	2.79 ± 0.026 aA	2.68 ± 0.022 aA	2.57 ± 0.022 aA
K = 1.5	East side	2.79 ± 0.028 aA	2.71 ± 0.024 aA	2.66 ± 0.016 aA	2.51 ± 0.023 aB
	South side	2.77 ± 0.013 aA	2.69 ± 0.034 aA	2.60 ± 0.025 aA	2.41 ± 0.036 bB
	West side	2.61 ± 0.017 bA	2.56 ± 0.017 bA	2.45 ± 0.038 bB	2.39 ± 0.024 bB
	North side	2.79 ± 0.014 aA	2.72 ± 0.035 aA	2.65 ± 0.019 aA	2.59 ± 0.016 aA
K = 1.0	East side	2.79 ± 0.026 aA	2.79 ± 0.013 aA	2.65 ± 0.024 aB	2.55 ± 0.052 aB
	South side	2.72 ± 0.021 aA	2.64 ± 0.024 bA	2.51 ± 0.022 bB	2.48 ± 0.046 aB
	West side	2.65 ± 0.014 bA	2.60 ± 0.022 bA	2.44 ± 0.012 bB	2.34 ± 0.026 bB
	North side	2.76 ± 0.011 aA	2.74 ± 0.019 aA	2.66 ± 0.026 aA	2.48 ± 0.022 aB
K = 0.5	East side	2.65 ± 0.026 aA	2.55 ± 0.023 aA	2.45 ± 0.019 aA	2.34 ± 0.015 aA
	South side	2.53 ± 0.031 bA	2.45 ± 0.016 bB	2.41 ± 0.011 aB	2.29 ± 0.025 aB
	West side	2.52 ± 0.022 bA	2.41 ± 0.027 bB	2.39 ± 0.015 aB	2.30 ± 0.022 aB
	North side	2.61 ± 0.025 aA	2.57 ± 0.031 aA	2.43 ± 0.027 aB	2.35 ± 0.037 aB

Note: Different lowercase letters in the same column indicate a significant difference at the 0.05 level among the heights of Korean pine trees with adjacent broadleaved trees at different orientations (positions) of east, south, west, and north within the same opening degree level. Different uppercase letters in the same row indicate a significant difference at the 0.05 level among the heights of Korean pine trees along with the height ranges of the adjacent broadleaved trees at the same orientations (positions) of the adjacent broadleaved trees relative to the target Korean pine trees.

2.2.3. ANOVA and Significant Difference Test

All data were processed and analyzed using Excel 2013. We calculated the mean, standard error, etc., and analyzed the correlations with SPSS17.0.

3. Results

3.1. Effect of the Position of Adjacent Broadleaved Trees on Growth of Pinus koraiensis Trees under the Canopy

3.1.1. Effect of Position of Adjacent Broadleaved Trees on Diameter Growth of Pinus koraiensis Trees under the Canopy

Generally, the average DBH (diameter at breast height) of Pinus koraiensis trees under the canopy of the secondary forest in the three investigation sites was higher when the adjacent broadleaved trees were located to the north or east sides of the P. koraiensis trees than when the adjacent broadleaved trees were located to the south or west sides at all DBH and height ranges and all opening degree levels (p < 0.05); however, there was no significant difference among the four orientations of adjacent broadleaved trees for the DBH ranges of 18.6–21.0 cm and 16.1–18.5 cm and the height ranges of 12.6–13.5 m and 13.6–14.5 m of the adjacent broadleaved trees in the case of the opening degree K = 0.5 (p > 0.05) (Table 3, Table 4, Table 5, Table 6, Table 7 and Table 8).

In the case of the opening degree K = 2.0, the average DBH of P. koraiensis trees under the canopy of the secondary forest was higher where the adjacent broadleaved trees were located on the north or east sides of the P. koraiensis trees than when the adjacent broadleaved trees were located on the south or west sides in all DBH and height ranges in the Shanhetun and Maoershan site (Table 3, Table 4, Table 5, Table 6, Table 7 and Table 8); the same is true of the Fangzheng site (Table 3, Table 4, Table 5, Table 6 and Table 8), with just two exceptions: there was no significant difference among the DBHs when the adjacent broadleaved trees were located on the south, west, and north sides for the DBH range of 11.0–13.5 cm; or when the adjacent broadleaved trees were located on the east, south, and north sides for the DBH range of 13.6–16.0 cm (Table 7); however, even so, the DBH of Korean pine trees with north-side broadleaved trees was higher than that of Korean pine trees with south- or west-side broadleaved trees in the DBH range of 11.0–13.5 cm. The DBH of Korean pine trees with north- or east-side broadleaved trees was higher than that of Korean pine trees with south-side broadleaved trees in the DBH range of 13.6–16.0 cm; this still conformed to the above trend but did not reach a significant level.

In the case of the opening degree K = 1.5, the general trends for the average DBH of P. koraiensis trees in the three areas were similar to those for K = 2.0 (Table 3, Table 4, Table 5, Table 6, Table 7 and Table 8) but with more exceptions; there was no significant difference among the DBHs of Korean pine trees with adjacent broadleaved trees located on the east, west, and north sides in the DBH range of 11.0–13.5 cm in the Shanhetun site and 13.6–16.0 cm in the Maoershan site, the east, south, and west sides in the DBH range of 16.1–18.5 cm in the Maoershan site, and DBH ranges of 11.0–13.5 cm, 13.6–16.0 cm, and 16.1–18.5 cm in the Fangzheng site. There was also no significant difference among the DBHs with adjacent broadleaved trees located on the east, south, and north sides in the height range of 12.6–13.5 m in the Shanhetun site and 10.5–11.5 m and 11.6–12.5 m in the Fangzheng site; the east, south, and west sides in the height ranges of 12.6–13.5 m and 13.6–14.5 m in the Maoershan site; and 13.6–16.0 cm and 16.1–18.5 cm in the Fangzheng site. Similarly, at K = 2.0, the DBHs of the Korean pine trees with adjacent broadleaved trees located on the east and/or north sides were larger than those of the Korean pine trees with adjacent broadleaved trees located to the south or west, but this difference did not reach a significant level.

In the case of the opening degree K = 1.0, the general trends for the average DBH of P. koraiensis trees in the three areas were similar to those for K = 2.0 (Table 3, Table 4, Table 5, Table 6, Table 7 and Table 8), without any exception.

In the case of the opening degree K = 0.5, the average DBHs of P. koraiensis trees under the canopy of the secondary forest were higher with adjacent broadleaved trees located on the north or east sides of P. koraiensis trees than those with adjacent broadleaved trees located on the south or west sides in the first and second DBH and height ranges in the three areas, but there was no significant difference among the DBHs of Korean pine trees with adjacent broadleaved trees located on the four sides in the third and forth DBH and height ranges in the three areas (Table 3, Table 4, Table 5, Table 6, Table 7 and Table 8), with several exceptions; i.e., there was no significant difference among the DBHs of Korean pine trees with adjacent broadleaved trees located on the east, west, and north sides in the DBH range of 11.0–13.5 cm and on the south, west, and north sides in the height range of 10.5–11.5 m in the Fangzheng site; the DBHs of Korean pine trees with adjacent broadleaved trees located on the south side in the DBH range of 16.1–18.5 cm and the height range of 12.6–13.5 m in the Fangzheng site were significantly lower than those with broadleaved trees on other sides.

3.1.2. Effect of the Position of Adjacent Broadleaved Trees on Height Growth of Pinus koraiensis Trees under the Canopy

Generally, the average height of the P. koraiensis trees under the canopy of the secondary forest at the three investigation sites was higher when the adjacent broadleaved trees were located on the north or east sides of the P. koraiensis trees than when the adjacent broadleaved trees were located on the south or west sides in all DBH and height ranges, but there were many more exceptions compared with those for the average DBH of P. koraiensis trees. On the other hand, the results were similar in that there were no significant difference in height among the four orientations of adjacent broadleaved trees in the DBH ranges of 18.6–21.0 cm and 16.1–18.5 cm and height range of 12.6–13.5 m and 13.6–14.5 m of the adjacent broadleaved trees in the case of the opening degree K = 0.5 (Table 9, Table 10, Table 11, Table 12, Table 13 and Table 14).

In the case of the opening degree K = 2.0, the average height of P. koraiensis trees under the canopy of the secondary forest were higher when the adjacent broadleaved trees were located on the north or east sides of P. koraiensis trees than where the adjacent broadleaved trees were located on the south or west sides in all DBH and height ranges in the Shanhetun and Fangzheng sites (Table 9, Table 10, Table 13 and Table 14); the same was true in the Maoershan site (Table 11 and Table 12) only in the DBH range of 11.0–13.5 cm and the height rang of 10.5–11.5 m, with the exceptions of equal or lower heights for Korean trees with east-side broadleaved trees in the DBH range of 13.6–16.0 cm or the height range of 11.6–12.5 m; lower heights for Korean trees with north-side or east-side broadleaved trees in the DBH range of 16.1–18.5 cm or the height range of 12.6–13.5 m; and lower heights for Korean trees with east- and north-side broadleaved trees in the DBH range of 18.6–21.0 cm and the height range of 13.6–14.5, even though some of these differences did not reach a significant level.

In the case of the opening degree K = 1.5, the average heights of P. koraiensis trees under the canopy of secondary forest were higher when the adjacent broadleaved trees were located on the north or east side of P. koraiensis trees than when the adjacent broadleaved trees were located on the south or west side in all DBH and height ranges in the three areas (Table 9, Table 10, Table 11, Table 12, Table 13 and Table 14), with just two exceptions: equal heights were found for Korean trees with north-side broadleaved trees in the DBH range of 13.6–16.0 cm, and a lower height was found for Korean trees with north-side broadleaved trees in the DBH range of 16.1–18.5 cm. However, many of the height differences in the case of the opening degree K = 1.5 did not reach a significant level.

In the case of the opening degree K = 1.0, the general trends for the average height of P. koraiensis trees in the three areas were similar to those for K = 1.5 (Table 9, Table 10, Table 11, Table 12, Table 13 and Table 14), without any exceptions. Many of the height differences also did not reach a significant level.

In the case of the opening degree K = 0.5, the average height of P. koraiensis trees under the canopy of secondary forest were higher when the adjacent broadleaved trees were located on the north or east side of P. koraiensis trees than when the adjacent broadleaved trees were located on the south or west side in the first and second DBH and height ranges in the three areas, but no significant difference was found among the DBHs of Korean pine trees when the adjacent broadleaved trees were located on the four sides in the third and fourth DBH and height ranges in the three areas (Table 9, Table 10, Table 11, Table 12, Table 13 and Table 14), with just one exception: a low height was found for Korean pine trees when the adjacent broadleaved trees were located on the east side in the DBH range of 13.6–16.0 cm (Table 11).

3.2. Effect of the Size of Adjacent Broadleaved Trees on Growth of Pinus koraiensis Trees under the Canopy

3.2.1. Effect of the Size of Adjacent Broadleaved Trees on DBH Growth of Pinus koraiensis Trees under the Canopy

Generally, the average DBH of P. koraiensis trees under the canopy of the secondary forest in the three investigation sites decreased with the increase in the DBH and height ranges of the adjacent broadleaved trees when the broadleaved trees were located in all directions from P. koraiensis trees at all opening degree levels, and its growth difference reached a significant level when the broadleaved tree size was in the DBH range of 18.6–21.0 cm and the height range of 13.6–14.5 m for most directions of broadleaved trees from P. koraiensis trees, especially on the south or/and west side; others did not reach significant levels (Table 3, Table 4, Table 5, Table 6, Table 7 and Table 8).

In the case of the opening degree K = 2.0, the growth difference in the average DBH of P. koraiensis trees under the canopy began to reach a significant level when the broadleaved trees were located to the south of P. koraiensis trees in the DBH range of 16.1–18.5 cm and the height range of 12.6–13.5 m in the Maoershan and Fangzheng sites, but not in the Shanhetun site; whereas it reached a significant level when the broadleaved trees were located on the south and west sides of P. koraiensis trees in the DBH range of 18.6–21.0 cm and the height range of 13.6–14.5 m in the three areas, with the exception of there being no significant difference in the height range of 13.6–14.5 m in the Shenhetun site and the west side in the height range of 13.6–14.5 in the Maoershan site (Table 3, Table 4, Table 5, Table 6, Table 7 and Table 8). The growth difference in the average DBH of P. koraiensis trees under the canopy also reached a significant level where the broadleaved trees were located on the east side of P. koraiensis trees in the DBH range of 18.6–21.0 cm and the height range of 13.6–14.5 m in the Maoershansite (Table 5 and Table 6).

In the case of the opening degree K = 1.5, the growth difference in the average DBH of Korean pine trees under the canopy reached a significant level when the broadleaved trees were located on the south and west side of P. koraiensis trees in the DBH range of 18.6–21.0 cm and the height range of 13.6–14.5 m in the three areas, without exception (Table 3, Table 4, Table 5, Table 6, Table 7 and Table 8).

In the case of the opening degree K = 1.0, the growth difference in the average DBH of P. koraiensis trees under the canopy began to reach a significant level when the broadleaved trees were located on the south side of P. koraiensis trees in the DBH range of 16.1-18.5 cm and the height range of 12.6–13.5 m in the Fangzheng site and the DBH range of 16.1–18.5 cm in the Maoershan site, as well as on the west side of P. koraiensis trees in the height range of 12.6–13.5 m in the Maoershan site; whereas it reached a significant level when the broadleaved trees were located on the south and west side of P. koraiensis trees in the DBH range of 18.6–21.0 cm and the height range of 13.6–14.5 m in the three areas, without exception (Table 3, Table 4, Table 5, Table 6, Table 7 and Table 8). The growth difference in the average DBH of P. koraiensis trees under the canopy also reached a significant level when the broadleaved trees were located on the east side of P. koraiensis trees in the DBH range of 18.6–21.0 cm and the height range of 13.6–14.5 m in the Shanhetun site (Table 3 and Table 4).

In the case of the opening degree K = 0.5, the growth difference in the average DBH of P. koraiensis trees under the canopy began to reach a significant level when the broadleaved trees were located on the south and west side of P. koraiensis trees in the DBH range of 13.6–16.0 cm and the height range of 11.6–12.5 m in the Shenhetun and Maoershan sites but not in the Fangzheng site; it reached a significant level when the broadleaved trees were located on the south, west, and east sides of P. koraiensis trees in the DBH ranges of 16.1–18.5 cm and 18.6–21.0 cm and the height ranges of 12.6–13.5 m and 13.6–14.5 m in the Shenhetun site; in the four orientations from P. koraiensis trees in the DBH ranges of 16.1–18.5 cm and 18.6–21.0 cm and the height ranges of 12.6–13.5 m and 13.6–14.5 m in the Maoershan area (Table 3, Table 4, Table 5 and Table 6), but only where they were on the south side of P. koraiensis trees in the DBH range of 16.1–18.5 cm and the height range of 12.6–13.5 m; and on the south and west sides of P. koraiensis trees in the DBH range of 18.6–21.0 cm and the height range of 13.6–14.5 m in the Fangzheng site (Table 7 and Table 8).

3.2.2. Effect of the Size of Adjacent Broadleaved Trees on Height Growth of Pinus koraiensis Trees under the Canopy

Generally, the average height of P. koraiensis trees under the canopy of the secondary forest at the three investigating sites decreased with the increase in the DBH and height range of the adjacent broadleaved trees when the broadleaved trees were located in all directions from P. koraiensis trees at all opening degree levels, and its growth difference reached a significant level when the broadleaved tree size was in the DBH range of 18.6–21.0 cm and the height range of 13.6–14.5 m, with most directions of broadleaved trees to P. koraiensis trees, and when the broadleaved tree size was in the DBH range of 16.1–18.5 cm and the height range of 12.6–13.5 m, with a south and/or west direction from broadleaved trees to P. koraiensis trees, except for the Shanhetun site and the Maoershan site in some cases; others did not reach a significant level (Table 9, Table 10, Table 11, Table 12, Table 13 and Table 14).

In the case of the opening degree K = 2.0, the growth difference in the average height of P. koraiensis trees under the canopy began to reach a significant level when the broadleaved trees were located to the south of P. koraiensis trees in the DBH ranges of 13.6–16.0 cm and 16.1–18.5 cm in the Maoershan and Fangzheng sites and in the height range of 12.6–13.5 m in the Fangzheng site, and reached a significant level when the broadleaved trees were located on the south and west side of P. koraiensis trees in the DBH range of 18.6–21.0 cm and the height range of 13.6–14.5 m in the Maoershan and Fangzheng sites (Table 9, Table 10, Table 11, Table 12, Table 13 and Table 14), with the exception of there being no significant difference at these DBH and height ranges in the Shenhetun site, instead of the east and/or north side (Table 9 and Table 10).

In the case of the opening degree K = 1.5, the growth difference in the average height of Korean pine trees under the canopy reached a significant level when the broadleaved trees were located on the south and west side of P. koraiensis trees in the DBH range of 18.6–21.0 cm and the height range of 13.6–14.5 m in the three areas, with the exception of the west side in the height range of 13.6–14.5 m in the Shanhetun site, the east side in the DBH range of 18.6–21.0 cm in the Fangzheng site, and the north side in the DBH range of 18.6–21.0 cm in the Maoershan site; a significant level as also reached when the broadleaved trees were located on the south and/or west side of P. koraiensis trees in the DBH range of 16.1–18.5 cm and the height range of 12.6–13.5 m in the three areas (Table 9, Table 10, Table 11, Table 12, Table 13 and Table 14).

In the case of the opening degree K = 1.0, the growth difference in the average height of P. koraiensis trees under the canopy reached a significant level when the broadleaved trees were located on the south side of P. koraiensis trees in the Shenhetun site, the south and west sides of P. koraiensis trees in the Maoershan site, and the south, west, and east sides of P. koraiensis trees in the Fangzheng site in the DBH ranges of 16.1–18.5 cm and 18.6–21.0 cm and the height ranges of 12.6–13.5 m and 13.6–14.5 m (Table 9, Table 10, Table 11, Table 12, Table 13 and Table 14), as well as on the east side of P. koraiensis trees in the height range of 13.6–14.5 m in the Maoershan site (Table 11 and Table 12) and the north side of P. koraiensis trees in the DBH range of 18.6–21.0 cm and the height range of 13.6–14.5 m in the Fangzheng site (Table 13 and Table 14).

In the case of the opening degree K = 0.5, the growth difference in the average height of P. koraiensis trees under the canopy began to reach a significant level when the broadleaved trees were located on the west side in the Maoershan site and the south and west sides in the Fangzheng site in the DBH range of 13.6–16.0 cm and the height range of 11.6–12.5 m, and they reached a significant level when the broadleaved trees were located on the south, west, and east sides of P. koraiensis trees in the DBH ranges of 16.1–18.5 cm and 18.6–21.0 cm and the height ranges of 12.6–13.5 m and 13.6–14.5 m in the Shenhetun and Fangzheng sites (Table 9, Table 10, Table 11, Table 12, Table 13 and Table 14), but there was almost no significant level difference at the K = 0.5 level in the Maoershan site (Table 9 and Table 10), probably because all of the DBH and height ranges at the K = 0.5 opening degree level caused the greatest suppression of normal growth in all Korean pine trees.

4. Conclusions

To promote the growth of planted P. koraiensis trees and regulate the species relationships in a mixed forest of P. koraiensis trees and naturally regenerated broadleaved trees, a quantitative regulation approach based on an opening degree index has been developed and has proven effective and feasible [8,9], but for more precision and better operational utilization of the approach, the possible factors influencing its effects needed to be checked, and the position and size of neighborhood broadleaved trees were considered to be factors of this type and were checked in this research.

4.1. Effect of Adjacent Broadleaved Tree Position on Growth of Pinus koraiensis Trees under the Canopy

The growth of Korean pine trees increased along with the enlargement of the opening degree [8,9], and, if the tending cutting interval is about 5 years and the age of the Korean pine tree is younger than 15–20 years, then the best opening degree is K = 1.0 < 2.0 [8]; if the tending cutting interval is about 10 years and the age of the Korean pine tree is older than 15–20 years, then the best opening degree is K = 2.0 and higher [9]. The opening degree is actually a quantitative measurement of the micro-space as well as the light environment of the target trees and is similar to the forest gap to some extent, and there is also the discovery that the plant growth in the gap is increased with gap size enlargement [16,19,20,21,22,23,24,25]. Some studies have also referred to the effect of the within-gap position; for example, the regenerated seedlings of P. thunbergii were distributed not in the center of the thinning gaps but at the north, west, and east edges of the gaps [20], and the photosynthesis capacity of 35-year-old Korean pine trees increased from the edge area to the center area of the gaps [21,22], but another study showed that the maximum net photosynthetic rate of the needles of Korean pine trees growing in the center of the gap was higher than in the transition and edge area [24]. Young trees of different tree species would be distributed in different position within the gaps [23], and the seedling survival, growth, and biomass accumulation were influenced by the within-gap position; the net photosynthetic rates of the Manchurian walnut were higher in gap centers and transitions than those at gap edges [25]. Our object trees of concern are different to those in these studies; our concern was the relative position of the neighborhood broadleaved trees to the target Korean pine trees, not the target tree position on the gaps as in the above studies [20,21,22,23,24,25], but the issues explored are similar. Our study results show that the mean DBH and height of P. koraiensis trees under the canopy were higher when the adjacent broadleaved trees were located to the north or east sides of P. koraiensis trees than when the adjacent broadleaved trees were located in the south or west sides of P. koraiensis trees for almost all opening degree levels (different gap size) except for K = 0.5 (small gap size); i.e., when the opening degree were bigger (here, bigger than 1.0), the effect of the orientation of neighborhood broadleaved trees on the light needs of the target P. koraiensis trees was bigger and should be considered when regulating the micro-habitat of the Korean pine trees—in other words, removing the interference trees on the south or/and west side; however, when the opening degree was smaller to a certain extent (here, 0.5), the light entrance to the stands was small, and the effect of the orientation of the broadleaved trees on the light needs of P. koraiensis trees was no longer significant and need not being considered when regulating the micro-habitat of the Korean pine trees.

4.2. Effect of Adjacent Broadleaved Tree Size on Growth of Pinus koraiensis Trees under the Canopy

Plant size is one of the main factors that determine the plant–plant interaction and it may change in response to neighborhood plants of either the same species or another species. Among the 330 papers related to this issue, 46.1% referred to plant growth, including height, diameter, biomass, and productivity [26]. Large trees control a large proportion of habitat resources; thus, the smaller neighborhood trees are denied these contested resources, especially the light resources in a mixed forest [26,27,28]. Crown size was strongly influenced by tree bole size, and crown radius was strongly dependent on tree DBH [28]. A few studies have been found which consider the influence of neighborhood tree size on the growth of the target Korean pine trees; Chen Dong et al. determined that the crown area of the neighborhood trees had a bigger influence on the young target Korean pine trees, and Wang Shuli et al. indicated that the height of the neighborhood upper trees had a significant influence on the growth of the target Korean pine trees [12]. This can be understood through the formula of the opening degree; when the horizontal distance from P. koraiensis trees to broadleaved trees was constant, the height of the broadleaved trees was greater and the opening degree was smaller, showing that the ability of P. koraiensis trees to receive light was worse and that the adjacent broadleaved trees affected the good light environment in the forest. In addition, the height and DBH of adjacent broadleaved trees reflects the size of the crown; this also changed the entry of light into the forest to affect the growth of P. koraiensis trees. No former study referring to the influence of the height and DBH of adjacent broadleaved trees on the individual growth situation of Korean pine trees has been found to account for size related to the opening degree and orientation to the target trees. Our study on the effect of the position, height range, and DBH range of adjacent broadleaved trees on the growth of the height and DBH of P. koraiensis trees was conducted at different opening degrees in different plots and showed that the mean DBH and height of P. koraiensis trees under the canopy decreased significantly with the increase in the DBH and height of the adjacent broadleaved trees when the broadleaved trees were located on the south or west sides of the P. koraiensis trees, but not when the broadleaved trees were located on the north or east sides.

In conclusion, the position and size of the adjacent broadleaved trees affected the growth of P. koraiensis trees to a certain degree: (1) south- and/or west-side adjacent broadleaved trees had a greater impact than north- and east-side trees on the diameter and height growth of P. koraiensis trees under the canopy in secondary forests; (2) the size (marked by DBH and height) of the adjacent broadleaved trees had a negative impact on the diameter and height growth of P. koraiensis trees under the canopy in secondary forests but only reached a significant level when it reached a large range and/or a low opening degree level. Therefore, the effect of the orientation and size of broadleaved trees on the growth of P. koraiensis trees should be considered in the case of a large opening degree but does not need to be considered when the opening degree is too small.

In terms of management implications, during the actual production process in forestry for P. koraiensis trees planted under a canopy, we should not only consider the opening degree but also the impact of various factors on the growth of P. koraiensis trees. P. koraiensis trees are recommended to be planted under an adjacent broadleaved tree with a smaller DBH and a lower tree height, and the appropriate removal of broadleaved trees located to the south and west sides should be undertaken during thinning cutting to provide good light conditions for the growth of P. koraiensis trees.

This study is of great significance to the regeneration and reconstruction of P. koraiensis tree in secondary forests. More than 1.2 million ha of mixed forest composed of planted Korean pine trees and natural broadleaved trees has been established, but much more such secondary forest (14.3 million ha estimated based on the data from the ninth national forest resource inventory of China) needs to be changed into this type of mixed forest. Therefore, these research results offer a firm perspective for practical applications.