Influence of Canopy Environmental Characteristics on Regen-eration of Nine Tree Species in Broadleaved Korean Pine Forests

Abstract

This study aimed to investigate the impact of local canopy environmental characteristics on the regeneration of common tree species in the understory of broadleaved Korean pine forests, thus deepening the understanding of species coexistence and forest growth cycle mechanisms. This study focused on nine tree species found in the Liangshui National Nature Reserve in Heilongjiang Province, China. We stratified trees by height and simulated the LAI distribution of each class using Voronoi polygons. These layers were overlaid to generate an integrated LAI spatial map. All these procedures were integrated into the self-developed R package Broadleaf.Korean.pine.LAI, which was used to calculate individual-level canopy environment indicators, including average local LAI, local LAI standard deviation, canopy percent, vertical distribution tendency degree, local coniferous LAI, and local broadleaf LAI. These indicators were then compared with the average values of uniformly distributed understory sampling points. A principal component analysis (PCA) was conducted to reduce the dimensionality of the local canopy environmental characteristics for both the uniformly distributed points and regeneration habitats of each tree species, resulting in comprehensive canopy environmental characteristics. Wilcoxon rank-sum tests were applied to assess the significance of differences between the regeneration habitats and the understory average, as well as between the regeneration habitats of seedlings and saplings within the same species. Cliff’s delta effect size was used to evaluate the impact of each environmental factor on the transition of regeneration from seedlings to saplings. The results showed that, based on both individual canopy environmental indicators and composite indices derived from principal component analysis, seedlings tended to regenerate in areas with higher canopy coverage, whereas saplings were more commonly established in relatively open habitats. Clear differences exist between the regeneration habitats of coniferous and broadleaf species, with coniferous species tending to regenerate in areas with higher local broadleaf LAIs compared with broadleaf species. The effect size analysis showed that canopy percent, vertical distribution tendency degree, average local LAI, and local coniferous LAI have greater impacts on the transition from seedlings to saplings, while the effect of local broadleaf LAI is relatively small. These findings suggest that strong shade tolerance allows species to establish seedling banks under canopy patches, while interspecific differences in growth response to microhabitats shape their roles in the forest growth cycle. Future research should explore the physiological responses and trait characteristics of tree regeneration under varying canopy patch environments. Long-term monitoring of regeneration processes—including invasion, growth, and mortality—across different canopy patches will help elucidate the mechanisms shaping understory spatial patterns.

1. Introduction

Patch dynamics theory is one of the most influential concepts in vegetation ecology, suggesting that small-scale disturbances in dominant vegetation create gaps, leading to the formation of plant community mosaics [1,2]. In climax-stage forest ecosystems, canopy gap disturbances are common small-scale endogenous disturbances closely linked to the self-maintenance of the ecosystem and the formation of the forest structure [3]. The formation of canopy gaps within the stands and subsequent regeneration tend toward a quasi-equilibrium landscape at the successional stage [4]. Due to the randomness of canopy gap disturbances, the development stages of gaps at specific locations within a forest stand are unpredictable, but the overall successional direction is predictable [5]. Furthermore, in climax forests, the number of patches at different stages of canopy gap development remains relatively constant, contributing to the dynamic mosaic steady state of these ecosystems [6,7].

In specific forest ecosystems, canopy gaps and patches at different developmental stages display distinct canopy characteristics, such as height, closure, and plant species composition. Therefore, the availability of resources, such as light, temperature, moisture, and soil nutrients, varies under different patches and patch interspersion zones, providing diverse resource niches for the regeneration of different tree species [8,9]. In temperate broadleaf–conifer mixed forests, light availability is higher during the early stages of canopy gap formation; temperatures are warmer, with a greater daily range; air humidity is lower; and soil moisture is higher. Increases in temperature and radiation promote the mineralization of litter and humus, making the local soil more fertile [10]. During the early stages of canopy gap development, light availability and soil moisture under broadleaf and mixed conifer–broadleaf patches are higher than those under conifer canopy patches in later stages [9,11]. Due to the directional nature of solar radiation, microhabitat characteristics in various areas within forest gaps and under adjacent forest canopies in different directions exhibit high spatial heterogeneity [11,12,13]. Environmental factors influence the growth and survival of the regeneration population and the formation of local regeneration communities, ultimately driving divergences in taxonomic composition and functional traits across distinct regeneration communities [14,15]. The regeneration of different tree species shows specific responses to local canopy environments. Some researchers suggest that tree species can be categorized into pioneer and climax groups based on their responses to canopy gaps [16,17]. Species in the pioneer group require gap environments for regeneration and growth, while climax species can establish seedling banks and grow under closed canopies. Other researchers, however, argue that tree regeneration does not depend on specific gaps or canopy patches [18]. Instead, growth rate differences among species under high light availability determine their role in the forest cycle, and these differences are influenced by trade-offs in plant traits [19]. These differences may arise from variations in the studied ecosystems and the analyzed gap areas. Regardless of the climax forest ecosystem type, such differences result from long-term natural selection and coevolution among multiple plant species [10]. The dynamic mechanisms of canopy patches in these systems underpin the coexistence of multiple tree species in forest ecosystems [3]. The dominant tree species in the local canopy primarily influence the canopy characteristics, which in turn affect understory resource availability and the structure of the regeneration community. Therefore, local patches in forest ecosystems can be seen as self-organizing systems [20]. During the construction of local regeneration communities in these patches, deterministic processes, such as environmental filtering by the canopy, play a key role. Analyzing how the regeneration of different tree species responds to the local canopy characteristics of their habitats provides a deeper understanding of forest ecosystem maintenance and community assembly mechanisms.

Broadleaved Korean pine forests are the dominant vegetation in the montane regions of Northeast Asia, characterized by patchy mosaics and an unevenly aged, multi-layered structure [10,21]. The formation of this structure results from the dynamics of canopy gaps. While some research has been conducted on the response of understory regeneration in broadleaved Korean pine forests to local canopy environments [22,23,24,25,26], most studies focus on binary canopy gap classifications, coniferous–broadleaf canopy patches, or large sampling windows. However, the process from gap formation to full infill is continuous, and the proportion of coniferous and broadleaf canopies in the local canopy is also continuous. Furthermore, following the establishment of regeneration individuals, their subsequent growth, development, and survival probability are influenced by fine-scale environmental filtering and intra- and interspecific interactions, which fall under the category of local processes [27,28]. These local processes are closely associated with the local canopy environment. However, quantitative descriptions of local canopy environmental characteristics remain insufficient in current research. For the broadleaved Korean pine forest ecosystem, we developed the R package Broadleaf.Korean.pine.LAI, which extracts detailed canopy characteristics. Based on this package, this study determined the local canopy environment for the regeneration seedlings and saplings of the nine most abundant tree species in the broadleaved Korean pine forest: Pinus koraiensis Siebold & Zucc., Picea koraiensis Nakai., Abies nephrolepis (Trautv. ex Maxim.) Maxim., Ulmus davidiana Planch., Acer mono Maxim., Acer tegmentosum Maxim., Acer ukurunduense Trautv. & C.A.Mey., Tilia amurensis Rupr. & Maxim., and Fraxinus mandshurica Rupr. This study aimed to analyze how the regeneration and growth of these species respond to the local canopy environment, thereby enhancing the understanding of species coexistence and gap dynamic mechanisms in broadleaved Korean pine forests. For this purpose, we propose the following scientific questions:

Question 1: What are the canopy environmental characteristics of regeneration habitats for seedlings and saplings of different tree species?

Question 2: Can and how do canopy structural characteristics influence the spatial distribution of regeneration seedlings and saplings?

Question 3: Can and how do canopy environmental characteristics influence the developmental transitions of regeneration stages?

2. Materials and Methods

2.1. Data Sets

This study was conducted in Liangshui National Nature Reserve, located in Heilongjiang Province, China (47°10′50″ N, 128°53′20″ E), on the southern slope of the Xiaoxing’an Mountains, where it serves as the core and most representative area of broadleaved Korean pine forest distribution [10]. The climate is a typical temperate continental monsoon climate, characterized by cold, dry winters and warm, rainy summers. The average annual temperature is −0.3 °C, the average annual maximum temperature is 7.5 °C, the average annual minimum temperature is −6.6 °C, and the total annual precipitation is approximately 676 mm, with the main precipitation period occurring from June to August. The snow cover period in the study area lasts approximately 130 to 150 days per year, with a frost-free period of about 100 to 120 days. The effective accumulated temperature ranges from 2200 °C to 2600 °C (Figure 1).

Four 50 m × 50 m plots were established along parallel contour lines on the middle and lower slopes of the broadleaved Korean pine forest in the core area of Liangshui Nature Reserve (with a closure error of less than 1/200). The middle and lower slopes were selected because they offer better water availability and higher tree species richness compared with the upper slopes, making them ideal sites for investigating tree coexistence mechanisms [10]. The four plots range in elevation from 340 m to 400 m, with slopes of 11° to 15°. The stand volume in the plots ranges from 261 m³/hm² to 344 m³/hm², with the dominant species, Pinus koraiensis, accounting for 50% to 70% of the total volume. Other codominant species include Abies nephrolepis, Picea koraiensis, Tilia amurensis, and Fraxinus mandshurica. The plot survey used the adjacent grid method [29], dividing each plot into one hundred 5 m × 5 m survey units. Trees with a diameter at breast height (DBH) of 5 cm or more were individually measured, with DBH measured using a caliper and tree height determined using a Vertex IV height meter (Haglöf Sweden AB, Långsele, Sweden, accuracy of 0.1 m). Referring to the classification of regeneration layers in China’s industry standard (LY: 1572-2000), trees with a height of 0.1 m ≤ h < 1.5 m in the broadleaved Korean pine forest were defined as regeneration seedlings [29], while trees taller than seedlings but with a DBH below the measurement threshold were defined as saplings [30]. Each regenerating tree was individually measured, and its species, height, and relative position within the small survey unit were recorded to later calculate its coordinates.

2.2. Regeneration Site Canopy Environment Measurement

The local canopy environmental characteristics of the regeneration habitat for seedlings and saplings were determined using the self-developed R package Broadleaf.Korean.pine.LAI, [31], which was developed with R version 3.6.3 and has been open-sourced on the platform GitHub. (The installation method for the R package Broadleaf.Korean.pine.LAI is as follows: library (devtools); install_github (“DuXinChina/Broadleaf.Korean.pine.LAI/Broadleaf.Korean.pine.LAI”). For package usage instructions, please refer to https://github.com/DuXinChina/Broadleaf.Korean.pine.LAI/tree/main/handbook. (accessed on 24 April 2025)) The basic steps for determining the local canopy environmental characteristics with this package are as follows: First, the trees in the survey plots were categorized into different height classes based on their height. This study divided trees with a DBH of 5 cm or more into five height classes, from shortest to tallest. The height classes are as follows: Class I, H < 10 m; Class II, 10 m ≤ H < 15 m; Class III, 15 m ≤ H < 20 m; Class IV, 20 m ≤ H < 25 m; and Class V, H ≥ 25 m. Next, in each height class, spatial sampling points were uniformly distributed using convex polygons from a Voronoi diagram. Regular hexagons were used to ensure that the radius of the circumscribed circle approximated half the average crown width of the trees in that class. In each height class, the nearest sampling point to each tree from the generated uniform sampling points was identified and removed. The cleaned uniform sampling points were then combined with the spatial coordinates of the trees to construct a tree distribution Voronoi diagram. This process ensures that the convex polygon surrounding each tree in that height class approximates its actual crown area. Based on the forest inventory data from the four plots, the average crown widths of trees in Classes I, II, III, IV, and V were 2.42 m, 3.78 m, 4.62 m, 5.90 m, and 6.50 m, respectively. Therefore, when constructing the canopy spatial distribution map for each height class, the radius of the circumscribed circle of the regular hexagons was set to 1.21 m, 1.89 m, 2.31 m, 2.95 m, and 3.25 m, respectively. Subsequently, based on the DBH, the leaf biomass of each tree was calculated using the leaf biomass equation. The total leaf area of each tree was then determined based on the specific leaf area of its species. Using the previously calculated area of the convex polygon occupied by the tree and the ratio of the tree’s total leaf area to this area, the LAI beneath each tree crown was determined. For the calculation method of LAI under tree crowns and the validation of its accuracy, refer to the relevant studies by Du et al. [31]. This enabled the creation of spatial distribution maps of LAIs for each height class. Finally, overlaying the LAI spatial distribution maps of different height classes generated a comprehensive LAI spatial distribution map for the entire forest stand. Based on this, the average LAI, standard deviation, local coniferous LAI, local broadleaf LAI, and vertical distribution tendency degree of LAI within a circular area of any given radius centered on any point in the stand can be calculated. Among these indices, the vertical distribution tendency degree quantifies the vertical distribution characteristics of the LAI within the local area. The formula for calculating this index is as follows:

$$V={\displaystyle \frac{{\sum }_{i=1}^n(n\times {LAI}_i)}{{\sum }_{i=1}^n\left(i\right)/n\times {\sum }_{i=1}^n{LAI}_i}}={\displaystyle \frac{{\sum }_{i=1}^n(n\times {LAI}_i)}{(n+1)/2\times {\sum }_{i=1}^n{LAI}_i}}$$

(1)

In this formula, n represents the total number of height classes from the lower to the upper layers in the survey plot, with the topmost layer of the canopy being assigned to height class n, and LAIi represents the LAI for the ith height class within the extracted local spatial range. When V > 1, the upper canopy contributes more to the LAI, while when V < 1, the lower canopy contributes more to the LAI.

To calculate the local canopy environmental characteristics of regeneration habitats for seedlings and saplings of different tree species, a 5 m inward buffer zone was set up for each plot to avoid edge effects. Seedlings and saplings within this buffer zone were excluded [32]. After excluding the boundary effect, the sample sizes for each tree species’ regeneration were as follows: for seedlings and saplings, Pinus koraiensis (129 and 54), Picea koraiensis (23 and 48), Abies nephrolepis (636 and 96), Ulmus davidiana (110 and 63), Acer mono (948 and 423), Acer tegmentosum (724 and 123), Acer ukurunduense (95 and 78), Tilia amurensis (105 and 60), and Fraxinus mandshurica (461 and 9). The circle radius used to calculate local canopy environmental characteristics was set to 2.792 m, representing the difference between the average radius of expanded gaps and the radius of canopy gaps [33]. This value was selected to define regeneration planting locations in open-canopy gaps, canopy gap interspersed zones, and closed-canopy forests. Furthermore, after excluding edge effects, the sampling points were uniformly distributed at 2 m intervals across the four broadleaved Korean pine forest plots, forming a grid of 21 rows × 21 columns. A total of 1764 sampling points were established across the four plots. The local canopy environmental characteristics at these uniformly distributed points were calculated using the same method as for the regeneration seedlings and saplings. This provided an average baseline for the broadleaved Korean pine forest understory, facilitating comparisons with the regeneration of each tree species.

After Z-score standardization of the canopy environmental characteristics at the uniformly distributed sampling points in the understory, a principal component analysis (PCA) was conducted to derive the formulas for the first two principal components representing the comprehensive canopy environmental characteristics. The local canopy environmental characteristics of regeneration habitats for seedlings and saplings of each tree species were standardized according to the same rules applied to the understory sampling points. The comprehensive canopy environmental characteristics of the regeneration habitats were then calculated based on the principal component formulas. After calculating the local LAI, canopy percent, vertical distribution tendency degree, local coniferous LAI, and local broadleaf LAI and comprehensive canopy environmental characteristics for each regeneration point and uniformly distributed understory point, we further computed the mean and standard deviation of each variable. These calculations provided a basis for addressing question 1—what are the canopy environmental characteristics of regeneration habitats for seedlings and saplings of different tree species?

2.3. Canopy Environmental Effects on Regeneration Spatial Distribution

To address question 2—whether and how canopy environmental characteristics influence the spatial distribution of regeneration seedlings and saplings—the Wilcoxon rank-sum test was performed for each tree species’ regeneration habitats and the uniformly distributed understory sampling points to compare the canopy environmental characteristics of the regeneration habitats with the average canopy environmental level in the understory. Furthermore, the ratio of average ranks for the canopy environmental characteristics in the regeneration habitats of each tree species and the uniformly distributed understory sampling points were calculated to compare their relative magnitudes [34].

2.4. Canopy Environmental Effects on Seedling-to-Sapling Transition

To address question 3—whether and how canopy environmental characteristics influence the developmental transitions of regeneration stages—the canopy environmental characteristics of regeneration habitats for seedlings and saplings of the same tree species were analyzed using the Wilcoxon rank-sum test to assess the significance of differences between developmental stages [35]. Furthermore, Cliff’s delta effect size was calculated separately for each local canopy characteristic indicator and the principal component of canopy environmental characteristics in the regeneration habitats of seedlings and saplings for each tree species. Based on Mangiafico’s classification criteria, the effect sizes were categorized as follows [36]: 0 ≤ |Cliff’s delta| < 0.11 for no significant effect; 0.11 ≤ |Cliff’s delta| < 0.28 for a small effect size; 0.28 ≤ |Cliff’s delta| < 0.43 for a medium effect size; and |Cliff’s delta| ≥ 0.43 for a large effect size.

3. Results

3.1. Canopy Environmental Factors’ Influence on Regeneration

The LAI across all plots exhibits strong spatial heterogeneity, with values ranging from 0 to 100 at different locations, demonstrating clear spatial clustering. The stand-level averages of local canopy characteristics, based on uniformly distributed understory sampling, were quantified as follows: the average local LAI was 9.176 ± 9.072, the standard deviation of the local LAI was 9.176 ± 9.072, the canopy percent was 0.477 ± 0.276, the vertical distribution tendency degree was 1.024 ± 0.361, the local coniferous LAI was 5.565 ± 8.310, and the local broadleaved LAI was 4.152 ± 4.492. After excluding edge zone samples, the regeneration of seedlings and saplings for each tree species was primarily found under canopy patches with high LAIs, in canopy gap interspersed zones, or within small gaps, while regeneration density was lower in the central areas of larger gaps. Overall, the regeneration of seedlings and saplings tended to cluster in areas with high LAIs (Figure 2).

The local canopy structure characteristic indicators for seedling and sapling regeneration habitats of each tree species show that the average local LAI for seedling habitats ranges from 9.153 to 15.857, while for sapling habitats, it ranges from 4.150 to 11.869. Except for Picea koraiensis, the average local LAI for sapling habitats is lower than that for the corresponding seedling habitats in all other species. The local LAI differences between seedling and sapling habitats for Abies nephrolepis, Ulmus davidiana, Acer mono, Acer tegmentosum, Tilia amurensis, and Fraxinus mandshurica are statistically significant (p < 0.05). The local LAI standard deviation for seedling habitats ranges from 9.240 to 12.839, while for saplings, it ranges from 6.665 to 10.728. Except for Acer ukurunduense, the local LAI standard deviation for seedling habitats is higher than that for sapling habitats in all other species. Among all tree species, significant differences in the local LAI standard deviation between seedlings and saplings (p < 0.05) are observed for Abies nephrolepis, Acer mono, Acer tegmentosum, Tilia amurensis, and Fraxinus mandshurica. The canopy percent in seedling habitats ranges from 0.505 to 0.636, while for sapling habitats, it ranges from 0.319 to 0.585. Only in Picea koraiensis does the canopy percent in sapling habitats exceed that in seedling habitats; for all other species, sapling habitats have lower canopy percents. Among these species, significant differences in the canopy percent between seedling and sapling habitats (p < 0.05) are observed in Ulmus davidiana, Acer mono, Acer tegmentosum, Acer ukurunduense, Tilia amurensis, and Fraxinus mandshurica. The vertical distribution tendency degree of seedling habitats for the nine tree species ranges from 0.954 to 1.194, with only Picea koraiensis having a value below 1, while that for sapling habitats ranges from 0.882 to 1.086, and except for Picea koraiensis, all other species have a lower vertical distribution tendency degree in sapling habitats compared with their seedling habitats. Significant differences in the vertical distribution tendency degree between seedling and sapling planting habitats were observed for Pinus koraiensis, Ulmus davidiana, Acer mono, Acer tegmentosum, and Tilia amurensis (p < 0.05). The local coniferous LAI for seedling planting habitats ranges from 3.253 to 11.664, while for sapling habitats, it ranges from 0.904 to 8.370. For Ulmus davidiana, Acer mono, Acer tegmentosum, Tilia amurensis, and Fraxinus mandshurica, the local coniferous LAI of sapling habitats is significantly lower than that of their corresponding seedling habitats (p < 0.05). The local broadleaf LAI for seedling and sapling planting habitats ranges from 2.791 to 6.739 and from 2.903 to 7.560, respectively. Only for Acer ukurunduense does the local broadleaf LAI between seedling and sapling planting habitats show significant differences (p < 0.05) (

Table 1. Canopy environmental factors of the colonization habitats of seedlings and saplings under broadleaved Korean pine forests.

Species	Average Local LAI		Local LAI Standard Deviation		Canopy Percent
	Seeding	Sapling	Seeding	Sapling	Seeding	Sapling
Pinus koraiensis	9.991 ± 7.333	8.464 ± 4.868	9.634 ± 4.226	9.600 ± 4.115	0.541 ± 0.247	0.492 ± 0.208
Picea koraiensis	10.897 ± 7.668	11.869 ± 8.832	10.740 ± 4.590	10.489 ± 4.26	0.570 ± 0.208	0.585 ± 0.250
Abies nephrolepis	10.344 ± 7.432	9.018 ± 7.933	10.396 ± 4.821	9.192 ± 4.361	0.505 ± 0.260	0.476 ± 0.255
Ulmus davidiana	10.300 ± 8.825	7.175 ± 6.040	9.266 ± 5.17 6	7.840 ± 4.530	0.572 ± 0.243	0.442 ± 0.255
Acer mono	13.574 ± 10.604	11.501 ± 9.965	11.900 ± 6.674	10.728 ± 6.537	0.605 ± 0.239	0.520 ± 0.265
Acer tegmentosum	12.598 ± 10.011	8.572 ± 7.823	11.367 ± 6.092	9.267 ± 4.852	0.558 ± 0.259	0.432 ± 0.254
Acer ukurunduense	9.153 ± 7.517	8.737 ± 7.039	9.240 ± 6.398	10.086 ± 5.599	0.556 ± 0.269	0.46 ± 0.256
Tilia amurensis	15.857 ± 11.677	11.067 ± 9.214	12.839 ± 7.166	9.733 ± 4.531	0.636 ± 0.261	0.544 ± 0.265
Fraxinus mandshurica	12.999 ± 11.296	4.150 ± 2.426	11.285 ± 6.259	6.665 ± 2.218	0.561 ± 0.243	0.319 ± 0.172
Species	Vertical Distribution Tendency Degree		Local Coniferous LAI		Local Broadleaved LAI
	Seeding	Sapling	Seeding	Sapling	Seeding	Sapling
Pinus koraiensis	1.011 ± 0.302	0.882 ± 0.254	3.253 ± 5.827	0.904 ± 1.617	6.739 ± 5.540	7.560 ± 4.807
Picea koraiensis	0.954 ± 0.372	1.033 ± 0.376	5.717 ± 7.657	6.940 ± 7.479	5.180 ± 4.279	4.929 ± 4.127
Abies nephrolepis	1.071 ± 0.341	1.002 ± 0.341	4.967 ± 6.883	4.069 ± 7.113	5.377 ± 5.229	4.949 ± 5.017
Ulmus davidiana	1.040 ± 0.343	0.885 ± 0.348	5.676 ± 8.651	3.648 ± 5.654	4.624 ± 4.140	3.527 ± 2.735
Acer mono	1.194 ± 0.345	1.086 ± 0.401	10.031 ± 10.463	8.370 ± 9.919	3.543 ± 4.236	3.131 ± 3.449
Acer tegmentosum	1.099 ± 0.353	0.992 ± 0.372	7.631 ± 9.796	4.171 ± 7.023	4.967 ± 4.687	4.401 ± 4.568
Acer ukurunduense	1.017 ± 0.346	0.956 ± 0.348	6.362 ± 7.441	4.943 ± 6.302	2.791 ± 3.240	3.793 ± 4.065
Tilia amurensis	1.187 ± 0.311	0.953 ± 0.344	11.664 ± 10.084	6.426 ± 8.873	4.194 ± 4.666	4.641 ± 3.828
Fraxinus mandshurica	1.090 ± 0.370	0.893 ± 0.364	7.488 ± 9.122	1.247 ± 1.882	5.511 ± 6.555	2.903 ± 1.872

Note: The bold text indicates significant differences between the canopy environmental factors of the same tree species’ seedling and sapling colonization habitats (p < 0.05).

).

Differences in the various canopy environmental characteristics between the seedling and sapling planting habitats of the nine tree species and the uniformly distributed understory sampling points are evident. For Pinus koraiensis, in the seedling planting habitat, the canopy percent and local broadleaved LAI are significantly higher than the understory average, while the local coniferous LAI is significantly lower; in the sapling planting habitat, the vertical distribution tendency degree and local coniferous LAI are significantly lower than the understory average, while the local broadleaved LAI is significantly higher (p < 0.05). For Picea koraiensis, in the seedling planting habitat, the local LAI standard deviation, canopy percent, and local broadleaved LAI are significantly higher than the understory average; in the sapling planting habitat, the average, local LAI standard deviation, canopy percent, and both coniferous and broadleaved LAI are all significantly higher than the understory average (p < 0.05). The average local LAI, local LAI standard deviation, vertical distribution tendency degree, and local broadleaved LAI in the seedling planting habitat of Abies nephrolepis are significantly higher than the understory average (p < 0.05); however, none of the local canopy environmental characteristics in its sapling planting habitat show significant differences from the understory average. Among the six broadleaf tree species, Acer mono, Acer tegmentosum, Tilia amurensis, and Fraxinus mandshurica exhibit similarities in their local canopy environment for seedling planting habitats. Their average local LAI, local LAI standard deviation, canopy percent, vertical distribution tendency degree, and local coniferous LAI are all significantly higher than the understory average (p < 0.05). The local broadleaved LAI shows some differences: for Acer ukurunduense, it is significantly lower than the understory average, while for Acer tegmentosum and Fraxinus mandshurica, it shows the opposite trend (p < 0.05). The local canopy environmental characteristics of Acer ukurunduense seedling planting habitats differ from those of other broadleaf species. Its canopy percent and local coniferous LAI are significantly higher than the understory average, while its local broadleaved LAI is significantly lower than the understory average (p < 0.05). Compared with seedling planting habitats, the local canopy environmental characteristics of sapling planting habitats for broadleaf species are significantly different from the understory average. For instance, the average local LAI and canopy percent of Acer tegmentosum and Fraxinus mandshurica sapling planting habitats shifted from being significantly higher than the understory average to being significantly lower (p < 0.05) (

Table 2. Significance of differences in the canopy environmental factors between the colonization habitats of seedlings and saplings and the uniformly distributed sample points.

Species	Average Local LAI				Local LAI Standard Deviation
	Seeding		Sapling		Seeding		Sapling
	Ratio of Average Rank	p-Value	Ratio of Average Rank	p-Value	Ratio of Average Rank	p-Value	Ratio of Average Rank	p-Value
Pinus koraiensis	1.077	0.075	1.003	0.486	1.050	0.174	1.064	0.214
Picea koraiensis	1.123	0.103	1.161	0.031 *	1.166	0.044 *	1.155	0.037 *
Abies nephrolepis	1.082	0.003 **	0.958	0.247	1.128	<0.001 ***	1.005	0.469
Ulmus davidiana	1.046	0.192	0.828	0.012 *	0.969	0.272	0.811	0.006 **
Acer mono	1.258	<0.001 ***	1.077	0.011 *	1.215	<0.001 ***	1.085	0.006 **
Acer tegmentosum	1.183	<0.001 ***	0.909	0.047 *	1.185	<0.001 ***	0.990	0.425
Acer ukurunduense	0.975	0.341	0.942	0.196	0.915	0.080	1.053	0.220
Tilia amurensis	1.321	<0.001 ***	1.080	0.156	1.264	<0.001 ***	1.046	0.281
Fraxinus mandshurica	1.182	<0.001 ***	0.574	0.013 *	1.167	<0.001 ***	0.699	0.059
Species	Canopy Percent				Vertical Distribution Tendency Degree
	Seeding		Sapling		Seeding		Sapling
	Ratio of Average Rank	p-Value	Ratio of Average Rank	p-Value	Ratio of Average Rank	p-Value	Ratio of Average Rank	p-Value
Pinus koraiensis	1.104	0.027 *	1.004	0.480	0.970	0.284	0.772	0.002 **
Picea koraiensis	1.174	0.036 *	1.200	0.010 *	0.886	0.118	1.018	0.417
Abies nephrolepis	1.028	0.167	0.960	0.259	1.077	0.004 **	0.970	0.313
Ulmus davidiana	1.170	0.001 **	0.889	0.073	1.015	0.388	0.771	0.001 **
Acer mono	1.276	<0.001 ***	1.059	0.041 *	1.312	<0.001 ***	1.103	0.001 **
Acer tegmentosum	1.150	<0.001 ***	0.866	0.006 **	1.125	<0.001 ***	0.941	0.139
Acer ukurunduense	1.138	0.013 *	0.930	0.150	0.982	0.381	0.895	0.060
Tilia amurensis	1.316	<0.001 ***	1.104	0.095	1.271	<0.001 ***	0.879	0.062
Fraxinus mandshurica	1.154	<0.001 ***	0.620	0.024 *	1.110	<0.001 ***	0.815	0.169
Species	Local Coniferous LAI				Local Broadleaved LAI
	Seeding		Sapling		Seeding		Sapling
	Ratio of Average Rank	p-Value	Ratio of Average Rank	p-Value	Ratio of Average Rank	p-Value	Ratio of Average Rank	p-Value
Pinus koraiensis	0.830	<0.001 ***	0.660	<0.001 ***	1.301	<0.001 ***	1.442	<0.001 ***
Picea koraiensis	1.090	0.173	1.160	0.030 *	1.210	0.015 *	1.162	0.030 *
Abies nephrolepis	0.992	0.390	0.912	0.071	1.111	<0.001 ***	1.071	0.124
Ulmus davidiana	1.035	0.251	0.826	0.010 *	1.099	0.030 *	1.000	0.500
Acer mono	1.364	<0.001 ***	1.158	<0.001 ***	0.880	<0.001 ***	0.865	<0.001 ***
Acer tegmentosum	1.129	<0.001 ***	0.912	0.050	1.100	<0.001 ***	1.001	0.490
Acer ukurunduense	1.106	0.039 *	1.000	0.499	0.780	<0.001 ***	0.960	0.277
Tilia amurensis	1.406	<0.001 ***	1.015	0.423	1.001	0.493	1.121	0.063
Fraxinus mandshurica	1.141	<0.001 ***	0.688	0.051	1.095	0.002 **	0.937	0.372

Note: The ratio of the average rank is the ratio between the average rank of the canopy structure characteristic index at the colonization positions of seedlings or saplings and the average rank of the same canopy structure characteristic index of the sample points evenly distributed under the forest. A ratio greater than 1 indicates that the corresponding canopy structure characteristic index at the colonization positions of seedlings or saplings is greater than that of the same canopy structure characteristic index of the sample points evenly distributed under the forest and vice versa. * p < 0.05, ** p <0.01, *** p <0.001; one-tailed test.

).

The calculation of Cliff’s delta effect size for various local canopy environmental indicators revealed that, except for Picea koraiensis, which shows some anomalies, the impact of the canopy environment on the transformation of seedling regeneration to sapling stages is relatively consistent across tree species. Higher vertical distribution tendency degrees, canopy percents, local coniferous and broadleaf LAIs, and LAIs tend to suppress the transformation of seedlings to saplings in most tree species. Additionally, it is worth noting that higher local broadleaf LAIs can promote growth in some species or exhibit a non-significant effect on seedling growth. The impact of various local canopy environmental characteristic indicators on the seedling-to-sapling transformation also differs. After excluding the highly variable Picea koraiensis, the rank sum of the effect of various canopy environmental factors on the transformation of seedlings to saplings was calculated, resulting in the following order of influence: canopy percent > vertical distribution tendency degree > average local LAI > local coniferous LAI > local LAI standard deviation > local broadleaved LAI. The influence of local canopy environmental indicators on the transformation of seedlings to saplings for different tree species also demonstrated strong specificity, with notable differences in the effect size rankings and orders for each species (Figure 3).

3.2. Composite Canopy Features’ Influence on Regeneration

After performing Z-score standardization on the local LAI standard deviation (LAI_sd), canopy percent (C_p), vertical distribution tendency degree (Ver), local coniferous LAI (N_LAI), and local broadleaved LAI (B_LAI) for the uniformly distributed understory sampling points, a principal component analysis (PCA) was used for dimensionality reduction. The formulas for the first and second principal components are as follows:

PC1 = 3.300 × LAI_sd + 3.037 × C_p + 3.076 × Ver +3.309 × N_LAI + 1.027 × B_LAI

(2)

PC2 = −0.254 × LAI_sd − 1.332 × C_p + 1.335 × Ver + 1.359 × N_LAI − 3.624 × B_LAI

(3)

The variance explained by PC1 and PC2 were 55.6% and 24.9%, respectively, with a cumulative explanation rate of 80.5%. After correcting the individual canopy environmental characteristic indicators of the understory regeneration habitats (seedling and sapling planting habitats) using the same standard as the uniformly distributed sampling points, the first and second principal component scores for both the uniformly distributed sampling points and the regeneration habitats of each tree species were calculated based on the aforementioned principal component formulas. These scores were then used as the first and second comprehensive canopy environmental characteristics.

Figure 4a,b illustrate the relationships between the first and second principal components of the canopy environment and the original local canopy characteristic indicators, as well as the distribution of comprehensive canopy environmental characteristics of the regeneration habitats for seedlings and saplings of each tree species. Based on an analysis of Figure 4a alongside the principal component loading formulas, it is evident that PC1 is most strongly influenced by the local LAI standard deviation, local coniferous LAI, vertical distribution tendency degree, and canopy percent, with a relatively minor influence from the local broadleaved LAI. In contrast, PC2 is predominantly influenced by the local broadleaved LAI. The regeneration environments of different tree species’ seedlings and saplings are distributed in the four quadrants defined by PC1 and PC2. Among these species, Acer tegmentosum seedlings and saplings, Acer ukurunduense seedlings, and Tilia amurensis seedlings are distributed in the first quadrant of the principal component space; Ulmus davidiana, Acer ukurunduense, and Fraxinus mandshurica saplings are distributed in the second quadrant; Pinus koraiensis seedlings and saplings, Abies nephrolepis saplings, and Acer mono saplings are distributed in the third quadrant; and Picea koraiensis seedlings and saplings, Abies nephrolepis seedlings, Ulmus davidiana seedlings, Acer mono seedlings, and Fraxinus mandshurica seedlings are distributed in the fourth quadrant. Except for Picea koraiensis, the PC1 scores for the local canopy environment in the sapling regeneration habitats of all tree species are lower than those in their respective seedling habitats. Among these environments, the seedling and sapling habitats show significant differences for Ulmus davidiana, Acer mono, Acer tegmentosum, Tilia amurensis, and Fraxinus mandshurica, all showing substantial Cliff’s delta effect sizes (p < 0.05). Among the nine tree species, the PC2 scores for the sapling habitats are higher than those for the corresponding seedling habitats of Picea koraiensis, Abies nephrolepis, Ulmus davidiana, Acer tegmentosum, and Fraxinus mandshurica. The tree species with decreased PC2 scores include Pinus koraiensis, Acer mono, Acer ukurunduense, and Tilia amurensis. Among all species, only Tilia amurensis exhibits a statistically significant difference in the PC2 scores between its seedling and sapling habitats, with a significant Cliff’s delta effect size (p < 0.05) (

Table 3. Significance of differences in the comprehensive canopy environmental characteristics of seedling and sapling planting habitats across tree species and their Cliff’s delta effect size.

Species	PC1 Score		PC2 Score
	p-Value	Cliff’s Delta	p-Value	Cliff’s Delta
Pinus koraiensis	0.096	−0.157	0.068	−0.172
Picea koraiensis	0.570	0.073	0.564	0.074
Abies nephrolepis	0.027 *	−0.140	0.914	0.007
Ulmus davidiana	0.001 **	−0.298	0.492	0.062
Acer mono	<0.001 ***	−0.160	0.689	−0.013
Acer tegmentosum	<0.001 ***	−0.263	0.683	0.023
Acer ukurunduense	0.440	−0.068	0.147	−0.128
Tilia amurensis	<0.001 ***	−0.329	0.021 *	−0.227
Fraxinus mandshurica	0.004 **	−0.556	0.640	0.091

Note: * p < 0.05, ** p < 0.01, *** p < 0.001; one-tailed test.

).

A further analysis was conducted to compare the differences between the PC1 and PC2 scores of the regeneration (including both seedlings and saplings) planting habitats and the understory average levels. The PC1 scores for the regeneration seedling habitats of each tree species are higher than the understory average. Among these scores, the differences for Abies nephrolepis, Acer mono, Acer tegmentosum, Tilia amurensis, and Fraxinus mandshurica are statistically significant compared to the understory average (p < 0.05). Additionally, for the nine tree species, the PC1 scores for the sapling planting habitats of Pinus koraiensis, Abies nephrolepis, Ulmus davidiana, Acer tegmentosum, Acer ukurunduense, and Fraxinus mandshurica are lower than the understory average, while Picea koraiensis, Acer mono, and Tilia amurensis have higher scores than the understory average. The differences in the PC1 scores between the sapling planting habitats of Ulmus davidiana, Acer mono, Acer tegmentosum, and Fraxinus mandshurica and the understory average are statistically significant (p < 0.05). Among the nine tree species, the PC2 scores for the regeneration seedlings of Acer mono, Acer ukurunduense, and Tilia amurensis are higher than the understory average, while those for the remaining species are lower. Among these species, the differences for Pinus koraiensis, Picea koraiensis, Abies nephrolepis, Ulmus davidiana, Acer mono, Acer ukurunduense, and Tilia amurensis are statistically significant (p < 0.05). For the regeneration saplings, the PC2 scores for the planting habitats of Acer mono, Acer ukurunduense, and Fraxinus mandshurica are higher than the understory average, while those for the remaining species are lower. Among these species, the differences for Pinus koraiensis and Acer mono are statistically significant (p < 0.05) (

Table 4. Significance of differences in a comprehensive canopy environment between the colonization habitats of seedlings and saplings and the uniformly distributed sample points under the broadleaved Korean pine forest.

Species	PC1 Score				PC2 Score
	Seeding		Sapling		Seeding		Sapling
	Ratio of Average Rank	p-Value	Ratio of Average Rank	p-Value	Ratio of Average Rank	p-Value	Ratio of Average Rank	p-Value
Pinus koraiensis	1.036	0.251	0.909	0.129	0.721	<0.001 ***	0.532	<0.001 ***
Picea koraiensis	1.060	0.269	1.133	0.063	0.791	0.014 *	0.861	0.051
Abies nephrolepis	1.084	0.002 **	0.960	0.258	0.924	0.003 **	0.933	0.138
Ulmus davidiana	1.046	0.190	0.789	0.003 **	0.912	0.044 *	0.94	0.215
Acer mono	1.326	<0.001 ***	1.104	0.001 **	1.184	<0.001 ***	1.174	<0.001 ***
Acer tegmentosum	1.179	<0.001 ***	0.908	0.044 *	0.965	0.099	0.988	0.413
Acer ukurunduense	1.024	0.348	0.947	0.218	1.112	0.034 *	1.002	0.487
Tilia amurensis	1.344	<0.001 ***	1.014	0.430	1.119	0.030 *	0.877	0.059
Fraxinus mandshurica	1.180	<0.001 ***	0.611	0.022 *	0.949	0.051	1.031	0.437

Note: The ratio of average rank is the ratio between the average rank of the principal component score of the colonization positions of seedlings and saplings and the average rank of the components of the sample points evenly distributed under the forest. * p < 0.05, ** p < 0.01, *** p < 0.001; one-tailed test.

).

4. Discussion

This study indicates that the local canopy environmental characteristics of the regeneration planting habitats of the nine tree species share certain common characteristics, though the differences among the species are also quite pronounced. Overall, understory regeneration (including both seedlings and saplings) tends to be distributed closer to the overstory trees, with fewer occurrences in the center of larger gaps (Figure 2). A further comparison of the canopy environmental characteristics of regeneration seedlings and saplings with the understory average shows that, at the seedling stage, understory regeneration is more likely to be distributed in habitats with higher local LAIs and canopy percents. This phenomenon may be attributed to the following reasons: ① The dispersal ability of the propagules of each tree species tends to concentrate regeneration close to the mother tree, resulting in a higher number of regeneration populations in canopy patches and canopy gap ecotones than in the center of larger gaps [37]. ② In the stands, areas with lower average local LAIs and canopy percents generally correspond to the early stages of gap development, during which herb, shrub, and vine species tend to invade, thereby altering local soil physicochemical properties and affecting plant diversity and community structure [38]. These herb, shrub, and vine species typically follow an R-strategy, which allows them to occupy the ecological niches in forest gaps before tree species regeneration and subsequently outcompete tree regeneration through rapid colonization of light resources [11,39,40]. ③ Certain environmental factors during the early stages of forest gap formation can filter regeneration. For example, the soil moisture content in forest gaps is higher than that under the forest canopy [41], and gaps typically have weaker water regulation capacity [42], which can result in waterlogging during the summer. The canopy acts as a thermal insulator and helps regulate the understory temperature [43]. However, temperature fluctuations—both diurnal and annual—are greater in forest gaps, and extreme temperature events are more frequent. The regeneration of conifer species in open gaps is more vulnerable to photo-oxidative damage [44,45]. These environmental filters may increase mortality rates for regeneration. ④ Areas with lower average local LAIs and canopy percents have more light availability, which can also promote the mineralization of coarse woody debris, litter, and humus, thereby making the soil more fertile [10]. This comprehensive effect of the microhabitat conditions enables regenerated seedlings to achieve better growth potential and rapid transition from seedlings to saplings. Among the nine tree species, except for Picea koraiensis, the average local LAI, canopy percent, vertical distribution tendency degree, and local coniferous LAI of sapling regeneration habitats are all lower compared with those of the seedling regeneration habitats of the corresponding species, which suggests that shaded understory environments and coniferous tree canopies suppress the rapid growth of regeneration populations for most species. In contrast to the above-mentioned canopy environmental characteristics, the local broadleaved LAIs between seedling and sapling regeneration habitats differ markedly across species. The local broadleaf LAI of seedling and sapling regeneration habitats varied among species without a consistent pattern, and only Acer ukurunduense showed a significant difference between stages. The local LAI standard deviation of the sapling regeneration habitats of the nine tree species is lower compared with that of the seedling regeneration habitats of the corresponding tree species, with significant differences observed in Abies nephrolepis, Acer mono, Acer tegmentosum, Tilia amurensis, and Fraxinus mandshurica, which suggests that the rapid growth of regeneration seedlings for each tree species requires specific canopy environmental conditions. Based on comparisons of other local canopy environmental characteristics, these conditions seem to correspond to a more open understory environment away from the tall coniferous tree canopies. Different types of canopy patches correspond to different stages of the forest growth cycle and gap development. These results indicate that the gap development stage plays a clear role in the growth and developmental transitions of regeneration populations. Similar patterns have been observed in many plant species and forest types where the growth rate of regeneration seedlings and saplings varies across different gap development stages [46,47,48].

According to the principal component loadings, the PC1 axis represents a coniferous–broadleaf composite environment dominated by conifer patches. A higher PC1 score indicates a habitat with a greater proportion of coniferous tree crowns, while a lower score suggests fewer coniferous trees. The PC2 axis is primarily influenced by the local broadleaf LAI, with a higher PC2 score indicating an open area that contains some coniferous tree crowns, while a lower score reflects patches dominated by broadleaf tree crowns. The PC1 scores of the regeneration planting habitats for seedlings and saplings of each tree species reflect their tolerance to shade from conifer-dominated canopy patches. Among the nine tree species, only Pinus koraiensis has a slightly lower average PC1 score for its seedling regeneration habitat compared with the understory average. However, when evaluated using average rank comparisons, the PC1 scores for the seedling regeneration habitats of all species are higher than the understory average. Significant differences (p < 0.05) are observed for Picea koraiensis, Acer mono, Acer tegmentosum, Tilia amurensis, and Fraxinus mandshurica, indicating that in the climax community of a broadleaved Korean pine forest, after long-term adaptation, each tree species develops the ability to establish seedling banks under shaded canopy patches. As shown in many studies, shade tolerance is a key characteristic for tree species in many climax forest communities, enabling them to establish and maintain seedling banks in shaded canopy environments [49,50]. With the exception of Picea koraiensis, the PC1 scores for the sapling regeneration habitats of the other species decreased to varying degrees compared with their respective seedling regeneration habitats. Significant declines in PC1 scores were observed for Pinus koraiensis, Abies nephrolepis, Ulmus davidiana, Acer mono, Acer tegmentosum, Tilia amurensis, and Fraxinus mandshurica (p < 0.05), suggesting that relatively open-canopy environments with a lower proportion of coniferous trees can facilitate regeneration. The PC2 scores of the regeneration habitats for seedlings and saplings of each tree species reflect their preference for environments with high canopy cover provided by broadleaf trees or low canopy cover with coniferous tree crowns. Overall, the PC2 scores for sapling regeneration habitats of the three conifer species are lower than those of the broadleaf species. Among all species, the PC2 scores of Pinus koraiensis for both seedling and sapling habitats are significantly lower than those of the uniformly distributed understory sample points, while the PC2 scores of Acer mono for both seedling and sapling habitats are significantly higher than those of the uniformly distributed understory sample points (p < 0.05). The potential reasons for this phenomenon include the following: ① Temporal niche differentiation in light resource competition between coniferous and broadleaf species: Previous studies have shown that the main carbon assimilation period for Pinus koraiensis regeneration occurs in early spring and late autumn, utilizing the period before the broadleaf trees fully leaf out and after they begin shedding their leaves for photosynthetic accumulation [51]. Similarly, Picea koraiensis and evergreen herb plants also share this adaptive mechanism [52,53]. ② The Janzen–Connell effect and similarity in limitations between related species: The regeneration of the same species and phylogenetically related species shares similar resource niches and enemy organisms (pathogens, invertebrates, and mammals) with the parent trees [54]. This may, to some extent, cause the “complementary” distribution of coniferous and broadleaf tree regeneration between the upper and lower canopy layers. ③ Differences in regeneration and growth potential between coniferous and broadleaf species in high-light-resource environments: Coniferous species typically have longer leaf lifespans, slow growth, and an S-shaped survival strategy, while broadleaf species have shorter leaf lifespans, fast growth, and an R-shaped survival strategy [39,55]. This allows broadleaf species to quickly occupy vacant ecological niches in early-stage forest gaps through rapid growth, shading out coniferous species [10]. Among the nine tree species, significant differences in PC1 scores were observed for the regeneration habitats of seedlings and saplings in many species, while only Pinus koraiensis and Tilia amurensis showed significant differences in PC2 scores between their seedling and sapling regeneration habitats (p < 0.05). This suggests that the composite canopy environmental factors, primarily driven by the local broadleaf LAI, have a relatively weaker effect on the growth of regeneration seedlings.

Compared with seedling regeneration, the local canopy environmental characteristics of sapling regeneration habitats offer a comprehensive reflection of the cumulative effects of long-term environmental filtering and differences in growth rates. The differences in local canopy environmental characteristics among the sapling regeneration habitats of different tree species more accurately reflect their habitat preferences. During the sapling stage, among the nine tree species, Fraxinus mandshurica regeneration habitats exhibit the lowest average local LAI and canopy percent, while Picea koraiensis regeneration habitats show the highest average local LAI and canopy percent. Xu [10] discussed the shade tolerance of various species in broadleaved Korean pine forests and pointed out that “among broadleaf species, Betula platyphylla and Populus davidiana are the least shade-tolerant, while Phellodendron amurense, Betula costata, Juglans mandshurica, Quercus mongolica, and Fraxinus mandshurica are less shade-tolerant than Pinus koraiensis. In contrast, Tilia amurensis, Ulmus davidiana, and Acer species are more shade-tolerant than Pinus koraiensis. Among conifers, the shade tolerance of Larix species is weaker than that of Pinus koraiensis, while Picea and Abies species are more shade-tolerant than Pinus koraiensis.” This shade tolerance ranking is also supported by other studies [56,57,58]. This study did not include pioneer tree species with strong light demands, such as Betula platyphylla and Populus davidiana, which typically regenerate after large-scale, high-intensity disturbances such as wildfires, because the canopy gap disturbances are not sufficient to meet their regeneration needs [21], nor did it include Betula costata, which relies entirely on fallen logs for regeneration [59]. However, the shade tolerance ranking of the tree species involved in this study, as mentioned by Xu, is closely related to the average local LAI and canopy density of their regeneration habitats. Among the nine tree species, Pinus koraiensis has the smallest local coniferous LAI in its regeneration habitat, significantly lower than that in the uniformly distributed understory sampling points (p < 0.05). In contrast, the local coniferous LAI of Abies nephrolepis in its regeneration habitat is slightly smaller than that in the uniformly distributed understory points, but no significant difference was found. The local coniferous LAI of Picea koraiensis in its regeneration habitat is significantly greater than in the uniformly distributed understory sampling points (p < 0.05). This suggests that among the three conifer species, Pinus koraiensis has the lowest shade tolerance and is unsuitable for growing under conifer canopy patches, while Picea koraiensis and Abies nephrolepis can grow normally under conifer canopy patches.

Based on Cliff’s delta effect size, the influence of various local canopy environmental characteristics on the transition from seedlings to saplings was assessed. The variability of the local broadleaf LAI was found to have a minor impact on the transition during the regeneration development stage, while environmental variables related to canopy closure and coniferous tree crown patches were key limiting factors for this transition. At the same time, PC1 score had a significantly higher influence on the transition during the regeneration development stage than PC2. Light availability under coniferous tree crowns is generally believed to be weaker than that under broadleaf tree crowns [9], and this low-light condition may limit the rapid growth of regeneration individuals. Based on Cliff’s delta effect size, except for Acer tegmentosum, the influence of PC1 on the transition during the regeneration development stage was notably higher for broadleaf tree species than for coniferous species. This phenomenon may be due to the fact that, in habitats with lower PC1 scores, the growth potential of broadleaf regeneration is higher than that of coniferous species. This is advantageous for broadleaf species to initially occupy the vacant ecological niches during the early stages of gap development, and since broadleaf species generally have shorter lifespans than coniferous species, this allows coniferous species to gain an advantage in the later stages of gap development [10,55].

Based on the above discussions and analyses, it can be concluded that the common tree species in the broadleaved Korean pine forest exhibit strong shade tolerance and are capable of establishing seedling banks under canopy patches. Notable differences can be seen in the understory planting habitats for the regeneration of coniferous and broadleaf species, reflecting the multiplicity of plant species’ survival strategies [60]. In general, coniferous species tend to be distributed under broadleaf canopy patches, while broadleaf regeneration populations show the opposite trend. Seedlings regenerating in gap areas or regions with a lower proportion of coniferous tree canopies are more likely to develop into saplings. A higher local proportion of coniferous tree canopies is the main limiting factor for the transition from regeneration seedlings to saplings, while the influence of broadleaf canopy proportion is relatively minor. Gap areas and regions with a lower proportion of coniferous tree canopies correspond to the early stages of the forest growth cycle, which provide higher light and fertility resource availability, promoting rapid growth of regeneration seedlings to occupy vacant ecological niches. Broadleaf tree regeneration may be more sensitive to such strong resources compared with coniferous regeneration. The above phenomenon is the result of the combined action of multiple ecological mechanisms [61]. Additionally, these findings suggest that the tree species in broadleaved Korean pine forests are generalist species rather than specialists adapted to gap or canopy niches, with the differences among species stemming from growth rate disparities in high-light environments and trade-offs in their lifespans.

5. Conclusions

Tree species in broadleaved Korean pine forests generally exhibit strong shade tolerance, allowing them to establish seedling banks under canopy patches. The differences in the roles that different tree species play in the forest growth cycle stem from disparities in their growth rates under varying microhabitat conditions. Future research should explore the physiological responses and trait characteristics of tree regeneration under varying canopy patch environments. Long-term monitoring of regeneration processes—including invasion, growth, and mortality—across different canopy patches will help elucidate the mechanisms shaping understory spatial patterns.