Effects of Strip Clearcutting and Replanting on the Soil Aggregate Composition and Stability in Cunninghamia lanceolata Plantations in Subtropical China

Abstract

Strip clearcutting and replanting are important methods for optimizing the structure of low-efficiency plantations, but their effects on soil aggregate properties remain unclear, especially in subtropical China, which experiences high levels of rainfall and high erosion risk. This study investigated changes in soil aggregate composition and stability through strip clearcutting and replanting treatments in Cunninghamia lanceolata plantations. The experimental treatments included clearcutting strips with widths of 10 m, 20 m, and 30 m and replanting with evergreen broadleaf Schima superba (SM10, SM20, and SM30) and deciduous broadleaf Liquidambar formosana (SF10, SF20, and SF30), respectively. The reserve belts were set at 15 m (S15), 30 m (S30), and 45 m (S45), with no clearcutting as the control (NT). The results indicated that soils of the treatment plots were dominated by >5 mm aggregates (57%–77%), however, lower than the control (NT) due to the clearcutting and replanting, except SF20 and S15 of reserve belts. The 20 m strip width with Liquidambar formosana replanting (SF20) demonstrated optimal soil structural stability, with significantly lower erodibility K values than the control. The content of >5 mm soil aggregates was significantly positively correlated with the mean weight diameter (MWD) and geometric mean diameter (GMD) and significantly negatively correlated with the erodibility factor (K). In contrast, the contents of the other particle sizes were significantly negatively correlated with the MWD and GMD and significantly positively correlated with the erodibility factor (K). This study demonstrates that 20 m strip clearcutting with Liquidambar formosana replanting (SF20) optimally maintains soil aggregate stability and reduces erosion risk, providing critical evidence for strip width configuration and species selection in ecological restoration of subtropical low-efficiency plantations.

1. Introduction

Soil aggregates, as the fundamental units and essential components of soil structure, play a crucial role in maintaining soil ecological functions [1]. A well-developed soil structure is beneficial for plant growth and promotes soil nutrient cycling [2]. Soil erodibility characterizes the ability of soil to resist erosion caused by rainfall and runoff and is typically measured by the soil erodibility factor (K value). A higher K value indicates weaker soil resistance to erosion, whereas a lower K value indicates stronger resistance [3].

Strip clearcutting is a widely adopted practice in forest management and involves the concentrated harvesting of all trees within a designated strip of a certain width in a forest, while keeping the forest in the reserve belts untouched to improve the growth conditions of the remaining trees [4]. After strip clearcutting, broadleaf tree species are often replanted within clearcut strips to accelerate forest recovery, enhance soil quality, or increase biodiversity, thereby optimizing stand structure and improving the biodiversity and ecological stability of the forest [5]. Strip clearcutting and the replanting of broadleaf trees indirectly affect the composition and stability of soil aggregates by influencing aboveground vegetation growth, soil hydrothermal conditions, and microbial activity [6].

Numerous studies have shown that strip clearcutting significantly promotes the growth of understory vegetation and increases plant diversity [7]. Changes in plant diversity affect the formation and stability of soil aggregates through mechanisms such as root exudates, organic matter input, and microbial activity [8]. It is generally believed that increased plant diversity improves soil structure and enhances aggregate stability, thereby reducing soil erodibility [7]. Additionally, strip clearcutting influences soil hydrothermal conditions and microbial activity by improving light availability and ventilation within the forest, which in turn affects the composition and stability of soil aggregates [9,10]. Typically, increased soil temperature and moisture content accelerate the decomposition of surface litter and roots by soil microorganisms [10], further promoting the formation and stability of soil aggregates. However, some scholars argue that the removal of the tree canopy after logging temporarily exposes the soil surface, increasing the risk of erosion from rainfall and wind, which reduces aggregate stability [11]. These findings collectively demonstrate that strip clearcutting exerts significant yet complex impacts on both the formation and stability of soil aggregates, with effects varying depending on vegetation recovery rates and local climatic conditions [10,12].

Replanting broadleaf trees also significantly influences the formation and structure of soil aggregates [13]. The introduction of broadleaf species increases litter diversity, providing more organic matter sources for the soil, while enhancing root exudates and microbial activity, which further promotes the formation of large aggregates [14]. Additionally, the replanting of broadleaf trees affects soil structure and stability by increasing vegetation cover and root networks [15]. Broadleaf trees typically have more extensive root systems, which enhance the mechanical stability of the soil and improve microbial activity through root exudates, thereby promoting aggregate stability and increasing soil resistance to erosion [8]. Moreover, the litter layer of broadleaf trees effectively reduces the direct impact of raindrops on the soil surface, lowering the risk of soil surface erosion [13]. Maintaining high soil aggregate stability is crucial for preserving soil productivity and reducing soil erosion and degradation [16].

In subtropical China, there is limited research on effects of strip clearcutting and replanting of broadleaf trees on aggregate stability and erodibility despite high rainfall and erosion risk [17,18]. As a dominant species in subtropical evergreen broadleaf forests of China, Cunninghamia lanceolata serves dual purposes as both valuable timber and soil ameliorator, with its deep root system significantly contributing to slope stabilization, while the pioneer species Schima superba demonstrates notable fire-resistance and aluminum tolerance, making it widely utilized in ecological restoration, while exhibiting exceptional litter decomposition rates [19]. Liquidambar formosana functions as a deciduous keystone species that provides seasonal nutrient inputs through leaf abscission, with its drought adaptability particularly suited for degraded land rehabilitation.

We studied the effects of strip clearcutting and broadleaf replanting on the composition and stability of soil aggregates in subtropical Cunninghamia lanceolata plantations. Specifically, we hypothesize that the replanting of Liquidambar formosana will enhance soil aggregate stability through its deciduous characteristics by regulating soil moisture content and pH, thereby mitigating aggregate disruption caused by wet–dry cycles and promoting macroaggregate formation [13]. To test this, we established strip clearcutting treatments with widths of 10 m, 20 m, and 30 m and replanted Schima superba (an evergreen broadleaf species) and Liquidambar formosana (a deciduous broadleaf species), as well as set reserve strips of 15 m, 30 m, and 45 m. This study aims to provide a theoretical foundation for the sustainable development and healthy management of Cunninghamia lanceolata plantations in subtropical China.

2. Materials and Methods

2.1. Study Area

The research area is located in the “rainy area of west China Ecological Research Station” at the Hongya Forest Farm, Sichuan Province (103°07′ E, 29°44′ N), within the low mountainous and hilly region of the Sichuan Basin. The average elevation ranges from 1200 to 1300 m, with an annual average temperature of 16.6 °C, an annual average sunshine duration of 1006.1 h, and an annual average precipitation of 1435.5 mm. The soil type is mountainous brown soils. The Cunninghamia lanceolata plantations in the study area is 16–20 years old. The experimental treatments were implemented in two distinct phases. Clearcutting operations were systematically conducted at the end of 2020, during which all merchantable trees within designated strips were harvested following sustainable forestry protocols. Subsequent replanting was carried out in February 2021 using a standardized spatial configuration, with seedlings planted at precise 5 m intervals to ensure optimal growing conditions and facilitate long-term monitoring. (Figure 1). The detailed physicochemical properties of the experimental soils (including bulk density, moisture content, pH, and organic matter content) are provided in Table S1. The replanted species included the evergreen broadleaf tree Schima superba and the deciduous broadleaf tree Liquidambar formosana. Currently, the area is rich in vegetation species with a complex community structure. The understory shrubs are dominated by Fargesia spathacea, Smilax glaucochina, Rubus corchorifolius, Litsea mollis Hemsl, and Schima superba, whereas the herbaceous layer consists primarily of pteridophytes (e.g., Acystopteris japonica, Diplopterygium glaucum, and Diplazium mettenianum).

2.2. Experimental Design

Conventional strip clearcutting methods have limited practicality due to their singular focus on determining optimal strip widths. To address this limitation while preventing soil erosion from excessive harvesting intensity, we implemented a controlled 40% stand conversion intensity. Within this framework, we systematically compared the ecological effects of different width configurations (clearcut strips: 10, 20, and 30 m; reserve strips: 15, 30, and 45 m) to identify the optimal strip clearcutting and replanting regime. Therefore, in the study area, a total of 9 treatments were established. The experimental design comprised strip clearcutting treatments of three different widths (10, 20, and 30 m) that were replanted with either Schima superba (designated as SM10, SM20, and SM30) or Liquidambar formosana (SF10, SF20, and SF30), paired with reserved strips of Cunninghamia lanceolata at three widths (15, 30, and 45 m, designated as S15, S30, and S45 respectively). The unthinned treatment (NT) served as the control. For each treatment, three 10 m × 10 m sample plots were established, resulting in a total of 30 sample plots. The distance between adjacent plots was more than 50 m.

2.3. Sample Collection and Processing

The soil samples were collected in August 2023. After surface litter and humus were removed, intact soil samples (0–20 cm depth) were extracted with aluminum specimen boxes. They were then placed in airtight plastic storage boxes and stored at 4 °C during transport to prevent microbial activity, resulting in a total of 30 intact soil samples. After air-drying naturally in the laboratory, the soil samples were gently broken along their natural structural planes into small clumps approximately 1 cm in diameter. Plant residues, gravel, and other debris were removed [20,21]. Preliminary results and literature review indicated non-significant differences (p > 0.05) in wet-sieving measurements across treatments. Therefore, dry sieving was then used to separate the soil aggregates of different particle sizes [22,23]. Aggregate separation was performed using the dry-sieving method on an SMJ-200 test sieve series. Soil sample weights ranged from 500 to 700 g. Sieves with aperture diameters of 5 mm, 2 mm, 1 mm, 0.5 mm, and 0.25 mm were employed. The test vibration amplitude was set at 0–3 mm, with a sieving duration of 4 min. The frequency remained constant throughout at 50 Hz. Finally, the aggregates were sieved into six size classes: >5 mm, 2–5 mm, 1–2 mm, 0.5–1 mm, 0.25–0.5 mm, and <0.25 mm.

2.4. Statistical Analysis

Based on previous studies and preliminary experiments, the research plots were dominated by >5 mm aggregates, rendering traditional erodibility indicators (e.g., dispersion ratio) less sensitive. Therefore, the following indicators were selected for analysis: content of aggregates >0.25 mm (WR_0.25), mean weight diameter (MWD), geometric mean diameter (GMD), and the soil erodibility factor (K). The formulas for calculating these indicators are as follows:

Content of aggregates >0.25 mm (WR_0.25) (%) [24]:

$${\mathrm{W}\mathrm{R}}_{0.25}={\displaystyle \frac{{\mathrm{M}}_{\mathrm{i}}>0.25}{{\mathrm{M}}_{\mathrm{T}}}}$$

(1)

M_i > 0.25: dry-sieved mass of aggregates with a diameter greater than 0.25 mm (g); M_T: total mass of all aggregates (g).

The mean weight diameter (MWD) and geometric mean diameter (GMD) are as follows [25]:

$$\mathrm{M}\mathrm{W}\mathrm{D}={\displaystyle \frac{{\sum }_{i=1}^n\left({\mathrm{W}}_{\mathrm{i}}\overline{{\mathrm{X}}_{\mathrm{i}}}\right)}{{\sum }_{i=1}^n{\mathrm{W}}_{\mathrm{i}}}}$$

(2)

$$\mathrm{G}\mathrm{M}\mathrm{D}=\mathrm{e}\mathrm{x}\mathrm{p}\left[{\displaystyle \frac{{\sum }_{i=1}^n\left({\mathrm{W}}_{\mathrm{i}}\mathrm{l}\mathrm{n}\overline{{\mathrm{X}}_{\mathrm{i}}}\right)}{{\sum }_{i=1}^n{\mathrm{W}}_{\mathrm{i}}}}\right]$$

(3)

$\overline{{\mathrm{X}}_{\mathrm{i}}}$: The mean diameter of the aggregates in the i-th size class (mm); W_i: the dry weight of the aggregates in the i-th size class (g).

The soil erodibility factor (K) [3] is as follows:

$$\mathrm{K}=7.954\times \left\{0.0017+0.0494\times \mathrm{e}\mathrm{x}\mathrm{p}\left[-0.5\times {\left({\displaystyle \frac{{\mathrm{l}\mathrm{o}\mathrm{g}}_{\mathrm{G}\mathrm{M}\mathrm{D}}+1.675}{0.6986}}\right)}^2\right]\right\}$$

(4)

One-way analysis of variance (One-Way ANOVA) was used to analyze the effects of strip clearcutting, replanting, and reserve strips on the soil aggregate composition, stability, and erodibility (p < 0.05). Linear regression was used to assess the relationships between soil aggregate stability and aggregate content. The dataset was analyzed with SPSS 27.0 (IBM Corp., Armonk, NY, USA) and processed using Origin 2021 (Origin Lab, Northampton, MA, USA).

3. Results

3.1. Changes in Soil Aggregate Composition

The content of soil aggregates of different particle sizes varied under the treatments of strip clearcutting, replanting, and reserve strips (Figure 2). Comparative analysis with the untreated control (NT) revealed that the SF20 and S15 treatments showed a tendency for higher >5 mm aggregate contents than NT, although these differences were not statistically significant; that both SF20 and S15 treatments contained significantly greater >5 mm aggregates compared to S30 and S45 treatments, while no significant differences were detected among other treatment pairs; and that six treatments (SM10, SF10, SM20, SM30, S30, and S45) demonstrated increased 2–5 mm aggregate contents relative to NT. The content of the 1–2 mm aggregates was higher under SM10, SF10, SM20, SM30, SF30, S15, S30, and S45 compared to NT. The content of 0.5–1 mm aggregates was greater under SF10, SM20, SM30, SF30, S30, and S45 than under NT. Among all treatments, only S30 showed significantly higher 2–5 mm aggregate content compared to NT. Similar patterns were observed for 1–2 mm and 0.5–1 mm fractions. The content of 0.25–0.5 mm aggregates was greater under SF10, SM20, SM30, SF30, S30, and S45 than under NT. Only under S45 was the content of <0.25 mm aggregates significantly greater than that under NT.

Under the different treatments of strip clearcutting and replanting, the proportion of aggregates with a particle size >5 mm was the greatest, accounting for 57%–77% of the total aggregates (Figure 3). The proportion of 2–5 mm aggregates ranged from 8.0% to 16.3%, whereas the proportion of 0.25–0.5 mm aggregates was relatively small (2%–5%). The proportion of aggregates with a particle size <0.25 mm was the smallest (2%–4%). As the width of the reserve strips increased, the proportion of >5 mm aggregates decreased. Overall, the distribution of soil aggregates under strip clearcutting and replanting was dominated by >5 mm aggregates. Replanting (except for SF20) and reserve strips (except for S15) generally reduced the percentage of >5 mm aggregates while increasing the percentage of <5 mm aggregates.

3.2. Changes in Soil Aggregate Stability

The effects of different treatments of strip clearcutting and replanting on soil aggregate stability varied (Figure 4). Overall, under the SF20 treatment, both the MWD and GMD were greater than those under the other treatments and were significantly different from those under S30 and S45. The WR_0.25 under the S45 treatment was significantly lower than that under the treatments. Compared to NT, treatments SF10, SM20, SM30, and SF30 showed an increasing trend in K values, while significant increases were observed under S30 and S45 treatments.

3.3. Correlation Between the Soil Aggregate Content and Aggregate Stability Indicators

Through fitting analysis of the soil aggregate stability indicators and the content of aggregates of different particle sizes, the MWD and GMD were significantly positively correlated with the content of >5 mm aggregates (Figure 5 and Figure 6). The MWD and GMD were significantly negatively correlated with the contents of 2–5 mm, 1–2 mm, 0.5–1 mm, 0.25–0.5 mm, and <0.25 mm aggregates. While K was negatively correlated with the >5 mm aggregates, it was positively correlated with the smaller aggregates (2–5 mm, 1–2 mm, 0.5–1 mm, 0.25–0.5 mm, and <0.25 mm) (Figure 7).

4. Discussion

4.1. Effects of Strip Clearcutting and Replanting on the Soil Aggregate Composition

This study found that in different treatments of Cunninghamia lanceolata plantations, aggregates larger than 5 mm were the dominant particle size. This result is consistent with the findings of Fang et al. [21] in coniferous forests in Jiangxi Province. The formation of soil aggregates is a complex process involving interactions among physical, chemical, and biological factors [26]. Strip clearcutting and replanting reduced stand density, improved light and ventilation conditions within the forest, and increased plant diversity [27,28]. Moreover, after strip clearcutting and replanting, the short-term changes inputs of litter and deciduous litter layer buffers soil moisture fluctuations, reducing disruptive wet–dry cycles [29], promoting the formation of large-sized aggregates and thus contributing to the stability of soil aggregates [14].

Moreover, this study revealed that the strip clearcutting, replanting, and reserve strip treatments (except for SF20 and S15) reduced the content and mass percentage of >5 mm aggregates, while increasing the content and mass percentage of <5 mm aggregates. One possible reason is that the mechanical operations during strip clearcutting and replanting (such as logging and replanting) increase physical disturbance to the soil, causing large aggregates (>5 mm) to break down and transform into small aggregates (<5 mm). Mechanical disturbance disrupts the original aggregate structure, breaking it down into smaller particles [30]. Second, strip clearcutting and replanting alter the stand structure and vegetation composition, thereby affecting the quantity and quality of litter. Changes in litter decomposition rates and organic matter inputs may not be sufficient to rapidly form stable large-sized aggregates [31]. Additionally, the root systems of newly planted broadleaf species are not yet fully developed, limiting their short-term contribution to soil organic matter accumulation and aggregate stability [32]. Third, changes in the microenvironment within the forest, such as light, temperature, and humidity, may affect soil moisture dynamics and microbial activity [33]. Accelerated or uneven water evaporation after strip clearcutting may lead to soil drying, disrupting the structure and stability of large aggregates [34].

4.2. Effects of Strip Clearcutting and Replanting on Soil Aggregate Stability

The mean weight diameter (MWD) and geometric mean diameter (GMD) are commonly used quantitative indicators for evaluating soil aggregate stability. Higher values of these indicators indicate a greater degree of soil aggregation and stability [25]. In this study, strip clearcutting and replanting had a limited impact on soil aggregate stability, as no significant changes were observed in the MWD or GMD. This suggests that, in the initial stages, strip clearcutting and replanting have a limited role in enhancing soil aggregate stability in Cunninghamia lanceolata plantations [35], which aligns with the findings of Ma et al. [8] in Larix principis-rupprechtii. This may be due to the young age of the forest during the initial stages of strip clearcutting and replanting, where human activities have a greater impact, and the understory vegetation is sparse, leading to greater soil bulk density and lower soil aggregation, resulting in less noticeable improvement effects [27].

However, as the number of years of enrichment planting increases, the soil structure is expected to improve, enabling the forest soil to better perform its ecological functions [12]. On the other hand, compared to NT, the S30 and S45 treatments significantly reduced soil aggregate stability while markedly increasing erodibility. This degradation effect is attributed to increased soil bulk density caused by excessively wide reserve strips, resulting in decreased soil porosity. Concurrently, reduced water content and loss of soil organic carbon further exacerbated these adverse impacts (Table S1) [35]. Implementing narrower vegetative strips in such areas serves dual purposes: (1) achieving optimal ground coverage density to effectively mitigate raindrop impact erosion, and (2) facilitating accelerated canopy closure that enhances precipitation interception capacity. This multi-layered vegetation configuration establishes a comprehensive erosion control system [36,37]. This affects the formation and stability of soil organic matter, reducing the amount of cementing substances in the soil and thereby decreasing soil aggregate stability and increasing soil erodibility [38]. Additionally, the wind speed in the reserve strips may have increased after clearcutting, accelerating the evaporation of surface soil moisture and leading to soil drying and the disruption of the aggregate structure [39,40]. Water loss also reduces the activity of soil microorganisms, affecting the decomposition of organic matter and cementation processes and further decreasing aggregate stability [41,42].

Vegetation cover in the reserve strips may temporarily decrease after clearcutting, reducing litter input and the soil-binding effects of roots. A reduction in litter decreases the source of organic matter, whereas weakening of the root network reduces the mechanical stability of the soil. Specifically, as noted in recent studies [43], the fine root systems of understory vegetation (particularly grasses and weeds) play a crucial role in surface soil protection by (1) forming dense networks that physically bind soil particles, (2) enhancing water infiltration to reduce surface runoff, and (3) exuding organic compounds that improve soil aggregation. These mechanisms collectively reduce soil erodibility during the vulnerable post-harvest period. These factors collectively contribute to the decline in aggregate stability and the increase in erodibility [31]. However, notably, this study revealed a significant positive correlation between the content of >5 mm aggregates and aggregate stability, whereas content of <5 mm aggregates was significantly negatively correlated with aggregate stability. This finding indicates that the formation of large aggregates is more conducive to improving aggregate stability. Compared to Pinus massoniana plantations in southern China [27], our SF20 treatment showed greater >5 mm aggregates. However, the limited treatment effects on smaller aggregates (<2 mm) contrast with reports from Quercus variabilis stands [44], possibly reflecting species-specific root exudation patterns. Notably, our results mirror the “structural hierarchy” concept proposed by Six et al. [45], where mid-sized aggregates (2–5 mm) proved most responsive to management interventions across studies.

In summary, the improvement in soil aggregate stability and reduction in erodibility were not significant in subtropical China Cunninghamia lanceolata plantations after strip clearcutting and replanting. However, the treatment with a 20 m strip width replanted with Liquidambar formosana presented the best soil aggregate stability and the greatest resistance to erosion. A 20 m strip width may provide moderate light, temperature, and humidity conditions, avoiding the excessive shading that might occur with a 10 m width and the overexposure that might occur with a 30 m width [46]. This moderate microenvironment is conducive to soil microbial activity and root growth, thereby promoting the formation and stability of aggregates [31]. The roots of Liquidambar formosana secrete more polysaccharides and have stronger penetration and binding capabilities, effectively promoting the formation of large-sized aggregates [47]. In contrast, the litter of Schima superba decomposes more slowly, resulting in less organic matter input, and its root characteristics may not be as effective as those of Liquidambar formosana in promoting aggregate formation, making its contribution to aggregate stability relatively weaker [48,49].

4.3. Limitations and Uncertainties

This study clarifies the effects of strip clearcutting and replanting on soil aggregate composition and stability in subtropical Cunninghamia lanceolata plantations, yet several important limitations remain. Although studies such as García et al. [50] have demonstrated that root development, organic carbon accumulation, and microbial activities collectively promote aggregate formation, our inability to measure fine root dynamics, organic matter composition, and microbial parameters in this study has resulted in incomplete interpretation of the biological driving mechanisms. The characteristic interannual precipitation–temperature fluctuations in subtropical regions may influence aggregate stability by regulating soil wet–dry cycle frequency [17,51], but this study failed to capture the temporal effects of these climatic dynamics. Although laboratory-based aggregate index evaluations were conducted, they require validation through field erosion monitoring, and the lack of clay mineralogy [52] and slopes [20] data also limits a comprehensive understanding of aggregate stabilization mechanisms. Future research should integrate observations of fine root biomass, microbial activity analysis, and long-term climate monitoring to more completely reveal the evolution of soil structure under thinning with broadleaf enrichment management.

5. Conclusions

Thinning has become a core issue in global sustainable forest management, particularly in subtropical China Cunninghamia lanceolata plantations, where there is an urgent need to explore how to balance forest ecology and soil protection through management measures. This study shows that in subtropical China Cunninghamia lanceolata plantations, >5 mm soil aggregates constitute the dominant fraction (>50%). However, both strip clearcutting and replanting treatments reduced the content of >5 mm aggregates. Notably, the S30 and S45 treatments significantly decreased aggregate stability, while the SF20 treatment (20 m strip + Liquidambar formosana planting) showed the highest stability and strongest erosion resistance, indicating its significant advantages in maintaining soil structure stability and reducing erosion risk.

The results yield three crucial management implications: First, the outstanding performance of 20 m strips suggests this width as an ecologically sustainable clearcutting standard for Cunninghamia lanceolata plantations, providing adequate light for understory regeneration while minimizing soil disturbance. Second, deciduous species such as Liquidambar formosana should be prioritized for enrichment planting due to their beneficial effects on soil structure. Finally, the short-term management period (<5 years) did not substantially affect overall stability, underscoring the necessity for long-term monitoring to fully elucidate the temporal dynamics of soil structure under strip clearcutting and replanting management regimes.