1 Introduction

Planting density and canopy structure directly affect the light, nutrient utilization, ventilation, pest and disease occurrence, and fruit development of fruit trees (Lordan et al., 2018; Haque and Sakimin, 2022). Appropriate planting density can optimize the tree structure and increase the yield per unit area. If combined with scientific canopy management, such as pruning, shaping and reasonable plant spacing, it can improve the light distribution and promote photosynthesis, thereby enabling fruits to accumulate more sugar and vitamin C (Kishore et al., 2023). Studies have found that although overly high density can increase yield in the short term, overly dense canopies and poor ventilation can lead to insufficient fruit coloring, decreased sugar-acid ratio, and poor flavor (Ladaniya et al., 2021). However, if the density is too low, it will reduce the land utilization rate and lead to a decrease in the per-unit yield. Therefore, a reasonable combination of density and canopy structure is the key to achieving high yield and quality (Anthony and Minas, 2021).

In traditional Changshan Huyou orchards, common problems are low planting density, loose canopy structure, and poor light utilization rate. As a result, the yield per unit area is not high and the fruit quality is unstable (Yano et al., 2022). With the increase of tree age, the tree canopy tends to be dense, ventilation and light penetration deteriorate, the incidence of pests and diseases rises, and fruit coloring and sugar accumulation are affected. Furthermore, under extensive management, the fruit sizes are uneven and the flavors fluctuate greatly, making it difficult to meet the market demand for high-quality fruits (Ladaniya et al., 2021; Haque and Sakimin, 2022). Therefore, it is necessary to enhance the productivity and fruit quality of orchards through reasonable planting density and canopy optimization.

This study summarized the effects of planting density and canopy structure on the yield and quality of Changshan Huyou, analyzed the problems of traditional planting methods, and proposed optimization strategies. This study aims to provide references for the management and industrial upgrading of Changshan Huyou orchards.

2 Planting Density in Changshan Huyou Orchards

2.1 Definitions and classifications of low-, medium-, and high-density planting

The planting density of fruit trees refers to the number of trees planted in one hectare and is one of the important parameters for orchard design and management. There is no unified national standard for the density classification of Changshan Huyou. The experience of citrus fruit trees can be referred to. It is generally divided into three categories: low density, medium density and high density (Figure 1). Low density means less than 400 plants per hectare, with a large spacing between plants, such as 5×5 meters or wider. The tree body has a large space, sufficient light, good ventilation and is convenient for mechanized operation. The medium density is approximately 400 to 800 plants per hectare, with moderate plant spacing (such as 4×3 meters or 5×2.5 meters), which can not only ensure the yield but also facilitate management. The high density is over 800 plants per hectare, with compact plant and row spacing (such as 3×2 meters), high early yield and high land utilization rate, but higher requirements for later pruning and management (Haque and Sakimin, 2022). The specific density should be adjusted according to the variety, rootstock, soil, climate and cultivation goals. Nowadays, some orchards even attempt ultra-high density (> 1 600 plants per hectare), but it requires dwarfing rootstocks and modern pruning techniques (Ladaniya et al., 2020).

2.2 Historical development and current practices in Changshan Huyou cultivation

Changshan Huyou has traditionally been mostly planted with low density, emphasizing strong tree body, long lifespan and good fruit quality (Huang et al., 2025). Later, with the shortage of land and the demand for early high yields, medium-density and high-density models began to emerge. Some orchards conducted experiments with different plant and row spacing to find the best combination (Haque and Sakimin, 2022). In recent years, high-density and even ultra-high-density cultivation has gradually been promoted. Combined with dwarf rootstocks, reasonable pruning and mechanized management, early high yield and efficient production can be achieved (Ladaniya et al., 2020). However, high density has also brought new problems, such as fierce competition among trees, high pressure from pests and diseases, and unstable fruit quality.

2.3 General effects of planting density on tree growth and orchard management

The influence of planting density on tree body, yield and quality is quite obvious (Figure 2). Under low density, the tree has a large growth space, more leaves and uniform light, which is conducive to fruit enlargement and sugar accumulation, and the quality is better. When the density is high, the competition among trees is intense, the growth of individual trees is restricted, the crowns overlap, ventilation and light penetration are poor, and it is easy for diseases and pests to occur. Although the yield increased rapidly in the early stage, with the increase of tree age, the yield per tree decreased, and the total yield may tend to stabilize or even decline. In terms of quality, low-density fruits have a higher sugar-acid ratio and better flavor. However, high density may lead to problems such as smaller fruits and decreased flavor. In addition, high-density orchards are difficult to manage, requiring stronger pruning and water and fertilizer management, as well as prevention of pests and diseases. Mechanized operations are also relatively limited. Therefore, although the high-density model can bring early high yield and higher land utilization rate, its long-term sustainability still needs to be comprehensively considered in combination with varieties, rootstocks and management techniques (Ladaniya et al., 2020; Haque and Sakimin, 2022).

3 Canopy Structure and Light Distribution

3.1 Tree architecture and canopy development patterns

The tree structure and crown development pattern of Changshan Huyou will directly affect the distribution of branches, leaf area index (LAI), and the effect of light energy interception and utilization. When the tree structure is reasonable, the canopy layers are clear and the ventilation and light transmission are good, which can better utilize light, help the fruits grow evenly and improve their quality. Studies have shown that compact tree shapes combined with reasonable pruning can make the distribution of branches and leaves more reasonable, reduce internal shading, make canopy light more uniform, which is conducive to sugar accumulation and fruit color improvement (Anthony and Minas, 2021; Liu et al., 2022; Zhang et al., 2025). Meanwhile, a good canopy structure can also improve the adaptability of fruit trees to high-density cultivation, enhance stress resistance and resource utilization rate (Li et al., 2021; Tian et al., 2024).

3.2 Influence of canopy density, height, and leaf area index on light interception

Canopy density, tree height and leaf area index are important parameters that determine the utilization of light. Moderately increasing the density can enhance the yield per unit area. However, when the density is too high, the canopy is prone to clotting, and the internal light is insufficient, which will affect the fruit development and quality (Ding et al., 2022; Lin et al., 2024). Studies have shown that LAI increases with density within a reasonable range, which can enhance light energy interception and biomass accumulation. However, if LAI is too high, it will increase internal shading, reduce photosynthetic efficiency and fruit quality (Li et al., 2021; Liu et al., 2022). By regulating the canopy height and hierarchical structure to ensure that leaves at different levels can receive sufficient light, leaf senescence can be delayed and the distribution of photosynthetic products to fruits can be improved (Huang et al., 2017; Duan et al., 2024). In addition, optimizing the canopy structure can also improve the utilization efficiency of water and nutrients, and enhance the stress resistance and yield stability of crops (Xu et al., 2017; Du et al., 2021).

3.3 Relationships between canopy microclimate and fruit growth

The canopy structure not only affects light but also alters the temperature, humidity and ventilation conditions within the tree crown, thereby influencing the growth and quality of fruits. Studies have found that the light and temperature in the outer and upper layers of the tree canopy are higher than those in the inner and lower layers. Therefore, the sugar content, dry matter and color of the fruits in the outer layer are better, while the quality of the fruits in the inner layer is often poorer (Wang et al., 2021; Ou et al., 2023). Good ventilation and light transmission can reduce pests and diseases and improve the uniform maturity of fruits (Collins et al., 2020; Zhang et al., 2025). In addition, the microclimate within the canopy also affects the accumulation of water, oil and vitamins in the fruit, thereby determining the final commercial quality (Grilo et al., 2021; Lu et al., 2022).

4 Effects of Planting Density on Yield Performance

4.1 Impact on flowering, fruit set, and yield per tree vs. per unit area

The planting density will directly affect the flowering quantity, fruit setting rate, yield per plant and yield per unit area of Changshan Huyou. Studies have found that when the density increases, the leaf area and the number of fruits per plant will decrease. However, due to the larger planting quantity, the total output will increase instead (Haque and Sakimin, 2022; Ibell et al., 2024). High-density planting can form row closure more quickly, increase canopy leaf area index (LAI), enhance photosynthesis, and thereby increase yield per unit area. However, under high density, the yield per plant and fruit size will decrease, and the fruit quality (such as soluble solids, sugar-acid ratio) may also be affected (Haque and Sakimin, 2022). Similar patterns are also reflected in citrus fruit trees such as limes and pomelos: high density increases the yield per unit area, but the yield and quality per plant decline (Ladaniya et al., 2020; Singh et al., 2020). Therefore, a reasonable density selection requires finding a balance between output and quality.

4.2 Inter-annual yield stability under different planting densities

The stability of interannual output is an important indicator for the long-term operation of orchards. High-density planting can significantly increase the yield in the early stage. However, as the tree age increases, the competition among trees intensifies, and there is insufficient light, the yield fluctuation will be more obvious (Singh et al., 2020). Some studies suggest that the ultra-high density model has a high yield in the first few years, but the yield stability decreases in the later stage due to growth restrictions and the aggravation of pests and diseases. Medium density is more suitable for maintaining a relatively stable interannual yield and reducing the occurrence of alternate years (Ladaniya et al., 2020; Ibell et al., 2024). In addition, reasonable canopy management measures such as pruning and thinning can alleviate the negative effects brought by high density and improve the stability of yield (Singh et al., 2020).

4.3 Long-term orchard productivity and sustainability

In the long run, excessive density will cause excessive competition among trees for light, nutrients and water, which not only affects the health of the trees, but also damages the orchard ecology (Haque and Sakimin, 2022). High-density orchards have high yields in the early stage, but they are prone to problems such as premature senescence, aggravated pests and diseases, and decreased quality in the later stage, which is not conducive to continuous production (Ladaniya et al., 2020; Singh et al., 2020). On the contrary, moderate density combined with scientific pruning, thinning and adjustment of row and plant spacing can help maintain the ecological balance of the orchard and improve the long-term stability of fruit quality and yield (Haque and Sakimin, 2022). Meanwhile, a reasonable density can also facilitate mechanized operation and pest and disease control, and improve the management efficiency and sustainable development capacity of the orchard.

5 Effects of Planting Density on Fruit Quality

5.1 Influence on fruit size, peel thickness, and external appearance

Research has found that low-density planting of Changshan Huyou is conducive to the uniformity of individual fruit weight, fruit diameter and fruit shape. The space between trees is larger, with more abundant light and nutrients. The fruit cells divide and expand more fully, so the fruit is larger, the shape is more regular, the peel is thicker and the color is brighter (Haque and Sakimin, 2022; Huang et al., 2025). When planted at high density, the competition among trees is intense and there is insufficient light. The fruits tend to become smaller, have irregular shapes, thinner fruit skins, and sometimes show dull color or surface defects (Lordan et al., 2018; Ladaniya et al., 2020; Singh et al., 2020). If the canopy is managed reasonably, such as thinning or pruning, it can improve the light distribution, reduce the negative impact brought by high density to a certain extent, and make the fruit appearance better (Bhusal et al., 2017; Kishore et al., 2023; Zhang et al., 2025).

5.2 Effects on internal quality: sugar-acid balance, juice content, aroma compounds

Low-density planting can also increase the soluble solids (TSS), total sugar, sugar-acid ratio and juice content of fruits, and the aromatic substances are also more abundant (Ke et al., 2025). At high density, the accumulation of sugar decreases, the acidity increases, the sugar-acid ratio drops, the flavor becomes miler, and there is less juice (Laužikė et al., 2021; Huang et al., 2025). Some studies suggest that choosing a moderate density and combining it with scientific pruning management can ensure yield while maintaining good flavor and nutrition (Haque and Sakimin, 2022; Kishore et al., 2023). In addition, density also affects the content of antioxidant substances (such as flavonoids and polyphenols) and vitamin C in the fruit. Low density is more conducive to the accumulation of these nutrients (Aubert et al., 2019; Ding et al., 2022).

5.3 Postharvest characteristics: storability, shelf life, and transport adaptability

The storability and transportation adaptability of fruits are also related to the planting density. Under low density, the fruit develops more completely, with thick skin and firm tissue. Therefore, it is more resistant to storage, less likely to be damaged during transportation, and has a longer shelf life (Huang et al., 2025). The fruit under high density has thin skin, loose tissue, is prone to injury or rot, has a short storage period and a reduced commercial rate (Djido et al., 2021; Haque and Sakimin, 2022). In addition, some studies have also pointed out that moderate density combined with reasonable fertilization and water management can improve fruit disease resistance and post-harvest quality (Ladaniya et al., 2020; Lindell et al., 2022).

6 Canopy Management and Its Role in Quality Improvement

6.1 Pruning and training systems for optimal light distribution

Pruning and shaping of fruit trees are important methods for regulating the canopy structure, improving light distribution and enhancing fruit quality. Through reasonable pruning and tree shape adjustment, the shading within the canopy can be reduced, making the light distribution more even and enhancing the efficiency of photosynthesis, thereby resulting in better fruit coloring, higher sugar content and better flavor. For instance, methods such as Y-shaped tree, spindle-shaped, and inclined trellis have been proven to significantly improve the utilization rate of light energy and the consistency of fruit quality on citrus, peach, apple and other fruit trees (Liu et al., 2022; Kishore et al., 2023). In the high-density cultivation of peach and apple trees, the adoption of thinner and two-dimensional tree shapes can enable the entire tree canopy to receive uniform light and reduce the difference in fruit quality (Liu et al., 2022). Pruning can also regulate the leaf area index, avoid overly dense canopies, and reduce poor ventilation and diseases (Collins et al., 2020; Zhang et al., 2025). Among pomelo trees such as Changshan Huyou, reasonable pruning and shaping not only improve the light conditions, but also promote the accumulation of sugar and vitamin C in the fruit (Anthony and Minas, 2021; Haque and Sakimin, 2022).

6.2 Effects of canopy ventilation and leaf-to-fruit ratio on fruit quality

The ventilation of the canopy and the leaf-to-fruit ratio are also important factors affecting fruit quality. Good ventilation can reduce the internal humidity of the tree canopy, decrease pests and diseases, and improve the appearance and internal quality of the fruit (Levy et al., 2023; Zhang et al., 2025). A reasonable leaf-to-fruit ratio can ensure that fruits obtain sufficient photosynthetic products, helping to accumulate sugar, vitamins and flavor substances (Collins et al., 2020; Lu et al., 2022). Research shows that among fruit trees such as peaches, apples and citrus fruits, a leaf-to-fruit ratio of 6 to 10 is the most suitable, which is conducive to fruit enlargement and quality improvement. An excessively high or low leaf-to-fruit ratio will affect the sugar-acid ratio, color and nutritional components (Anthony and Minas, 2021; Liu et al., 2022). In pomelo fruit trees, improving ventilation and regulating the leaf-to-fruit ratio can significantly increase the contents of soluble solids, antioxidants and flavor substances in the fruits (Haque and Sakimin, 2022; Kishore et al., 2023).

6.3 Integration of canopy management with planting density regulation

The combination of canopy management and the regulation of planting density is an effective way to increase the yield and quality of orchards. High-density planting can increase the yield per unit area. However, without scientific canopy management, it is prone to canopy closure, insufficient light and decline in fruit quality (Duan et al., 2024). Combining pruning, shaping, reasonable planting density and ventilation can improve fruit quality while ensuring high yield (Ladaniya et al., 2020; Li et al., 2021). In citrus and pomelo fruit trees, when high-density cultivation is carried out, tree shapes such as spindle-shaped, Y-shaped, and inclined trellis, combined with pruning and regulation of leaf-to-fruit ratio, can simultaneously achieve yield and quality improvement (Anthony and Minas, 2021; Zhang et al., 2021; Kishore et al., 2023). In addition, reasonable dense planting and canopy management can also improve the mechanization level and resource utilization rate of orchards (Li et al., 2021; Haque and Sakimin, 2022; Duan et al., 2024).

7 Case Study: Changshan Huyou Orchard with Different Planting Densities

7.1 Study background: location, climate, and orchard setup

Changshan Huyou is mainly produced in the Changshan area of Quzhou City, Zhejiang Province. The local area has a subtropical monsoon climate, with an average annual temperature of about 17℃, an annual precipitation of about 1 500 millimeters, and good light conditions, which are suitable for the growth of pomelo fruit trees. Most of the local orchards are built on hills and plains, and the soil is mostly loam or sandy loam with good drainage. In recent years, with the promotion of high-density planting and canopy optimization concepts, orchard layout has gradually shifted from the traditional wide rows and large plant spacing to high-density, compact and diverse canopies. The orchard of this case covers an area of approximately 10 hectares, runs from north to south, and has a flat terrain, which is convenient for mechanized management. Different density test areas were set up in the orchard, including conventional density (4×5 meters), medium density (3×4 meters), and high density (2.5×3 meters), to compare the effects of density and canopy structure on yield and quality (Ladaniya et al., 2020; Haque and Sakimin, 2022; Portarena et al., 2024).

7.2 Research methods: planting density treatments, canopy measurements, yield and quality evaluation

The study adopted a random block design, with each density treatment zone covering an area of 1 hectare, and each was repeated three times. After the fruit trees are planted, they are managed uniformly, with consistent measures for fertilization, irrigation and pest control. Canopy structure was determined by parameters such as leaf area index (LAI), canopy volume, leaf distribution and photosynthetically active radiation (PAR), and was regularly monitored using portable canopy analyzers and photosynthesis analyzers (Kishore et al., 2023). In terms of yield, statistics were made on the yield per plant, yield per unit area, number of fruits and average fruit weight per plant. In terms of quality, indicators such as soluble solids (TSS), total acid, vitamin C, peel color and pulp cell structure were determined using the unified laboratory method (Haque and Sakimin, 2022). In addition, combined with economic benefit analysis, the input-output ratio and net income under different densities were calculated (Ladaniya et al., 2020; Portarena et al., 2024).

7.3 Results: yield response, fruit quality differences, and economic implications

The results indicated that the yield per unit area in the high-density (2.5×3 m) area was more than 30% higher than that in the conventional density, but the yield per plant and the weight per fruit decreased (Ladaniya et al., 2020). Medium density (3×4 meters) not only ensures a higher yield per unit area, but also offers better yield per plant and fruit uniformity than high density. After canopy optimization, the LAI value in the medium-high density area was between 3.5 and 4.2, and the light energy distribution was more uniform, which was conducive to fruit development (Kishore et al., 2023). The TSS, vitamin C and flavor scores of the fruits in the high-density area were relatively low. The color of the fruit skin became pale, the cell structure of the pulp was loose, and the proportion of small fruits and deformed fruits increased. The fruits in the medium-density area are of the best quality, with a TSS of 12.5%, moderate total acidity, bright skin color and rich flavor. Although the fruit quality in the conventional density area is high, the yield per unit area is low and the economic benefits are limited (Ladaniya et al., 2020; Haque and Sakimin, 2022; Portarena et al., 2024).

In terms of economic benefits, the high-density area requires a relatively large initial investment. However, due to its high output, the benefits in the first three years are significantly better than those of the conventional density area. However, during the peak fruiting period, due to the decline in fruit quality and the increased pressure of pruning and pest management, the growth rate of net income slowed down. Overall, the medium-density area has the best balance among output, quality and cost, the highest input-output ratio, and the best economic benefits (Ladaniya et al., 2020; Portarena et al., 2024).

8 Challenges and Research Gaps

8.1 Lack of standardized planting density guidelines for Changshan Huyou

At present, there is no unified standard or industry guideline for the optimal planting density of Changshan Huyou. Although high-density cultivation has been proven to increase yield per unit area in citrus and other fruit trees, the optimal density varies greatly under different varieties, rootstocks, climates and management methods (Ibell et al., 2024). Some studies have found that although excessively high density can increase yield in the short term, it is also prone to cause overly dense canopy, poor ventilation and light transmission, thereby affecting fruit quality and stress resistance (Ladaniya et al., 2020; Ladon et al., 2024). Due to the lack of unified standards, density selection in actual production is often rather arbitrary, which limits the high-quality development of the industry (Haque and Sakimin, 2022; Ibell et al., 2024).

8.2 Trade-offs between high yield and optimal fruit quality

High-density cultivation generally increases the total yield per unit area, but the qualities such as single fruit weight, sugar content and flavor often decline. It may also lead to problems such as aggravated pests and diseases and poor fruit coloring (Karpe et al., 2024). Optimizing the canopy structure through pruning and tree shape adjustment can improve light conditions and enhance fruit quality. However, if planted too sparsely, the total output will decrease (Mahmud et al., 2023). How to strike a balance between high yield and high quality remains a core issue in current management (Anthony and Minas, 2021; Haque and Sakimin, 2022; Ladon et al., 2024).

8.3 Limited long-term ecological and economic evaluation studies

Most of the existing research focuses on short-term yield and quality performance, while there is insufficient study on the long-term ecological effects of high-density planting and canopy optimization. For instance, there is still a lack of systematic assessment of the impacts on soil health, ecological niche changes of pests and diseases, and carbon sink capacity (Ladaniya et al., 2020). Some literature points out that excessive density may lead to accelerated consumption of soil nutrients, increased risk of diseases, and ultimately affect the stability of orchard ecosystems (Ibell et al., 2024). In addition, economic factors such as the long-term input-output ratio, labor demand and mechanization adaptability should also be taken into comprehensive consideration (Anthony and Minas, 2021; Gao et al., 2024).

9 Future Perspectives

9.1 Precision orchard management combining density and canopy optimization

As the fruit tree industry pursues efficiency, quality and sustainability, precise management is becoming an important direction for the production of Changshan Huyou. By leveraging digital monitoring, remote sensing and model prediction, orchards can dynamically regulate planting density, canopy structure, light and water and fertilizer management, thereby enhancing resource utilization and fruit uniformity. Studies show that reasonable planting density combined with canopy optimization can increase the yield per unit area, but if the density is too high, the fruit quality will decline. Therefore, it is necessary to maintain the balance between output and quality through precise regulation (Liu et al., 2022; Lin et al., 2024; Duan et al., 2025). Three-dimensional modeling and photosynthetic parameter monitoring can help managers adjust pruning and fruit thinning more scientifically, improve light distribution, and enhance quality indicators such as fruit sugar content and vitamin C (Kviklys et al., 2022; Kishore et al., 2023; Zhang et al., 2025). In the future, the intelligent orchard system integrating sensors, unmanned aerial vehicles, the Internet of Things and big data will promote the optimization of density and canopy structure, achieving high-yield, high-quality and green low-carbon development (Underwood et al., 2016; Mahmud et al., 2023).

9.2 Breeding and training systems for density-adapted varieties

High-density planting places higher demands on varieties. Future breeding should focus on selecting tree species that are compact, dwarf, have high photosynthetic efficiency and strong resistance to diseases and pests to adapt to spatial competition and resource allocation (Tian et al., 2024; Ye et al., 2025). Meanwhile, it is necessary to innovate shaping and pruning methods, such as spindle shape, Y shape, wall shape, etc., to improve ventilation and light transmission, promote the distribution of photosynthetic products to fruits, and enhance fruit uniformity and commercialization rate (Lordan et al., 2018; Gao et al., 2024; Zhang et al., 2025). Studies have found that reasonable pruning can not only control tree growth and extend economic life, but also reduce pests and diseases and improve the consistency of fruit quality (Liu et al., 2020; Anthony and Minas, 2021; Kishore et al., 2023). In the future, collaborative research on varieties, tree shapes and densities should be strengthened to form a high-density cultivation technology system suitable for different ecological zones.

9.3 Digital tools and modeling approaches for predicting yield-quality outcomes

Digitalization and modeling technologies have also provided new ideas for the yield and quality prediction of Changshan Huyou. Parameters such as canopy volume, leaf area and flower and fruit distribution can be monitored through remote sensing, lidar and three-dimensional imaging, and models can be established with yield and quality data (Underwood et al., 2016). Function-structural plant models (FSPM) and photosynthetically growing simulations can quantify the distribution of light energy, carbon allocation and fruit development under different densities and canopy structures, providing a scientific basis for management (Li et al., 2021; Duan et al., 2024). Artificial intelligence and big data can integrate multi-source data to conduct dynamic prediction and early warning of yield, quality and pests and diseases (Underwood et al., 2016; Mahmud et al., 2023). In the future, digital twin orchards and intelligent decision-making systems will become important tools for enhancing the intelligence and refinement of the Changshan Huyou industry.

Acknowledgments

The authors appreciate the comments from two anonymous peer reviewers on the manuscript of this study. The authors thank the members of the research group for their assistance during the research process.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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