Impacts of Tree Thinning on Overall Productivity in Densely Planted Walnut Orchards

Abstract

To effectively address the issues of poor ventilation, light deficiency, increased pest and disease pressure, and declining fruit quality in closed-canopy walnut orchards, this study was conducted in a standard, densely planted ‘Xinwen 185’ walnut orchard. Three treatments were established: an unthinned control (CK), a 1-year thinning treatment (T1), and a 2-year thinning treatment (T2). All parameters were uniformly investigated during the 2023 growing season to analyze the effects of thinning on orchard population structure, microenvironment, leaf physiological characteristics, fruit quality, and yield. The results demonstrated that tree thinning significantly optimized the population structure: crown width expanded by 6.22–6.76 m, light transmittance increased to 27.74–33.64%, and orchard coverage decreased from 100% to 75.94–80.51%. The microenvironment was improved: inter-row temperature increased by 2.34–4.08 °C, light intensity increased by 5.38–25.29%, and relative humidity decreased by 2.15–3.30%. Furthermore, leaf physiological functions were activated: in the T2 treatment, the chlorophyll content in outer-canopy leaves increased by 15.23% and 12.45% at the kernel-hardening and maturity stages, respectively; the leaf carbon-to-nitrogen ratio increased by 18.67%; the net photosynthetic rate (Pn) during fruit expansion increased by 34.21–46.10%; and the intercellular CO₂ concentration (Ci) decreased by 10.18–10.31%. Fruit quality and yield were synergistically enhanced: single fruit weight increased by 23.39~37.94%, and kernel weight increased by 26.79–41.13%. The total sugar content in inner-canopy fruits increased by 16.50–16.67%, while the protein and fat content in outer-canopy fruits increased by 0.69–12.50% and 0.60–2.18%, respectively. Yield exhibited a “short-term adjustment and long-term gain” pattern: the T2 treatment (after 2 years of thinning) achieved a yield of 5.26 t·ha⁻¹, which was 20.38% higher than the CK. The rates of diseased fruit and empty shells decreased by 65.71% and 93.22%, respectively, and the premium fruit rate reached 90.60%. This study confirms that tree thinning is an effective measure for improving the growing environment and enhancing overall productivity in closed-canopy walnut orchards, providing a scientific basis for sustainable orchard management and increased orchard profitability.

1. Introduction

Walnut (Juglans regia L.), one of the world’s four major dried fruits, is widely cultivated globally due to its rich nutritional value and high economic benefits [1]. In recent years, with the rapid development of the walnut industry, its planting area and yield have increased year by year. Particularly in China, walnut cultivation has become an important way to increase farmers’ income in many regions [2,3]. However, during the development of the walnut industry, although the early high-density planting model (≥600 trees·ha⁻¹) was commonly used in production to pursue early and high yields [4], compared with conventional density (200–330 trees·ha⁻¹), high-density orchards, despite being able to recover establishment costs within 3–5 years after planting, face problems such as poor ventilation and light transmission, intensified pests and diseases, and decreased fruit quality as tree age increases and canopy volume expands. This leads to increased control costs and severely impacts the economic benefits of the orchard [5,6].

To alleviate the problems of closed orchards, measures such as heading-back, trunk lifting, branch thinning, and tree thinning are often used in production for efficient transformation [7,8,9,10,11]. Among them, tree thinning, as a classical measure for regulating stand or orchard population density, has been proven in various woody plants including timber forests [12,13,14], apples [8,15,16], pears [17], and persimmon trees [18] to significantly optimize canopy light distribution, improve microclimate, and enhance dry matter accumulation. However, there is still relatively little reported systematic research on thinning in high-density walnut orchards.

Based on the above background, this study selected three 9-year-old high-density ‘Xinwen 185’ walnut orchards in the same area, designated as follows: ① unthinned control (CK); ② current-year alternate-tree thinning (T1); and ③ alternate-tree thinning maintained for one year (T2). By simultaneously comparing these three orchard management strategies, the study systematically analyzes the effects of thinning on orchard population structure, microenvironment, leaf physiology, and fruit quality, aiming to provide a scientific basis for the sustainable management of high-density walnut orchards.

2. Materials and Methods

2.1. Study Site and Materials

2.1.1. Study Site

The experiment was conducted in the Third Regiment of the First Division, Xinjiang Production and Construction Corps. This region is located on the northwestern edge of the Taklimakan Desert, with geographical coordinates of 40°23′30″ N latitude and 80°03′45″ E longitude, at an altitude of 1049 m above sea level.

The region features a warm temperate continental climate (Köppen climate classification BSk), characterized by abundant sunlight and heat resources. The mean annual temperature is 11 °C, with extreme maximum and minimum temperatures of 43.9 °C and −27.1 °C, respectively. The average frost-free period is 207 days. The annual accumulated temperature (≥0 °C) is 4620.8 °C. The total annual solar radiation amounts to 142 kcal·cm⁻², with an average annual sunshine duration of 2793.4 h. The mean annual wind speed is 1.08 m·s⁻¹. The mean annual precipitation is 65 mm, while the mean annual evaporation is significantly higher at 2337.5 mm. The average annual relative humidity is 53.1%.

The climatic conditions in this area are suitable for walnut growth. However, due to the initial high-density planting, the orchards have become severely closed, providing an ideal experimental environment for this study.

2.1.2. Experimental Materials

The experimental walnut orchard was established in 2014 with a planting spacing of 3 m × 5 m and an east–west row orientation. The rootstock employed was ‘33’, a drought-tolerant local variety, onto which ‘Xinwen 185’ was grafted in 2015. The soil type is sandy loam (pH 7.8, organic matter content 1.2%, available nitrogen 85 mg/kg, available phosphorus 25 mg/kg, available potassium 110 mg/kg), and the annual irrigation volume is approximately 9000 m³·ha⁻¹. The trees were 9 years old, with a pre-thinning density of 667 trees·ha⁻¹. Prior to the experiment, the orchard exhibited widespread canopy closure: tree heights generally exceeded 4 m, ventilation and light transmission were poor, inner-canopy areas were bare, incidence of pest and disease damage on fruits was high, fruit size was small, fruit quality was inferior, and both yield and economic returns had significantly declined. Orchard management practices (e.g., irrigation, fertilization, pest and disease control) were consistent across all treatment groups to ensure the reliability of the experimental results.

2.2. Experimental Design

To investigate the effects of thinning on a closed-canopy walnut orchard, three treatments were established in this study: the orchard was divided into three sections. One section remained unthinned and served as the control (CK); another section underwent alternate-tree thinning in 2022 (T2); and a third section underwent alternate-tree thinning in 2023 (T1). All observations and data collection were conducted uniformly during the 2023 growing season. The specific experimental design is shown in

Table 1. Experimental design.

Treatment	Thinning Time	Plant Spacing Before Thinning	Plant Spacing After Thinning	Density (Plants·ha−1)	Notes
T1	Mid-April 2023	3 m × 5 m	6 m × 5 m	334	Alternate-plant thinning for 1 year
T2	Mid-April 2022	3 m × 5 m	6 m × 5 m	334	Alternate-plant thinning for 2 years
CK	No thinning	3 m × 5 m	3 m × 5 m	667	Control without thinning

. The thinning pattern and sampling areas are illustrated in Figure 1.

2.3. Measurements

A randomized complete block design was employed. Each treatment contained five replicate plots, with four uniformly growing and healthy trees fixed in each plot, totaling 60 experimental trees.

The canopy space was divided into the following: ① inner canopy (≤120 cm from the trunk); and ② outer canopy (>120 cm from the trunk).

The observation schedule was as follows:

• Fruit expansion stage: Mid-May
• Kernel hardening stage: Mid-June
• Oil accumulation stage: Mid-July
• Fruit maturation stage: Mid-August
• Fruit samples: Harvested uniformly in late August

Measurements of population structure, microenvironment, and leaf physiological indicators were completed between 09:00 and 11:00 on consecutive sunny days in early August. The parameters discussed in the next section were measured.

2.3.1. Determination of Tree Structure and Light Environment

Crown width: The east–west and north–south crown diameters (the widest part of the crown projection) were measured. The vertical projection area of a single tree crown was calculated using the formula: S = π × (R_east × R_south), where R_east and R_south are the crown radii in the east and south directions, respectively.

Orchard coverage: This was calculated as: (Vertical projection area of a single tree crown × number of planted trees)/total ground area occupied by plants × 100%.

Light transmittance: On sunny days from 12:00 to 14:00, a 50 cm × 50 cm white checkerboard cloth was laid under the canopy. The proportion of light spot area was calculated, with three replicates per treatment (Formula: Light transmittance = Light spot area/total grid area × 100%).

Leaf area index (LAI) and canopy openness (DIFN): These were measured using an LAI-2200C plant canopy analyzer (LI-COR Biosciences, Lincoln, NE, USA) on cloudy days or in the early morning in four directions (east, south, west, north) around the tree. The extinction coefficient (K) was calculated as K = −ln(DIFN)/LAI.

2.3.2. Determination of Microenvironment Indicators

Temperature and humidity: Air parameters were measured at three canopy layers (50 cm above the canopy top, middle of the canopy, and 1 m above the ground) as well as in inter-row and inter-plant spaces using a digital thermohygrometer (Testo 635-2, Testo SE & Co. KGaA, Lenzkirch, Germany). For each treatment, five trees were measured as replicates, with three repeated measurements per tree.

Light intensity: Light intensity (lux) at each layer was measured synchronously using a handheld illuminance meter (TES-1332A, TES Electrical Electronic Corp., Taiwan, China). The average value from five trees per treatment was calculated.

2.3.3. Determination of Leaf Physiological Characteristics

Chlorophyll content: The chlorophyll content was determined using the 95% ethanol extraction method. Absorbance at 665 nm and 649 nm was measured with a UV–vis spectrophotometer (UV-2550, Shimadzu Corporation, Kyoto, Japan), and the contents of chlorophyll a and b were calculated.

Photosynthetic parameters: Photosynthetic parameters were measured on sunny days between 11:00 and 13:00 using a Li-6400x portable photosynthesis system (LI-COR Biosciences, Lincoln, NE, USA). Mature outer-canopy leaves were selected for the measurement of net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO₂ concentration (Ci), and transpiration rate (Tr).

Measurement conditions: Photosynthetic photon flux density (PPFD) was maintained at 1200 μmol·m⁻²·s⁻¹, CO₂ concentration at 400 μmol·mol⁻¹, and leaf temperature at 25 ± 2 °C. Thirty-five leaves per treatment were measured as replicates.

2.3.4. Determination of Fruit Quality and Yield

Appearance quality:

Single nut weight and kernel weight: Thirty fruits were randomly harvested from each treatment. Single nut weight was measured using an electronic balance (accuracy: 0.01 g). After shelling, the kernel weight was measured, and the kernel percentage was calculated as (kernel weight/single nut weight) × 100%.

Shell thickness and nut shape index: Shell thickness was measured using a digital vernier caliper (mean of three measurements). The nut shape index was calculated as transverse diameter/longitudinal diameter.

Suture tightness: Determined using a texture analyzer (TMS-PRO, FTC Inc., Sterling, VA, USA).

Internal quality:

Total sugar/reducing sugar/cellulose: Determined by anthrone colorimetry/3,5-dinitrosalicylic acid (DNS) method, expressed in mg·g⁻¹.

Protein: Determined by Coomassie Brilliant Blue G-250 staining method, expressed in mg·g⁻¹.

Tannin: Determined by ultraviolet–visible spectrophotometry, expressed in mg·g⁻¹.

Total phenols: Determined by Folin–Ciocalteu method, expressed in mg·g⁻¹.

Fat: Determined using a fully automatic fat analyzer, expressed as %.

Yield and pest-related indicators:

Yield per plant: The total fruit weight per plant was recorded during the harvest period and converted to yield per hectare (t·ha⁻¹).

Diseased fruit rate/empty shell rate: Three hundred fruits were randomly investigated. The diseased fruit rate was calculated as (number of diseased fruits/total number of fruits) × 100%, and the empty shell rate was calculated as (number of empty shells/total number of fruits) × 100%.

Premium fruit rate/inferior fruit rate: After harvest, 300 walnuts were randomly selected. Premium and inferior fruits were screened according to the walnut nut quality grade standard.

2.3.5. Data Statistics and Analysis

Experimental data were statistically analyzed using SPSS 22.0 software. One-way analysis of variance (ANOVA) was used to compare differences among treatments, with a significance level of p ≤ 0.05. Multiple comparisons were performed using Tukey’s HSD test. Data were expressed as mean ± standard error (SE), and graphs were plotted using Origin 2021.

3. Results and Analysis

3.1. Effects of Tree Thinning on Canopy Structure Parameters

3.1.1. Effect of Tree Thinning on Tree Structure in Closed Walnut Orchards

Thinning significantly optimized the population structure of closed walnut orchards (

Table 2. Effect of tree thinning on population structure indexes of a closed walnut orchard.

Treatment	Plant Spacing (m)	East–West Crown Width (m)	North–South Crown Width (m)	Projection Area per Plant (m2)	Orchard Coverage (%)	Orchard Light Transmittance (%)
CK	3 × 5	5.41 ± 0.95 a	3.91 ± 0.57 a	17.36 ± 5.32 a	100	18.70 ± 2.11 a
T1	6 × 5	6.22 ± 1.08 b	4.55 ± 0.78 ab	23.21 ± 6.76 b	77.37 ± 0.21 b	33.64 ± 3.44 c
T2	6 × 5	6.76 ± 0.27 b	4.87 ± 0.78 b	26.70 ± 4.28 b	89.01 ± 0.14 ab	27.74 ± 1.11 b

Note: Data are presented as mean ± SE (n = 5). Different lowercase letters within a column indicate significant differences among treatments at p ≤ 0.05 according to Tukey’s HSD test. Due to canopy overlap, the actual orchard coverage may exceed 100%. Such values are uniformly displayed as 100% in the table.

). With regard to individual growth advantages, significant results were found:

Crown expansion: The east–west crown width reached 6.76 m in T2 (a 25.0% increase compared to CK), while the north–south crown width reached 4.87 m in T2 (a 24.6% increase compared to CK). The projection area per plant increased to 26.55 m² in T2 (a 77.0% increase compared to CK), indicating that thinning released the growing space for individual trees.

With regard to the alleviation of population canopy closure, the following results were found:

Improved light transmittance: The light transmittance in T1 (33.64%) and T2 (27.74%) increased by 79.9% and 48.3%, respectively, compared to CK (18.70%).

Reduced orchard coverage: The orchard coverage decreased from 100% in CK to 75.94% in T1 and 80.51% in T2.

3.1.2. Variations in LAI, DIFN, and Extinction Coefficient (K) Under Tree Thinning Treatments

Thinning achieved optimization of canopy structure parameters by regulating the spatial distribution of population leaves (

Table 3. Effects of tree thinning on canopy structure parameters of a closed walnut orchard.

Growth Stage	Parameter	CK	T1	T2
Fruit expansion stage	LAI	3.30 ± 0.18 a	2.58 ± 0.07 c	2.79 ± 0.15 b
	DIFN	0.12 ± 0.01 b	0.16 ± 0.02 a	0.15 ± 0.02 a
	K	0.65 ± 0.02 c	0.72 ± 0.02 a	0.68 ± 0.01 b
Fruit hardening stage	LAI	3.52 ± 0.45 a	3.08 ± 0.33 ab	3.03 ± 0.16 b
	DIFN	0.10 ± 0.01 ac	0.12 ± 0.01 a	0.12 ± 0.01 ab
	K	0.65 ± 0.03 a	0.69 ± 0.10 a	0.69 ± 0.03 a
Oil accumulation stage	LAI	3.99 ± 0.27 a	3.22 ± 0.25 b	3.17 ± 0.26 b
	DIFN	0.08 ± 0.01 b	0.10 ± 0.01 a	0.10 ± 0.01 a
	K	0.65 ± 0.04 a	0.72 ± 0.06 a	0.72 ± 0.09 a
Fruit maturation stage	LAI	3.62 ± 0.11 a	2.88 ± 0.25 b	2.75 ± 0.15 b
	DIFN	0.10 ± 0.01 b	0.14 ± 0.00 a	0.14 ± 0.00 a
	K	0.63 ± 0.03 b	0.69 ± 0.04 ab	0.71 ± 0.05 a

Note: Different lowercase letters after data in the same row indicate significant differences (p ≤ 0.05); LAI = leaf area index, DIFN = crown openness, K = extinction coefficient.

). The leaf area index (LAI) decreased by 19–24% overall, while the crown openness (DIFN) increased by 27–38%. The extinction coefficient (K) increased by 8–20%, indicating that thinning enhanced the efficiency of light energy utilization. It alleviated the “light competition” caused by canopy overlap, enabling the distribution of light resources to the middle and lower leaves, thus laying a structural foundation for the improvement of photosynthetic efficiency.

3.2. Effects of Tree Thinning on Orchard Microclimate

Thinning improves the microenvironment of closed walnut orchards through structural adjustment (

Table 4. Effects of tree thinning on orchard temperature.

Treatment	Near Base (°C)	Inter-Row (°C)	Inter-Plant (°C)	Upper Canopy (°C)	Middle Canopy (°C)	Lower Canopy (°C)
CK	28.59 ± 0.68 b	28.51 ± 0.41 c	27.84 ± 0.36 c	30.39 ± 0.57 b	28.81 ± 0.71 c	27.84 ± 0.36 c
T1	32.23 ± 2.56 a	32.59 ± 1.65 a	32.43 ± 2.08 a	33.30 ± 1.59 a	32.39 ± 1.53 a	31.94 ± 1.66 a
T2	30.08 ± 1.42 b	30.85 ± 1.67 b	30.48 ± 0.90 b	31.10 ± 0.59 b	30.63 ± 0.58 b	30.24 ± 0.90 b

Note: Data are presented as mean ± SE (n = 5). Different lowercase letters within a column indicate significant differences among treatments at p ≤ 0.05 according to Tukey’s HSD test.

,

Table 5. Effects of tree thinning on the relative humidity of the orchards.

Treatment	Near Base (%)	Inter-Row (%)	Inter-Plant (%)	Upper Canopy (%)	Middle Canopy (%)	Lower Canopy (%)
CK	42.19 ± 0.85 a	42.68 ± 0.78 a	43.16 ± 1.08 a	41.63 ± 0.92 a	43.09 ± 1.02 a	45.11 ± 0.86 a
T1	38.89 ± 0.78 b	38.46 ± 0.64 c	38.74 ± 0.54 c	37.78 ± 0.78 c	38.73 ± 1.00 c	39.54 ± 0.54 c
T2	40.04 ± 4.98 ab	40.05 ± 0.67 b	41.01 ± 0.52 b	39.15 ± 0.58 b	40.36 ± 1.87 b	42.23 ± 1.82 b

Note: Data are presented as mean ± SE (n = 5). Different lowercase letters within a column indicate significant differences among treatments at p ≤ 0.05 according to Tukey’s HSD test.

and

Table 6. Effects of tree thinning on light intensity in the orchards.

Treatment	Near Base (lux)	Inter-Row (lux)	Inter-Plant (lux)	Upper Canopy (lux)	Middle Canopy (lux)	Lower Canopy (lux)
CK	37,776 ± 2281 b	46,520 ± 3181 b	37,252 ± 3110 b	53,522 ± 3243 b	45,923 ± 4190 b	34,911 ± 2593 b
T1	47,339 ± 2080 a	52,780 ± 4999 a	48,011 ± 3670 a	59,993 ± 4567 a	50,338 ± 2151 a	42,977 ± 2463 a
T2	48,792 ± 3110 a	49,133 ± 2718 ab	46,125 ± 1972 a	61,152 ± 3098 a	50,888 ± 2458 a	44,313 ± 2842 a

Note: Data are presented as mean ± SE (n = 5). Different lowercase letters within a column indicate significant differences among treatments at p ≤ 0.05 according to Tukey’s HSD test.

).

After tree thinning, the temperature in all parts of the orchard increased overall. The temperature in all parts of T1 was significantly higher than that of CK, and the temperature in all parts of T2 was also significantly higher than that of CK. For all treatments, the temperature followed the order: inter-row > inter-plant, and the canopy temperature decreased from top to bottom. At the same canopy position, the temperature order was consistently T1 > T2 > CK.

Except for the inter-row of T2, the light intensity of all treatments showed the order: inter-row > inter-plant, and the canopy light intensity decreased from top to bottom. At the same canopy position, the light intensity order was consistently T2 > T1 > CK. The light intensity of T1 and T2 in all parts was significantly higher than that of CK. This indicates that thinning enhanced the penetration depth of light radiation by reducing canopy shading.

Thinning significantly reduced the relative humidity of the orchard. The relative humidity of T1 near the base was significantly lower than that of CK; the relative humidity in inter-row, inter-plant, and all canopy layers of T1 and T2 was significantly lower than that of CK. For all treatments, the relative humidity followed the order: inter-row < inter-plant, and the canopy relative humidity increased from top to bottom. At the same canopy position, the relative humidity order was consistently CK > T2 > T1. This reflects that the air circulation was enhanced after thinning, accelerating transpiration cooling and water vapor diffusion.

3.3. Effects of Tree Thinning on Leaf Photosynthetic Physiology

3.3.1. Effects of Tree Thinning on Photosynthetic Pigment Content

As shown in Figure 2, thinning significantly increased the chlorophyll content of leaves. Except for T1 during the fruit expansion stage, the chlorophyll content of T1 and T2 was higher than that of the control (CK) in all growth stages. Among them, the chlorophyll content of outer leaves in T2 showed extremely significant difference with CK at the hardening stage and significant difference at the maturity stage. The chlorophyll content of all treatments reached the maximum at the fruit maturity stage; T1 and T2 had the lowest content at the oil accumulation stage, while CK had the lowest content at the hardening stage. This reflects the adaptation of chloroplast development to the continuous improvement of light conditions.

As can be seen from Figure 3, thinning significantly increased the leaf carbon–nitrogen ratio. The order of leaf carbon–nitrogen ratio among treatments was: T2 outer canopy > T2 inner canopy > T1 outer canopy > CK outer canopy > T1 inner canopy > CK inner canopy, and for each treatment, the ratio of outer canopy was higher than that of inner canopy. Among them, the C/N ratio of leaves in T2 outer and inner canopies was extremely significantly higher than that in CK. This indicates that the enhancement of carbon assimilation is coordinated with nitrogen metabolism, which may be related to the accumulation of photosynthetic products driven by light.

3.3.2. Effects of Tree Thinning on Photosynthetic Parameters

Tree thinning had impacts on multiple leaf photosynthetic parameters by optimizing the canopy light environment (

Table 7. Effects of tree thinning on the net photosynthetic rate (Pn) of walnut leaves.

Treatment	Fruit Expansion Stage	Fruit Hardening Stage	Oil Accumulation Stage	Fruit Maturation Stage
CK	7.57 ± 1.28 b	11.54 ± 2.28 b	7.95 ± 1.30 b	6.63 ± 1.14 b
T1	11.06 ± 2.22 a	14.95 ± 2.92 a	11.49 ± 1.93 a	9.79 ± 1.87 a
T2	10.16 ± 2.74 a	14.80 ± 2.55 a	10.59 ± 2.59 a	9.02 ± 1.60 a

Note: Different lowercase letters after data in the same column indicate significant differences (p ≤ 0.05). Unit: μmol·m⁻²·s⁻¹.

,

Table 8. Effects of tree thinning on the stomatal conductance (Gs) of walnut leaves.

Treatment	Fruit Expansion Stage	Fruit Hardening Stage	Oil Accumulation Stage	Fruit Maturation Stage
CK	0.14 ± 0.06 b	0.22 ± 0.06 a	0.15 ± 0.09 b	0.21 ± 0.06 a
T1	0.17 ± 0.05 a	0.17 ± 0.04 b	0.25 ± 0.05 a	0.16 ± 0.04 b
T2	0.19 ± 0.05 a	0.16 ± 0.05 b	0.19 ± 0.04 b	0.15 ± 0.05 b

Note: Different lowercase letters after data in the same column indicate significant differences (p ≤ 0.05). Unit: mol·m⁻²·s⁻¹.

,

Table 9. Effects of tree thinning on the intercellular CO₂ concentration (Ci) in walnut leaves.

Treatment	Fruit Expansion Stage	Fruit Hardening Stage	Oil Accumulation Stage	Fruit Maturation Stage
CK	208.39 ± 17.96 b	237.17 ± 22.09 a	261.47 ± 26.19 a	225.74 ± 29.97 a
T1	207.70 ± 25.29 b	212.72 ± 24.80 b	255.15 ± 26.71 a	201.12 ± 31.39 b
T2	232.56 ± 27.66 a	213.03 ± 25.88 b	259.80 ± 24.64 a	211.01 ± 23.59 ab

Note: Different lowercase letters after data in the same column indicate significant differences (p ≤ 0.05). Unit: μmol·mol⁻¹.

and

Table 10. Effects of tree thinning on the transpiration rate (Tr) of walnut leaves.

Treatment	Fruit Expansion Stage	Fruit Hardening Stage	Oil Accumulation Stage	Fruit Maturation Stage
CK	4.23 ± 0.40 a	2.84 ± 0.57 b	3.46 ± 1.97 b	3.24 ± 1.82 a
T1	3.61 ± 0.84 a	3.83 ± 1.05 a	5.77 ± 1.45 a	3.64 ± 1.70 a
T2	4.08 ± 0.90 a	3.71 ± 0.96 ab	5.16 ± 1.95 a	4.11 ± 2.15 a

Note: Different lowercase letters after data in the same column indicate significant differences (p ≤ 0.05). Unit: μmol·m⁻²·s⁻¹.

). As a core indicator of light energy utilization, the net photosynthetic rate (Pn) reached its peak during the fruit expansion stage, with T1 and T2 increasing by 46.1% and 34.2% compared with CK, respectively. Moreover, the increase remained at 26.4~28.3% during the hardening stage, indicating that the sustained improvement of photosynthetic efficiency after thinning was directly related to the increase in stomatal conductance (Gs). The Gs of T1 and T2 increased by 35.7~42.9% compared with CK, promoting CO₂ absorption and leading to a 10.18~10.31% decrease in intercellular CO₂ concentration (Ci), forming a positive feedback of “increased Gs → decreased Ci → improved carbon assimilation efficiency”.

The change of transpiration rate (Tr) was closely related to microclimate regulation and canopy water balance. During the fruit hardening stage, the Tr of T1 (3.83 ± 1.05) was significantly higher than that of CK (2.84 ± 0.57), while T2 (3.71 ± 0.96) had no significant difference with CK or T1. Thinning treatments (T1, T2) promoted leaf stomatal opening due to improved light conditions (Table 6) (Gs of T1 and T2 increased by 13.3~26.7% compared with CK in Table 8), accelerating water transpiration to meet the water demand for fruit hardening development, while CK had a lower transpiration rate due to canopy closure and insufficient light. During the oil accumulation stage, the Tr of T1 (5.77 ± 1.45) and T2 (5.16 ± 1.95) was significantly higher than that of CK (3.64 ± 1.70), with no significant difference between T1 and T2. During this period, the temperature was relatively high (Table 4), and the improved canopy ventilation in thinning treatments (coverage reduced to 75.94~80.51%) accelerated leaf water evaporation; meanwhile, the increased photosynthetic rate (Table 7) drove the enhancement of transpiration, forming a positive correlation between photosynthesis and transpiration. In contrast, CK had poor air circulation due to closure, resulting in inhibited transpiration rate. There was no significant difference in Tr during the fruit expansion stage and fruit maturity stage.

It is worth noting that Pn was significantly positively correlated with Gs and negatively correlated with Ci, indicating that thinning achieved the synergistic improvement of photosynthetic efficiency through the optimization of carbon supply dominated by stomatal factors and the enhancement of light energy capture driven by non-stomatal factors (such as chloroplast development).

3.4. Effects of Tree Thinning on Fruit Quality and Yield

Synergistic improvement in fruit appearance quality, internal quality, and yield components was achieved through optimization of the population structure and microenvironment (

Table 11. Effects of thinning on fruit morphological characteristics (appearance quality) of walnut.

Treatment	Transverse Diameter (mm)	Suture Diameter (mm)	Longitudinal Diameter (mm)	Fruit Shape Index	Shell Thickness (mm)	Single Fruit Weight (g)	Kernel Weight (g)	Kernel Rate (%)	Suture Tightness (N)
CK Inner	29.37 ± 1.38 c	29.09 ± 1.87 c	32.52 ± 2.05 c	0.91 ± 0.06 b	0.99 ± 0.15 c	7.68 ± 1.01 c	5.13 ± 0.79 c	65.30 ± 6.7 a	205.01 ± 55.9 ab
CK Outer	29.94 ± 4.90 c	28.95 ± 1.48 c	33.30 ± 2.19 c	0.90 ± 0.14 b	1.01 ± 0.16 bc	7.96 ± 1.38 c	5.15 ± 0.90 c	64.79 ± 5.08 a	219.82 ± 68.01 a
T1 Inner	31.82 ± 2.17 b	31.15 ± 2.13 b	36.31 ± 2.64 b	0.88 ± 0.05 b	1.04 ± 0.14 abc	9.86 ± 1.81 b	6.53 ± 1.04 b	65.50 ± 6.12 a	168.15 ± 36.35 b
T1 Outer	33.14 ± 2.77 a	31.75 ± 2.70 ab	37.39 ± 3.36 a	1.05 ± 0.07 a	1.08 ± 0.22 a	10.75 ± 2.42 a	7.15 ± 1.25 a	66.95 ± 5.32 a	168.5 ± 57.57 b
T2 Inner	31.73 ± 1.43 b	31.43 ± 1.76 ab	36.42 ± 1.99 ab	0.87 ± 0.05 b	1.02 ± 0.12 abc	9.94 ± 1.17 b	6.62 ± 1.40 b	67.02 ± 4.09 a	201.31 ± 50.76 ab
T2 Outer	33.01 ± 1.88 a	32.09 ± 1.79 a	37.45 ± 2.10 a	0.89 ± 0.05 b	1.08 ± 0.18 ab	10.98 ± 1.63 a	7.24 ± 1.85 a	67.25 ± 5.89 a	190.79 ± 52.48 ab

Note: Data are presented as mean ± SE (n = 5). Different lowercase letters within a column indicate significant differences among treatments at p ≤ 0.05 according to Tukey’s HSD test.

,

Table 12. Effects of tree thinning on nutritional composition (internal quality) of walnut kernels.

Treatment	Total Sugar Content (mg/g)	Reducing Sugar Content (mg/g)	Protein Content (mg/g)	Tannin Content (mg/g)	Total Phenol Content (mg/g)	Cellulose Content (%)	Crude Fat Content (%)
CK Inner	12.60 ± 0.61 b	9.13 ± 0.51 b	1.37 ± 0.21 a	16.26 ± 0.88 a	22.10 ± 2.80 a	0.98 ± 0.12 ab	61.33 ± 0.90 b
CK Outer	14.17 ± 1.16 a	9.29 ± 0.47 b	1.44 ± 0.15 a	16.18 ± 0.95 a	19.32 ± 2.44 a	1.09 ± 0.01 a	61.91 ± 1.83 ab
T1 Inner	14.05 ± 0.41 a	9.19 ± 0.42 b	1.34 ± 0.09 a	15.58 ± 1.14 ab	17.42 ± 1.76 a	0.86 ± 0.10 b	61.49 ± 1.81 ab
T1 Outer	15.21 ± 0.37 a	10.07 ± 0.91 ab	1.45 ± 0.12 a	14.28 ± 2.29 ab	17.64 ± 4.32 a	0.96 ± 0.02 ab	62.28 ± 0.27 ab
T2 Inner	14.70 ± 0.71 a	9.95 ± 1.80 ab	1.51 ± 0.12 a	13.28 ± 1.4 b	18.03 ± 2.87 a	0.92 ± 0.08 b	63.61 ± 0.39 a
T2 Outer	15.13 ± 1.20 a	11.17 ± 0.72 a	1.62 ± 0.13 a	13.03 ± 0.76 b	19.33 ± 5.27 a	0.86 ± 0.03 b	63.26 ± 0.44 ab

Note: Data are presented as mean ± SE (n = 5). Different lowercase letters within a column indicate significant differences among treatments at p ≤ 0.05 according to Tukey’s HSD test.

,

Table 13. Effects of tree thinning on yield per plant and per unit area in walnut orchards.

Treatment	Plant Density (plants·ha−1)	Yield Per Plant (kg)	Yield (t·ha−1)
CK	44.47	6.55	4.37
T1	22.22	11.25	3.75
T2	22.22	15.78	5.26

and

Table 14. Effects of tree thinning on the rates of diseased fruit, empty shells, and premium fruit in walnut orchards.

Treatment	Diseased Fruit Rate (%)	Empty Shell Rate (%)	High-Quality Fruit Rate (%)	Inferior Fruit Rate (%)
CK	7.00 ± 0.53 a	4.87 ± 0.61 a	71.40 ± 2.45 c	28.60 ± 2.45 a
T1	3.07 ± 0.31 b	2.00 ± 0.35 b	83.60 ± 1.28 b	16.40 ± 1.28 b
T2	2.40 ± 0.20 b	0.33 ± 0.31 c	90.60 ± 0.91 a	9.40 ± 0.91 c

Note: Data are presented as mean ± SE (n = 5). Different lowercase letters within a column indicate significant differences among treatments at p ≤ 0.05 according to Tukey’s HSD test.

).

3.4.1. Effects of Tree Thinning on Fruit Quality

Tree thinning significantly improved the commercial appearance and internal nutritional level of walnut fruits. In terms of appearance quality (Table 11), single fruit weight in T1 and T2 treatments increased by 23.39~35.05% and 29.43~37.94% compared with the control (CK), respectively; kernel weight increased by 26.79% and 41.13%; transverse and longitudinal diameters increased by 8.3~12.4%. Among them, the fruit shape index of outer fruits in T1 reached 1.05 ± 0.07, indicating a significant improvement in fruit plumpness.

In terms of internal quality, carbon and nitrogen metabolites in walnut fruits accumulated significantly after thinning. The total sugar content in inner canopy fruits increased by 16.50% and 16.67% in T1 and T2, respectively; protein content in outer-canopy fruits increased by 0.69% and 12.50%; fat content increased by 0.60~2.18% (Table 12). It is worth noting that the protein content in outer fruits of T2 reached 1.62 ± 0.13 mg/g, which was metabolically linked to the high carbon–nitrogen ratio (15.8 ± 0.9) in outer leaves of T2 (Figure 3) and the increase in net photosynthetic rate (Pn + 34.2%) in Table 7, confirming the physiological pathway of “enhanced photosynthetic efficiency → accelerated nitrogen assimilation → protein accumulation”. In addition, the content of anti-nutritional factors such as tannin and total phenols decreased by 8.7~19.8%, which may be related to the inhibition of secondary metabolite synthesis by enhanced light after thinning.

3.4.2. Effects of Tree Thinning on Yield Components and Fruit Quality Maintenance

The T1 treatment resulted in a 14.2% decrease in yield per mu due to a halving of plant number, but the yield per plant increased by 71.8%; the T2 treatment achieved a yield per plant of 15.78 kg (140.9% higher than that of CK), with the overall yield exceeding that of CK by 20.4%, achieving a compensatory increase in yield (Table 13). This was closely related to the further expansion of the crown width in T2 compared to T1 (Table 2), indicating that long-term thinning achieved yield recovery through compensatory growth in tree morphology.

In terms of pest control effect and improvement of high-quality fruit rate, the diseased fruit rate of T1 and T2 decreased by 56.14% and 65.71% compared with CK, the empty shell rate decreased by 58.93% and 93.22%, and the high-quality fruit rate increased to 83.60% and 90.60% (Table 14). Combined with the 4.22% decrease in inter-row humidity of T1 in Table 5 and the improvement of light uniformity in Table 6, it can be seen that microenvironment optimization significantly inhibited the breeding of pests and diseases, and the comprehensive control effect of T2 was 21.8% higher than that of T1.

4. Discussion

The early high-density planting model, while pursuing early and high yields, also laid hidden risks for orchard canopy closure. Through a systematic evaluation, this study confirms that alternate-tree thinning can significantly optimize the population structure of walnut orchards and effectively alleviate crown overlap and light competition. This finding is consistent with the rehabilitation practices of various fruit trees. Although research specifically on walnuts is relatively scarce, our results highly align with the report by Arreola Ávila et al. [19] in closed-canopy pecan (Carya illinoensis) orchards, whose study similarly showed that a 50% thinning treatment persistently increased the photosynthetically active radiation (PAR) within the canopy by more than 44%. Furthermore, the significant expansion in crown width (+25.0%) and the increase in the extinction coefficient (K) indicate a strong morphological and physiological plasticity of walnut trees in response to the released growing space. This structural optimization is a prerequisite for microenvironment improvement and the activation of physiological functions, whose importance has been widely demonstrated in the rehabilitation of closed-canopy orchards of species such as apple [20,21,22] and pear [17]. This study extends its validity to walnut, an important economic tree species.

The redistribution of light and heat resources driven by tree thinning directly activates leaf photosynthetic function. The significant increase in the net photosynthetic rate (Pn) and stomatal conductance (Gs), coupled with the decrease in intercellular CO₂ concentration (Ci), constitutes an initial photosynthetic response pattern dominated by stomatal factors. This physiological response mechanism is universal across tree species and has been extensively confirmed in apples [23,24] and walnut [25,26]. This study confirms that this pattern also holds true for walnuts, demonstrating that their photosynthetic apparatus is equally sensitive and efficient in responding to the improved light environment, forming the common physiological basis for yield and quality formation.

The core of this study lies in revealing the “short-term adjustment, long-term gain” pattern of thinning on walnut yield. The substantial compensatory effect on yield per plant in the T2 treatment (+140.9%) led to the total output surpassing the control in the second year. This finding mutually supports the conclusion by Arreola Ávila et al. [19] that a “gradual thinning” strategy can buffer yield losses. More importantly, this study also systematically reports the synergistic enhancement effect on the internal nutritional quality of walnuts (e.g., fat, protein), rather than being limited to external marketability. This indicates that thinning can redirect more photoassimilates to fruit instead of excessive vegetative growth, achieving improvement in both fruit quantity and quality. Concurrently, the substantial decrease in the rates of diseased fruit and empty shells, along with the increase in the premium fruit rate, significantly enhances the economic value of thinning.

In conclusion, thinning is an effective approach for achieving sustainable high yield and quality in closed-canopy walnut orchards. It is recommended to adopt tree thinning in high-density, closed orchards in production to fully exploit the long-term benefits. This study successfully extends the thinning theory, originally derived from apples, pears, and other fruit trees, to walnuts and enriches its implications for nut quality regulation. Future research could focus on: (1) A comparative economic analysis of different thinning intensities and patterns; (2) Applying the model developed in this study to other main production regions and walnut cultivars.

5. Conclusions

Based on observation data from the year of thinning and the two subsequent years, this study demonstrates that a one-time alternate-tree thinning intervention (reducing planting density from 667 to 334 trees·ha⁻¹) effectively alleviates canopy closure, improves light penetration, and enhances both fruit quality and yield in high-density walnut orchards. This practice significantly improves the overall productivity of closed-canopy orchards. Therefore, its application is highly recommended in the management of such orchards to achieve high, stable yields and sustainable management.