Chinese chestnut (Castanea mollissima
Blume) is an important woody food crop in China, with its nuts containing sugar, starch, protein, fat, vitamins, and minerals [1
]. Its biological properties include drought resistance, the ability to grow in poor soil, heliophile characteristics, and high ecological and economic value [2
]. Chestnut quality is closely related to the cultivar [4
], and different environmental factors in each area also affect chestnut quality [6
]. Therefore, chestnut quality is determined by the cultivar and the environment, such as light intensity, temperature, and precipitation. The different climates across China mean that chestnut production areas can be divided into six regions, including the Northeast region, the North region, the Northwest region, the middle and lower reaches of the Yangtze River region, the Southeast coastal region, and the Yunnan–Guizhou Plateau region. The North region and the middle and lower reaches of Yangtze River region are the most suitable areas for chestnut cultivation [7
]. Chestnut quality is affected by environmental factors, meaning that it varies widely across different regions. For example, the single nut weight from chestnut cultivars of South China is greater than the nut weight produced in the North region, while the quality of North chestnuts is better than the quality of the chestnuts grown in the South of China [8
In the late 1980s and early 1990s, Chinese chestnuts began to be planted in high density to improve nut-bearing and yield [9
]. However, improper canopy management began when chestnut trees did not bear a large amount of fruit, causing the canopy to close and thereby leading to poor light penetration. This resulted in a low yield and a decrease in nut quality [10
]. Pruning is a common canopy management practice that improves fruit yield and quality [11
] by modifying the canopy microclimate [13
]. Therefore, increasing our understanding of the canopy microclimate is necessary if chestnut pruning practices are to improve.
The canopy microclimate is the climatic condition of a small area formed by external meteorological factors after canopy filtration. These factors include light, temperature, and humidity, which affect almost all physiological processes [15
]. For example, light distribution within a canopy plays an important role in the photosynthetic capacity of leaves [16
]. In a single tree crown, light availability depends on the position within canopy. Light intensity and penetration in the inner canopy are always lower than in the outer canopy [17
]. Canopy temperature increases from the bottom to the top [19
], even when the spatial distance within the canopy is short [20
]. The temperature in the outer canopy is higher than in the inner canopy [21
], which is in contrast to the trend of humidity [22
]. In addition, the number, proportion, and spatial distribution of the branches and leaves making up the canopy structure also influences the canopy microclimate. For example, light absorption, reflection, and transmission within a canopy are easily affected by leaf interception [23
]. Appropriate canopy light, temperature, and humidity levels improve the distribution and composition of the branches and leaves, which increases the ability of the tree to continuously bear fruit.
Fruit yield and quality vary greatly within the canopy due to the different microclimates that are present. Light is essential for crop growth, yield, and quality and adequate light distribution improves fruit yield and quality, whereas fruit growth, quality, and yield are poor in canopy positions where light levels are low [24
]. The soluble sugar and starch contents of chestnuts in the outer canopy are significantly higher than in the inner canopy due to the high light levels that enhance photosynthesis [6
]. Furthermore, the nut set percentage of peripheral branches of chestnuts with round heads is as high as 95.45%, whereas it is only 4.55% for inner branches [25
]. In olive trees, over 60% of the total tree production came from fruit growing in the middle-outer and upper parts of the canopy [26
], with the fruit growing in the higher layers being richer in phenol components and saturated fatty acids [27
]. Fruit yield and soluble solids contents in d’Anjou pears increased with canopy height, and more anthocyanins were found in fruit growing in higher canopies because the high levels of light intensity enhanced the photosynthetic activity of the fruit, thereby promoting anthocyanin formation [18
Canopy temperature and humidity also have significant effects on fruit characteristics [28
]. More figs with lower acidity exist in the exterior canopy positions compared to the interior due to the high temperatures, which enhance fruit enlargement and decrease the acidity of fig juice [29
]. In contrast, d’Anjou pear fruit grown in high-temperature areas of the canopy had high percentages of fruit blush, attributed to increased enzyme activity in the anthocyanin synthesis pathway caused by high temperatures [18
]. It was reported that lower temperature and higher humidity positions within apple tree canopies increased Ca transportation from the xylem to the fruit, leading to a reduction in bitter pits and better fruit quality [30
]. In contrast, low Ca concentrations in chestnut leaves may reduce yields [31
Previous studies on chestnut mainly focused on its photosynthetic characteristics [32
], distribution, and nut yields of different chestnut tree structures [6
]. These studies only provided details regarding light; they did not include other canopy microclimate factors, such as temperature and humidity. In this study, we chose cultivar Zun Da chestnut trees from the Yanshan region as the material and determined the light intensities, temperatures, and humidities of different canopy positions from the blossoming to ripening stages. Nut quality and yield and the number of different branch types were also measured at these positions. The purposes of this study were (i) to investigate the distribution of microclimate, branches, and nut yield and quality, (ii) to obtain correlations between the microclimate and nut yield and quality, and (iii) to propose suitable canopy microclimate conditions during the blossoming to ripening stages of Chinese chestnut trees.
2. Materials and Methods
2.1. Experimental Site Description and Plant Materials
The experiment was conducted at the Fine Cultivar Breeding Center of Chestnut located at Weijinghe forest farm in Zhunhua, China (39°55′–40°22′ N, 117°34′–118°14′ E, 173 m in altitude). The experimental site is the core area for planting high-quality chestnut in China. The annual rainfall was 760 mm and the average temperature was 0.40 °C, with an average temperature ranging from −7.5 °C in January to 25.4 °C in July. The soils were leached cinnamon soil and meadow drab soil at 50–100 cm depth. The soil parent materials were sandy and slight acidic. The soil characteristics were as follows: pH, 6.5; organic matter content, 1%; available nitrogen, 30 mg kg−1; available phosphorus, 2–4 mg kg−1; available potassium, 30–45 mg kg−1. There were also areas with good drainage. According to our investigation, the single-plant yield of chestnut low-yield orchard was 50 gm−3 and the single-plant yield of high-yield orchard was 200 gm−3.
Cultivar Zun Da chestnut trees aged 10 years with round heads were used for the experiments. The trees were pruned into a round-head shape, and were 3 m high, spaced at 2 m × 3 m, and had a crown diameter of 2.0 × 2.0 m. Six chestnut trees with strong growth and no disease and pest problems were chosen for this study. All of the trees were grown under the same site conditions, and had the same growth strength, shape, and canopy structure.
2.2. Canopy Microclimate
Centered on the trunk, the tree canopy was divided into eight directions, namely, south, southwest, west, northwest, north, northeast, east, and southeast. Each crown was divided into horizontal and vertical directions as well. The horizontal direction was then divided into inner (0–60 cm away from the center of the trunk) and outer canopies (60–100 cm away from the center of the trunk) (Figure 1
a). The canopy was considered to start at 120 cm from the base of the trunk. A vertical layer was identified at every 60 cm. The vertical extent was divided into three layers, which were the lower layer (0–60 cm), middle layer (60–120 cm), and top layer (120–180 cm) (Figure 1
b). There were 48 areas in total.
Light intensity, temperature, and humidity levels were measured every 7 days from blossoming to the ripened nut stage of the trees on wind-free, clear days. The canopy microclimate shown in Figure 1
was recorded from 08:00 to 17:00 at hourly intervals throughout the observation days. The measurements were repeated three times at any fixed time and the averages were taken. The averages of six repetitions were used to analyze the differences in microclimates of the different canopy areas during the fruit development period. An LI-250 light meter (Li-COR, lnc.; Lincoln, NE, USA) was used to determine the light intensity, while the temperature and humidity were measured using a portable meteorograph (Kestrel 4000, Nielsen Kellerman, Boothwyn, PA, USA).
2.3. Distribution of Different Branch Types
All of the basal diameters and lengths of the 1-year-old branches were measured in the areas shown in Figure 1
and classified as follows: a diameter and length of >0.5 cm and 20 cm, respectively, indicated a strong branch, a diameter and length of <0.3 cm and 10 cm, respectively, indicated a weak branch, and those with intermediate values were classified as moderate branches [35
]. The numbers of each type of branch were then counted. An electronic vernier caliper (TESA-CAL IP67, TESA Tech., Renens, Switzerland) and diameter tape were used to determine the basal diameters and lengths, respectively.
2.4. Flesh Nut Yield and Quality
The nuts were picked at the maturation stage from the areas shown in Figure 1
, and the flesh yield, weight, and nutritional quality of the nuts were determined. Multiple different parameters were used to evaluate the quality of the nuts, including starch, fat, total sugar, and protein. These parameters were determined using enzymatic hydrolysis, soxhlet extractions, direct titration, and the Kjeldahl method, respectively, in accordance with Chinese National Standards GB/T5009.9-2003, GB/T5009.6-2003, GB/T5009.7-2003, and GB/T5009.5-2003. Four repetitions for each of the nut quality parameters were recorded and the averages were taken.
2.5. Statistical Analysis
Differences in the microclimate, nut yield, and quality between the different areas were analyzed using one-way ANOVA (SPSS 18.0, SPSS Inc., Chicago, IL, USA). Multiple comparisons of the means were conducted using an LSD test at α = 0.01. Design expert (version 8.0) software (Stat-Ease Inc., Minneapolis, MN, USA) was used to draw the distributions for the nut yield, the canopy microclimate, and the different types of branches in the top, middle, and lower layers. Pearson’s (parametric) test was used for all of the correlations, with additional analysis using SPSS.