Causes of Biennial Bearing in Oil Tea Camellia and Measures to Overcome It

Causes of Biennial Bearing in Oil Tea Camellia and Measures to Overcome It

1 Causes of Biennial Bearing in Oil Tea Camellia

The causes of biennial bearing in oil tea camellia include both physiological factors and external environmental influences.

1.1 Physiological factors

Physiological factors are the leading cause of biennial bearing in oil tea camellia.

1.1.1 Vegetative growth of branches and leaves versus flowering and fruiting

Good vegetative growth of branches and leaves is the foundation for flowering and fruiting of the tree. Early fruiting and high yield in young trees require a certain amount of branches and leaf area so that the tree can transition from vegetative growth to reproductive growth and gain the ability to form flowers and set fruit; high-quality and stable yields in mature trees require a certain leaf-to-fruit ratio to ensure a balance between tree growth and fruiting. At the Lin Guo Planting Cooperative in Dashi Township, Taihu County, 173.33 hm2 of oil tea camellia was planted early on; because supporting cultivation techniques such as improved varieties and strong seedlings, applying basal fertilizer at planting, and pruning and training were not adopted, by 2007, 8-year-old oil tea camellia produced only 35 kg of crude tea oil per 667 sq m. Due to excessive vigorous growth of the tree, excessive nutrient consumption, and reduced accumulation of stored nutrients in the tree, flower bud differentiation and floral organ development in the following year were affected; in 2008 the fruit set was low and fruit quality was poor. From Table 1 it can be seen that too few and too weak roots, branches, and leaves will not do, but too many and too vigorous will not do either. Long-term field observations suggest that the best tree condition for oil tea camellia fruiting is a growth vigor that is medium to slightly strong.

1.1.2 Quality of flower bud differentiation versus flowering and fruiting

Table 1 Relationship among leaves, fruits, and buds in different fruiting types of oil tea camellia

Note: Survey records from an 8-year-old oil tea camellia forest at Taling, Dashi Lin Guo Planting Cooperative, Taihu County.

Fruiting of the tree is directly affected by the quality of flower bud formation; flower buds that are large and plump lead to a high fruit-setting rate after flowering, and the fruit also develops quickly; conversely, flower buds that are thin and flat are prone to flower and fruit drop. How many flower buds a tree forms and how good their quality is are mainly affected by the following factors.

(1) Quality of shoot development and flower bud differentiation. The potential ability of oil tea camellia leaf buds to form flower buds is directly related to the development of the shoot itself. Oil tea camellia flower buds generally occur on spring shoots of the current year, and there are very few flower buds on summer shoots. According to observations in an 8-year-old oil tea camellia forest of the Dashi Farmers' Planting Cooperative in Taihu County, thin and weak spring shoots have only a few flower buds at the tip, while thick and strong spring shoots not only have dense flower buds at the tip but usually also have 1-2 flower buds at bud positions in the middle and lower parts. The flower buds developed at the top of oil tea camellia are large and plump, decreasing progressively downward, and flower buds on the main shoot are plumper than those on side branches.

(2) Nutrient supply for fruiting and flower bud differentiation. The initial stage of flower bud differentiation in oil tea camellia is from mid-May to early June after spring shoot growth stops, but flower bud differentiation in oil tea camellia exactly overlaps with the fruit development period, and the growth habit of oil tea camellia is that tree nutrients first meet the needs of fruit growth and development. Because of excessive fruiting, nutrients are insufficient, and flower bud growth and development are affected and they drop; every year in August, flower bud drop is severe, accounting for about 10% of the total bud drop. According to observations, when oil tea camellia bears too much fruit, nutrient consumption is excessive, flower buds are small and not plump, and spring shoot development is also affected, as shown in Table 2. Therefore, the leaf-to-fruit ratio must be adjusted through summer pruning so that nutrient distribution within the tree is relatively reasonable; this is the foundation and key to ensuring flower bud differentiation in the tree and eliminating biennial bearing.

(3) Vigorous root activity is beneficial to flower bud differentiation. The flower bud differentiation period of oil tea camellia is exactly the first growth peak period of its root system, and the two promote each other. The root system not only provides inorganic nutrients for the materials produced by the aboveground branches and leaves, but can also use the photosynthetic products of leaves as nutrients to directly synthesize some structural substances and regulatory substances needed for flower bud differentiation. Therefore, fertilization measures implemented during the flower bud differentiation period should all help enhance root physiological function and promote vigorous root activity.

Table 2 Effect of fruiting amount of oil tea camellia on spring shoot growth

Note: Experimental records from the Wuchong oil tea camellia forest in Shiling Village, Dashi Lin Guo Planting Cooperative, Taihu County.

1.2 Influence of the external environment

1.2.1 Unsuitable site conditions and poor growth and development of oil tea camellia

The growth of oil tea camellia requires external environmental conditions suitable for its own growth and development. Because regional landforms and topography are variable, the overall environment is diverse, changing with different landform conditions such as altitude, slope, slope aspect, and slope position; in particular, light, temperature, moisture, and soil properties vary even more, thereby affecting the growth and fruiting of oil tea camellia. For example, oil tea camellia at too high an altitude or on shady slopes grows poorly and yields low, because this type of site has short sunshine duration, strong wind, and low temperature, creating unfavorable factors for flower bud differentiation and causing a large amount of flower and fruit drop. On mountain lands with severe soil erosion, shallow soil layers, and poor water and fertilizer conditions, oil tea camellia has poor growth vigor and low yield. The middle and lower-middle parts of slopes have strong photosynthetic capacity, which is beneficial to the growth and development of oil tea camellia.

1.2.2 Mixed varieties (types) and many low-yielding varieties (types)

Excellent varieties (types) are the foundation for high quality and high yield in oil tea camellia. The oil tea camellia cultivated in various parts of Anqing is mainly common oil tea camellia; due to long-term seedling cultivation and cross-pollination, varieties (types) with diverse flowering periods, maturity periods, fruit shapes, and fruit colors have formed. Different varieties (types) also have different resistance; some varieties (types) or individual trees with weak disease resistance are easily infected by anthracnose and soft rot, causing severe flower and fruit drop. Different flowering periods also have different effects on fruit set in oil tea camellia. The earliest flowering period of oil tea camellia begins in mid-September, and the latest begins in early December, with a difference of more than 2 months in the start of flowering. Because the climatic conditions during the flowering season differ, the fruit-setting rate, overwintering fruit retention rate, and yield are all affected. Oil tea camellia is an insect-pollinated flower; the level of fruit set, apart from the development status of the flower buds themselves, depends on the pollination conditions at the time of flowering. For varieties (types) with early flowering periods, because there are few flowering plants, they cannot obtain sufficient cross-pollination conditions, or they flower too early and the flower buds themselves are not well developed, so the fruit-setting rate is not high; if flowering is too late, even if they can flower, because the climate gradually becomes colder, insects cannot pollinate, or pollen is difficult to germinate and pollination cannot be completed, also resulting in low and unstable yield.

1.2.3 Extensive management and poor tree nutrition

In the long-term process of growth and development, oil tea camellia has formed characteristics different from other forest trees and fruit trees, namely that the spring shoots of the first year are the fruiting branches of the second year; extensive management and insufficient nutrition will inevitably cause poor growth and development of spring shoots, fewer fruits in the second year, and low yield. March to April is the vigorous season for oil tea camellia to sprout spring shoots; if it bears too much fruit, it also consumes more nutrients, and because the tree's nutrients are preferentially supplied to meet the growth needs of young fruits, spring shoot growth is restricted; not only is the number small, but the spring shoots that emerge are also short and weak, especially in old oil tea camellia forests with poor water and fertilizer conditions, where this is even more obvious. By June to July, during the flower bud differentiation period, if the tree bears too much fruit, it will also affect flower bud differentiation, resulting in few flower buds; sometimes although some flower buds differentiate, by the stage when tea fruits are growing and producing oil, because nutrients are preferentially supplied to meet fruit growth and development, flower bud growth and development are affected and they drop early, and in severe cases this also affects plant growth, causing leaves to turn yellow and dry and a large amount of leaf drop, which similarly affects the sprouting of spring shoots the following year and fruiting in the third year. Therefore, a 3-year cycle gradually forms, with a heavy year in the first year, a light year in the second year, and an average year in the third year, or a biennial bearing phenomenon with a 2-year cycle.

1.2.4 Poor climate can also widen the difference between heavy and light years

For example, during June to July, when oil tea camellia flower buds differentiate, if the weather is dry and water is lacking, absorption of inorganic nutrients is difficult, synthesized nutrients are insufficient, few flower buds are formed, and a light year appears in the following year.

2 Management measures

Biennial bearing in oil tea camellia can be overcome if management is proper and measures are scientific. The Jinghui Oil Tea Camellia Professional Cooperative in Huangpu Town, Qianshan County adopted comprehensive cultivation techniques such as improved varieties and strong seedlings, fine land preparation, plastic film mulching, sufficient basal fertilizer, appropriate topdressing, pruning and training, and timely harvesting, creating better ecological and physiological conditions for the growth and development of oil tea camellia. In a 33.33 hm2 demonstration forest planted in 2010, fruiting began in 2013, with fresh fruit yield that year of 75 kg/667 sq m; in 2014 the fresh fruit yield was 410 kg/667 sq m; in 2015, under yield measurement organized by experts from the provincial forestry department, fresh fruit yield reached 628 kg/667 sq m. In 2016, although dry weather occurred, according to yield measurement by the county forestry bureau, fresh fruit yield reached 714 kg/667 sq m, with an output value of 1,927 yuan; both yield and output value increased by 14.51%, achieving early fruiting, high yield, and stable high yield. Analysis of survey data from various places suggests that in order to overcome biennial bearing in oil tea camellia or narrow the yield gap between heavy and light years, multiple management measures should be adopted.

2.1 Selecting improved varieties and strong seedlings for afforestation

Using improved varieties and strong seedlings for afforestation is a fundamental measure to overcome biennial bearing in oil tea camellia. According to surveys, oil tea camellia forests with relatively high yields and relatively stable economic traits all have relatively good varieties (types). In recent years, Anqing City has comprehensively promoted and applied the Changlin series introduced from Jiangxi, as well as the province's Dabieshan No. 1-4 and the local Wanqian No. 1-2, with good planting results. The farmers' planting cooperative in Dashi Township, Taihu County began cultivating oil tea camellia in 2010, with 350 hm2, all mixed plantations of Changlin series varieties (types), of which the early-planted 173.33 hm2 had already flowered and set fruit, and 8-year-old oil tea camellia forests averaged 35 kg of tea oil per 667 sq m, achieving large-area high and stable yield.

2.2 Deep plowing and reclamation

Deep digging and reclamation is a basic technical measure for high-yield cultivation of oil tea camellia. Through reclamation, not only are weeds and shrubs in the forest land removed, the forest land cleaned, and the occurrence of pests and diseases reduced, but dead branches, fallen leaves, and weeds are also turned into the soil, thereby improving the soil's economic structure and physical properties, promoting soil maturation, increasing soil organic matter and trace elements, improving soil water and fertilizer conditions, and favoring root growth. A vigorous root system increases the absorption of nutrients, which is beneficial to the supply of nutrients and substances required for flower bud differentiation.

Deep plowing is mostly carried out in winter. According to the soil structure conditions, landforms, and slope of the oil tea camellia forest land, different reclamation methods are adopted; generally, along the horizontal direction of the mountain on contour terraces, the method of half digging and half filling is used, shaping the slope surface into horizontal terrace strips, higher on the outside and lower on the inside, to facilitate water storage and soil conservation.

2.3 Rational fertilization

Oil tea camellia has the physiological characteristic of "bearing fruit while carrying buds," with flowers and fruits on the branches throughout the year, consuming a large amount of water and fertilizer; therefore, fertilization must be suited to the forest and the tree, scientific and rational, to supplement the insufficiency of soil nutrients, so as not to affect flower bud differentiation, spring shoot growth, and flower, bud, and fruit drop. The types of fertilizer applied should be determined by season, site, and tree; generally, in a heavy year, phosphorus and potassium fertilizers are mainly used, while in a light year, nitrogen fertilizer or compound fertilizer is added; in autumn and winter, organic fertilizer is mainly used, and in spring and summer, quick-acting fertilizer can be applied. Fertilization should be combined with intertillage and reclamation.

2.4 Pruning and training

Through appropriate pruning, the oil tea camellia tree structure is made good, with ventilation and light penetration, concentrated nutrients, and reduced pests and diseases. In particular, the flower bud differentiation period of oil tea camellia overlaps with the fruit growth and development period, and there is obvious competition in nutrient demand; if pruning is not carried out in time to regulate this, both will be damaged. At the same time, adjusting the leaf-to-fruit ratio through summer pruning is beneficial to flower bud differentiation of the tree and lays the foundation for increased production and a bumper harvest the following year. Practice shows that pruning not only improves the relationship between the tree and the environment, but also improves the rational utilization of nutrients within the tree; in production, techniques such as heading back, renewal pruning, flower and fruit thinning, branch twisting, and girdling are commonly used to control the flow of nutrients between vegetative organs and reproductive organs, so as to regulate the balance of nutrient supply among flower bud differentiation, fruit development, and vegetative shoot growth.

2.5 Cultivating and utilizing pollinating insects

Oil tea camellia is a cross-pollinated, insect-pollinated tree species. The fruit-setting rate of cross-pollination reaches 20%-35%, and the level of fruit set is closely related to pollinating insects. The pollinating insects in oil tea camellia forests mainly include more than 40 kinds such as flies, ants, horseflies, butterflies, and bees, among which the best pollination effect is from native wild bees, with Andrena magna, Andrena camellia, and Andrena striata being good. This type of bee colony is widely distributed, numerous, and highly resistant, and in physiology, ecology, life, and reproduction, it can adapt to the ecological environment of oil tea camellia forests and the flowering and pollination habits of oil tea camellia. Therefore, to increase the fruit-setting rate of oil tea camellia and narrow the gap between heavy and light years, cultivating, introducing, releasing, and protecting pollinating bees is an important measure.

The reason ground bees can pollinate oil tea camellia is that during their activities they collect large amounts of pollen to make bee bread and prepare food for their offspring. When collecting pollen, they frequently transfer pollen from different plants, playing a role in multiple mixed pollination from many flowers, increasing the fruit-setting rate and promoting high and stable yield of oil tea camellia.

2.6 Strengthening pest and disease control

Pests and diseases of oil tea camellia are widespread and severe. According to surveys, the incidence rate of oil tea camellia leaf gall disease in key production areas such as Taihu, Qianshan, and Susong in 2016 reached more than 30%, causing a large amount of leaf, fruit, and bud drop. The flower and fruit drop rate of oil tea camellia is as high as 70%-80%, of which diseases account for 1/3, and anthracnose is the main cause of fruit drop in oil tea camellia. Pest and disease control in oil tea camellia should focus on integrated management and prevention, combined with tree management; in the early stage of pest and disease occurrence, fungicides and insecticides should be sprayed in time to achieve "prevent early, prevent small, prevent completely," reducing tree damage and yield loss.