The root system is the organ that the plant directly contacts with the soil. It is a form of the biological energy of the terrestrial ecosystem and constitutes the main part of the plant. The acquisition and use of the materials and energy in the soil by trees is achieved through the root system. Therefore, the distribution characteristics of the forest root system reflect the possibility of the use of the soil's material and energy and its productivity. On the one hand, the root system continuously obtains nutrients and water from the soil to meet the growth and development of the plant; on the other hand, the root system (whether live root or dead root) directly participates in the two major ecological processes of material circulation and energy flow in the soil and improves the soil structure. The development of fertility and the exertion of soil productivity play an important role. As the main body of terrestrial ecosystems, the role of forests in material production, maintaining the orderly structure of ecosystems, and environmental protection has been increasingly recognized and valued by people.
The dynamic process of forest-soil interaction has been one of the focuses that the international soil science and ecology circles have paid close attention to since the 1990s. With the deepening of research, people are paying more and more attention to the study of the forest root system as part of the forest ecosystem. At the same time, modern instruments such as root analyzers and root analysis systems are widely used in the study of forest root systems. What are the factors affecting the distribution of forest root systems? Which factors affect the distribution of roots such as trees?
1 Root morphology and classification studies
Numerous studies have shown that the absorption of water and nutrients from the soil is one of the important functions of the plant root system. The ability of plants to absorb soil nutrients and water depends largely on the morphology of the root system. Dr#Nobo-ru Karizumi classified the root morphology into drooping roots, oblique roots, and horizontal roots. Hanging roots are classified into shallow roots, middle and deep roots; horizontal roots are divided into scattered, intermediate and concentrated types. The root distribution patterns of the main afforestation tree species in the Xishan area of ​​Beijing were divided into five basic types: horizontal root type, vertical root type, oblique root type, compound root type, and metamorphic root type. It is believed that the root type of the afforestation tree is special. Influenced by external conditions. This root classification method is basically suitable for cuttings and burial of forests grown from afforestation.
Song Chaoshu et al. (1964) initially divided the root system into straight roots, pillar roots, horizontal pillar roots, and vertical pillar roots. According to Shi Qing et al. (1981), in root research of afforestation tree species, roots were divided into horizontal roots, primary roots, secondary roots, sagging roots, oblique roots, and cores according to the site of root growth and its extension in the soil. Roots and roots, etc.
2 Factors affecting the distribution of root morphology
2.1 Site Conditions
The morphology and distribution of the forest root system are first determined by the genetic characteristics of the tree itself, and are strongly influenced by the soil ecological environment conditions, especially the moisture and ventilation conditions. Zhang Guosheng et al. (1999) studied root distribution and root volume of Sabina vulgaris in the Mu Us Sandy Land. It was found that the root volume and diameter distribution of Sabina vary depending on site conditions and soil depth. Zhao Zhong et al. (2000) confirmed that different site conditions in the Loess Plateau have significant effects on the distribution characteristics of Robinia pseudoacacia root, and the difference in soil water status of forest land is the key to this effect. The root system has a great deal of plasticity. It has a straight root system in fertile and deep soils, a long lateral root and a short main root in viscous soils, and a main root in stone soils. . Nicoll (1996) studied the adaptive growth of the root system of the 46a seedling spruce, and found that the vertical distribution of the tree roots was limited by the height of the groundwater table, and the root shape was also affected by the wind. He believes that the adaptive growth of the roots of trees to the wind enhances the resistance of trees to the wind. Zhang Yuqing et al. (2002) found that root characteristics of Tamarix ramosissima on two types of site conditions are based on the generalization of root morphology by K.Lemke and the classification method to Shih-qing. The characteristics of deep rooting, but the horizontal distribution range of the roots of H. kaempferi in the shade slope is much greater than that of the sunny slope. The proportion of roots absorbed is larger, and the growth and development status of the root system is significantly better than that of the sunny slope, resulting in a large negative impact on the terraced crops. It is recommended that the tamarisk be placed on the ridge of Yangpo terraced field.
Some foreign studies have also confirmed that, under normal circumstances, deep-rooted species are more productive than shallow-rooted species, especially on poor site conditions. Ren Anzhi et al. found that the population distribution of Salix chinensis populations in different sand habitats differs in the distribution characteristics of the root system in the soil. The semi-fixed sand dune is the most developed root system (with deep root distribution and large numbers). The yellow sand in the fixed sand dune is clearly inferior to the former, while the root system of the yellow sand will be the least developed in the lowland between hills. The distribution of coarse roots (5\5mm) was significantly correlated with soil moisture and soil bulk density, and the distribution of fine roots (5<5mm) was extremely significantly related to soil bulk density and compactness. According to He Weiming's study on the distribution characteristics of Sabina root in different habitats, the depth coefficient (B), maximum root depth (Rmax), and root depth with 50% of the total root area of ​​Sabina chinensis in mobile and fixed sandy lands ( R50). The root depth (R90) values ​​with 90% of the total root area are very close, especially the B value, and they are all greater than the corresponding values ​​of the sandy shoreland, especially Rmax and R90. These results indicate that the depth patterns of the sandy roots in sandy and fixed sandy lands are similar, but there is a difference in the root depths of the sandy lands. It also means that the distribution of the roots of the sandy cypress on sandy lands is relatively shallow, while the mobile sands are The roots of fixed sands are relatively deep.
2.2 Forest age
The influence of tree age and soil characteristics on the distribution and morphology of roots can be reflected in the differences in the vertical distribution characteristics of the roots. Lya and Hoffmanna believe that in the study, the maximum value of the vertical distribution of roots is generally reached at the seedling stage, and the root distribution of the acacia is common. In 4 years it can reach a depth of 317m. Coile's study also confirmed that although the root density increases with age, extremes of horizontal distribution and vertical distribution can be achieved at a certain age. Wang Wenquan et al. (1997) studied the distribution of the root system of Populus tremuloides, and found that the rate of increase of root volume in different age stages showed the trend of rushing to the first place and slowing down to 0. At a distance of 50cm from the planting point, the root volume of 9a raw forest is 37% higher than that of 7a, and the average annual increase is 18162%, while 14a is only 14% higher than 9a, and the average annual increase is 218%.
2.3 Mixed mode
Due to the heterogeneity of the natural environment and the differentiation of tree species niches, due to the competition among tree species in the mixed forest, the roots compete for soil nutrients and water firstly. Therefore, mixed tree species will cause differences in root morphology and distribution. The study on the morphological characteristics and distribution of roots of mixed forests of Cunninghamia lanceolata and Cunninghamia lanceolata by Qu Mingpu, Liu Chunjiang, etc., and Li Zhenwen, all considered that the roots of mixed forests were more uniform and reasonable than those of pure forests. The roots of absorption are distributed deep into the soil, and the roots of the two tree species are intertwined with each other and are in mosaic distribution. The soil nutrients can be used in a coordinated manner, which is conducive to improving the land utilization rate and productivity of forest stands. However, because of the different competition ability among tree species, unreasonable mixing can reduce the distribution range of the roots of the disadvantaged tree species and reduce the root volume. According to studies by Yan Mingpu et al., the mixture of Pinus tabulaeformis and Quercus variabilis reduced the fine root and root mass of Pinus tabulaeformis, indicating that Quercus variabilis is more competitive than Pinus tabulaeformis in this habitat.
Fang Zhiwei (2001) found that Eucalyptus urophylla had significant effects on promoting root growth of Cunninghamia lanceolata, showing that the root volume per plant of Cunninghamia lanceolata was significantly higher than that of pure Chinese fir in pure forest. Xie Wenhua et al. studied the root system of mixed forest of Pinus massoniana and Eucommia. It was found that mixed Pinus massoniana and Eucommia ulmoides promoted the vertical development of the root system of Pinus massoniana and the horizontal development of the root system of Eucommia ulmoides. The two were interspersed with each other, so that the horizontal and vertical distributions of the roots of the two tree species were compared. Uniform and reasonable, especially the fine root volume distribution has a good spatial configuration, effectively utilizing soil nutrients and moisture, and relatively improving the growth conditions of the two tree species. According to Zhang Yandong et al.'s study on the root system of L. chinensis under mixed conditions, the competition will have a certain influence on the root distribution of tree species in mixed forests. The tree species with strong competitiveness have a wide range of root distribution in the mixed forest, and the tree species with weak competitiveness The root distribution is rejected. Ash has a larger root system, strong competitiveness, and relatively weak larch competitiveness. When Ash and Larch were mixed, the root growth of Fraxinus mandshurica accelerated, while the growth of Larch roots decreased.
According to Shi Peili et al.'s study on the root system of A. cypress mixed forest, the mixed growth of cypress and cypress promoted the growth of cypress roots. The roots of eucalyptus and cypress were mixed in the mixed forest, and the cedar roots were deep-separated. This avoided excessive competition among root systems, and took full advantage of the difference. The water and nutrients in the layered soil laid the foundation. Under the conditions of low nitrogen and phosphorus content in the purple soil of limestone purple soil in hilly areas of Sichuan Province, the introduction of nitrogen-fixing species of eucalyptus and native species of cypress were mixed to improve the nutrition space and growth conditions of cypress and improve the conditions. Fertility, while increasing the productivity, resistance and stability of plantation ecosystems. Cui Langjun et al. studied the mixed forest of seabuckthorn and poplar, indicating that the mixed structure of seabuckthorn-poplar mixed forest and the poplar pure forest under the same conditions have changed the distribution pattern of the community, forest structure and biomass. Biomass increased significantly, and soil physical properties, moisture, and nutrient status were improved. The roots of pure root forest with root diameters of <3 5 mm and 3 5 ~ 10 mm are mainly distributed within 0-30 cm of the soil surface, accounting for about 90% of the total, while the roots of mixed forests are distributed within 0-105 cm. The reduction is relatively deeper and more evenly distributed. This way, the poplars in mixed forests can absorb more deeper nutrients and moisture in the soil and participate in the material cycle, which ultimately promotes the productivity of forest trees.
3 Issues related to the need for further research
With the continuous deepening of the research on the structure, function and productivity of forest ecosystems, the study of forest roots has received increasing attention. In the future, research in the following areas should be strengthened.
(1) Continue to obtain more quantitative data on natural ecological conditions. These ecological conditions include those that favor root growth and are not conducive to root growth, such as the response of the root system to soil compaction, soil aeration, or soil moisture status. Further discover some pioneer species that can adapt to the environment.
(2) The effectiveness of root system to improve soil physical and biochemical characteristics. The content involved the comparison of soil physicochemical characteristics in rhizosphere and non-rhizosphere, and the comparative study of soil physical and chemical characteristics in root zone and rootless zone. It can provide a theoretical basis for revealing the dynamic process of root soil interaction.
(3) The effects of competition on the morphology, distribution and bio-production of mixed tree species under different site conditions. Including the target tree species and mixed tree species root system morphology, distribution and biological yield in space, time differences in the law. Further explore the ecological coordination mechanism of tree species in mixed forests.
(4) Mathematical simulation technology will play a more active role in root research. It is more necessary to carry out root research on the basis of dynamics, because for the root system is only measured once and the research data obtained, the value is very limited.
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