**7. Application of mycorrhizal fungi in forestry**

Once a plant enters a new growth environment, the introduction fails due to lack of symbiotic mycorrhizal fungi or suitable mycorrhizal fungi. This phenomenon is common in all continents. For example, Puerto Rico introduced foreign pine, and annual introduction failed repeatedly until the introduction of mycorrhiza fungi from the origin of the successful introduction. The survival rate of imported pine reached above by mycorrhizal fungi inoculation and afforestation with mycorrhizal seedlings in China. Mycorrhizal seedling afforestation can not only improve the ability of trees to absorb and utilize nutrients but also improve the survival rate of afforestation and timber yield. Moreover, inoculation at seedling stage can also greatly reduce the amount of fungicides. To this end, the United States also established mycorrhizal technology companies. Mycorrhizal biological agents were specially provided for mycorrhizal forest trees. The mycorrhizal research and development center of Chinese Academy of Forestry has also carried out research on the production and application technology of mycorrhizal biological agents, has been popularized in various provinces and regions throughout the country, and achieved remarkable results. A large number of studies have shown that mycorrhizal fungi not only prevent the invasion of pathogenic bacteria in structure but also secrete antibiotics to inhibit the growth and development of pathogenic bacteria, which can improve the disease resistance of forest trees and play an important role in the biological control of forest root diseases. At present, the inoculation of *Larix gmelinii* with Boletus has been successfully used to prevent and control root rot caused by *Pythium fulvus*.

#### **7.1 Application of mycorrhizal fungi in tree breeding**

When introducing new tree species, corresponding mycorrhizal species should be introduced, especially pine and other specialized mycorrhizal species. As early as in the early 1950s, after the failure of introducing eucalyptus, Iraq introduced undisinfected soil and finally introduced it successfully and significantly promoted seedling growth [65]. In 2001, Hua et al. put forward the concept of forest mycorrhizal bioengineering or forest mycorrhizal biotechnology [66], that mycorrhizal trees and the whole forest ecosystem is of great significance, that not only can improve forestry productivity but also provide a sustainable development path for forestry development.

In the current study on the effects of mycorrhizal fungi on forest growth, most of the experiments inoculated mycorrhizal fungi on forest seedlings to observe the changes in growth indicators, namely mycorrhizal forest. Zhang et al. test showed that the survival rate of mycorrhizal fungi in *Pinus elliottii* was above 90% [67]. The ground diameter and height of mycorrhizal *P. sylvestris* seedlings were 37% and 39% higher, respectively, and the survival rate of seedlings under drought conditions was improved [68]. Zhang found that inoculation with mycorrhizal fungi could promote seedling height growth, ground diameter growth, and biomass growth of poplar through experiments [69]. Some studies also inoculated seven mycorrhizal fungi of Russula on *Pinus massoniana* seedlings [70], which could not only form mycorrhizal fungi with *P. massoniana* seedlings but also promote the growth of seedlings

to varying degrees. He et al. treated the rhizosphere of *P. massoniana* seedlings with mycorrhizal fungi by non-sterile inoculation method [71]. The height, ground diameter, and crown width of mycorrhizal *P. massoniana* seedlings were 29%, 51.7%, and 46.6% higher than those of the control group, respectively. Wu et al. conducted pot experiments after soil sterilization, and the results showed that exogenous mycorrhizal fungi could promote the growth and development of *P. tabulaeformis* seedlings and the accumulation of phosphorus *in vivo* and could significantly increase the basal diameter and leaf length of plants [72]. At the same time, it was concluded that the difference between natural forest and artificial forest in the natural regeneration of seedlings was due to the composition and diversity of exogenous mycorrhizal communities. Therefore, mycorrhizal fungi played an important role in the natural regeneration and growth of *P. tabulaeformis*.

Changes in plant water use efficiency, stomatal conductance, and CO2 absorption efficiency can also affect the distribution of nitrogen content. Mycorrhiza can change the photosynthetic efficiency and thus affect the growth of trees through the above methods [73]. Experiments show that mycorrhizal *P. elliottii* can increase chlorophyll content, thereby increasing photosynthesis [67]. Zhu et al. found that the net photosynthetic rate, intercellular CO2 concentration, stomatal conductance, seedling height, and ground diameter of *Zelkova schneideriana* were significantly improved after inoculation with mycorrhizal fungi [74].

Mycorrhiza promoted root growth of seedlings. Wang (2013) inoculated exogenous mycorrhiza on *P. massoniana* seedlings [75]. The results showed that the main root length, lateral root length, and lateral root number of mycorrhizal seedlings were higher than those of control group. Song et al. found that the establishment of symbiotic relationship between *P. cathayana* seedlings and VA mycorrhizal fungi could promote the development of seedling roots, increase the root surface absorption area, increase the ratio of active root absorption area, and enhance the activity of polyphenol oxidase in roots [76]. Mycorrhiza can indirectly promote forest growth by promoting root absorption of nutrients in soil. Lu found that after AM fungi infected roots, the growth of host plants changed significantly, and seedling height and root length changed significantly in the early stage of seedlings [77]. Jin (2019) inoculated different strains of *Quercus* spp., and the inoculated mycorrhizal fungi showed different degrees of growth-promoting effects on the host and the ability to significantly regulate root architecture [78]. Ditengou et al. found that *C. bicolor* could secrete sesquiterpenoid small-molecule substances, which had a significant regulatory effect on poplar roots, resulting in an increase in the number of secondary roots, expanding the root area of poplar, thus making poplar absorb nutrients [79]. Wu inoculated *R. irregularis* in arbuscular mycorrhizal fungi on *Populus deltoides* and found that inoculation with mycorrhizal fungi could increase the contents of iron, copper, manganese, and phosphorus in roots at high nitrogen levels [80]. It was speculated that *R. irregularis* could alleviate the nutritional imbalance caused by high nitrogen stress by promoting the absorption of nutrients and the content of elements in roots of *P. deltoides*. At the same time, this experiment found that mycorrhizal poplars might absorb nitrogen through mycorrhizal pathways and downregulate the expression of nitrogen transport-related genes in roots.

#### **7.2 Application of mycorrhiza in forestry nursery**

With the continuous understanding and in-depth study of mycorrhiza, mycorrhiza seedling technology is more and more widely used in forestry. From

#### *Plant Microbiome and Mycorrhizal Fungi DOI: http://dx.doi.org/10.5772/intechopen.107373*

introduction, mycorrhizal seedling to afforestation in ecologically fragile areas, the application of mycorrhizal has achieved initial results. For mycorrhizal plants, countries or regions should introduce corresponding mycorrhizal fungi while introducing a new plant. There are many examples of failed introduction due to the lack of mycorrhizal fungi. For example, Puerto Rico in South America introduced pine tree species from all countries in the world in the 1930s, which failed. It was not until the introduction of mycorrhizal fungi from the origin that the afforestation was successful in 1955. Iraq's migration to different tree species was also unsuccessful. In the early 20th century, Iraq 's move to different eucalyptus species was also unsuccessful. Later, the unsterilized soil in the introduced eucalyptus forest was introduced and applied to the roots of the seedlings. Finally, it was successful and significantly promoted seedling growth. Finally, it succeeded and significantly promoted the growth of seedlings. Three kinds of pine trees, *P. elliottii*, *P. caribbeanensis*, and *Pinus taeda*, were introduced by Guangdong Forestry Research Institute in China. However, due to the lack of sufficient mycorrhizal fungi, afforestation was declared to be unsuccessful. Subsequently, the seedlings inoculated with mycorrhizal fungi were used for afforestation so that the survival rates of these three pines reached, and a large number of *P. yunnanensis* trees were introduced into Hainan Island in China. Due to the adoption of mycorrhizal fungi inoculation measures, the seedlings grew vigorously, and the introduction and afforestation were also successful. Chen (2013) reported the role of Australian and domestic exogenous mycorrhizal fungi in the introduction of Australian tail leaves, and the results showed that the inoculation of exogenous mycorrhizal fungi could improve the plant growth [81].

There are many advantages of mycorrhizal seedlings compared with ordinary afforestation technology, which can greatly improve the survival rate of tree afforestation, promote the growth of seedlings, increase the absorption capacity of plants to equal greenhouse gases, improve the carbon fixation ability of plants, improve the ability of seedlings to absorb nutrients such as nitrogen, phosphorus, and potassium, and secrete a variety of enzymes to activate soil components to form soil aggregates and improve soil rhizosphere microenvironment. At present, many countries and regions in the world stipulate that afforestation in some ecologically fragile areas must adopt mycorrhizal seedling technology.

American mycorrhizal development has been in the forefront of the world and set up a special mycorrhizal technology company to provide special mycorrhizal agents for forest mycorrhizal. The relevant departments of the United States have stipulated that seedling cultivation and afforestation in wet grassland areas must be inoculated before the Soviet Union stipulated in the forest steppe zone, and the mycorrhizal inoculation technology must be used to establish nursery for the seedlings. Mycorrhizal inoculation of roots at the early stage of seedlings was not only beneficial to mycorrhizal formation but also had less usage and obvious effect.

Forestry scientists in China have been working on mycorrhizal seedling technology in recent years. Mycorrhizal agent developed by China Academy of Forestry is mainly used for seedling cultivation of *P. massoniana*, which can increase seedling height, ground diameter, biomass, lateral root number, yield of qualified seedlings, and survival growth rate of afforestation. Therefore, it can solve the problems of low survival rate and slow growth in the early stage of *P. massoniana* and has been popularized in various provinces, cities, and regions nationwide, and remarkable results have been achieved. Using mycorrhizal technology to develop *P. elliottii*, studies on seedling cultivation and seedling ectomycorrhizal rate up to the above indicated that mycorrhizal treatment of seedlings of *P. massoniana* and *P. elliottii* could promote root growth of seedlings, increase seedling growth, increase ground diameter by one, and grow vigorously.

In areas where conventional afforestation is difficult, selecting suitable mycorrhizal fungi and adopting mycorrhizal technology according to the principle of suitable land and tree can greatly improve the survival rate of seedlings and afforestation effect, accelerate the recovery ability of vegetation, prevent further deterioration of the environment, and promote ecological balance. Mycorrhizal technology will play an important role in the restoration and reconstruction of ecological environment in China and other countries.

In summary, mycorrhizal technology has been applied in a variety of tree species, and pine mycorrhizal research has also been widely concerned. Many scientists have done some statistical analysis on the mycorrhizal formation of a tree species or a mycorrhizal fungi, described the mycorrhizal fungi of American pine seedlings, and described the necessary species of ectomycorrhizal roots for radiation pine seedlings. The exophytic mycorrhizas of P. radiata were divided into subclasses and subsubclasses. A series of morphological, histological, and cytological studies on the mycorrhiza were carried out. A key to the mycorrhiza of *Platycladus erinaceus*, *Picea norwayensis*, and Pine New Year's Eve was developed, from which the 10 genera of Boletus, such as Russula, Lactarius, Bacteroides, Boletus, Cephalophyllum, Chlamydomonas, and so on, which formed mycorrhiza, can be retrieved. It is a comprehensive reference for the identification of mycorrhiza. *P. tabulaeformis* is the second largest species of pine in planting area and quantity. The application value and prospect of fine mycorrhizal fungi breeding and propagation technology for *P. tabulaeformis* are quite extensive.

The inoculation of *P. tabulaeformis* with mycorrhizal technology can not only provide excellent nursery stocks but also improve the ecological environment in ecologically fragile areas, control soil erosion, and improve soil fertility. In landscaping, it can increase the absorption capacity of equivalent greenhouse gases, that is carbon sequestration capacity is very strong, and can slow down the heat island effect in the city. In today's global warming, without sacrificing the premise of economic development, this technology makes a certain contribution to the social energy conservation and emission reduction. Using mycorrhizal inoculation technology to cultivate fine *P. tabulaeformis* seedlings, carrying out industrial production, establishing nursery, and applying it to afforestation under difficult site conditions will greatly improve the survival rate of seedlings and forest growth, accelerate the pace of afforestation in obstacle areas, and promote a new step in afforestation in China. It plays an important role in improving the ecological environment, improving the productivity of forest land, controlling desertification land, and preventing soil erosion.

### **8. Future perspectives**

With the continuous strengthening of people's awareness of forest resource protection, the ecological value of forestry has attracted more and more attention. Mycorrhizal fungi, as an important role in forestry, play a positive role in the growth and development of trees and the whole forestry ecological environment. It is mainly manifested in the introduction of new species of trees, breeding and afforestation, promoting the growth and development of trees, affecting the photosynthesis of trees, strengthening the absorption of soil nutrients by tree roots and the resistance to adverse environments, and improving the soil and the whole forestry ecology.

### *Plant Microbiome and Mycorrhizal Fungi DOI: http://dx.doi.org/10.5772/intechopen.107373*

Although the research on mycorrhizal science is deepening, there are still shortcomings. In order to better apply mycorrhizal science to forestry development and provide a theoretical basis for the sustainable development of forestry economy, it is suggested to carry out research from the following aspects. At present, there are few statistical studies on the influence of geographical environment on mycorrhizal fungi. By analyzing the distribution characteristics of mycorrhizal fungi in various mountainous areas and forest areas under the influence of environmental factors, the dominant mycorrhizal fungi in various forest areas can be selected for investigation, analysis and in-depth study. The classification of mycorrhizal fungi needs to be further refined, which can be associated with peripheral factors such as symbiotic trees, surrounding undergrowth vegetation, and other microorganisms in soil and soil element content, to more comprehensively analyze the role of mycorrhizal fungi in forest land ecology. Molecular biology and omics are rarely used in mycorrhizal fungi, and the study on the dynamic changes of forest molecular level under the action of mycorrhizal fungi is rare, including omics such as molecules, proteins, and metabolism, which can strengthen the study on molecular level and further reveal the mechanism of symbiosis between mycorrhizal fungi and forest.
