Liu Shirong, Academician of the Chinese Academy of Engineering: How to make good use of the rich mine of forests to achieve "double carbon"?

Since 1979, when China designated March 12 as Arbor Day, with the continuous efforts of several generations, China's forest area and volume have maintained double growth for more than 30 consecutive years, with a total of 68 million hectares of afforestation, contributing about 1/4 of the new green area to the world, and becoming the country with the largest growth of forest resources and the largest planted forest area in the world. However, there are also hidden concerns behind the results. Due to the single tree species planted, a considerable number of plantations have been killed due to the invasion of alien species. In addition, ecological problems such as forest fires and soil fertility decline also pose many threats to artificial afforestation. In the face of the arduous task of carbon peak and carbon neutrality, how should we develop forests, how should we make choices about planting plantations and protecting natural forests, and can we find the key force to achieve the "double carbon" goal from forests?

Deforestation has had a huge impact on the ecology

Humans have relied on forests since their birth, especially during agricultural periods, where they have been the source of their livelihoods. Although the forest was exploited at the time, the damage was relatively limited. In the industrial age, population growth and industrialization exacerbated the consumption of natural resources. In the early days of the founding of the People's Republic of China, forest resources became an important pillar of our economy, and timber, as a key forest product, played a key role in industrial development and the improvement of people's lives. As a result, forests are regarded as valuable assets, which has led to the establishment of many forest industry bureaus to harvest timber in exchange for foreign exchange and to help build the country. Later we realized that forests are not only a source of timber, but also an ecosystem, and that overuse can weaken the timber production capacity and other ecological values of forests. Therefore, it is now time to avoid the mistakes of the past, such as large-scale clear-cutting that undermines the ability of forests to regenerate. Although some forests were planted after the early clear-cutting process, the economic tree species planted for timber production had limited ecological value, leading to an ecological crisis. After the 70s of the 20th century, sandstorms occurred frequently in northern China, which was related to the excessive use of land and the destruction of grassland. In response to this problem, we have implemented the "Three Norths" shelterbelt project, which aims to build a great green wall and suppress sandstorms. China is one of the countries with the most serious degree of desertification in the world, known as the "Three Norths" in the northwest, north and northeast regions, which are distributed in China's eight deserts, four major sandy lands and the vast Gobi, and the desertified land accounts for about 90% of the country's desertified land area, which is the core area of desertification prevention and control in China. Over the past 40 years, the three major sandy lands of Mu Usu, Horqin and Hulunbuir have all been reversed, and the forest stock in the "Three Norths" area has increased from 613 million cubic meters to 3.042 billion cubic meters. In 1998, the major floods in the north and south of China had a huge impact on production and life, and after in-depth analysis, it was found that the root cause of the floods was that a large number of forests in the upper reaches of the rivers were logged down and destroyed in order to produce food, and the deforestation led to serious soil erosion. In order to solve this problem, in 1998 we implemented the Natural Conservation Forest Project to protect natural forests, and successively implemented the project of returning farmland to forests, which is prone to soil erosion, and converted sloping farmland that is prone to soil erosion into forest land.

The protection of natural forests is closely related to biodiversity

Ecological prosperity leads to civilization prosperity. The history of human development is closely related to forests, and forest degradation will lead to ecological degradation and eventually the disappearance of civilization. We are now increasingly aware of the importance of natural forests, most of which are located in the upper reaches of major rivers, and that in order to protect the future of mankind, natural forests must be protected, which can reduce soil erosion, increase water conservation, and protect biodiversity. Biodiversity, including the diversity of plants, animals and microorganisms and their complex relationships, is the foundation of human survival and future development, providing endless wealth to mankind, without which there would be no ecosystem, which would eventually lead to the extinction of humanity.

From Hainan Island to the Great Khing'an Mountains, there are different types of forests in various places and are composed of different tree species. The composition of a particular forest is the result of long-term evolution, and they are mutually conditional, interdependent, and have complete stability. Understanding the natural succession of forests is essential for determining the appropriate composition and type of forests. The loss of natural forests leads to a lack of understanding of future afforestation and its structure, and once biodiversity is lost, it cannot be restored, such as the loss of many species. Humans can make steel and industrial materials, but they can't create new species in the natural succession process. If natural ecosystems are completely replaced by artificial, they will lose their adaptability, resistance and resilience to environmental changes. Therefore, people and ecosystems are inseparable, and our management and use of forests cannot simply imitate agricultural systems or traditional agricultural practices. Plantation management should minimize disturbance to nature and avoid excessive fertilization and pesticide application to maintain ecological balance and human health. Taking eucalyptus as an example, although wood can be obtained by planting a large number of trees in the south, it requires fertilization and pesticides, and once excessive pesticides and fertilizers enter rivers and groundwater, they will pollute the water environment and even pose a threat to human health. Therefore, it is necessary to recognize the evolution of forest structure and function and respect the natural carrying capacity when using forests. Although China has a vast territory, there are many arid and semi-arid areas and ecologically fragile areas, with a large population and diverse needs, and the construction of ecological environment is a permanent topic.

What are the problems with the afforestation of a single tree species?

At present, China's forest area is 231 million hectares, with a coverage rate of 24.02%, ranking first in the world. China has made significant contributions to forest protection, vegetation construction and restoration, accounting for 25% of the global contribution to greening. China's forestry development for more than 40 years has not only benefited the country, especially in improving the environment of the northern provinces, but also promoted the international ecological construction.

Although great achievements have been made in the construction of long-term plantations, they are also facing new problems. The pattern of pure artificial forests dominating the world with "Shajiabang in the south, Yang Jiajiang in the north, and larch in the northeast" has brought some new challenges, such as the decline of soil fertility, the intensification of forest diseases and pests, and the deterioration of the ability to adapt to changes in the ecological environment. China's large-scale land greening is now dominated by a single tree species, and 10 tree species from the north and south together account for nearly 70% of the entire plantation tree species, which has the weakness of low resistance to environmental change and poor ecological service function from the perspective of diversity. For example, pine wood nematode was introduced to China from the United States in 1982 and was first discovered in Nanjing, Jiangsu Province. Because there are no natural predators and native pine trees cannot resist it, and we have created a large area of artificial coniferous pure forest, it is easy to die in a large area after contracting this disease. In this way, a lot of manpower and material resources are wasted, and it will also lead to problems in our wood and pine resin production, and will also affect the ability of forest water conservation and regulation.

The so-called monoculture plantations in southern China often appear as "green deserts". When people see the plantation, they will have a feeling of green and lush, but if the litter and shrubs under the forest are removed, the woodland is very clean, and there is a phenomenon of "looking green from afar, and seeing water and soil flow from close", resulting in a great reduction in the ecological function of the forest. Science magazine has a report on "Death in Germany's Forests" It is pointed out that Germany is the most advanced country in the field of forestry scientific research, and the theory and practice are ahead of the world, at that time, it was also for timber production, planting a large number of pure European pine and Norwegian spruce plantations, with high economic value, but under the influence of global climate change, the superimposed impact of heat waves and drought led to damage to the growth of plantations, coupled with the infection of small silverfish, resulting in the death of about 300,000 hectares of Norwegian spruce and European pine, resulting in huge economic losses for forest farmers. Also in North America, Canada saw the death of 400,000 hectares of white spruce.

How will our forests be created in the future?

From a long-term perspective, to promote China's ecological construction, we need to think about how to build our forests in the future? What kind of tree species and forest structure should be selected to maintain the material, ecological, and cultural functions of forests? How to restore natural forests and manage the species that exist in them? This is the challenge we are facing. In the past, we were relatively blind and didn't know where to start, but just protected it, circled it, and let it recover naturally. After long-term positioning studies, it was found that even if tropical forests were restored for 50 years, their species richness was only 70%-80% of that of primary forests, and the species composition was far from that of original natural forests. To accelerate the restoration of natural forests, it is necessary to find the ecologically critical species for the restoration process. A large number of studies have shown that although there are few rare species in tropical complex natural forests, their ecological functions are important. It promotes niche diversification through non-random competition with common species, which is conducive to maintaining and maintaining the structure and function of ecosystems. For example, the slope of the tropical rainforest on Hainan Island (Hopea hainanensis Merr. et Chun), which has high economic value and important ecological functions. In the case of insufficient natural regeneration, the ecosystem function and natural forest can be quickly restored within a limited time and space by promoting natural regeneration and artificial replanting by promoting natural regeneration and artificial replanting, so that the slope barrier can be accelerated to regenerate and enter the main forest layer in a restricted or degraded state. In addition, the continuous planting of coniferous forests of the same species often leads to the decline of biomass and soil nutrients from generation to generation. In the case of Chinese fir trees, the biomass of the second generation will decrease by 24% compared to the first generation, and the third generation will decrease by 40% compared to the second generation. In the long run, plantations experience soil fertility decline and long-term productivity decline.

Of course, we should not blindly emphasize the ecological value of forests and ignore their material production functions, so how can we take into account multiple functions? How should we rebuild a healthy, stable, and resilient forest that can resist natural risks? This requires that in the process of cultivating forests, we should first take forests as an ecosystem, study clearly and link up the functional relationships between species, so as to form ecological complementarity and multi-functionality. In Guangxi, we found that the species diversity of pure forests of P. massoniana was low, which was not conducive to maintaining long-term timber production. Therefore, we introduced Castanopsis hystrix Miq., a high-value, shade-tolerant tree species, and planted it randomly or in clusters by appropriately thinning pure forests of Pinus massoniana. The root structure of red cone and P. massoniana is complementary, and nutrient absorption is not in direct competition. Moreover, the needles of Pinus massoniana combined with the broad-leaved litter of C. rubrum accelerated the decomposition through microbial action, making it easier for nutrients to enter the soil and be absorbed by plants, and promoting the co-growth of the two tree species. This ecological synergy transforms a single tree species into a multi-species mix, improving timber quality and economic value, while avoiding past clear-cutting operations and making the forest land a long-term cover selective forest.

Artificial greening should not only select suitable seeds, but also improve quality

Forests have a huge capacity to store carbon. Protecting and restoring forests and grasslands can increase the absorption of carbon dioxide, that is, carbon sequestration, which plays an important role in slowing down climate warming. It has been estimated that for every 100 million cubic meters of forest stock, an additional 160 million tons of carbon dioxide can be fixed. There are four main ways to use forests to mitigate climate change and achieve the "dual carbon" goal.

The first is to protect the forests. Forest vegetation and soil have a huge carbon storage capacity, from the annual carbon sequestration capacity, all kinds of ecosystems together, forests can account for about 80%, so the protection of about 65% of China's natural forests is also a major contribution to the response to global climate change.

The second is to actively create and expand the area of green space. China has been afforestation and reforestation for more than 40 years, and the achievements are remarkable, but the space for afforestation in the future is limited, and by 2030, our goal of forest coverage rate will increase to 25%, and eventually it may reach 26%-28%. The area of existing suitable forest land is limited, and most of them are in the western region, and the limitation of water resources will affect the afforestation effect and increase the cost of afforestation, and will also aggravate the contradiction of limited water resource utilization. This requires the selection of suitable tree species on limited land, the study of drought resistance and habitat adaptability, and the adaptation of tree species to climate change scenarios. For example, the increase of precipitation in the western region in recent years has increased the possibility of afforestation in some areas, which needs to be scientifically evaluated. In addition, improving forest quality is also the key to improving forest carbon sequestration capacity, including the quality of natural forests and planted forests. In natural forests, different tree species have different regulatory effects, and ecologically critical species and rare species contribute greatly to carbon sequestration, so these tree species should be replanted or supplemented in degraded secondary forests to improve carbon sequestration function.

Therefore, we must first protect natural forests and, of course, planted forests. Different tree species in plantations have different carbon sequestration capacities, different carbon content in different organs, different carbon sequestration rates and carbon densities, and tree species with high carbon sequestration need to be screened. In addition to increasing the aboveground carbon storage, it is also necessary to dig deeper into the soil carbon sink function. For example, some tree species are highly rooted, and the formation of corks in the root system can cause a large amount of carbon sequestered by plants to enter the soil, especially deep soils. It is also possible to cultivate high carbon sequestration tree species by means of genetic breeding, and improve the carbon sequestration capacity of forests by combining aboveground and underground. At the same time, the rotation period of the forest can be adjusted, for example, the rotation period of fir trees can be extended from 22-25 years to 30 or even 35 years, which will help restore the balance of soil nutrients and ensure that nitrogen and phosphorus remain balanced and stable. That's why we need to adopt the concept of ecological rotation to ensure ecosystem integrity, health and sustainability, while producing high-value large-diameter timber. It is also possible to improve the carbon sequestration capacity by changing the management mode of plantations and using the mixed breeding of different tree species, especially the young forests and natural secondary forests in China, which have great potential to increase sinks. Of course, all of this is premised on the fact that we must protect, restore and use these forests so that we can continue to contribute to carbon neutrality and global warming through nature-based forest carbon sequestration pathways.

The third is the multifunctional and sustainable management of forests. Given the cyclical nature of forest growth, it is necessary to study how to invest efficiently to maximize the cost-effectiveness of forest construction. Prevent the adverse effects of climate change, drought, warming, pests and diseases on forests, and select tree species that are adapted to climate change. Ensure the stability of tree species construction and forest community structure, improve the carbon sequestration capacity of aboveground and underground soils, and achieve high carbon sequestration function. At present, China's terrestrial ecosystem can absorb about 10%-15% of China's carbon dioxide emissions, which is an economical and effective solution, and enhancing terrestrial carbon sinks, especially the use of forest carbon sinks, can also provide more space for industrial development.

The fourth is to do carbon substitution. Wood products are a renewable resource, and if the house appliances are made entirely of wood, the carbon emissions generated by reinforced concrete can be avoided. Timber buildings contribute to the natural cycle and achieve net-zero emissions, and biomass products can be used to replace high-fossil energy emitting materials, insisting that carbon reduction and carbon sequestration go hand in hand. In short, under the "dual carbon" goal, there is a good prospect for improving the carbon sequestration function of forests, which is the most economical and effective way to improve the production and living environment of human beings. Therefore, we need to do a better job of protecting, restoring and using forests.