Das Kapitel diskutiert die entscheidende Rolle der Wälder im Kohlenstoffkreislauf und ihre Bedeutung für die Eindämmung des Klimawandels. Sie untersucht den historischen Kontext der Waldentwicklung, die Auswirkungen der Entwaldung und das Potenzial der Wiederaufforstung in tropischen und subtropischen Regionen. Der Text beleuchtet verschiedene Wiederaufforstungsstrategien, darunter natürliche Regeneration, Aufforstung und Agroforstwirtschaft, und präsentiert Fallstudien aus China, Ägypten, Äthiopien, Nigeria und Ecuador. Er betont auch die Bedeutung einer nachhaltigen Waldbewirtschaftung und den sozioökonomischen Nutzen von Aufforstungsinitiativen.
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Abstract
There are only two ways to slow down the increase of the carbon dioxide content of the atmosphere effectively: On the one hand, CO2 emissions from fossil sources and from land use changes can be reduced, and on the other hand, terrestrial CO2 reservoirs can be increased, mainly by increasing forest cover. The latter, forest increase is particularly effective in terms of carbon ecology if it takes place in the tropics and subtropics, where the growth and thus also the CO2 reduction potential is particularly high. Afforestation- provided it is carried out with site-appropriate adapted tree species- has a number of other positive effects, such as the prevention of erosion and flooding or the creation of work and recreational opportunities.
About 4 billion years ago, the primordial atmosphere was still free of oxygen, but had a very high carbon dioxide (CO2) content. Over millions of years, large amounts of CO2 were dissolved from the atmosphere into the oceans and stored in sediments (Schönwiese, 2020). Only through photosynthesis—initially in cyanobacteria, later also by land plants (Hanson & Rice, 2014)—was oxygen added to the atmosphere in large quantities and CO2 removed to the same extent for the build-up of biomass (Schönwiese, 2020). For photosynthesis, organisms need light. The higher land plants grow, the more sunlight they can use compared to other, nearby plants. Trees gain this competitive advantage by building long, woody stems. Forests therefore dominate the earth’s vegetation insofar as the climate allows trees to grow.
Forest and Atmosphere
The existence of trees can be traced back 300 million years (Boenigk & Wodniok, 2014). During this period, there have been repeated major climate fluctuations and corresponding changes in forest area and distribution. Forests and atmosphere are in dynamic equilibrium. If forests expand, large quantities of CO2 are removed from the atmosphere and stored in the wood of trees. Conversely, when wood rots or burns, CO2 is released into the atmosphere.
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Forests also affect the energy exchange at the Earth’s surface and thereby influence the atmosphere’s energy balance. Forests take energy from the atmosphere and use it mainly to evaporate water; the air above the forest cools. Conversely, the loss of forests usually leads to a warming of the air layer above the affected areas (Duveiller et al., 2018) (cf. article by Schwarzer starting on p. 255).
After the last ice age, forests had spread over half the global land area. Caused by climate fluctuations, they changed their shape several times, but retained their areal extent for a long time. It was only with the increase in human population and the expansion of agriculture that their decline began. The loss of forests affected the various parts of the world in waves: Europe in the millennium before last, North America in the century before last, and the tropics since the middle of the twentieth century. Today, forests cover 4 billion hectares, or merely one-third of the world’s land area (FAO, 2020a). In addition to virgin (primary) forests and commercial forests, degraded forests exist over a large area—in the humid and semi-humid tropics alone they occupy an area nearly the size of Australia (ITTO, 2020). These forests, which have been exploited in the past, are severely altered in structure and reduced in function. They can no longer adequately fulfill their role in the natural balance. In addition, there are roughly 450 million hectares of abandoned agricultural land worldwide (Campbell et al., 2008), most of which was previously forest. All these areas must be taken into consideration for purposes of influencing the CO2 content of the atmosphere to counteract climate change.
Currently, forests, together with the other land-based ecosystems, bind 31% of anthropogenic CO2 emissions; the oceans absorb an additional 23%. As in the past millions of years, vegetation and the oceans act as a “giant pump” (Schönwiese, 2020) that removes CO2 from the atmosphere. In recent decades, therefore, the atmosphere has been polluted with only a portion (46%) of global man-made CO2 emissions (IPCC, 2021) Even from this simple overview of the Earth’s carbon budget (Fig. 1), it is clear that there are essentially only two levers for reducing the anthropogenic CO2 load on the atmosphere: the burning of fossil fuels can be reduced or carbon reservoirs can be activated, with vegetation (minimizing forest losses and expanding forest area) playing an important role.
Fig. 1
The global carbon cycle. (Data from Friedlingstein et al., 2020). The largest carbon stocks on earth are found in the sediments, in the ocean, and in fossil deposits. In land-based ecosystems, most carbon is stored in the soil—especially in the permafrost soils of the Northern Hemisphere. More than half of the above-ground carbon is in the biomass of forests in the tropics and subtropics. Mankind pollutes the atmosphere primarily through the combustion of fossil fuels, the production of cement, and through the clearing of forests with carbon emissions, mainly CO2. Since currently only about half of all these emissions can be stored by vegetation and the oceans, the CO2 content of the atmosphere is continuously increasing
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Forest Management
For managing existing forests there are the following options:
Primary forests usually contain very large carbon reservoirs and a high level of biodiversity. These must be safeguarded. Silvicultural interventions in these forests can only be justified if clearing can be prevented through sustainable forestry use or if the adaptation of forests to climate change is actively supported.
On a regional scale, commercial forests can achieve wood stocks comparable to those of primary forests. They should therefore be managed so that carbon stocks remain stable and good quality wood is sustainably produced. This enables long-term removal of CO2 from the atmosphere, storing it in wood products. The impact improves with less energy used in further processing, with better raw wood quality, and with longer life and better recyclability of the wood products. At the same time, wood can replace other energy-intensive raw materials such as reinforced concrete (see also chapter “Bauhaus Earth”).
In degraded forests, the cause of the impairment (e.g., grazing) must first be eliminated. If these forests are placed under full protection, they will, over an extended period, revert to structured, fully functional forests. Through silvicultural measures, restoration programs can accelerate the restoration of forests and their CO2 sequestration. Forests can be converted into commercial forests so that the CO2-ecologically beneficial effects of wood use can be realized. An important management objective for these forests would be to build up the wood and carbon stocks of the stands.
Regarding currently forest-free areas that are to bear forest again, the following possibilities arise:
On cleared areas, forests will often re-establish themselves sooner or later without human intervention. So-called secondary forests are created through natural seeding and the sprouting of roots and tree trunks. Site conditions and existing landscape structures determine the speed at which natural reforestation and thus CO2 sequestration take place (Poorter et al., 2016). Silvicultural measures can accelerate natural reforestation, by reducing game feeding on young plants, for example, or by promoting the wider spreading of roots.
The artificial establishment of forests, when carried out in a professional silvicultural manner, is the fastest form of reforestation. New forest areas are created by planting and choosing the composition of tree species. Most afforestation restores forests in naturally suitable areas. But artificial irrigation makes it possible to create forests also beyond these growth limits.
Either way, if the newly created forests are left to their own devices, parts of the carbon stored in them will be released again through decomposition and fire. In the case of forestry use, the proportion of wood put to long-term use can be expanded by silvicultural measures, and a large part of the sequestered carbon can be removed from the atmosphere through use of the wood as a raw material. This avoids CO2 emissions by substituting fossil energies and helps to save climate friendly energies (solar, wind) for reasonable purposes.
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Forest in the Tropics and Subtropics
The tropics extend between the latitudes of 23.5 degrees North and 23.5 degrees South. Frost occurs there only in the mountains. At the equator, there is an ever-humid diurnal climate, which means that the temperature differences between day and night exceed those among months of the year. Seasons as we know them do not exist in the ever-humid inner tropics. The natural forests are evergreen deciduous forests with very high productivity and diversity. The further one moves from the equator to the tropics, the longer the dry seasons become. Many trees react to the dry season with leaf fall and an adaptation to the increasing risk of forest fires—through thicker bark, for example. With greater length of the dry season, forests become increasingly unproductive and sparse.
Bordering the tropics to the North and South are the subtropics. There frost may occasionally occur in the lowlands. Large amounts of rain fall on the eastern sides of the continents. This area of the subtropics is humid and well suited for tree growth. In the so-called Mediterranean subtropics, which are located on the Western sides of the continents, precipitation falls mainly in winter. As a result, the summers are dry. Trees must adapt to this dryness in summer and are thereby limited in their productivity. In the center of the continents, the subtropics are dry almost year-round. There forests can often barely survive without artificial irrigation.
The greatest destruction of forests is currently taking place in the tropics and subtropics (see also chapter “Stop Rainforest Deforestation”). The release of large amounts of carbon could be prevented if it were possible to stop this forest destruction. Enormous carbon sequestration potential exists where forest areas were cleared but not converted into permanently managed agricultural land. The ever-humid tropics and subtropics support very high productivity, so that reforestation would enable sequestration of large quantities of CO2 within a few years (Fig. 2). Furthermore, measurements show that reforestation in these regions has cooling effects (Alkama & Cescatti, 2016). Reforestation would therefore make positive biochemical and biophysical contributions to limiting global warming.
Fig. 2
Range of values for the annual CO2 reduction (positive values) or release (negative values) in the first 20 years after afforestation. The studies show that a large absorption effect can be expected from the build-up of biomass, especially in the tropics. (Data from Paul et al., 2009)
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With large-scale afforestation, large quantities of valuable timber could be produced in an economically profitable way. There would thus be an economic incentive to promote afforestation even without financial subsidies. Trees could be integrated into both industrial and smallholder agriculture. Due to the large share of agricultural land, the resulting agroforestry systems not only have a large CO2 sequestration potential, but also offer many other social, ecological, and economic advantages.
For a 3-degree warmer world, climate models predict for the tropics and subtropics a moderate warming and a redistribution of precipitation (Gutiérrez et al., 2021). For the Sahara, higher precipitation is expected, so that parts of this desert may possibly green up (IPCC, 2021). Tropical and subtropical vegetation would spread into the current desert areas. Afforestation could support this process, and unexpected dry periods could be buffered by artificial irrigation. Conversely, precipitation is expected to decrease in the Amazon region. Its current vegetation is not adapted to this, leading to an increase in forest fires with release of large quantities of CO2. Comprehensive forest management could remedy this situation. Existing forests would have to be protected from man-made fires. Forest development on sensitive sites could be specifically adapted to the expected climate conditions. Climate-adapted afforestation and rehabilitation measures on deforested or degraded forest areas could achieve quite fast reforestation. This would also relieve the pressure of timber use on the still intact tropical forests. Insofar as these activities yield qualitatively valuable wood, the cost of those efforts can be offset, and CO2 can be sequestered in wood products.
Practical Examples
The foregoing reflections provide the global framework. Planning and implementation of concrete measures require, however, a regional approach due to the highly variable and complex local ecological, technical, and socio-economic conditions. In what follows, reforestation in different regions of the tropics and subtropics is therefore presented in five case studies that were prepared with the Institute of Silviculture at the Technical University of Munich during the time when the author was director of this institute.
Reforestation of Subtropical Forests in Central China
China is home to about one-tenth of the world’s tree species, and half of it would by nature be densely forested (Ahrends et al., 2017; Wenhua, 2004). Agriculture developed here more than 10,000 years ago, and forest land gradually became agricultural land. By the twentieth century, the forest area had decreased to about one tenth of the national territory (Ahrends et al., 2017; Miao et al. 2016). In this century, the remaining forests, mostly in mountain areas, were systematically cleared to use the wood for economic development and to create agricultural land for the population (Summa, 2013). On the now predominantly arable land, precipitation increasingly ran off the surface, causing soil erosion. This resulted in greater sediment loads and higher flood peaks in water bodies, with corresponding adverse effects on people, infrastructure, and drinking water supply. Frequent catastrophic floods finally led to a change in China’s land use policy in the 1990s, with the aim of compensating for the enormous loss of forest land (Ahrends et al., 2017). Agricultural use of slopes steeper than 25 degrees was outlawed nationwide in 1991 (Liu et al., 2011). The 1998 Forest Conservation Program aimed to rehabilitate degraded forests and stop their exploitation. With the help of afforestation, soil and water conservation were supposed to be promoted and timber production to be increased again (Wenhua, 2004). As a result, China’s forest area temporarily expanded by 4 million hectares annually. Today, once again, almost a quarter of the country’s area is forested. With 80 million hectares (roughly the area of Turkey), China currently has the largest area of forest plantations in the world (Ahrends et al., 2017). As a result, the carbon stock in the biomass of Chinese forests has since 1990 increased by 4.2 billion metric tons or nearly doubled (FAO, 2020a, b). However, due to the still limited use of wood and the still young forest plantations, there is still a nationwide wood shortage, which the Chinese are trying to mitigate through extensive wood imports. In many cases, other materials are substituted for wood in construction, so that the centuries-old Chinese tradition of timber construction is at present barely practiced.
Located in the center of China, the Qin Ling Mountains have a rich flora and fauna. Among other things, they are home to the famous giant panda (Ailuropoda melanoleuca). The mountain range forms the climatic divide between the dry, temperate Northern parts of China and the humid (sub-)tropical Southern China, which is characterized by the summer monsoon. The mountains are an important water source for Northern China, which tends to be short of water. National resource planning therefore provides that this region supply water to China’s economic centers in the dry North.
The abrupt abandonment of agricultural land in steep terrain created in the Qin Ling Mountains large areas that are now mainly covered by grass and shrub vegetation. From older protected areas, where agricultural use had been prohibited for a long time, it is known that rich, stable mixed forests can in due course develop on former agricultural lands. This process of reforestation can be accelerated by afforestation. There is great interest therefore in using afforestation to restore forest to formerly cleared Qin Ling Mountain areas as quickly as possible. This should lead to a more consistent water supply and reduce the high sediment loads in the rivers. One focus is on the creation of stable, near-natural mixed forests of native tree species.
In the course of a German-Chinese cooperation, reforestation areas with native tree species were established in the Qin Ling Mountains on agricultural land that had been set aside some years ago. In 2007, the tree species Pinus tabuliformis (Chinese pine), Quercus variabilis (Chinese cork oak), Acer truncatum (Chinese Norway maple) and Pistacia chinensis (Chinese pistachio) were planted. The aim of the study was to determine the survival rate and productivity of these tree species on formerly agricultural lands (Summa, 2013).
The tree species chosen are native to the area and therefore well adapted to the prevailing climate and soil conditions. They enable the production of valuable wood. Furthermore, seeds and plant parts are used in the pharmaceutical industry or for vegetable oil production. It was very difficult, however, to acquire suitable planting material of the above-mentioned tree species which, in regard to its genetic characteristics, meets the requirements for establishing near-natural forests. For this purpose, the seeds for growing the young forest plants should ideally come from recognized seed stocks so as to ensure large genetic diversity as well as favorable quality characteristics of the future forests. In this case, too, a globally common problem manifested itself: for larger afforestation efforts, we still need to identify a sufficient number of tested resources for forest seeds and establish a continuous testing system.
The experimental afforestation areas of 2007 have meanwhile developed into dense forest and completely replaced the previous grass vegetation. Rainwater can now seep into the soil and erosion hardly occurs anymore, even on steep slopes (Kägler, 2019). This is associated with rapid storage of CO2 in tree biomass and soil. Compared to other forms of land use, the establishment of forests proves to be the best measure for erosion control (Fig. 3).
Fig. 3
Surface runoff and erosion when covered with forest, grassland, maize field, and young tea bushes in combination with root crop cultivation. Among all land uses, forests provide the best protection against erosion and increased surface runoff. (Data from El Kateb et al., 2013)
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In an accompanying study, the local population was asked about their acceptance of afforestation. Due to the loss of agricultural land, many local residents need additional sources of income. Afforestation is therefore always of interest to the population if it can improve their economic situation. This underlines the importance of ensuring that reforestation can also contribute to securing the livelihood of the rural population (Wang et al., 2015).
Reforestation of Desert Areas in Egypt
Egypt is located in the dry subtropics. The country consists mainly of desert and is thus almost devoid of vegetation. Forests cover less than 0.1% of the country (FAO, 2020c). Remnants of natural forests exist only in the protected mountain region of Elba in the south of the country. The fertile land areas along the Nile are fully used for agriculture and increasingly as settlements for the rapidly growing population. However, Egypt has had a national forest administration since the eleventh century and has thus been able to systematically establish plantation forests since the 1970s. Within the framework of the “National Program for the Safe Use of Treated Wastewater for Afforestation,” attempts have been made since the 1990s to use the large amount of urban wastewater for the cultivation of a variety of tree species in desert areas. The aim is to establish large-scale afforestation to bind CO2, to protect cities and agricultural land from sandstorms, to protect the soil from wind erosion, to produce wood and bioenergy, and to create green spaces and new jobs for the local population.
Highly productive afforestation in the desert through irrigation with pre-cleaned (and otherwise unusable) wastewater has great potential for CO2 sequestration. Large knowledge gaps still exist, however, in the field of silviculture and sustainable management. These gaps concern the selection of suitable tree species and seeds, matters of forest care, and technical-administrative challenges in the construction and maintenance of wastewater treatment and irrigation facilities. In a long-term practical trial, artificially irrigated afforestation areas were therefore established at three locations in Egypt with 14 native and foreign tree species from tested forest genetic resources. The aim of the studies was to resolve the following questions: what are the survival rates and productivities of the planted tree species under different irrigation regimes? And how does their wood quality develop?
The results show that the tree species Eucalyptus camadulensis (red eucalyptus), Corymbia citriodora (lemon eucalyptus), Casuariana equisetifolia (horsetail-leaved casuarina) and Khaya senegalensis (African mahogany) show exceptionally high productivity with an optimal supply of the nutrient-rich effluents and predominantly also develop good wood quality. At the age of 15 years, the stands can store 50 metric tons of CO2 per hectare annually (El Kateb et al., 2022). Other tree species tested, however, did not cope with the conditions in the desert. Due to the social upheavals in recent years, securing irrigation was a major problem.
The experiments show that, where urban wastewater is available, desert afforestation has great potential for CO2 sequestration. As the population continues to grow, settlements are increasingly relocated to desert areas. These newly emerging cities could form the core of new afforestation areas that store CO2 and reduce the widespread wood scarcity. Due to the dry climate, wooden buildings in the desert have an extremely long lifespan. In addition, there are from the field of climate modelling indications that support further desert afforestation: under expected global climate conditions, the Sahara could receive more precipitation, and desert afforestation could promote cloud formation and precipitation at the regional level (Branch & Wulfmeyer, 2019).
Reforestation and Rehabilitation of Degraded Primary Forests in Ethiopia
In the tropical and subtropical areas of Africa, forests are dwindling (FAO, 2020d). The main causes of this development are poverty of the population, armed conflicts, and deficiencies in forestry legislation and supervision. Growing population density and rising world market prices for food and energy will exacerbate this situation. The increasing development of the continent enables growing access to previously untouched resources and forest areas. In view of the weaknesses of public administrations, which are expected to continue, careful management of the tropical forest can be achieved only if the sustainable use of forests offers improved income opportunities for the local population.
Ethiopia is among the longest-settled regions on earth. Despite its long history of settlement, the country has a diverse tropical flora and fauna and is one of the most important centers of biodiversity in the world. The Ethiopian flora comprises about 7000 higher plant species, 15% of which are endemic (Gebretsadik, 2016). The country has a varied topography and geology which produce a diversity of different habitats. The climate is shaped by a dry season, as is typical in the marginal tropics. At the beginning of the twentieth century, over one-third of Ethiopia was still forested (Dessie & Christiansson, 2008). Today, only 16% of the country is covered by forest (FAO, 2020d). Ethiopia’s forest loss is currently above average among African countries.
One of the largest closed forest areas is the Munessa-Shashamene forest in the center of Ethiopia, covering approximately 35,000 hectares. The natural vegetation of the area is tropical mountain forest. Its structure is dominated in the canopy by the tree species Podocarpus falcatus, which is intermixed with the tree species Syzygium guineense, Prunus africana and Croton macrostachyus depending on the location (Strobl, 2011). Based on a cooperation with European research institutions, agricultural land and natural forest areas were converted into forest plantations in the past. Since the 1960s, the Shashamene Forest Industry, which is responsible for the forest area, has planted pure stands of the foreign tree species Eucalyptus saligna, Cupressus lusitanica and Pinus patula on 7000 hectares (Strobl, 2011). These monocultures have an annual timber growth of 20 to 25 cubic meters per hectare at a stand age of 20 years, with stocks averaging 300 cubic meters per hectare. Per hectare, these forest stands have thus stored about 300 metric tons of CO2 in the wood of the trunks alone and are currently sequestering another 20 metric tons annually. However, pure stands are regularly attacked by insect pests. The local population has lost its traditional usage rights in the plantations. They practice agriculture and livestock breeding on the remaining agricultural land. Since the afforestation, remnants of natural forest have been used even more intensively for firewood and as forest pasture. Young trees are thus browsed or cleared by cattle. This gives the former primeval forest areas a park-like character. These forests are degraded over a large area and will dissolve completely over the years.
The example of the Munessa-Shashamene forest area clearly shows how problematic it is simply to transfer practices of nineteenth century European forestry to tropical Africa. The future lies more in developing sustainable utilization practices for the remaining and mostly overused natural forest remnants and in converting unnatural timber plantations back into near-natural, multifunctional forests.
Doing this requires researching the characteristics and dynamics of the natural forest. This was done in a cooperation between Ethiopian and German universities. Since 2006, a joint experimental station has been established in the Munessa-Shashamene forest area. This station has been the basis for numerous studies investigating the reaction of natural forests and plantation stands to different ecological factors and their control through silvicultural interventions. The silvicultural studies pursue two main questions: how can the overexploited natural forest be regenerated and made attractive for sustainable use by enriching it with lucrative native tree species? And how can the functions of the non-natural plantations be stabilized and improved by intensive thinning and regeneration with native tree species?
Due to overexploitation, many gaps have appeared in the canopy layer of the natural forest. At the same time, subsequent tree generations fail due to browsing and firewood use. Therefore, the native tree species Cordia africana, Juniperus procera, Prunus africana and Podocarpus falcatus were planted in these gaps as an experiment (Birru et al., 2011). The long-term aim was to determine which native tree species could be introduced into the region’s overexploited natural forests under which light conditions. First results showed that especially Juniperus procera and Podocarpus falcatus can be established very successfully by planting. These two tree species were therefore recommended for the rehabilitation of degraded forests in the Ethiopian highlands.
Coniferous afforestation is highly productive but has, due to deficient forest management, developed into susceptible monocultures with poor timber quality. Currently, these stands are cut down according to plan at the age of 30 to 35 years. In a series of experiments, it was investigated how the growth of valuable wood and the stability of the stands can be improved through targeted promotion of the most vital and highest-quality trees. For this purpose, trees that impede the growth of the most promising trees were felled in thinning operations. These interventions also allow additional light to reach the soil so that native tree species can reestablish themselves and the stands can develop from pure stands to near-natural mixed stands (Strobl, 2011).
Further studies show that conversion of plantations back to more natural stands reverses adverse changes in the topsoil that accompanied the cultivation of the pure coniferous stands that were far away from close-to-nature (Ashagrie et al., 2007).
The studies in the Munessa-Shashamene forest area provided fundamental insights into the management of tropical forests. They also provided positive impulses for sustainable forest development in Ethiopia, which have been incorporated into the country’s forest management guidelines. The research station and the study plots were handed over to the largely underfunded Ethiopian universities after project funding was completed. A visit 10 years later showed that the research station had fallen into disrepair, the study plots were neglected, and usable materials were found in the households of the surrounding villages. This underlines the need to involve in such projects not only scientific expertise and practical experience in afforestation and forest rehabilitation measures, but also the local population, administration, and businesses. Only in this way can such projects be successful in the long term.
The reforestation activities initiated by Prime Minister Abiy Ahmed can also be seen as an indicator of more sustainable forest development in Ethiopia. According to government figures, 350 million trees were planted in 2019. With this number of plantings, a region the size of Saarland’s forest area can be covered. The government in Addis Ababa does indeed have large areas in mind: Ethiopia’s participation in the African Forest Landscape Restoration Initiative (AFR100, 2021) for the restoration of 100 million hectares of land in Africa and the Great Green Wall Project (Mirzabaev et al., 2022) for the restoration of a further 100 million hectares in the Sahel are proof of this.
Rehabilitation of Degraded Forest Areas in Nigeria
Nigeria stretches from the dry forests of the Sahel to the ever-humid rainforests of the Atlantic. Nigeria is home to 220 million people. The country has one of the highest rates of deforestation. Since 1990, 20% of Nigeria’s forest area has been lost. Today, forests cover 23% of the country’s surface (FAO, 2020e).
In Nigeria, forest planning days for the production of timber have been observed since the beginning of the twentieth century, especially in the tropical-humid South of the country. In the Oluwa and Omo forest reserves, large-scale afforestation of degraded primary forest areas began in the 1960s. Initially, the areas were planted using the Taungya system, which originated in Asia. In this agroforestry system, the manual cultivation of tropical timber is usually carried out by contractually bound farmers or agricultural workers who are given the right to temporarily grow agricultural plants next to the still young trees. After one or two years, the tree canopy closes and the farmers cultivate a new plot of land. After a few years, the forest plantation is cleared, and the cycle begins anew. From the 1980s onwards, mechanization prevailed and replaced the taungya system.
In the beginning, mainly native tree species such as Nauclea diderrichii, Entandrophragma spp., Guarea spp., Terminalia spp., Khaya spp. and Lophira alata were used. From the 1960s onwards, however, foreign tree species dominated—especially Gmelina arborea (trade name Gmelina) and Tectona grandis (trade name Teak). In the Oluwa and Omo Forest Reserve, Gmelina arborea dominates today with a share of about 90%. The original goal was to produce pulp with a production period of 10 years. For this purpose, the soil was turned over, after which Gmelina arborea was planted and competing ground vegetation was fought with great effort. In the following years, the plantations were protected from fire. No measures were taken to increase the quality of the wood, as it was to be used exclusively as mass for the pulp industry. The only problem was that the pulp mill, which was supposed to process the wood, did not work. Therefore, the focus shifted toward producing valuable wood in longer production periods of 15 to 20 years, and the plantations were systematically thinned to increase the proportion of valuable trees and to accelerate their growth.
The sustainability of forest plantations is often questioned. Central to such criticisms are the threat of soil damage and the loss of biodiversity, which are comparable to those accompanying conversion to agricultural land. For this reason, the Technical University in Akure, in cooperation with German universities, conducted studies on Gmelina arborea plantations in the Oluwa and Omo Forest Reserves to answer the following questions (Onyekwelu et al., 2006): Does the cultivation of Gmelina arborea lead to a loss of soil quality relative to primary forest areas? And is a loss of biodiversity to be expected with the cultivation of Gemelina arborea?
The results show that nutrient stocks in young and middle-aged plantations are somewhat lower than in the natural forest. However, there are no significant differences across all ages. On the contrary, soil conditions improve with the age of the plantations so that Gmelina forest plantations can be operated in the long term, over several generations, without loss in soil fertility. Due to the canopy closure, to the warm and moist conditions in the plantation, and to the activity of decomposers (such as earthworms), organic as well as mineral nutrients are incorporated into the soil. Organic carbon accumulates in the soil and sequesters long-term the atmospheric CO2 that is bound by the plants. At the same time, the trees’ nutrient supply and hence their productivity and CO2-reducing effect is maintained or even improved.
The question is then not whether, but how forest plantations can be run sustainably. Much damage is caused, for example, by the cultivation of unsuitable conifer species, by soil compaction and humus losses during mechanized soil preparation, by burning off residues from clearing, and by inappropriate timber harvesting techniques. If these practices are avoided and if, in addition, the nutrient-rich leaves and twigs are left on the plantation after the timber harvest and the production period for valuable roundwood is increased to 25 years, then soil quality in the Gmelina plantations can be maintained at its original level over several tree generations. The studies show that suitable management of forest plantations can avoid the kind of a long-term loss of soil quality occurring after conversions from forest to agricultural land.
The older Gmelina plantations, which have already been intensively studied, were compared in terms of their biodiversity with degraded forests in the Oluwa Forest Reserve and with a primary forest in the neighboring Akure Conservation Area (Onyekwelu & Olabiwonnu, 2016). The results show that in regard to the older trees in the forest plantation, biodiversity is limited both in terms of the plant families and species involved. This is not surprising, as the sole management objective of the plantation was to grow Gmelina arborea. It is remarkable, however, that, since planting began 26 years ago, eight tree species from seven different families have naturally established themselves in the plantation in addition to the planted Gmelina arborea. Two of these tree species are classified as endangered.
The biodiversity of seedlings and young trees below the crown layer, however, is comparable in all three forest types. In the Gmelina plantation, 13 tree species from 17 plant families are found in the young trees. In the seedlings, there are 24 tree species from 17 plant families—including several economically important native tree species such as Cola gigantea, Celtis zenkeri, Bridelia ferruginea, Pterygota macrocarpa, Cleistopholis patens, Sterculia rhinopetala and Strombosia pustulata.
The woody layer below the canopy of the planted tree species is thus crucial for assessing the biodiversity of forest plantations. Older plantations, especially, can contribute to the preservation of biodiversity and endangered tree species. Furthermore, the natural regeneration of forests plays a crucial role in the dynamics of forest ecosystems and their long-term development. After the harvest of Gmelina arborea, the rich natural regeneration of the plantation stands would develop into a mixed stand consisting of native tree species, which would also contain various economically lucrative tree species. These stands could therefore continue to be managed in a near-natural way.
In summary, it can be concluded from these studies that, if properly managed, forest plantations can contribute to the preservation of soil fertility and biodiversity. At the same time, they produce valuable wood and sequester CO2 long-term. In the medium term, forest plantations can also be converted into near-naturally managed commercial forests and ultimately support the restoration of natural forest areas. However, the development of the study area also showed that forest management is possible only in conjunction with adequate structures for processing and selling the wood. The development of new forest areas therefore requires cross-sectoral regional planning.
Reforestation of Tropical Mountain Rainforests in Ecuador
Ecuador is among the most biodiverse countries on Earth. Much of this biodiversity is found in Ecuador’s forests, which are, however, massively threatened by one of Latin America’s highest rates of deforestation (Mosandl et al., 2008).
Whereas in other South American countries, such as Chile, forest area is continuously increasing through afforestation, it is steadily decreasing in Ecuador (between 2010 and 2020 by 53,000 hectares annually, according FAO, 2020f). This trend is due to low afforestation activities and steady deforestation. As part of a large-scale research project of the German Research Foundation (DFG) in the mountain rainforest of Southern Ecuador, the processes underlying forest loss were investigated in detail (Beck et al., 2008; Mosandl & Günter, 2008; Bendix et al., 2013). The main driver of deforestation turned out to be the clearing of natural forests to obtain pastureland. As a result of the invasion of grazing areas by other vegetation (mainly bracken), which strongly impaired grazing and limited grazing periods, new grazing areas had to be created continually. This caused substantial expansion of abandoned pasture areas, which in turn spawned the thought that these degraded areas should be used in some way to reduce the clearing pressure on the still existing natural forests. Within the framework of the DFG research project, experiments were conducted to make these areas fit to be grazed again, and various reforestation experiments were established toward restoring the natural forest.
On the areas earmarked for afforestation, there were only very few naturally accrued woody plants, so that natural reforestation was not to be expected within the foreseeable future. Without afforestation, neither restoration of biodiversity nor a possibility of future use could be expected. The procurement of suitable planting material for the afforestation proved to be a major difficulty (Stimm et al., 2008). The lack of tree nurseries, seeds of indigenous trees, and knowledge about fruiting and cultivation of forest plants delayed the start of afforestation considerably. Apart from seeds of exotics such as Pinus patula and Eucalyptus saligna, there was practically nothing available on the market. Studies on the fructification of natural forest tree species, own seed harvests, and greenhouse trials were necessary to produce the planting material needed for the afforestation. Already 2 years after the successful planting on the afforestation plots, large differences between native and exotic tree species became apparent. The native tree species had great difficulty surviving on the strongly sunlit open areas; only Tabebuia chrysantha (current name Handroanthus chrysanthus) was able to keep up with the exotics Pinus patula and Eucalyptus saligna by achieving similarly good survival rates (Fig. 4). The growth of the native tree species was also very limited in the open areas. Only Alnus acuminata, as a pioneer tree species, could reach heights of over one meter as the two exotic tree species.
Fig. 4
Survival rate and growth heights of all trees 2 years after afforestation. (Data from Aguirre, 2007) (Aa = Alnus cuminata, Ha = Heliocarpus americanus, Cm = Cedrela montana, Jn = Juglans neotropica, Tc = Tabebuia chrysantha, Es = Eucalyptus saligna, Pp = Pinus patula)
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Compared to the unsatisfactory growth of the native trees in open areas, native trees planted in gaps in the natural forest (enrichment planting) or under the thinned-out canopy of pine plantations developed splendidly. This ultimately led to the recommendation for forestry practice in Ecuador to work in difficult afforestation sites (especially in open areas) with easy-to-establish pine (Pinus patula) or alder (Alnus acuminata) plantations. Under the thinned-out canopy of pines or alders, the native tree species can then later be raised without problems.
Conclusions
Forests are natural carbon sinks, removing substantial amounts of CO2 from the atmosphere. This absorption effect of forests can be maintained or increased by protecting existing forest areas from clearing, by forest restoration, and through sustainable management. In the medium term, however, the restoration of former forest areas through reforestation is necessary for achieving a stabilizing effect on the atmosphere’s CO2 content (IPCC, 2018). Forest restoration is especially efficient in the humid tropics and subtropics, as the trees there are highly productive and hence absorb a lot of CO2 (Strassburg et al., 2020).
The build-up of carbon stocks in newly planted forest areas takes place over many decades. However, the annual CO2 sequestration rate of forest areas in the subtropics and tropics reaches a maximum after just a few years and then decreases steadily. The only way to sustainably maintain the high sequestration potential of these forest areas is through the targeted renewal of forests with the help of silvicultural measures. Wood obtained in this way can then permanently store the sequestered carbon in the form of durable wood products. Wood can replace raw materials with a negative CO2 balance and thus contribute to reducing CO2 emissions. These substitution effects must be taken into account in evaluating forest-based solutions to protect the atmosphere. Even wood from damaging events can usually still be used and then generate such substitution effects. The sustainable production and efficient use of wood should consequently be part of any decarbonization strategy.
The case studies have shown that the restoration of forests in many regions of the subtropics and tropics can contribute to stabilizing the natural balance (e.g., through protection of soil, water resources, and biodiversity) and can provide many positive impulses for rural and economic development. These significant side effects must be taken into account when evaluating forest-based measures. After all, sustainable forest management will only succeed if the local people benefit from the forest. Due to heterogeneous local and cultural conditions, implementation should therefore always involve the local population. The regionally varying climate changes also require a regionally adapted approach (IPCC, 2021).
Forest-based measures to reduce atmospheric CO2 emissions therefore go far beyond the planting of trees (Girardin et al., 2021). They encompass nature and society. They are part of global efforts to moderate the rise of atmospheric CO2 through “nature-based solutions” (Seddon et al., 2021). The global potential of nature-based solutions is estimated at 24 billion metric tons of CO2 in relief for the atmosphere annually, taking into account the areas needed for agricultural production and the preservation of biodiversity (Griscom et al., 2017). However, this potential can only be realized if the costs for the metric ton of CO2 saved are not taken into account. If costs, e.g. for planting and other forest operations are taken into account, the savings potential is reduced. At a maximum price of €100 per metric ton of CO2, however, savings would still amount to 11 billion metric tons of CO2 annually (Griscom et al., 2017)—equivalent to about 30% of the emissions caused by humans. “Forest-based solutions” can make a decisive contribution here (Roe et al., 2019; IPCC, 2018; Griscom et al., 2017; Felbermeier et al., 2016; Canadell & Raupach, 2008; Burschel & Fabian, 1999)—especially in the tropics and subtropics (Strassburg et al., 2020). Even if there is still high uncertainty in the estimates of CO2 sequestration (Griscom et al., 2017) and even if it is to be feared that afforestation will often favor the wrong tree species (as in Cambodia, cf. Scheidel & Work, 2018) or the wrong sites (as in the upper reaches of the Paramo in Ecuador, cf. Quiroz Dahik et al., 2021)—there are nonetheless enough suitable areas and tree species as well as promising technologies available to restore the forests. Whether the areas estimated to be available worldwide (about 1 billion hectares, which roughly corresponds to the area of Europe) will actually be afforested and whether afforestation has the potential of binding 3–4 billion metric tons of CO2 annually (Canadell & Raupach, 2008), are irrelevant for the afforestation efforts that are necessary now. What seems of foremost importance is that the forest-based proposals for combating the global climate crisis—formulated as early as 1999 by leading forest scientists in a Forest-Wood Manifesto—be finally taken up (Burschel & Fabian, 1999). Measures of forest/carbon management that have been shown to be carbon-ecologically effective, must be implemented on a larger scale and not be discredited from the outset as “modern trade in absolutions” or as “nature-based distractions.”
Forest-based solutions offer great opportunities for successful bioeconomic development. By establishing efficient and socially balanced value chains—from ecological forest plant production and sustainable forest management to careful harvesting of wood that takes biodiversity aspects into account to the production and sale of wood products—we can achieve many positive effects that are lacking in the previous economy based on fossil resources. Taking account of the interests of the local population and making use of market mechanisms, it should be possible with appropriate economic incentives to restore large forest areas. Here the emitters of CO2 from fossil fuels should also be involved by paying for the external costs of their economic activities and providing funds for forest restoration. However, this must not come at the expense of the priority tasks of CO2 avoidance and the conversion to renewable energies.
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