Suitability of large-scale tree plantation models in Africa, Asia and Latin America for forest landscape restoration objectives

Today, tree plantations play a crucial role in supplying wood and wood-based products. They supply almost half of global demand, as well as supporting a diversity of ecosystem services. In tropical and subtropical areas, where tree growth is optimum and large tracts of land are available, forest restoration is presented as one of the most effective strategies for climate change mitigation. For these reasons, large-scale tree plantations are being encouraged in Africa, Asia and Latin America. Based on a review of the literature and of public databases on forest plantations, we drew up a typology of large-scale tree plantations in Latin America, Africa and Asia using four criteria: the management objective (production versus protection), number of species planted (multi-species versus mono-species), origin of species (exotic versus indigenous) and management status (industrial companies, private smallholders, state). Our analysis identified seven main plantation types and reveals that the two most common types represent almost 60% of the total planted area: (1) private mono-species plantations using exotic species; and (2) public production-oriented mono/multi-species plantations of indigenous trees. Numerous experimental studies were conducted in the 1950s and 1960s with a wide range of tree plantation models. However, few were adopted by operators because the production rates and financial returns were considered low. The dominant tree plantation types are failing to meet most of the forest restoration objectives set out in the Bonn Challenge (i.e., productivity, carbon storage, biodiversity conservation, rural livelihoods). Alternative large-scale tree plantation models could be promoted by focusing on the other goods and services that plantations can provide. This could be achieved if more diverse stakeholders were involved in plantation design and management processes, and if appropriate technical, financial, and institutional incentives were developed.


Suitability of large-scale tree plantation models in Africa, Asia and Latin America for forest landscape restoration objectives
Today, tree plantations play a crucial role in supplying wood and wood-based products. They supply almost half of global demand, as well as supporting a diversity of ecosystem services. In tropical and subtropical areas, where tree growth is optimum and large tracts of land are available, forest restoration is presented as one of the most effective strategies for climate change mitigation. For these reasons, large-scale tree plantations are being encouraged in Africa, Asia and Latin America. Based on a review of the literature and of public databases on forest plantations, we drew up a typology of large-scale tree plantations in Latin America, Africa and Asia using four criteria: the management objective (production versus protection), number of species planted (multi-species versus mono-species), origin of species (exotic versus indigenous) and management status (industrial companies, private smallholders, state). Our analysis identified seven main plantation types and reveals that the two most common types represent almost 60% of the total planted area: (1) private mono-species plantations using exotic species; and (2) public production-oriented mono/multi-species plantations of indigenous trees. Numerous experimental studies were conducted in the 1950s and 1960s with a wide range of tree plantation models. However, few were adopted by operators because the production rates and financial returns were considered low. The dominant tree plantation types are failing to meet most of the forest restoration objectives set out in the Bonn Challenge (i.e., productivity, carbon storage, biodiversity conservation, rural livelihoods). Alternative large-scale tree plantation models could be promoted by focusing on the other goods and services that plantations can provide. This could be achieved if more diverse stakeholders were involved in plantation design and management processes, and if appropriate technical, financial, and institutional incentives were developed.

Introduction
In 2020, forests covered 31% of the global land area (4.06 billion hectares) and the vast majority (93%) were considered natural (FAO, 2020). There was a net decrease of 3% in the global forest area between 1990 and 2015. More specifically, the natural forest area worldwide decreased from 3,961 million hectares (M ha) in 1990 to 3,721 M ha in 2015. Most forest loss occurred in tropical regions in Central and South America, South and Southeast Asia and Africa (Keenan et al., 2015). In contrast, the area of planted forests has risen from 4% of the world's total forest area in 1990 to 7% in 2020 (FAO, 2020). In absolute terms, planted forests increased from 167.5 to 294 M ha over the period 1990-2015 (Payn et al., 2015;FAO, 2020). Therefore, planted forests now play a crucial role in roundwood production. They supply 47% of world demand for roundwood (including timber, woodfuel and pulp production) and provide a diversity of ecosystem services (FAO, 2017;Baral et al., 2016).
In the context of climate change and biodiversity loss, well managed tree plantations can reduce pressure on natural forests, capture CO 2 , enhance biodiversity conservation, restore degraded land or ecosystems, and improve food security and nutrition for rural populations, by providing a source of income, employment and economic growth (Chazdon, 2008;HLPE, 2017).
According to Bastin et al. (2019), "the restoration of trees remains among the most effective strategies for climate change mitigation". Thus, several international processes have been launched, such as the REDD+ process or the Bonn Challenge to support and promote tree plantations around the world, especially in Africa, Asia and Latin America. Forest restoration is also a central component of national commitments to the Paris Agreement and of the United Nations' Sustainable Development Goals and Decade on Ecosystem Restoration (2021-2030). Nowadays, planting programmes are receiving more financial, political and societal support than ever (Holl and Brancalion, 2020). These initiatives largely focus on tropical and subtropical areas (photo 1), where tree growth is faster  and larger tracts of land are available for tree planting (Bastin et al., 2019). However, according to Chazdon and Brancalion (2019), "enormous gaps remain between high-level focus on restoration and implementation on the ground". This disconnection can be partly explained by the fact that theoretical models of forest restoration are not adapted to the real constraints -particularly economic -encountered by tree plantation managers (Lopez-Sampson et al., 2021). As a matter of fact, it is from a better understanding of actual large-scale tree plantation models and their historical evolution that we can analyse their ability to evolve and apply adapted forms of forest restoration. A better understanding and characterisation of the planted forests in Africa, Asia and Latin America would allow a more detailed analysis of their functions, advantages and limitations, and finally of their suitability to support reforestation efforts in tropical countries (Batra and Pirard, 2015;Malkamäki et al., 2018). In this perspective, based on a literature review, this article aims to: (1) draw up and describe a typology of large-scale tree plantations in Africa, Asia and Latin America; (2) explore the regional dynamics of large-scale tree plantations; and (3) discuss their capacity to contribute to forest restoration.

Main concepts
A forest is a "land spanning more than 0.5 ha with trees higher than 5 m and a canopy cover of more than 10%, or trees are able to reach these thresholds in situ" (FAO, 2018). The FAO (2018) defines a "Forest Plantation" as a "Planted Forest that is intensively managed and meets all the following criteria at planting and stand maturity: one or two species, even age class, and regular spacing". A planted forest is defined as a "forest composed mainly of trees established by planting and/or deliberate seeding, where planted and/or seeded trees constitute more than 50% of the mature growing stock". The FAO definition of a "Forest Plantation" is recent, and no statistics are available for this specific land-use category. Therefore, when referring to planted forests, this article does not differentiate between "planted forest", "forest plantations" or "tree plantations". We define a "large-scale tree plantation" as a tree plantation of at least 50 ha on a single plot of land or as a series of small plantations, which may add up to hundreds or thousands of hectares, spread over different plots in the same territory, as the example of the Mampu plantation in Democratic Republic of Congo (photos 2) (Bisiaux et al., 2009). These large-scale plantations can be mono or pluri-specific and even-age or not.
We do not include naturally regenerating forests in our analysis as they are defined by FAO (2018) as "forest predominantly composed of trees established through natural regeneration".
Finally, forest restoration is defined as a "planned process that aims to regain ecological functionality and enhance human well-being in deforested or degraded landscapes" (Gann et al., 2019). Therefore, a tree plantation is one option for forest restoration among others, such as natural regeneration, assisted natural regeneration, agroforestry, or rehabilitation/ reclamation (Chazdon, 2008). Note that according to Lewis et al. (2019) and Romijn et al. (2019) forest plantations and agroforestry systems represent 79% of forest restoration committed to by 24 countries through 2019.

Geographical scope
This study focuses on countries of Africa, Asia and Latin America. It is in these three continents that we find most of the Bonn Challenge commitments  and where larger tracts of land are available for tree planting (Bastin et al., 2019).

Literature Review
This study is based on a review of existing literature, including peer-reviewed and grey literature from 1990 to the present. The work began with the analysis of 50 references gathered by the co-authors during their past research. This preliminary database was supplemented by additional research using Google Scholar, which yielded 189 documents. The initial database made it possible to establish a first plantation typology and an overview of different geographical situations. Then, additional research was carried out in the second phase, which made it possible to obtain more specific documents by country/geographical region and/or type of plantation and/or specific theme (e.g., yields, pests, questions about sustainability, etc.). After a preliminary reading, 109 documents were discarded because they lacked information or specific relevance to the topic. Ultimately, 79 documents were selected (48 articles, 23 technical reports, 5 conference papers and 3 books).

Choosing variables for defining a typology for large-scale tree plantations
Two steps were followed in selecting the most suitable variables to design a large-scale tree plantation typology: (i) a review of existing tree plantation typologies and a Google Scholar search to identify criteria in order to determine the different types of large-scale plantation for our database, and (ii) identification of the most relevant discriminating factors under the constraint that they can be documented for each tree plantation type.
We identified three existing typologies (CIFOR, 2002;Brockerhoff et al., 2008;D'Amato et al., 2017). The typology proposed by D'Amato et al. (2017) is the most complete and uses a coding system of 11 criteria. In comparison, the CIFOR (2002) and Brockerhoff et al. (2008) typologies are based on three discriminating factors. These typologies are not specific to large-scale plantations.
All three typologies use the following criteria to define plantation type: the nature of the plantation (species composition: monoculture or mixed and native or exotic), its purpose and management intensity. While the first two criteria are easy to determine, the third is less clear-cut. Consequently, we chose the nature of the plantation (tree type) and the purpose of the plantation as discriminating factors to define the typology for large-scale plantations.
Beyond these three factors, D'Amato et al. (2017) stress the importance of other variables, such as: (a) species composition (monoculture or mixed); (b) a plantation's intended use (provision, regulation and cultural services); (c) land ownership (public and private); (d) management responsibility (public and private); (e) scale (large-medium-small) and composition (monoculture/mixed) in landscape; (f) original initiator of plantation (external and internal); and (g) level of institutional arrangements. Criteria (a) and (d) are easy Photos 2. The Mampu plantation, in the Democratic Republic of Congo, is mostly constituted by Acacia auriculiformis plots belonging to hundreds of smallholders (a) and dedicated to woodfuel and cassava production (b). Photos A. Péroches. a b to evaluate and have been included in our typology, along with the nature and purpose of the plantation. Criterion (b) is already included in the typologies of CIFOR (2002) and Brockerhoff et al. (2008). Criteria (c), (f) and (g) are not easy to determine and were therefore excluded from our definition. Lastly, criterion (e) is not relevant because it does not relate to large-scale tree plantations.
Based on the analysis, four criteria were selected for our typology: 1. Management responsibility (or forest ownership) -The latest FRA report (FAO, 2020) and D'Amato et al. (2017) distinguish two main types of forest ownership: public or private. Szulecka et al. (2014) state that these two categories are also valid for tree plantations. Del Lungo et al. (2006) go further, by separating the private sector into two categories: companies and smallholders. As explained by D' Amato et al. (2017), the nature of "ownership and management are relevant to identifying plantation types because it can exert both positive and negative forces on local communities, for instance by influencing the status of, and access to plantation land and surrounding ecosystems and related services, and by influencing community life and relations, livelihoods, education and employment opportunities". Three categories are distinguished: public plantations, company plantations and plantations owned by smallholders. In the case of private plantations (companies or smallholders), the land may belong to the private person in charge of the plantation or to the state. 2. Planted species -In Africa, Asia and Latin America, most tree plantations are based on even-aged short rotation stands of exotic (non-indigenous) species and intensive management methods (Tassin, 2011;Jürgensen et al., 2014;Payn et al., 2015). In most cases, the main species planted are pine, eucalyptus, acacia and teak (Cossalter and Pye-Smith, 2003;Louppe, 2011;Cateau et al., 2018). These are fast growing exotic species, their seeds are easy to obtain, and their silvicultural techniques are known (Louppe, 2011;INDUFOR, 2017). 3. Species composition -Species can be planted as monocultures or in mixed stands, although mixed plantations are uncommon (Lopez-Sampson et al., 2021). This makes a significant difference in terms of biodiversity and economic returns (D'Amato et al., 2017). 4. Purpose of the plantation -Louppe (2011) points out that the primary objective of planting exotic species is to produce pulp or wood energy, followed by timber. Del Lungo et al. (2006) and Szulecka et al. (2014) also mentioned that some plantations have protection objectives (protection of soil and water resources, carbon storage, etc.). The fourth discriminating factor used was the objective of the plantation (i.e., timber, pulp or woodfuel production, ecosystem services-oriented or multi-purpose).

Determination of relative weight of each type of plantation
Firstly, in the cited bibliographic sources, all references to the areas covered by large-scale plantations were collected and classified by plantation type, by country and finally by continent. It is on the basis of these figures that the areas covered, and the relative weight of each type were estimated by continent. When the estimated areas were presented as a range, the middle of the class was retained. Regarding the precision of the available literature, the scale of analysis that seems the most relevant to us is the continent, even if contrasting dynamics can sometimes exist within the same continent.

Relevant types of large-scale tree plantations in Africa, Asia and Latin America
Based on the four discriminating factors, a "decision tree" was constructed (figure 1). According to figure 1, 60 "theoretical" types of large-scale tree plantations were identified. However, most of these theoretical types do not actually exist or are negligible. The types not mentioned in the literature, or which only cover very small areas (a few thousand hectares) were considered negligible. For example, exotic multi-species stands occupied less than 0.1% of all large-scale plantations worldwide in 2006 (Nichols et al., 2006), and no recent source mentioning the existence of this type of tree plantations has been identified. After eliminating the non-existent theoretical types, we identified seven groups of large-scale tree plantations in Africa, Asia and Latin America (figure 1): Type 1 = Private and production-oriented plantations of monospecific exotic species; Type 2 = Private and production-oriented plantations of mono/multi-specific indigenous species; Type 3 = Public and ecosystem services-oriented plantations of monospecific exotic species; Type 4 = Public and production-oriented plantations of monospecific exotic species; Type 5 = Public and production-oriented plantations of mono/multi-specific indigenous species; Type 6 = Public and ecosystem services-oriented plantations of mono/multi-specific indigenous species; Type 7 = Production-oriented plantations of monospecific exotic species managed by smallholders. The main characteristics of these types are provided in tables I to VII.

Previous dominant land use
Generally degraded forest areas.

Main geographical areas
Asia. Africa.

Main species planted
Several depending on geographical areas. Largely dominated by teak (in native countries) in Asia.

Change in area over the last 30 years
An increase in teak plantations in Asia (particularly India), stable due to the low economic interest for other species.

Sensitivity to pests and diseases
Few documented. Lepidoptera attacks noticed on the genus Khaya in Africa.

Age of stands in 2020 (average)
15 to 25 years.

Social and environmental impacts
Model highly criticised for its environmental Virtuous model in terms of environmental concerns because it could potentially reduce harvesting in the natural environment, specifically for certain species, particularly in Central Africa. Few visible impacts on local populations due to the absence of very large-scale plantations, except in the case of teak. Potential land conflicts and value-added sharing identified in the development of this model.

Previous dominant land use
Mainly degraded land and savanna areas.
6 to 7% of the surfaces were deforested for plantation installation. This is particularly true in Chile and Indonesia.

Change in area over the last 30 years
Significant increase, particularly over the last 15 years. It is the dominant model in southern countries.

Sensitivity to pests and diseases
Significant pressure from pests and diseases linked to monoculture and the cultivation of clones outside their area of origin.
For example, over 50 types of pests and diseases, which attack acacias and eucalyptus, have been listed in Vietnam alone.

Age of stands in 2020 (average)
Most aged from 5 to 10 years, with a harvesting period ranging from 8 to 30 years.

Social and environmental impacts
Model highly criticised for its environmental and social impact:  High chemical input use  Low biodiversity  Soil depletion  Pollution and over-consumption of water  Frequent conflicts with local populations, particularly over land  Low job creation per hectare and increasing urban migration  Considered as the secondary cause of deforestation in Southeast Asia and South America. However, this plantation type can alleviate pressure on natural forests due to its high productivity.

Previous dominant land use
Areas that are degraded / threatened by desertification.

Main geographical areas
Asia (Mainly China and India). Africa (Mainly Tunisia).

Change in area over the last 30 years
Significant increase in the late 1990s and 2000s, particularly with Chinese government programmes. Significant increase in Tunisia between 1990 and 2015. Still increasing in Asia and Tunisia.

Sensitivity to pests and diseases
In China, some indigenous and exotic forest parasites, such as pine caterpillars, fall worms, spring cankerworms, nematodes, pine mealybugs and rodents destroy large areas of plantations. In Tunisia, eucalyptus plantations are frequently attacked by four types of insects from Australia, including two species of wood borers and two species of gall insects.

Age of stands in 2020 (average)
15 to 15 years.

Social and environmental impacts
Some projects compete with local populations' traditional and agricultural practices, creating tensions. This type of plantation limits biodiversity due to the monoculture of exotic species. Nevertheless, it can help restore very degraded areas and protect some soils from erosion. In addition, it contributes to carbon storage. Risks of the spread of invasive species are occasionally identified.

Table IV.
Type 4: Large-scale plantation characteristics -Public and production-oriented plantations of monospecific exotic species.

Previous dominant land use
Degraded or agricultural areas.

Main species planted
Eucalyptus, acacia, pine and teak.

Change in area over the last 30 years
Significant growth, particularly in Asia until the 2000s. Since then, growth rate has slowed down because states generally favour the private sector for this type of plantation.

Sensitivity to pests and diseases
In China, some indigenous and exotic forest parasites, such as pine caterpillars, fall worms, spring cankerworms, nematodes, pine mealybugs, and rodents destroy large areas of plantations.

Social and environmental impacts
Some projects compete with local populations' traditional and agricultural practices, creating tensions. This type of plantation limits biodiversity due to the monoculture of exotic species (even if it can participate to local species recolonization). In addition, the regular export of woody material can further deplete poor soils. However, it can protect some soils from erosion, contribute to carbon storage and reduce pressure on natural forests.

Previous dominant land use
Agricultural land, savanna and degraded forest areas.

Main geographical areas
Asia (Mainly China and India). Africa (Mainly Sudan, over a few tens of thousands of hectares in Gabon or Côte d'Ivoire).

Main species planted
Chinese fir trees, poplars, acacias from Africa, teak.

Change in area over the last 30 years
Significant increase, particularly in China since 1990. Weak dynamics in Sudan, where most plantations are old.

Sensitivity to pests and diseases
In Sudan, most damage to acacias is due to goat and camel alimentation (pruning). Parasite attacks are secondary. Quite variable depending on country and species. Monocultures are generally more sensitive.

Age of stands in 2020 (average)
15 to 30 years.

Social and environmental impacts
Some projects compete with local populations' traditional and agricultural practices, creating tensions. Monospecific plantations (85% of plantations in China), limit biodiversity. Conversely, multi-species plantations have a positive impact on biodiversity. This plantation type can help restore degraded areas, provide carbon storage and provide local resources to the population.

Previous dominant land use
Degraded forest and/or agricultural areas.

Main geographical areas concerned
Mainly East Asia (China and India).

Main species planted
Chinese fir tree, poplar.

Change in area over the last 30 years
Significant increase, particularly in China, as a result of government programmes.

Sensitivity to pests and diseases
This plantation type does not seem to have too many problems.

Age of stands in 2020 (average)
Most range from 10 to 20 years.

Social and environmental impacts
Beneficial effects on soil protection, water resource protection and carbon storage.

Latin America
Today, Latin America has just over 21.5 M ha of tree plantations (FAO, 2020), of which 80-90% are planted with exotic species (Payn et al., 2015;FAO, 2020). The main planted species are eucalyptus (70%) and pine (25%) (photo 3) (ITTO, 2009;Payn et al., 2015). In this region, the area covered by tree plantations increased by 3.2% per year for the decade 2000-2010. This growth was driven by the private sector and supported by state land and tax policy incentives (ITTO, 2009;EFIATLANTIC et al., 2013). Overall, Type 1 plantations are the most represented category of large-scale plantations.
Wood-based products obtained from South American plantations are generally exported (Cossalter and Pye-Smith, 2003;ITTO, 2009;Cateau et al., 2018). Brazil and Chile have the largest areas of plantations (Jürgensen et al., 2014;Payn et al., 2015), which account for two-thirds of the region's total plantation area (EFIATLANTIC et al., 2013;Payn et al., 2015;Cateau et al., 2018). In both countries, tree plantations produce greater volumes of wood-based products than natural forests (James and Del Lungo, 2005). In Chile, large-scale tree plantations were initially responsible for the deforestation of native forests. However, it seems that since 2001, a shift in tree plantation management combined with changes in forestry policy, have reduced pressure on native forests over time (Heilmayr et al., 2016).

Asia
Asia is the leading continent in terms of forest plantation area, with nearly 123 M ha (EFIATLANTIC et al., 2013). Unlike Latin America, in Asia, plantations of native species are the dominant model (Payn et al., 2015). These native species plantations, such as industrial plantations of teak in India (a native species in the region), are managed according to the same intensive model used for exotic species (ITTO, 2009).
In Asia, historically, the development of tree plantations was based on public projects and funding. One of the main objectives of the plantations was to protect soil and water resources (EFIATLANTIC et al., 2013;Wolosin, 2017). According to FAO (2010a), nearly 30% of the planted area in Asia is geared towards the protection of soil, water resources or biodiversity, more than in any other region of the world. Therefore, a significant proportion of plantations are public and correspond to plantation Types 3, 4, 5 and 6 Del Lungo et al., 2006;Wolosin, 2017). Nevertheless, as in most southern countries, since the late 1990s and early 2000s, there has been growing interest in and a major development of industrial production-oriented plantations, particularly in Southeast Asian countries, such as Indonesia and Thailand (Michon, 2003;Enters et al., 2004;Szulecka et al., 2014;Wolosin, 2017). In China, the leading country in terms of planted area, the state has maintained control over plantations (Martin, 2014), which are mainly intended for protection purposes Wolosin, 2017).
Then, in Asia, two contrasting situations co-exist: (i) the Chinese model, mostly based on Types 3, 4, 5 and 6; and (ii) the Southeast Asian model, where Type 1 is predominant, as it is in Latin America.

Previous dominant land use
Degraded forests, wooded savannas.

Main geographical areas
Africa and Asia.

Main species planted
Eucalyptus, pine and acacia.

Change in area over the last 30 years
Little development globally. Development is strongly correlated with the initial public, private or NGO investment.

Sensitivity to pests and diseases
Little documentation available.
Age of stands in 2020 (average) 5 to 10 years based on rotations.

Social and environmental impacts
In many cases, exporting a large part of the woody material at regular intervals can reduce soil fertility (especially in the case of woodfuel production). Given that erosion is usually the main factor involved in soil degradation, this plantation type has positive features. It generally improves the incomes of poor households.

Average yield Africa
Africa has 11.4 M ha of tree plantations (FAO, 2020), of which 80% are indigenous species, mainly Acacia senegal and Acacia nilotica 1 (Gafaar, 2011;Jacovelli, 2014;Payn et al., 2015). The rate of increase in plantation area on the African continent is one of the lowest in the world (Payn et al., 2015). Apart from the specific case of South Africa, most African plantations are state-owned (EFIATLANTIC et al., 2013). Large planting campaigns in the 1970s and 1980s failed to meet expectations. Since then, there have been few planting campaigns because of a lack of interest or state withdrawal in the 1990s (Marien and Mallet, 2004;Louppe, 2011;Hamel and Dameron, 2011;Chevalier, 2018).
Sudan (and South Sudan) and South Africa are the top two countries in terms of tree plantation area (Jacovelli, 2014;Cateau et al., 2018). However, their large-scale plantation strategies are very different. For example, in Sudan, tree plantations are public plantations of indigenous species to produce gum arabic (Type 5). In South Africa, private industrial plantations of exotic species (pine, eucalyptus and acacia) (Type 1) are the predominant model (Del Lungo et al., 2006;Jacovelli, 2014;Cateau et al., 2018). However, in this country, expansion of Type 1 plantations is limited with planted area remaining stable (Jürgensen et al., 2014) because of poor soil quality, lack of water and competition with other land uses in the country. Tunisia comes in third in terms of tree plantation area, with Eucalyptus being the main species planted to reduce desertification (Type 3) (Jacovelli, 2014). The remaining African countries have less than 0.7 million hectares of plantations combined.
1 Established by direct sowing and in low density.

Discussion
The typology of tree plantations established in this article is based on the state of available knowledge to provide an operational tool for classifying plantations. Nevertheless, the four criteria retained are relevant for discussing the place of these tree plantation models in relation to forest restoration objectives: 1-Management responsibility (or forest ownership) -This first criterion has indirect impacts on forest restoration objectives. The status, management and access to the plantation land and its related services (ecosystem, social, economic) have an impact on local communities' well-being (D'Amato et al., 2017). 2-Planted species -The recovery of ecological functionality of an area is better in native species tree plantations than in exotic species plantations . 3-Species composition -The recovery of ecological functionality of an area is better in mixed tree plantations than in monoculture tree plantations (D'Amato et al., 2017). 4-Purpose of the plantation -The four retained objectives of tree plantations (i.e., timber, pulp or woodfuel production, ecosystem services-oriented or multi-purpose) have direct impacts on biodiversity and human well-being.
Considering the three continents analysed, the dominant model for large-scale tree plantations is private industrial monospecific plantations of exotic species (eucalyptus, pine, acacia and teak in non-native areas), which corresponds to our Type 1 (figure 2). These plantations are largely dominant in Latin America and Southeast Asia. In addition, the development of these plantations has been particularly Relative weight of large-scale plantation typologies worldwide (as %): Type 1 = Private and ecosystem servicesoriented plantations of monospecific exotic species; Type 2 = Private and production-oriented plantations of mono/ multi-specific indigenous species; Type 3 = Public and ecosystem services-oriented plantations of monospecific exotic species; Type 4 = Public and production-oriented plantations of monospecific exotic species; Type 5 = Public and production-oriented plantations of mono/multi-specific indigenous species; Type 6 = Public and ecosystem servicesoriented plantations of mono/multi-specific indigenous species; Type 7 = Production-oriented plantations of monospecific exotic species managed by smallholders. dynamic in Brazil, Southeast Asia and China over the last two decades. This plantation category has various advantages: (i) it stores carbon and produces large quantities of wood products; (ii) it can rehabilitate degraded soils; and (iii) it probably reduces pressure on natural forests -this last point being questionable according to Pirard et al. (2016) (Cossalter and Pye-Smith, 2003;Buongiorno and Zhu, 2014;Martin, 2014;Cateau et al., 2018). However, Malkamäki et al. (2018) conclude, based on a few examples, that industrial exotic monoculture tree plantations often have a negative environmental and social impact. The main criticisms of this model are summed up in table VIII. Therefore, Type 1 fails to meet the necessary conditions -particularly environmental -for forest restoration objectives in accordance with recent international commitments (Bonn Challenge, UN Decade on Ecosystem Restoration or national commitments to the Paris Agreement). According to Malkamäki et al. (2018), these environmental and social impacts of Type 1 plantations should be studied more. This affirmation is confirmed by the study of Pirard et al. (2017), which shows that in Indonesia, these plantations can be perceived as either positive or negative by local populations, and may or may not have social impacts on those populations. In the same way, Tassin et al. (2011) explain that planted on degraded land near to natural areas, cloned eucalyptus can facilitate the recolonization of native flora and fauna. That is why a better understanding of various situations could improve the outcomes of large-scale tree plantations on local populations and/or the environment.
This synthesis of literature also shows that regardless of the purpose of a tree plantation, all models are controversial. For example, Bremer and Farley (2010) discuss Type 3, sometimes qualifying it as "green-desert". The conclusions of their study "suggest that plantations are most likely to contribute to biodiversity when established on degraded lands rather than replacing natural ecosystems, such as forests, grasslands and shrublands, and when indigenous tree species are used rather than exotic species". Then, the debate is nearer to the one about Type 1, when dedicated to intensive wood production.

Sustainability parameters
Contribution to the preservation of biodiversity.
Carbon storage.
Preservation of soil quality.
Preservation of the quality of water resources.
Social and economic development of production areas.
Maintaining production in the long term.

Weaknesses of the dominant model
Monocultures and pesticide applications are harmful to biodiversity, which is low in plantations. The use of exotic species can cause imbalances (e.g., invasions) in natural biological processes. Fragmentation of natural forest stands is harmful to the survival of sensitive species. In some cases, new plantations may be responsible for the deforestation of natural forests.
Land is harvested and cleared for replanting (typically once a decade), which releases carbon as a result of decomposition of plantation waste and products (mostly paper and woodchip boards).
The use of pesticides and intense production methods depletes the soil over time (chemical pollution, compaction and erosion).
The use of chemical pesticides pollutes the water resource. Water resources are overexploited in some cases to irrigate plantations, which reduces river flow rate.
Low job creation per hectare compared to other types of plantation Frequent source of social conflict, particularly relating to land issues. Displacement of local population.
From the early 20th century until the 1980s, very diverse mixed plantation models were designed and tested in Africa, Asia and Latin America (Lopez-Sampson et al., 2021). However, they were rarely adopted, primarily because they would probably give low financial returns. Financial returns are documented for exotic monocultures as Eucalyptus spp. or Pinus spp. (Cubbage et al., 2014), however, financial performance and operation costs of mixed plantations are limited or absent from the literature (Lopez-Sampson et al. 2021). According to Cubbage et al. (2014), environmental regulations and land rent to governments or local populations reduce plantation investment returns. Then, to promote more socially and environmentally diverse and more efficient tree plantation models, non-financial aspects should be factored in, and the involvement of governments (in both producer and consumer countries) and other stakeholders should be encouraged.

What are the alternatives to Type 1 large-scale tree plantations?
Alternative tree plantation models may be more effective for forest restoration in Africa, Asia and Latin America. According to Brockerhoff et al. (2008), native species and long rotation cycles are prerequisites for sustainability. Numerous authors have demonstrated that multi-species plantations improve the sustainability performance of large-scale tree plantations. They optimise environmental resource use, increase productivity and CO 2 storage per hectare, and improve resilience to climatic and biological hazards (Erskine et al., 2005(Erskine et al., , 2006Nichols et al. 2006;Hung et al., 2011;Louppe, 2011;Pryde et al., 2015;Kelty, 2006;Liu et al., 2018).
Developing alternative plantation types requires funding that will accept a higher risk factor. Funds could be provided by governments or public sources, as illustrated in China (Xu et al., 2004), or by a mix of public incentives, including subsidies, tax benefits and preferential access to credit for private societies as in the Chilean example (Heilmayr et al., 2020). As shown in figures 3 and 4, large-scale tree plantation types are more diverse in Asia and Africa. This can be explained partially by the fact that public funds were allocated to establish plantations. This was not the case in Latin America, where private investment is dominant. Thus, past experiences show the importance of public funds in parallel with or in addition to private investments for developing diverse models of large-scale tree plantations and for considering general interest.
However, alternative tree plantation types have also been criticised. Each type has specific advantages and disadvantages, which are summarised in table IX. Plantation types 1, 4 and 7, which are based on the monoculture of exotic species, may significantly reduce pressure on natural forests, but they are criticised for their frequent (though not systematic) negative ecological impacts, particularly on biodiversity (Holl and Brancalion, 2020). Conversely, types 2, 5 and 6 are more resilient, have fewer negative environmental impacts and promote job creation. However, they are less productive, and mechanisation and processing are more complex.
The diversity of large-scale tree plantation models shows that there are various approaches to achieving the goal of forest restoration. The choice of the appropriate model depends on the results expected from forest restoration, as well as the context. In response to the substantial development of Type 1, current international reforestation initiatives are placing greater emphasis on environmental and social considerations. We now have the knowledge to anticipate these impacts, at least generically, and better prepare new tree plantation projects (Warman, 2014). Today, the major difficulty involved in promoting sustainable tree plantations is clearly identifying the context in terms of decision making and implementation. It is important to consider stakeholders' needs and available knowledge regarding the financial, technical and institutional capacities actually available.

Conclusion
In Africa, Asia and Latin America, large-scale tree plantations are developed in order to: (i) produce forest products (ligneous or non-ligneous); (ii) preserve ecosystems and biodiversity; (iii) act as carbon sinks; and (iv) play a positive social, economic and cultural role, which includes equitable income distribution between the various stakeholders (Cossalter and Pye-Smith, 2003;Louppe, 2011;Payn et al., 2015;Pryde et al., 2015;HLPE, 2017;Cateau et al., 2018;Malkamäki et al., 2018;Bastin et al., 2019;Lewis et al., 2019). None of the existing large-scale tree plantation models can meet these objectives simultaneously, because as Holl and Brancalion (2020) explain "a single tree planting project may achieve multiple goals, but it is rarely possible to simultaneously maximize them all because goals often conflict, and prioritizing one goal may result in other undesirable outcomes". For instance, the dominant plantation models -private monospecific plantations with exotic species -contribute significantly to roundwood production, but probably generate negative ecological and social externalities in many cases . Conversely, less intensive models have low yields and financial returns. As Chazdon (2008) and Holl and Brancalion (2020) recall, the choice of a tree plantation model depends not only on the plantation's main goal, but also on the degree of degradation of the forest ecosystem, the local population's needs, the available financial and technical resources, and access to land.
Various existing tree plantation management schemes can improve the social, economic, and environmental integration of large-scale tree plantations in landscapes (Pirard et al., 2017). In addition, selecting the most appropriate tree plantation model will be possible if better knowledge about all plantation types is available. For types 1, 3, 4 and 7, studies should be conducted on their social and environmental impacts , and for the remaining types, more studies are needed on technical and financial performance (Lopez-Sampson et al., 2021).
Rather than focusing on high performance plantations, a change of scale is also possible. Developing mixed-species plantations and including them in multifunctional forest landscapes offer an increasingly popular framework, which combines different types of tree plantations, strengthens their synergies and trade-offs, and involves a broader range of stakeholders, including smallholders Chazdon, 2008;Barua et al., 2014;Payn et al., 2015;WWF, 2015;Lewis et al., 2019). This approach is still difficult to implement because it requires complex institutional planning and organisation (Holl and Brancalion, 2020). However, if tree plantations, and more particularly, large-scale tree plantations, are not integrated at landscape scale, they will continue to respond only to market signals (Pirard et al., 2016) and fail to contribute in forest restoration goals.

Table IX.
Evaluation of positive and negative impacts of large-scale tree plantations in Africa, Asia and Latin America (--= very negative impact; -= negative impact; + = positive impact; ++ = very positive impact).