**1. Introduction**

In 2015, all signatory countries of the landmark Paris Agreement pledged to reduce their national greenhouse gas (GHG) emissions and enhance resilience to climate change. The Nationally Determined Contributions (NDCs) are the blueprints outlining mitigation and adaptation efforts, and the Paris Agreement requires each party to prepare, communicate, and pursue their NDCs with domestic or international support. NDCs describe post-2020 climate programs, and parties can prioritize sectors which substantially contribute to their national emissions. Most signatory countries submitted their first NDCs to the United Nations Framework Convention on Climate Change (UNFCCC) by 2016 [1], with the option to amend before a final submission in 2020.

Agriculture features prominently in the NDCs of non-Annex I countries, which are mostly developing countries [2]. About 40% of 148 non-Annex I countries include mitigation measures for the agriculture sector in their NDCs, with half of these mentioning integrated systems such as agroforestry [3]. However, the NDCs of many countries do not elaborate these mitigation measures into concrete actions and associated targets.

Agroforestry, an integrated agricultural system with crops and trees, can substantially reduce GHG emissions through carbon sequestration [4,5]. Agroforestry also increases farmer adaptation to climate change through, e.g., diversified products and sources of income, resource use efficiency, and improved micro-climates [6–9]. Yet the absence of reliable data, including on types and distribution of existing practices and their carbon storage, partially constrain estimation of mitigation and adaptation potential from agroforestry at national scale [10].

Agriculture is a significant source of emissions in Vietnam. Total GHG emissions from Vietnam's agricultural sector reached an 88.3 million ton CO<sup>2</sup> equivalent (mil tCO2e) in 2010, accounting for 35.8% of total national emissions [11], and are projected to increase to 109 mil tCO2e by 2030 under baseline conditions. Vietnam's first NDC submitted to the UNFCCC in 2016 identified 15 mitigation measures for the agriculture sector but excluded agroforestry. The measures focused on improving the efficiency of inputs, plot management practices, such as transforming conventional water management in rice to alternate wetting drying for reducing methane production, and waste treatment, such as converting livestock waste into biogas, and are expected to reduce the projected 2030 baseline emissions by 6–42% [12]. Vietnam has recently included agroforestry in its revised NDC submitted to the UNFCCC in 2020 as part of the Land Use, Land Use Change, and Forestry (LULUCF) sector. However, the revised NDC only specifies the purpose of agroforestry measure, namely for "enhancing carbon stocks and conserving lands", without elaboration on activities and associated mitigation or adaptation targets.

A methodological framework for estimating mitigation and adaptation potential of agroforestry and associated targets at a national or sub-national scale for climate programs such as NDCs is necessary. In this paper, focusing on the mitigation potential, we describe such a framework that we used to provide a technical and economic estimate of mitigation potential from agroforestry in Vietnam by 2030 by assessing its capacity for offsetting GHG emissions through carbon sequestration. We took into account above-ground carbon (AGC), below-ground carbon (BGC), and soil carbon (SOC) sequestered in existing areas and potential expansion areas of agroforestry across the country. We considered the impact of future climate change scenarios on the expansion area and carbon sequestration potential from agroforestry. The economic estimate accounts for the investment cost required to fulfill the mitigation potential over 2021–2030.

#### **2. Materials and Methods**

#### *2.1. Methodological Framework*

Agroforestry mitigation potential was estimated at a national scale and can be disaggregated into a sub-national scale using eight ecological regions (Figure 1a), differentiated based on geographical characteristics, topographical features, ecosystem types, and climate [13]. These eight regions are North West, North East, Red River Delta, North Central Coast, South Central Coast, Central Highlands, South East, and Mekong River Delta. The detailed characteristics of each region, including soil condition and dominant land uses, are given in [13].

**Figure 1.** (**a**) Eight regions of Vietnam. (**b**) Methodological framework for estimating mitigation potential from agroforestry (AF) in Vietnam. **Figure 1.** (**a**) Eight regions of Vietnam. (**b**) Methodological framework for estimating mitigation potential from agroforestry (AF) in Vietnam.

The assessment of agroforestry mitigation potential consists of six steps (Figure 1b). The first two steps pertain to existing agroforestry areas and the following four pertain to potential expansion areas for agroforestry. The sequestered carbon in the potential expansion areas was assessed over a ten-year period (2021–2030), and the impact of future climate scenarios on extent areas for agroforestry system expansion was investigated using representative concentration pathway (RCP) 4.5 and RCP 8.5 scenarios. The aggregate of sequestered carbon across existing and potential agroforestry areas constitutes the total carbon contribution from agroforestry by 2030. We used the information on types and distribution of existing agroforestry practices in Vietnam The assessment of agroforestry mitigation potential consists of six steps (Figure 1b). The first two steps pertain to existing agroforestry areas and the following four pertain to potential expansion areas for agroforestry. The sequestered carbon in the potential expansion areas was assessed over a ten-year period (2021–2030), and the impact of future climate scenarios on extent areas for agroforestry system expansion was investigated using representative concentration pathway (RCP) 4.5 and RCP 8.5 scenarios. The aggregate of sequestered carbon across existing and potential agroforestry areas constitutes the total carbon contribution from agroforestry by 2030.

from the Spatially Characterized Agroforestry (SCAF) database (http://scafs.worldagroforestry.org/), as a basis for estimating sequestration contributions from existing areas of agroforestry and for selecting agroforestry systems for expansion. For the carbon estimation, we used input carbon storage data reported in the literature or estimated carbon storage per land area using relevant allometric equations, stem diameter, and crop density data from the literature. The input AGC data are speciesspecific and mostly specific to Vietnam. On the other hand, the input BGC and SOC are system- but not species-specific. For example, the BGC of most of the assessed agroforestry systems was estimated using the BGC/AGC partitioning factor for agroforestry from [14], and the SOC using the SOC sequestration rate from [10]. We used the information on types and distribution of existing agroforestry practices in Vietnam from the Spatially Characterized Agroforestry (SCAF) database (http://scafs.worldagroforestry.org/), as a basis for estimating sequestration contributions from existing areas of agroforestry and for selecting agroforestry systems for expansion. For the carbon estimation, we used input carbon storage data reported in the literature or estimated carbon storage per land area using relevant allometric equations, stem diameter, and crop density data from the literature. The input AGC data are species-specific and mostly specific to Vietnam. On the other hand, the input BGC and SOC are system- but not species-specific. For example, the BGC of most of the assessed agroforestry systems was estimated using the BGC/AGC partitioning factor for agroforestry from [14], and the SOC using the SOC sequestration rate from [10].

#### 2.1.1. Step 1: Determine Key Existing Agroforestry Systems in Vietnam The SCAF database provides information on 48 agroforestry practices observed in 2013–2014 2.1.1. Step 1: Determine Key Existing Agroforestry Systems in Vietnam

across 42 out of the 63 provinces in Vietnam. Most of the 48 practices can be classified into eight key systems based on their main perennial crop component, excluding the "other systems" (Table 1). The total area of the eight key systems is 820,000 hectares (ha) or about 91% of the total agroforestry system area. Examples of the eight key systems are illustrated in Figure 2. We cannot find a more recent database than SCAF for existing agroforestry systems in Vietnam. The SCAF database provides information on 48 agroforestry practices observed in 2013–2014 across 42 out of the 63 provinces in Vietnam. Most of the 48 practices can be classified into eight key systems based on their main perennial crop component, excluding the "other systems" (Table 1). The total area of the eight key systems is 820,000 hectares (ha) or about 91% of the total agroforestry system area. Examples of the eight key systems are illustrated in Figure 2. We cannot find a more recent database than SCAF for existing agroforestry systems in Vietnam.

**Agroforestry** 

Melaleuca (*Melaleuca cajuputi*)-based **Total Area** 


#### **Table 1.** Existing agroforestry systems in Vietnam. 245.5 Fresh-water inland forest with paddy rice, sugarcane, bananas, and fish Mekong River Delta

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**Table 1.** Existing agroforestry systems in Vietnam.

**(103 ha) \*\* Common System Components Main Regions** 

\* Ordered by total area; \*\* Total area in the country based on the Spatially Characterized Agroforestry (SCAF) database. database.

**Figure 2.** *Cont.*

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**Figure 2.** Examples of key common agroforestry systems in Vietnam: (**a**) Arabica and leucaena in the North West region, (**b**) robusta, cassia, and black pepper in the Central Highlands, (**c**) tea and cassia in the North Central Coast, (**d**) acacia and cassava in the North East, (**e**) rubber, potato, and maize in the North Central Coast, (**f**) cashew, robusta, and black pepper in the South East, (**g**) melaleuca and rice in the Mekong River Delta, and (**h**) Rhizophora and shrimp farming in the Mekong River Delta. Source of photos, (**a**–**g**) (SCAF), (**h**) (https://nongnghiep.vn/tom---rung-voi-tang-truong-xanh-**Figure 2.** Examples of key common agroforestry systems in Vietnam: (**a**) Arabica and leucaena in the North West region, (**b**) robusta, cassia, and black pepper in the Central Highlands, (**c**) tea and cassia in the North Central Coast, (**d**) acacia and cassava in the North East, (**e**) rubber, potato, and maize in the North Central Coast, (**f**) cashew, robusta, and black pepper in the South East, (**g**) melaleuca and rice in the Mekong River Delta, and (**h**) Rhizophora and shrimp farming in the Mekong River Delta. Source of photos, (**a**–**g**) (SCAF), (**h**) (https://nongnghiep.vn/tom---rung-voi-tang-truong-xanh-d233998.html).

#### d233998.html). 2.1.2. Step 2: Estimate Sequestered Carbon in Existing Areas of Agroforestry

2.1.2. Step 2: Estimate Sequestered Carbon in Existing Areas of Agroforestry The existing areas of agroforestry in Vietnam refer to the 820,000 ha occupied by the eight key agroforestry systems. Due to a lack of information on crop ages, the total AGC sequestered in these areas was estimated using time-average AGCs obtained from the literature (Table A1). For the six key agroforestry systems beside the two in wetlands, we generally used the BGC/AGC biomass partitioning factor for agroforestry from [14] to estimate the BGC, and the response ratio from [10] to estimate the time-average SOC assuming all areas of agroforestry were converted from logged-over forests. For the two systems in wetlands, species-specific BGC and SOC are available from the literature (Table A1). The aggregate of AGC, BGC, and SOC from the eight key agroforestry systems constitutes the carbon contribution from existing areas of agroforestry. We assumed the total area The existing areas of agroforestry in Vietnam refer to the 820,000 ha occupied by the eight key agroforestry systems. Due to a lack of information on crop ages, the total AGC sequestered in these areas was estimated using time-average AGCs obtained from the literature (Table A1). For the six key agroforestry systems beside the two in wetlands, we generally used the BGC/AGC biomass partitioning factor for agroforestry from [14] to estimate the BGC, and the response ratio from [10] to estimate the time-average SOC assuming all areas of agroforestry were converted from logged-over forests. For the two systems in wetlands, species-specific BGC and SOC are available from the literature (Table A1). The aggregate of AGC, BGC, and SOC from the eight key agroforestry systems constitutes the carbon contribution from existing areas of agroforestry. We assumed the total area and sequestered carbon in the existing agroforestry systems are constant until 2030.

#### and sequestered carbon in the existing agroforestry systems are constant until 2030. 2.1.3. Step 3: Determine Potential Areas for Agroforestry Expansion

2.1.3. Step 3: Determine Potential Areas for Agroforestry Expansion Due to the lack of a recent national land cover map from a reliable institution in Vietnam, we used Vietnam's 2018 land cover map from [15] to determine potential areas for agroforestry expansion. Among other land cover types in the map, we selected croplands as potential areas for agroforestry expansion. The croplands are defined by [15] as "lands with herbaceous and shrubby crops followed by harvest and bare soil period". They exclude orchards, annual crops with trees, forest plantations, and wet and low-land paddy fields. We excluded wet paddy lands because they are the main source of staple food for Vietnam, forest plantations and other tree-based systems because they are high-biomass land uses, and barren forest lands because they can be used for forest restoration. Croplands are spread across the country and have a total area of about 3.6 million ha (Figure 3a). Due to the absence of a spatial boundary around existing agroforestry areas, and because croplands exclude all systems with trees, we assumed the existing areas of agroforestry and Due to the lack of a recent national land cover map from a reliable institution in Vietnam, we used Vietnam's 2018 land cover map from [15] to determine potential areas for agroforestry expansion. Among other land cover types in the map, we selected croplands as potential areas for agroforestry expansion. The croplands are defined by [15] as "lands with herbaceous and shrubby crops followed by harvest and bare soil period". They exclude orchards, annual crops with trees, forest plantations, and wet and low-land paddy fields. We excluded wet paddy lands because they are the main source of staple food for Vietnam, forest plantations and other tree-based systems because they are high-biomass land uses, and barren forest lands because they can be used for forest restoration. Croplands are spread across the country and have a total area of about 3.6 million ha (Figure 3a). Due to the absence of a spatial boundary around existing agroforestry areas, and because croplands exclude all systems with trees, we assumed the existing areas of agroforestry and croplands (hereafter called the expansion domain) are thoroughly separated.

croplands (hereafter called the expansion domain) are thoroughly separated.

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(**d**)

**Figure 3.** (**a**) Potential expansion domain of agroforestry in Vietnam; (**b**) baseline average annual temperature; (**c**) baseline average annual precipitation; (**d**) changes in temperature and precipitation under representative concentration pathway (RCP) 4.5 and RCP 8.5. **Figure 3.** (**a**) Potential expansion domain of agroforestry in Vietnam; (**b**) baseline average annual temperature; (**c**) baseline average annual precipitation; (**d**) changes in temperature and precipitation under representative concentration pathway (RCP) 4.5 and RCP 8.5.

#### 2.1.4. Step 4: Select Agroforestry Systems for Expansion and Land Suitability Analysis 2.1.4. Step 4: Select Agroforestry Systems for Expansion and Land Suitability Analysis

We selected five out of the eight key agroforestry systems for expansion, excluding rubber-, Rhizophora-, and Melaleuca-based systems. For rubber, the Vietnam's Master Plan on Agricultural Production Development to 2020 vision to 2030 focuses on strengthening the processing industry instead of area expansion. This orientation also applies to robusta coffee and tea; however, coffeeand tea-based agroforestry provide diverse products such as timber, nuts, or fruits, for which the country is still unable to meet national demand. We excluded Rhizophora- and Melaleuca-based systems because considerable further research is necessary. Spatial data for determining their potential expansion areas is scarce and we lack information such as inundation frequency, water salinity, and tide intensity, that is crucial for land suitability analysis of wetland systems e.g., see [16]. Moreover, we could not find facts about the suitable growing conditions of these two systems from a reliable institution in the country. The most popular acacia-based system in Vietnam is the shortrotation (3–5 years) type for pulp and paper with annual crops such as cassava in the first or second We selected five out of the eight key agroforestry systems for expansion, excluding rubber-, Rhizophora-, and Melaleuca-based systems. For rubber, the Vietnam's Master Plan on Agricultural Production Development to 2020 vision to 2030 focuses on strengthening the processing industry instead of area expansion. This orientation also applies to robusta coffee and tea; however, coffee- and tea-based agroforestry provide diverse products such as timber, nuts, or fruits, for which the country is still unable to meet national demand. We excluded Rhizophora- and Melaleuca-based systems because considerable further research is necessary. Spatial data for determining their potential expansion areas is scarce and we lack information such as inundation frequency, water salinity, and tide intensity, that is crucial for land suitability analysis of wetland systems e.g., see [16]. Moreover, we could not find facts about the suitable growing conditions of these two systems from a reliable institution in the country. The most popular acacia-based system in Vietnam is the short-rotation (3–5 years) type for pulp and paper with annual crops such as cassava in the first or second year after tree planting [17]. However, the system for expansion is long-rotation (8–12 years) for timber purposes to help minimize Vietnam's dependence on timber importation. For the cashew, the selected system for expansion is alley cropping

with annual crops such as maize, not the perennial shade system with coffee. In the alley cropping system, cashew plants have a higher density to maximize production for national and export markets.

Based on the guideline of land evaluation from [18,19] and the available spatial data, the land suitability analysis for each agroforestry system for expansion considered the topographical, soil, and climatic conditions within the expansion domain. We used slope as an indicator for topographical conditions, soil depth and type for soil conditions, and average annual temperature and precipitation for climatic conditions. The suitability of areas within the expansion domain depends on the indicator values. The assignment of suitability levels involved two steps: first, the indicator values within the expansion domain were compared with thresholds of the growing conditions of the agroforestry systems. We utilized thresholds from a three-tiered ranking of growing conditions reported by reliable institutions in Vietnam (Table 2). Ranking s2 implies little to no limitation of enabling factors for sustaining crop productivity; s1 indicates moderate to severe limitation of such factors, requiring modest or substantial additional inputs or plot management practices for sustaining crop productivity; and s0 describes conditions in which the specified crop cannot grow even with any additional inputs or plot management practices. Subsequently, each area was classified as "highly suitable" if all its indicators met the s2 growing condition, "not suitable" if all indicators met the s0 growing condition, and "less suitable" if all indicators met the s1 growing condition, or if not all indicators met the s2 or s0 condition. For simplicity, we only assessed the suitability of the main perennial crop species in each system, e.g., tea for the tea-based system.


**Table 2.** Growing condition of the main crop species of selected agroforestry systems for expansion.

\* Common thresholds for *Acacia mangium*, *Acacia auriculiformis*, *Acacia crassicarpa*, or acacia hybrid, \*\* Vietnam National Institute of Agricultural Planning and Projection (NIAPP), \*\*\* Ac: acrisols; Fe: ferrasols; RhFe: rhodic ferralsols; XaFe: xantic ferralsols; Fl: fluvisols; Gl: gleysols; HuFe: humic ferralsols.

We used the 1960–1990 climate data from WorldClim 1.4 [22] as the baseline climate (Table 3). The World Meteorological Organization recommended 1960–1990 as the baseline period [23]. To investigate the impact of future climate on suitable areas for agroforestry expansion, we selected the CNRM-CM5 climate model under RCP 4.5 and RCP 8.5 for the period 2041–2060, also from the WorldClim 1.4, which is available at 30 arc seconds resolution. WorldClim 2.0 provides climate data

for the period 2021–2040 but at coarser resolution, and a stronger impact of climate change on the area for expansion and carbon sequestration potential from agroforestry can be expected using the 2041–2060 data. CNRM-CM5 is one of the best climate models for South East Asia [24]. Because we only considered the main perennial crop component of each agroforestry system for expansion in the land suitability analysis, the impact of future climate was also assessed for the main crop component only to represent the impact on each agroforestry system, neglecting the potential of agroforestry to modify micro-climates through an integration of multiple plant components that can reduce the intensity of climate change impact. Due to the absence of a spatial boundary to existing areas of agroforestry, we only investigated the impact of future climate on the potential expansion areas. All spatial inputs were standardized into one arc second, considered as a suitable resolution for country analysis.



Based on the baseline climate data from WorldClim 1.4, the northern part of the country generally has lower average annual temperatures and precipitation than the central and southern parts (Figure 3b,c). A similar pattern was reported by, e.g., [13]. This pattern will potentially change in the future, especially under the RCP 8.5 scenario, due to substantial increases in temperature and precipitation in the northern part of the country (Figure 3d).

#### 2.1.5. Step 5: Estimate Sequestered Carbon in Potential Expansion Areas

We estimated the carbon sequestered through agroforestry expansion by subtracting the AGC and BGC stored in croplands from the total carbon accumulated over ten years in areas suitable for agroforestry expansion. We used the average AGC of croplands of 5 tC ha−<sup>1</sup> for "cropland containing annual crops" according to the Intergovernmental Panel on Climate Change (IPCC) [25] and a BGC/AGC partitioning factor of 0.05–0.2 for "permanent cropland" [14]. The SOC is 53–158 tC ha−<sup>1</sup> for "permanent cropland" [14]. We assumed that the conversion of croplands into agroforestry retained the SOC in the soil, but we excluded this SOC from the estimation of accumulated SOC in the ten-year agroforestry expansion. All input AGC, BGC, and SOC data for the five agroforestry systems for expansion are given in Table A1.

There were five scenarios of agroforestry expansion, one for each agroforestry system. In each scenario, the system expands into highly suitable areas only, or into 10% and, at most, 25% of the less suitable areas. We assumed that agroforestry systems that have expanded into less suitable areas accumulate AGC, BGC, and SOC at the same rates as in highly suitable areas.

The highly and less suitable areas for expansion were gradually converted into agroforestry across the ten years at a constant conversion rate. Therefore, in the first year, only 10% of suitable areas was converted into agroforestry, to reach the total suitable area in the tenth year. For simplification, we applied a constant carbon accumulation rate across the years for AGC, BGC, and SOC.

To assess the advantages of using agroforestry to offset the GHG emissions from the agriculture sector, we compared the accumulated carbon levels in the expansion domains from agroforestry and from sole crop plantations. We used three commodities in the comparison, namely arabica coffee, robusta coffee, and tea. Acacia and cashew crops were not considered because the sole plantation of the two commodities are forestry, not agricultural systems. The comparison was made for highly suitable areas and AGC only because the BGC and SOC input data are not species-specific. All estimated carbon values were converted into CO2e with a factor of 3.67.
