2.1.6. Step 6: Estimate the Cost Efficiency of Agroforestry Expansion

We defined cost efficiency as the investment cost required to sequester one ton of CO<sup>2</sup> equivalent. We calculated cost efficiency using the total investment cost to establish and maintain agroforestry systems for ten years under the baseline climate conditions. The cost efficiency will not change under the future climate conditions because the change in investment cost is proportionate to that in the potential expansion area. We estimated all investment costs according to their monetary worth in 2019 (Table 4) by using the annual inflation rate of Vietnam. We compared cost efficiency among agroforestry expansion scenarios, and between agroforestry and sole plantations of arabica, robusta, and tea.



#### **3. Results 3. Results**

#### *3.1. Sequestered Carbon in Existing Areas of Agroforestry 3.1. Sequestered Carbon in Existing Areas of Agroforestry*

*3.2. Suitable Areas for Agroforestry Expansion* 

The total (TOC) of sequestered AGC, BGC, and SOC in the existing areas of agroforestry, from the eight key systems in Vietnam, reaches 1346 ± 92 mil tCO2e. The two systems in wetlands, namely Rhizophora- and Melaleuca-based, contribute about 82% to the TOC (Figure 4) thanks to their high carbon storage per hectare and large areas. About 52% of the TOC is SOC, 27% is BGC, and 21% is AGC. The share of TOC from BGC is higher than from AGC due to a large contribution from the root biomass of the Rhizophora-based system. The AGC, BGC, and SOC sequestered by agroforestry systems are shown in Table A2. The total (TOC) of sequestered AGC, BGC, and SOC in the existing areas of agroforestry, from the eight key systems in Vietnam, reaches 1346 ± 92 mil tCO2e. The two systems in wetlands, namely Rhizophora- and Melaleuca-based, contribute about 82% to the TOC (Figure 4) thanks to their high carbon storage per hectare and large areas. About 52% of the TOC is SOC, 27% is BGC, and 21% is AGC. The share of TOC from BGC is higher than from AGC due to a large contribution from the root biomass of the Rhizophora-based system. The AGC, BGC, and SOC sequestered by agroforestry systems are shown in Table A2.


**Figure 4.** Main contributions to the total sequestered carbon in existing areas of agroforestry. **Figure 4.** Main contributions to the total sequestered carbon in existing areas of agroforestry.

**Table 5.** Suitable areas for agroforestry expansion under baseline and future climate conditions.  **Acacia Arabica Robusta Cashew Tea**  Highly suitable (thousand ha) Baseline 419 54 189 24 181 RCP 4.5 435 15 36 21 170 RCP 8.5 389 6 32 17 127 Combined (highly and less) suitable (thousand ha) Baseline 2407 937 1985 1789 1787

reduced by 34% and 48% in 2050 under RCP 4.5 and RCP 8.5, respectively.

Highly suitable areas for the five agroforestry systems for expansion range from 24 to 419 thousand ha nationwide under the baseline climate (Table 5). Cashew has the most limited area, while acacia has the largest. However, in terms of aggregate of highly and less suitable areas, the most limited is arabica. Neglecting the capacity of agroforestry system to modify micro-climate that can reduce the impact of climate change especially warming climate, the highly suitable areas for agroforestry expansion were substantially affected by the potential change in climate (Table 5). For example, large areas across the country are projected to have an increase in average annual temperature by 1.8–2 °C by 2050 under RCP 8.5, and this reduces the highly suitable area of the arabica coffee-based system by 89% compared to baseline. For most of the agroforestry systems, the change in climate transformed the highly suitable areas into less suitable areas for expansion. The two coffee-based systems are the most severely affected, while acacia is relatively resistant to the climate change. On average, for all agroforestry systems, the highly suitable areas are potentially

#### *3.2. Suitable Areas for Agroforestry Expansion*

Highly suitable areas for the five agroforestry systems for expansion range from 24 to 419 thousand ha nationwide under the baseline climate (Table 5). Cashew has the most limited area, while acacia has the largest. However, in terms of aggregate of highly and less suitable areas, the most limited is arabica. Neglecting the capacity of agroforestry system to modify micro-climate that can reduce the impact of climate change especially warming climate, the highly suitable areas for agroforestry expansion were substantially affected by the potential change in climate (Table 5). For example, large areas across the country are projected to have an increase in average annual temperature by 1.8–2 ◦C by 2050 under RCP 8.5, and this reduces the highly suitable area of the arabica coffee-based system by 89% compared to baseline. For most of the agroforestry systems, the change in climate transformed the highly suitable areas into less suitable areas for expansion. The two coffee-based systems are the most severely affected, while acacia is relatively resistant to the climate change. On average, for all agroforestry systems, the highly suitable areas are potentially reduced by 34% and 48% in 2050 under RCP 4.5 and RCP 8.5, respectively.


**Table 5.** Suitable areas for agroforestry expansion under baseline and future climate conditions.

Among the eight regions, Central Highlands contains the largest area that is highly suitable for agroforestry expansion (Figure 5a) due to favorable soil and climate conditions for crop cultivation. The South Central Coast and Mekong River Delta have the smallest suitable areas due to higher temperatures, lower precipitation, or unsuitable soil types for agroforestry systems' expansion. These two regions, along with the Red River Delta, also have the smallest suitable area for agroforestry expansion when both highly and less suitable areas are combined (Figure 5b). Suitable areas for agroforestry systems organized by region under baseline and future climate conditions are given in Table A3. Among the eight regions, Central Highlands contains the largest area that is highly suitable for agroforestry expansion (Figure 5a) due to favorable soil and climate conditions for crop cultivation. The South Central Coast and Mekong River Delta have the smallest suitable areas due to higher temperatures, lower precipitation, or unsuitable soil types for agroforestry systems' expansion. These two regions, along with the Red River Delta, also have the smallest suitable area for agroforestry expansion when both highly and less suitable areas are combined (Figure 5b). Suitable areas for agroforestry systems organized by region under baseline and future climate conditions are given in Table A3.

**Figure 5.** (**a**) Highly and (**b**) combined (highly and less suitable) areas by species and region. **Figure 5.** (**a**) Highly and (**b**) combined (highly and less suitable) areas by species and region.

climate conditions (Figure 6). Acacia-based AF accumulated the highest TOC that reaches 44 ± 4.5 mil tCO2e by 2030. The inclusion of 10% or 25% of the less suitable areas substantially increased the TOC on average by a factor of 3.3 and 6.8, respectively (Figure 6b,c), compared to the TOC from highly suitable areas only. The sequestered TOC values under baseline and future climate conditions with

*3.3. Sequestered Carbon in the Agroforestry Expansion Areas* 

or without the inclusion of less suitable areas are provided in Table A4.

#### *3.3. Sequestered Carbon in the Agroforestry Expansion Areas*

Among the five scenarios, expansion using arabica- and cashew-based agroforestry in highly suitable areas accumulated the smallest TOC over the ten-year expansion period under baseline climate conditions (Figure 6). Acacia-based AF accumulated the highest TOC that reaches 44 ± 4.5 mil tCO2e by 2030. The inclusion of 10% or 25% of the less suitable areas substantially increased the TOC on average by a factor of 3.3 and 6.8, respectively (Figure 6b,c), compared to the TOC from highly suitable areas only. The sequestered TOC values under baseline and future climate conditions with or without the inclusion of less suitable areas are provided in Table *Land* **2020**, *9*, x FOR PEER REVIEW A4. 12 of 26

**Figure 6.** Sequestered total carbon (TOC) over ten-year agroforestry expansion in (**a**) highly suitable and (**b**) highly suitable and 10% or (**c**) 25% of less suitable areas under baseline climate conditions. **Figure 6.** Sequestered total carbon (TOC) over ten-year agroforestry expansion in (**a**) highly suitable and (**b**) highly suitable and 10% or (**c**) 25% of less suitable areas under baseline climate conditions.

#### *3.4. Cost Efficiency of Agroforestry Expansion 3.4. Cost E*ffi*ciency of Agroforestry Expansion*

plantations.

The investment cost for agroforestry expansion in the highly suitable areas under baseline climate conditions ranges from USD 28 to 2790 million for the ten-year period (Figure 7a). The cost doubled when the combined suitable areas were included. Among the five agroforestry systems, the expansion of tea-based systems into the total highly suitable area of 180,000 ha requires the largest investment cost. For the purpose of sequestering carbon, neglecting potential economic returns, acacia- and cashew-based systems are the most cost efficient among the five agroforestry systems for expansion. The cost efficiency of the two systems is USD 8–12 per tCO2e. The cost efficiencies of sole The investment cost for agroforestry expansion in the highly suitable areas under baseline climate conditions ranges from USD 28 to 2790 million for the ten-year period (Figure 7a). The cost doubled when the combined suitable areas were included. Among the five agroforestry systems, the expansion of tea-based systems into the total highly suitable area of 180,000 ha requires the largest investment cost. For the purpose of sequestering carbon, neglecting potential economic returns, acacia- and cashew-based systems are the most cost efficient among the five agroforestry

systems for expansion. The cost efficiency of the two systems is USD 8–12 per tCO2e. The cost efficiencies of sole crop plantations are USD 9121, 1041, and 934 per tCO2e for arabica, robusta, and tea, respectively. Agroforestry expansion is 1.3–17 times more cost-efficient for sequestering carbon than sole crop plantations. *Land* **2020**, *9*, x FOR PEER REVIEW 13 of 26

**Figure 7.** (**a**) Total investment cost and cost efficiency of the five agroforestry expansion scenarios, (**b**) mitigation contribution from agroforestry expansion for removing greenhouse gas (GHG) emission of Vietnam's agriculture sector. **Figure 7.** (**a**) Total investment cost and cost efficiency of the five agroforestry expansion scenarios, (**b**) mitigation contribution from agroforestry expansion for removing greenhouse gas (GHG) emission of Vietnam's agriculture sector.

#### *3.5. Mitigation Contribution to Agriculture Sector 3.5. Mitigation Contribution to Agriculture Sector*

considered in the assessment.

The sequestered TOC in the existing areas of agroforestry that reaches 1346 ± 92 mil tCO2e can thoroughly offset the projected GHG emissions of the agriculture sector by 2030. However, if only the carbon contribution from post-2020 programs is considered, the sequestered TOC in agroforestry expansion areas under baseline climate can remove 15–88% of the total GHG emissions of the agriculture sector compared to if 25% of the less suitable areas is included in the carbon assessment (Figure 7b). The acacia-based expansion provides the largest contribution, while the arabica-based provides the smallest. If the 15 mitigation measures in Vietnam's first NDC provide the minimum The sequestered TOC in the existing areas of agroforestry that reaches 1346 ± 92 mil tCO2e can thoroughly offset the projected GHG emissions of the agriculture sector by 2030. However, if only the carbon contribution from post-2020 programs is considered, the sequestered TOC in agroforestry expansion areas under baseline climate can remove 15–88% of the total GHG emissions of the agriculture sector compared to if 25% of the less suitable areas is included in the carbon assessment (Figure 7b). The acacia-based expansion provides the largest contribution, while the arabica-based provides the smallest. If the 15 mitigation measures in Vietnam's first NDC provide the minimum contribution of

6%, about 6–79% of projected emissions by 2030 would remain unremoved. A higher contribution from agroforestry can be expected if more than 25% of the less suitable areas were considered in the assessment.

#### **4. Discussion**
