Tree Plantations (on Degraded Land)
Project Drawdown defines tree plantation on degraded land as the cultivation of trees for timber or other biomass uses on degraded land. Climate mitigation is achieved through biosequestration in soils, biomass, and timber. This practice replaces annual cropping on active cropland, and other uses on degraded grassland and forest. Tree plantation on degraded land also aims to reduce emissions from deforestation by providing an alternative source of timber, though this impact is not modeled here.
Tree plantation on degraded land has been widely promoted as a land-based mitigation strategy due in part to its high sequestration rates. Project Drawdown's tree plantation on degraded land scenarios are more modest than many. This is because other tree-focused solutions with high sequestration rates are given higher priority, including tree intercropping, silvopasture, multistrata agroforestry, perennial staple crops, tropical forest restoration, and temperate forest restoration. Nonetheless, tree plantation on degraded land is of critical importance for mitigation, building material, and restoration of degraded lands.
Methodology
Total Land Area
The total area allocated for tree plantation on degraded land is 718 million hectares, and is composed of degraded grassland and forest. The degraded forest represents only the current adoption of the solution, while all new adoption of the solution is allocated on the degraded grassland area. Current adoption[1] is estimated at 294 million hectares, based on data from the Food and Agriculture Organization (FAO, 2015).[2]
Adoption Scenarios
Projected adoption of tree plantation on degraded land is based on historic country-level growth rates from FAO (2015), aggregated by region and based on the model projection from recent literature (Evans, 2009; Kreidenweis et al., 2016). Eight custom scenarios are developed.
Impacts of increased adoption of tree plantation on degraded land from 2020 to 2050 were generated based on two growth scenarios. These were assessed in comparison with a Reference Scenario, in which the solution’s market share was fixed at the current levels.
- Scenario 1: Scenario analysis shows tree plantation on degraded land adoption on 406.62 million hectares of degraded land.
- Scenario 2: Aggressive adoption yields adoption of tree plantation on degraded land on 467.85 million hectares of the allocated degraded land area.
Emissions, Sequestration, and Yield Model
The sequestration rate is 3.3 tons of carbon per hectare per year, based on 29 data points from 13 sources. This is in line with Intergovernmental Panel on Climate Change (IPCC) estimates (Watson, 2007). It is assumed that all sequestered carbon is re-emitted at harvest, except for carbon stored in timber. Average timber yield per hectare per year was based on meta-analysis. An average lifespan of 26 years was considered for a tree plantation on degraded land plantation, based on the meta-analysis of 36 data points from five sources.
Financial Model
First cost of tree plantation on degraded land is US$668.57 per hectare,[3] based on meta-analysis of 22 data points from nine sources. It is assumed that first costs for the land use that tree plantation on degraded land is replacing have already been paid, because the land is already in production. Net profit per hectare is calculated at US$593.96 per year for the solution (based on meta-analysis of 16 data points from 13 sources), compared with US$37.84 per year for the conventional practice (based on 20 data points from 15 sources).[4] Annual operational cost per hectare is calculated at US$123.37 for the solution (based on meta-analysis of 18 data points from 12 sources), compared with US$80.64 for the conventional practice (based on nine data points from seven sources).[5] The financial variables for conventional practice are weighted by the percent degraded grassland area to the total land area because it is assumed that not all degraded grassland area is under conventional practice.
Integration
Project Drawdown's Agro-Ecological Zone model allocates current and projected adoption of solutions to the planet’s forest, grassland, rainfed cropland, and irrigated cropland areas. Adoption of tree plantation on degraded land was constrained by Project Drawdown's higher prioritization of food production and forest restoration. In degraded grassland, tree plantation on degraded land was the second-highest priority.
Results
Total adoption in Scenario 1 is 406.62 million hectares in 2050, representing 57 percent of the total available land. Of this, 112.52 million hectares are adopted from 2020 to 2050. The sequestration impact of this scenario is 22.2 gigatons of carbon dioxide-equivalent greenhouse gases by 2050. Net cost is US$17 billion and lifetime operational cost is US$164.8 billion. Lifetime saving in net profit is US$2.1 trillion
Total adoption in Scenario 2 is 467.85 million hectares in 2050, representing 65 percent of the total available land. Of this, 173.75 million hectares are adopted from 2020 to 2050. The impact of this scenario is 35.9 gigatons of carbon dioxide-equivalent by 2050. Net cost is US$72 billion, and lifetime operational cost is US$259.5 billion. Lifetime saving in net profit is US$3.3 trillion.
Discussion
Benchmarks
The IPCC provides a benchmark of 4.0 gigatons of carbon dioxide-equivalent per year in 2030 from afforestation, given a price of $100 per ton of carbon dioxide (Metz, 2007). The Project Drawdown model shows 0.67–1.09 gigatons of carbon dioxide-equivalent per year in 2030. These impact are substantially lower, because this study has much lower adoption than most studies due both to higher prioritization of other land uses and to not modeling a price on carbon dioxide. When combined with the bamboo production and perennial staple crops solutions, however, emissions reduction in 2030 increased to 1.33–2.45 gigatons of carbon dioxide-equivalent per year in 2030.
Limitations
The study has several limitations. One limitation is the use of a single sequestration rate across all climates. The current version of the model does not account for albedo impacts at temperate and boreal latitudes. This will be addressed in future versions. It would also be desirable to model the impacts of timber replacing carbon and steel in construction, as these materials are emissions-intensive.
Conclusions
Tree plantation on degraded land is already practiced on a wide scale, and represents an important high-carbon land use. It produces products of critical importance and can help reduce pressure on intact forests. Though not a "silver bullet," it is an essential component of land-based mitigation efforts.
[1] Current adoption is defined as the amount of functional demand supplied by the solution in the base year of study. This study uses 2018 as the base year due to the availability of global adoption data for all Project Drawdown solutions evaluated.
[2] Determining the total available land for a solution is a two-part process. The technical potential is based on the suitability of climate, soils, and slopes, and on degraded or nondegraded status. In the second stage, land is allocated using the Project Drawdown Agro-Ecological Zone model, based on priorities for each class of land. The total land allocated for each solution is capped at the solution’s maximum adoption in the Optimum Scenario. Thus, in most cases the total available land is less than the technical potential.
[3] All monetary values are presented in 2014 US$.
[4] Tropical staple trees are not as labor-efficient as annual crops, in a mechanized context. However, 175 million hectares of the world’s farms are smallholders with little mechanization. The net profit per hectare figure shows that these crops are economically viable despite higher labor costs.
[5] Tropical staple trees are not as labor-efficient as annual crops, in a mechanized context. However, 175 million hectares of the world’s farms are smallholders with little mechanization. The net profit per hectare figure shows that these crops are economically viable despite higher labor costs.