Tropical Staple Trees
Project Drawdown defines tropical staple trees as: the production of trees and other perennial crops for staple protein, fats, and starch. This solution replaces conventional annual crop production in humid and semi-arid tropics.
Annual cropping systems are a major contributor of emissions from agriculture. The great majority of world cropland is used to produce annual staple crops like maize, wheat, potatoes, and soybeans. Annuals are not the only crops producing staple food, however – in the tropics, many tropical staple trees are already fully domesticated and widely grown, and yield as well or better than their annual staple crop competitors.
These tropical staple trees sequester impressive carbon in soils and aboveground biomass, like any tree. Their sequestration rates are much higher than any annual cropping system, though they present other tradeoffs and challenges.
One critical assumption of this study is that all tropical staple trees adoption would be on degraded land, with no forest clearing, despite the current situation in which much forest is cleared for staple tree crops like avocado and oil palm. It is of great importance to note that if forest (particularly peatland) is cleared for tropical staple tree planting, the result is net emissions regardless of sequestration. Thus, this solution involves planting on degraded land or convert cropland to realize its powerful mitigation potential.
This solution has received very little attention in the climate change mitigation literature. Its high sequestration rate, high current adoption, and rapid growth rate indicate its impressive potential.
Methodology
Total Land Area [1]
Total available land for this solution is 193 million hectares. Current adoption [2] is 47 million hectares, according to the Food and Agriculture Organization Statistical Service (FAOSS). Current growth is very high for crops like oil palm and avocado, but those are often planted on land cleared for the purpose. This solution assumes that the rate of growth will continue, but planting will exclusively be on degraded lands with no forest clearing. [3]
Adoption Scenarios [4]
Future adoption is based on a linear projection of regional data from 1962-2012 (FAOSS). Five custom adoption scenarios were developed based on the low and high historic adoption rates, including some with early adoption of the solution (i.e. 60 percent adoption by 2030).
Impacts of increased adoption of tropical staple trees from 2020-2050 were generated based on three growth scenarios, which were assessed in comparison to a Reference Scenario where the solution’s market share was fixed at the current levels.
- Plausible Scenario: In this scenario, adoption of tropical staple trees is on 110.6 million hectares of degraded land.
- Drawdown Scenario: The high carbon sequestration potential of this solution leads to an aggressive adoption of 143.2 million hectares by 2050.
- Optimum Scenario: The most aggressive adoption scenario shows adoption of tropical staple trees on 192.9 million hectares of degraded land by 2050.
Emissions, Sequestration, and Yield Model
Carbon sequestration rates are set at 4.7 tons per hectare per year, based on 13 data points from 9 sources. It is assumed that all sequestered carbon is re-emitted at the end of an orchard or plantation’s useful life, which here is set at 37.5 years.
The weighted average yield of tropical staple trees is 2.4 times greater than that of annual staples, based on analysis of data from 7 perennials and 15 annuals (FAOSS).
Financial Model
First costs are US$1,887.29 per hectare, based on meta-analysis of 11 data points from 6 sources. [5] For all agricultural solutions it is assumed that there is no conventional first cost, as agriculture is already in place on the land. Net profit per hectare is calculated at US$943.87 per year for the solution (based on meta-analysis of 11 data points from 9 sources), compared to US$30.47 per year for the conventional practice (based on 6 data points from 6 sources). [6]
Integration [7]
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 tropical staple trees was the second-highest priority for degraded grassland and the sixth-highest for degraded cropland.
Results
Total adoption in the Plausible Scenario is 110.6 million hectares in 2050, representing 57.3 percent of the total suitable land. Of this, 63.6 million hectares are adopted from 2020-2050. The emissions impact of this scenario is 20.2 gigatons of carbon dioxide-equivalent by 2050. Net cost is $120.1 billion. Net savings is $627.0 billion.
Total adoption in the Drawdown Scenario is 143.2 million hectares in 2050, representing 74.1 percent of the total suitable land. Of this, 96.2 million hectares are adopted from 2020-2050. The impact of this scenario is 31.5 gigatons of carbon dioxide-equivalent by 2050.
Total adoption in the Optimum Scenario is 192.9 million hectares in 2050, representing 99.9 percent of the total suitable land. Of this, 145.9 million hectares are adopted from 2020-2050. The impact of this scenario is 46.7 gigatons of carbon dioxide-equivalent by 2050.
Discussion
Benchmarks
Benchmarks for this solution are unavailable, and mitigation benchmarks for tree crops of any kind are rare. This suggests a need for a new area of research. Projections for afforestation can be used as a rough comparison. The Intergovernmental Panel on Climate Change estimates an impact of 4.0 gigatons of carbon dioxide-equivalent per year by 2030 from afforestation, given a carbon dioxide price of US$100 per ton (Metz, 2007). The tropical staple trees model shows 0.5-0.6 gigatons of carbon dioxide-equivalent per year by 2030, much lower. When combined with Drawdown's bamboo and afforestation solutions, however, emissions reductions increase to 1.0-2.5 gigatons of carbon dioxide-equivalent per year by 2030.
Limitations
Additional data on financials, sequestration rates, and yields would improve this study. The potential adoption area could be increased to include arid regions, as many tropical staple trees like mesquite are suited to arid conditions.
Conclusions
Perennial cropping solutions like multistrata agroforestry and tropical staple trees can offer the high sequestration rates of afforestation and forest restoration while providing food. These somewhat neglected "edible afforestation" solutions are worthy of a place at the center of land-based mitigation efforts. It should also be noted that there are staple trees for cold climates, though their yields are not yet competitive with annual crops. Drawdown hopes to inspire further research and widespread adoption of this promising solution.
[1] To learn more about the Total Land Area for the Food Sector, click the Sector Summary: Food link below.
[2] Current adoption is defined as the amount of functional demand supplied by the solution in the base year of study. This study uses 2014 as the base year due to the availability of global adoption data for all Project Drawdown solutions evaluated.
[3] 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 non-degraded status. In the second stage, land is allocated using the 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.
[4] To learn more about Project Drawdown’s three growth scenarios, click the Scenarios link below. For information on Land Use Sector-specific scenarios, click the Sector Summary: Food link.
[5] All monetary values are presented in US2014$.
[6] 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.
[7] For more on Project Drawdown’s Food Sector integration model, click the Sector Summary: Food link below.
