Aspiration Tree Planting Program Overview

At the core of our tree planting program is a commitment that every project we support delivers benefits for both critical ecosystems and the people who live there.

For details and FAQs of our tree planting program, please visit: Planting the seeds for a livable future - Aspiration Tree Planting Program.

 

Current Tree CO2 Sequestration Rate Estimation Methodology

Our current tree restoration partner, veritree, has developed a sequestration rate estimation methodology that we have opted to use as well for the sake of consistency.

For details on that methodology, please review the relevant FAQs on their website here.

 

Historical Tree CO2 Sequestration Rate Estimation Methodology*

*The following methodology was used prior to March 2024; we have left it up to provide full transparency to anyone interested in our historical methodology and basis for past claims.

 

Carbon dioxide (CO2) sequestration rates of forests vary widely as a result of tree species, climate, planting density, soil composition, survival rate, tree lifetime, and other factors. As such, there is inherent uncertainty when estimating future CO2 sequestration at a per tree level, with increasing confidence at the per hectare level as sequestration rates can be averaged across many trees. Aspiration took these factors into consideration and used the best available science as well as our tree planting partner data to develop conservative estimates of CO2 sequestration rates for our tree planting program.

 

Forest CO2 sequestration is composed of three main parts: the above-ground biomass (a), below-ground biomass (b), and the soil CO2 sequestration (u) (for our purposes dead wood and litter were not included). The total forest CO2 sequestration is the sum of the CO2 sequestration from all three of these parts and can be computed for each forest climate region and dominant tree species in that forest. The forest CO2 sequestration can then be normalized spatially and temporally to estimate an annualized forest CO2 sequestration for each hectare. To estimate the rate of CO2 sequestration (R) for an individual tree species (s) and location (l), the annualized forest CO2 sequestration rate can be normalized by the average living tree density (d) for that climate region and forest’s dominant tree species. This is shown in equation (1) below.

Next, to calculate the estimated lifetime tree CO2 sequestration (L), the annualized rate of CO2 sequestration for a living tree can be multiplied by the sequestration lifetime (t), probability of survival (p), and the fraction of trees that reach maturity that are destroyed by natural or unnatural events (r), typically called the reversal buffer. This is shown in equation (2) below.

For Aspiration’s tree planting program portfolio, the above steps were completed with two significant limitations: (1) tree species were grouped together into either “planted forests and woodlots” or “mangroves”, (2) and locational-differences are not accounted for. On (1), data limitations prevent species-level sequestration estimations so grouped-species with comparable absorption characteristics are used. For (2), the majority (~95%) of Aspiration’s tree planting program portfolio is from mangroves which primarily live in humid regions with smaller sequestration rate differences across locations, thus a location-agnostic rate was used. Future improvements addressing both items are planned when data on tree species and planting location becomes available. For reporting and claim purposes, Aspiration calculates a tree-planted weighted average of tree lifetime CO2 sequestration (L) across tree species group based on the number of trees planted by Aspiration’s tree planting partners (n). 

The following sections describe the data sources and details used to follow the aforementioned calculation steps. 

 

Forest CO2 Sequestration Rate Estimation

As mentioned above, forest CO2 sequestration rates are the sum of above-ground, below-ground, and soil sequestration rates for a particular forest species (or species group) and locations (for our purposes dead wood and litter were not included). 

 

Biomass sequestration: Our above- and below-ground biomass calculations at the hectare scale are based on the seminal Bernal et al. Winrock International study (2018). Bernal et al. reviewed over 330 forest landscape restoration (FLR) studies to determine average CO2 absorption rates for different FLR types based on climatic regions, species, or species groups. Specifically, the FLR categories included: 

• Planted forests and woodlots
• Natural regeneration
• Agroforestry
• Mangroves

 

The Bernal et al. (2018) study FLR categories overlap with Aspiration’s reforestation program. To obtain sequestration rates for species groups at a hectare level, Aspiration reforestation species were categorized and mapped to either the mangrove category or the “planted forests and woodlots” category. For planted forests and woodlots, we removed eucalyptus from the observed biomass sequestration rates as Aspiration does not plant eucalyptus trees, which are typically used for timber and are not native in most regions.

 

Soil sequestration: For mangrove soil carbon sequestration, Salmo et al. (2019) studied a newly regenerated mangrove habitat after an earthquake event resulted in new soil habitat availability for the mangroves to expand. They were able to measure soil carbon stocks across five different sites and determine an average soil carbon sequestration rate for mangroves. 

 

Specifically, per Salmo et al. (2019), the annualized total carbon (C) sequestration rate was 24 +/- 1 Mg C/ha/yr. The study states that out of the 24 ± 1 Mg C/ha/yr, 13% was from above-ground biomass, 5% from below-ground biomass, and 82% from soils. Thus, the carbon stock of soil is 19.68 Mg C/h/yr, above-ground biomass is 2.56 Mg C/ha/yr, and below-ground biomass is 0.13 Mg C/ha/yr. As the mean cumulative soil carbon stock to 100 cm depth was computed at 19.68 Mg C/h/yr, the possible C before the earthquake needed to be subtracted. We assumed this to be similar to sediment C observed at 40-100 cm depth. Therefore, the mean rate of sediment C accumulation was estimated at 8 Mg C/ha/yr, which ultimately yields 29.33 tCO2/ha/yr.

 

Soil sequestration of planted forests and woodlots is not currently included in our calculations as the soil carbon in non-mangrove forests is typically very low compared to aboveground carbon. Thus, the sequestration for these types of trees is more conservative as any potential CO2 sequestration from the soil is not currently included in our calculations. 

 

Single tree scale sequestration: Next, the forest level CO2 sequestration values were normalized by tree density (number of trees/hectare) to obtain a single-tree scale sequestration rate for each tree species group. Bernal et al. (2018) was used to obtain the appropriate tree densities. To determine the average planting density for planted forests and woodlots and mangroves, we reviewed all of the studies referenced and averaged the reported planting densities if the information was available.

 

Once we determined the average planting densities for planted forests and woodlots and mangroves from the studies, these values were used to normalize the total biomass and soil sequestration rates for each of the two categories, per equation (1).

 

Single Tree Lifetime CO2 Sequestration Estimation

Single tree scale Lifetime CO2 sequestration can be estimated based on the single tree sequestration rate, combined with Aspiration-specific data on survival rate, reversal buffer, and the sequestration duration considered, as described in equation (2). 

 

Aspiration’s reforestation partners provided average survival rates for trees planted to date for terrestrial species (80.1%) and mangroves (81.35%). Next, the sequestration timeline we assumed was 20 years because the sequestration rates obtained from Bernal et al. (2018) were based on the first 20 years of life for each tree species group. While forests will continue to grow and sequester some amount of carbon over the remaining lifetime, we truncate our sequestration calculations at 20 years for additional conservatism. Finally, the buffer rate for Aspiration’s specific reforestation portfolio is estimated to be 26% using the VCS Non-Permanence Risk Tool (Verra 2019). This value is used for all tree species groups in Aspiration’s current portfolio.

 

Aspiration Average Single Tree Lifetime CO2 Sequestration Estimation

To estimate a representative value of tree lifetime CO2 sequestration across Aspiration’s tree planting portfolio, we need to compute a weighted average of tree sequestration based on the number of trees planted for each species group. We worked with our reforestation partners to determine their average planting densities and the amount of hectares currently planted and the total area to be planted at our sites. We also determined what percentage of our trees planted overall were categorized as planted forests and woodlots or mangroves. Thus, we had total trees planted by tree species group in our reforestation portfolio and we used equation (3) to estimate an average tree lifetime CO2 sequestration. 

 

Future Updates

We plan to update these estimates over time as our planting site composition will continue to evolve and this will allow us to reflect the most accurate CO2 absorption rate for Aspiration’s reforestation program. Additionally, we plan to improve the resolution of our CO2 estimation techniques by estimating the CO2 sequestration and survival rates at a climate location level in the next update to these calculations.

 

1 Since we are using single tree sequestration rates based on literature data combined with  Aspiration-specific reforestation survival rates, we are assuming that an incremental planting density leads to a linearly proportional value of CO2 sequestration at both the biomass and soil levels.

 

References

Bernal, Blanca, Lara T. Murray, and Timothy R. H. Pearson. 2018. “Global Carbon Dioxide Removal Rates from Forest Landscape Restoration Activities.” Carbon Balance and Management 13 (1): 22. https://doi.org/10.1186/s13021-018-0110-8.

Salmo, Severino G., Vanessa Malapit, Maria Carmela A. Garcia, and Homer M. Pagkalinawan. 2019. “Establishing Rates of Carbon Sequestration in Mangroves from an Earthquake Uplift Event.” Biology Letters 15 (3): 20180799. https://doi.org/10.1098/rsbl.2018.0799.

Verra. 2019. “Risk Report Calculation Tool: VCS Version 4.” https://verra.org/wp-content/uploads/2023/01/VCS-Risk-Report-Calculation-Tool-v4.0.xls.