How does one estimate biomass accumulation without cutting down trees?

Fabrizio's main point in his previous post on carbon financing highlights the difficulties in ensuring that funds from a carbon-offset programme actually reach the participating community members. The costs of implementing a tiered evaluation of carbon sequestration (UN-REDD Programme, 2013), the certification process and the costs of simply developing the project in the first place are too large for any slicing of the pie to bring social benefits.

The innovative way around this is to minimise these demands by using a non-destructive method of evaluating biomass, and therefore carbon sequestered, and using a different system for accountability relying on geo-referenced locations of individual trees.

Allometry refers to the relationships between various characteristics of an organism and, in our case, these characteristics are related to tree size. In "normal" conditions, trees within the same plantation have similar development; this means that proportions between easily measured variables (diameter, height, wood density) and difficult-to-obtain variables (biomass, volume) follow patterns that are similar for all trees growing under the same conditions. As a result, allometric equations can be used to determine biomass production, which is important for climate change mitigation studies.

As one might guess, the number of allometric equations available is as large as the number of tree species and the myriad ways and places they can be planted. So how might one go about choosing the appropriate equation for a given species? To start with, researchers, foresters and other stakeholders who need to measure tree biomass have developed and tested a number of equations. By reviewing the extensive academic literature, the options can be narrowed down to a few pertinent equations. From that point, average biomass values, per individual tree of a given species, or per hectare can be obtained. With each tree being geo-referenced and verifiably present, the risk of over-estimating is limited.

The region in Madagascar where Alamanga farmers are planting for Greenamity has an average annual rainfall of < 1500 mm. This classifies the tropical region as "dry", which is an important parameter to consider when measuring tree growth and biomass accumulation. The focal species are Anacardium Occidentale (cashew), Moringa Oleifera (benzoil tree), Acacia Mangium (black wattle), Delonix Regia (flamboyant tree), Khaya Madagascarensis (Madagascar mahogany) and Jacaranda Mimosifolia.

While some species, particularly A. Mangium and M. Oleifera, have been widely studied for their biomass (and also have models describing their growth under plantation conditions) the other species have fewer published data on allometric equations. In an effort to remain consistent, the allometric model that will be used for all species comes from Chave et al.'s (2005) allometric equation for dry forests in the pan-tropics, using height, diameter at breast height (DBH) and wood specific gravity as variables:

W = exp(-2.187 + 0.916 * ln(ρ * DBH^2 * H)),
where ρ: wood specific gravity (g/m^3), DBH: diameter at breast height (cm), and H: tree height (m).

This equation was tested by Vieilledent et al. (2012) in Madagascar's dry forest and found to match values from destructive sampling most closely.

For example, a 5-year old A. Mangium tree with DBH = 14.80 cm, height = 5.64 m, and wood specific gravity = 0.507 (Zanne et al., 2009), will have a biomass of approximately 27 kg of carbon, and the CO2 equivalent is ~ 100 kg per tree. In a 1 ha plantation with trees of the same age, spaced at 3 m by 3 m, there will be about 1089 trees, and the biomass per hectare in that plot will amount to approximately 1089 x 100 kg or 108.9 tons of CO2 captured per ha. Due to the exponential nature of the equation, trees with double the dimensions (29.60 DBH, 11.28 h) would stock more than six times as much CO2.

Data to calculate average biomass values come always from published literature, or from forestry databases. See references and descriptions at the end of the post for further information. Species-specific allometric equations will be used when available.

There are some drawbacks to this method: firstly, most equations are useful for a narrow range of DBH, typically 5 cm onwards. This means that trees must attain a minimum age for the estimate to be done. Secondly, unless the equation was developed from a similar region to where the trees are being planted, some error is unavoidable. However, the species selected are typically fast-growing species, and attain sufficient girth earlier than typical old-growth trees. Also, as more and more people are interested in questions on biomass and carbon sequestration, there are teams of scientists working in several regions trying to fill the data gaps, and making these data available to the interested public.

Another major point is that as the offset is obtained at the end of the 10-year period, the carbon-dioxide sequestered in a 10-year old plantation needs to be estimated. When DBH and height measurements aren't available from an already existing 10-year-old plantation, then the DBH and heights will need to be estimated, using an equation for growth rates. Establishing a relationship between tree DBH and age is very difficult for the tropics. As a result, when DBH values are obtained for tree species, it isn't always possible to know the average age of the plantation (or indeed the individual tree).

We know that trees show an exponential growth, with an important slope (linear) in the early years that tapers off after a certain age. Since the plantation owners participating in Greenamity's projects are obliged to maintain their stands for a minimum of 10 years, we hypothesise that their trees will still be within the "linear" part of their growth. Thus, when data on tree DBH and height are obtained from personal communication with field researchers, an equation for growth rate can be obtained and used to estimate the average DBH and height which those trees would reach at a certain age. This would evidently be an estimation and no tree will have that exact value, but as we are considering big figures (thousands), the errors (over and under estimation) will cancel each other out. As trees younger than 10 years are still within the linear part of the growth curve, a linear equation can be used to estimate DBH and height accumulation per year.

So the equations will be of the form:

DBH (cm) = a * (age in months) + b,
where a and b are parameters obtained by an ordinary least squares regression.

Thus far, published literature on A. Mangium plantations comes from much wetter regions, and growth rate is expected to be influenced by that, so measurements have been taken on the field for trees aged 1 to 5 years. But when data from the field are obtained and the plantation age is unknown, using an average value of growth calculated from field sites in Madagascar allows one to estimate the age of the plantation based on the known relationship between DBH and age.

For calculating total biomass (i.e. above-ground + below-ground biomass) we used appropriate root-shoot ratios from the study of Mokany et al. (2006), which is also referenced by the IPCC.

Greenamity is also in contact with researchers, foresters and other practitioners in the forestry sector in Madagascar and worldwide — as new data and methods become available, the estimations will be refined.

Key references and resources:

Henry, M., Bombelli, A., Trotta, C., Alessandrini, A., Birigazzi, L., Sola, G., et al. (2013). GlobAllomeTree: international platform for tree allometric equations to support volume, biomass and carbon assessment. iForest - Biogeosciences and Forestry, 6(5), 326–330.

This article explains how the repository for allometric equations on tree growth works. Data are free to individuals or organisations using the information for non-profit, scientific purposes.

Chave, J., Andalo, C., Brown, S., Cairns, M. A., Chambers, J. Q., Eamus, D., et al. (2005). Tree allometry and improved estimation of carbon stocks and balance in tropical forests. Oecologia, 145(1), 87–99.

This is a seminal paper that developed pan-tropical allometric equations for estimating biomass, based on forest type (dry, moist, wet, and mangrove).

Vieilledent, G., Vaudry, R., Andriamanohisoa, S. F., Rakotonarivo, O. S., Randrianasolo, H. Z., Razafindrabe, H. N., et al. (2012). A universal approach to estimate biomass and carbon stock in tropical forests using generic allometric models. Ecological Applications, 22(2), 572–583.

In general terms, these researchers compared different allometric equations for measuring biomass, using destructive sampling from a dry region in Madagascar. Their conclusions show that the Chave et al. (2005) equations, for dry forests including DBH and height as variables are most accurate. This, coupled with a similar study conducted in the DRC (Fayolle, 2012) suggests that until species-specific equations are found, for various age-classes, using the dry forest equation, shown above, is the most accurate.

Zanne, A. E., Lopez-Gonzalez, G., Coomes, D. A., Ilic, J., Jansen, S., Lewis, S. L., et al. (2009). Data from: Towards a worldwide wood economics spectrum. Dryad Digital Repository.

This is an open-source database with tree density (a.k.a wood specific gravity) data for several species of trees. This is an important variable in the allometric equations used to calculate above-ground biomass.

Edward, E., Chamshama, S. A., Ngaga, Y. M., & Mndolwa, M. A. (2014). Survival, growth and biomass production of Moringa oleifera provenances at Gairo inland plateau and Ruvu Coastal Region in Tanzania. African Journal of Plant Science, 8(1), 54–64.

This article discusses the growth rates and biomass accumulation of M. Oleifera individuals that had different origins, and were planted in two different sites in Tanzania, with varying precipitation conditions. It appears that seedling provenance does impact growth and biomass accumulation, in drier and wetter regions. While it is not always possible to know the provenance of saplings, the average values of DBH and height provided in the article allows Greenamity to estimate an average biomass, based on age-classes.

Mokany, K., Raison, R. J., & Prokushkin, A. S. (2006). Critical analysis of root : shoot ratios in terrestrial biomes. Global Change Biology, 12(1), 84–96.

This study reviews and categorises root/shoot ratio for different climate regions, which can be used to estimate the below-ground biomass based on the above-ground value.

UN-REDD Programme. (2013). National Forest Monitoring Systems: Monitoring and Measurement, Reporting and Verification (M & MRV) in the context of REDD+ Activities.

This document estimates the costs of evaluating a carbon sequestration project according to United Nations' Reducing Emissions from Deforestation and forest Degradation guidelines.