Following on from the last blog post, this post will look at another method for measuring aboveground biomass in forests is through the use of allometric models which relate tree dimensions to biomass. This is a good time to discuss this approach, since I’ve spent the past few days comparing allometric models used in two different regional carbon assessments of cocoa farms in Indonesia.
Direct tree harvesting allows biomass to be calculated directly however it is destructive and impractical for large scale assessments. A non-destructive forest inventory approach allows ground-based measurements of tree diameter at breast height (DBH), sometimes in combination with tree height, to be correlated to forest carbon stocks. Generalised allometric equations have been developed for broad forest types of ecological zones (stand tables), with a lot of attention on tropical forests (Anitha et al., 2015; Brown, 1997; Chave et al., 2005; Gibbs and Brown, 2007).
(Brown, 1997) outlined available methods for estimating biomass per hectare of tropical forests using existing data (based on existing volume data and stand tables) and presents biomass estimates for developing countries based on existing inventories. Figure 1 shows the relationship between oven-dry biomass of tropical trees and DBH for a range of climatic zones (for trees with DBH between 5 to 40 cm). Subsequent efforts (Brown et al., 2001) used GIS to incorporate spatial databases of climatic, edaphic, and geomorphic indices and vegetation to estimate and develop a map of potential carbon densities of forest in thirteen Southeast Asian countries.
Figure 1 – Relationship between DBH and biomass from Brown, 1997.
(Chave et al., 2005) developed generalized allometric equations for the pan-tropics based on a large dataset. The assumption that a single pan-tropical allometry could be used in aboveground estimation procedures was tested. It was found that overestimates prevailed, giving a bias of 0.5–6.5% when errors were averaged across all stands. It was also found that the most important predictors of AGB for a tree were, in decreasing order of importance, trunk diameter, wood specific gravity, total height, and forest type (dry, moist, wet), and that developing species or location specific allometric relationships does not typically improve the accuracy of the assessment but occasionally a localized relationship is warranted. (Gibbs et al., 2007) agrees with this and suggests that DBH accounts for the majority of the variation in aboveground biomass, and that generalised allometric models are effective estimations for tropical forests. However some literature does recommend developing specific biomass models for each region and forest (Segura and Kanninen, 2005; Stas, 2014) and that allometric equations are only most useful in uniform forests or plantations with similar aged stands (Kumar and Mutanga, 2017).
In the assessments I have been looking at for example, some selected equations are generic models which are based on large numbers of tree harvest data (such as the Chave et al. 2005 equations) and some are more species specific based on smaller numbers of trees of single or similar species. Careful consideration of the most appropriate allometric equation is needed as discussed above, since different equations can lead to significant differences in the estimated biomass and thus carbon stored in the trees.