Knowledge reference for national forest assessments - modeling for estimation and monitoring
4. Biomass estimation
Biomass is defined as the total mass of living plant organic matter expressed as oven-dry tons or oven dry tons per unit area. Estimates may be restricted to the aboveground portion of the vegetation, or to trees only, or to tree components (such as foliage, wood, etc.).
Biomass is the mass of living plant organic material, and it is often proportional to volume and basal area at the stand level, and diameter and height at the individual plant level. Biomass is also related to water, nutrient, and energy requirements of individual plants and stands. The carbon content of vegetation is directly related to biomass as discussed in the following section.
4.1 Biomass Components
Biomass may be estimated in total for stands or portions of stands as noted, but information on biomass distribution by plant component is often needed. Biomass components may be divided as necessary for a given application, but often include categories such as stem wood, branch wood, foliage, bark, roots, etc., with more or fewer subdivisions as needed. A common constraint is that the sum of the component biomass estimates must equal the total biomass for the stands or portions of stands of interest.
In many applications, only above ground biomass estimates are used. There are obviously belowground components to biomass (such as coarse roots, fine roots, etc.), but studies quantifying these values are difficult to conduct, are available for only a small number of species and ecosystems, and typically have low precision in the data.
At the stand level, biomass may be estimated for the overstory, shrubs, herbs, lichens, moss, etc. In forested situations, the overstory biomass usually dominates. There are cases where tree cover is low and overstory or tree biomass is smaller compared to that of other ecosystem components. The decision on which biomass components are necessary to consider is dependent on the ecosystems to be surveyed and the intended use of the resulting information.
Cannell (1982) presents a compendium of worldwide data that includes ratios for various biomass components for many forest types. These are measured data from many forest types, and often include multiple observations from similar forest types. Jenkins et al. (2003) present two sets of equations (one for temperate deciduous and one for temperate coniferous species) giving the ratio of total biomass present in foliage, coarse roots, stem bark, and stem wood. These equations take the form:where ri is the fraction of total biomass in the ith biomass component and D is diameter at breast height.
4.2 Biomass Equations
Like volume equations, biomass equations usually express biomass as a function of tree dimensions (diameter and height) or stand-level variables such as basal area. Equations are usually developed for particular species or species groups, and may be developed with data collected from narrow geographic ranges. There are some examples described below where more widely-applicable equations have been developed through a synthesis of other published studies.
Cannell (1984) presented equations to estimate stand level woody biomass from total stand basal area and average tree height for a wide range of temperate and tropical stand types; most of the equations are for temperate coniferous forest types. These equations are simple to apply since they use variables commonly obtained during field data collection, and are applied at the stand level rather than at the individual tree level.
At the individual tree level, Jenkins et al. (2003) give composite equations applicable for temperate species across North America. These equations could be applied, with appropriate qualification, to other temperate forest types. Total aboveground biomass is estimated for individual trees based on an allometric relationship with diameter at breast height:
where B is total aboveground biomass (kg) for trees 2.5 cm and larger in diameter at breast height (D). Coefficients are given for both deciduous and coniferous species groups throughout all regions of the United States. Broad species groupings are utilized (Pine, for instance, and Spruce, with a total of five coniferous and four deciduous species groups).
Brown (1997) presents similar equations for individual trees in tropical forests. For broadleaved species, two equations are presented for Tropical Dry forests, two for Tropical Moist forests, and one for Tropical Wet forests. In addition, one equation is presented for palms and another for Tropical Conifer forests. All of these equations express individual tree biomass as a function of diameter and height, though different specific equation forms are used in different applications.