Forest-Based Biomass Basic Computations - Mathematics and Statistics for the Environment - page 331

13.3.5 b iomass e quations *
The intensity of forest utilization has increased in recent years because of whole-tree harvesting
and the use of wood for energy. Actually, estimating tree biomass (weight) based on parameters that
are easily measured in the field is becoming a fundamental task in forestry and forest-based bio-
mass technology. Traditionally, cubic-foot or board-foot volume of merchantable products, such as
sawlogs or pulpwood, adequately described forest stands; however, the intensity of forest utilization
has increased in recent years because of whole-tree harvesting and the use of wood for energy. All
aboveground branches, leaves, bark, small trees, and trees of poor form or vigor are now commonly
included in the harvested product and are listed as biomass of whole trees (WT) or individual com-
ponents. With this increasing emphasis on complete tree utilization and use of wood as a source of
energy, tables and equations have been developed to show the whole tree biomass as weights of total
trees and their components.
Numerous equations for estimating tree biomass from dry weight in kilograms and DBH (tree
diameter at breast height, in centimeters) have been developed by researchers based on local and
tree species. For example, Landis and Mogren (1975) developed an equation for estimating the bio-
mass of individual Engelmann spruce trees employing the following model:

Y = b 0 + b 1 × DBH 2 (13.7)

where Y is tree component dry weight in kilograms, b 0 and b 1 are regression coefficients, and DBH
is the tree diameter at breast height in centimeters.
Similar sets of equations have been developed for other species and locations. Regression equa-
tions are used to estimate tree biomass in both forestry and ecosystem studies. Examples of many
common equations used in the northeastern United States are presented below. These equations
are typically developed in the following way: Samples of major tree species are chosen for study,
selected dimensions of each tree are recorded, the tree is felled and weighed either whole or in
pieces, and subsamples are oven-dried and weighed again to determine tree moisture content. (Tree
green weights are converted to dry weights by using moisture content values.) Because biomass is
related to tree dimensions, regression analysis is used to estimate the constants or regression coef-
ficients required for the actual calculation of biomass. The resultant regression equations may be
used to estimate the biomass, by species, of all trees for which dimensional data are available. The
equations that follow are of several different forms; they can be used to predict biomass from DBH
or DBH and height. The most common forms used are allometric, exponential, and quadratic.

13.3.5.1 Key to Abbreviations Used in Example Biomass Equations
Br Branch biomass
DBH Diameter at breast height (1.37 m) measured in inches (in), millimeters (mm), or
centimeters (cm)
DdBr Dead branch biomass
ht Tree height
Lf Leaf biomass
Lf + Tw Leaf and twig biomass
ln

Natural logarithm to the base e

log Logarithm to the base 10

Rt Root and stump biomass

St Stem biomass

St + Br Steam and branch biomass but not foliage

* Based on material contained in Tritton, L.M. and Hornbeck, J.W., Biomass Equation for Major Tree Species of the
Northeast , U.S. Department of Agriculture, Washington, D.C., 1982.