188.8.131.52.2. Total tree biomass
For live tree biomass, diameters of a sample of trees are measured and converted
to biomass and carbon estimates using allometric biomass regression equations.
Such equations exist for many forest types; some are species-specific, whereas
others, particularly in the tropics, are more generic in nature (e.g., Alves
et al., 1997; Brown, 1997; Schroeder et al., 1997). Cutting and
weighing a sufficient number of trees to represent the size and species distribution
in a forest to generate local allometric regression equations with high precision,
particularly in complex tropical forests, is extremely time-consuming and costly
and may be beyond the means of most projects. The advantage of using generic
equations, stratified by ecological zones (e.g., dry, moist, and wet; see Brown,
1997), is that they tend to be based on a larger number of trees (Brown, 1997)
and span a larger range of diameters; these factors increase the precision of
the equations. A disadvantage is that the generic equations may not accurately
reflect the true biomass of trees in the project. Relatively inexpensive field
measurements performed at the beginning of a project can be used to check the
validity of the generic equations, however. It is very important that the database
for regression equations contain large-diameter trees because such trees tend
to account for more than 30 percent of the aboveground biomass in mature tropical
forests (Brown and Lugo, 1992; Pinard and Putz, 1996). For plantation or agroforestry
projects, developing or acquiring local biomass regression equations is less
problematic because much work is done on plantation forestry (e.g., Lugo, 1997).
Dead wood, both lying and standing, is an important carbon pool in forests that
should be measured for an accurate representation of carbon stocks. Methods
have been developed for this component and tested in many forest types. The
non-tree component of the vegetation (understory, shrubs, mosses, lichens) may
also be important in some forests and should be included in measurements.
The carbon content per unit mass of plant tissue varies little within a species
and tissue type but can vary significantly between tissue types (e.g., fruits
vs. wood) and function groups of plants (e.g., trees vs. grasses). Default values
generally can be used, but they should be supported by a validated sample.
Roots are an important part of the carbon cycle because they transfer large
amounts of carbon directly into the soil, where it may be stored for a long
time. Most of the below-ground biomass of forests is contained in coarse roots-generally
defined as >2 mm-but most of the annual growth is allocated to fine roots (Deans,
1981; Jackson et al., 1997). Part of the carbon in roots is used to increase
biomass, but carbon is also lost through exudation, respiration, and decomposition.
Although some roots may extend to great depths (Canadell et al., 1996),
the overwhelming proportion of the total root biomass is generally found within
30 cm of the soil surface (Jackson et al., 1996). Measuring the amounts
of biomass in roots and their turnover is an extremely costly exercise. Therefore,
regression equations are often used to extrapolate aboveground biomass to whole-tree
biomass (Kurz et al., 1996; Cairns et al., 1997). The problem
with this approach is that deforestation and harvests (as well as changing environmental
factors) may change the relationship between aboveground and below-ground biomass.
On the other hand, below-ground carbon might still be assessed from a known
history of aboveground vegetation.