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.
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