1.1 PREVIOUS ESTIMATES OF FOREST BIOMASS
Forests have traditionally been used for many products, including timber, fuel, and fodder. Determining the biomass of forests is a useful way of providing estimates of the quantity of these components. Typically, the quantity of sawtimber has been assessed by making volume estimations, but this ignores the other useful components such as smaller size wood for fuel use. Furthermore, very few to no assessments have been made of the quantity of wood present in forests that appear to have no potential for sawtimber production. Assessing the total aboveground biomass of forests, defined as biomass density when expressed as dry weight per unit area (see Section 2.2 for expanded definitions), is a useful way of quantifying the amount of resource available for all traditional uses. It either gives the quantity of total biomass directly or the quantity by each component (e.g., leaves, branches, and bole) because their biomass tends to vary systematically with the total biomass. However, the way the biomass of each forest component varies with total biomass varies by forest type, such as natural or planted forests and closed or open forests. For example, leaves for fodder are about 3-5% and merchantable bole is about 60% of the total aboveground biomass of closed forests.
The quantity of biomass in a forest is a result of the difference between production through photosynthesis and consumption by respiration and harvest processes. Thus it is a useful measure for assessing changes in forest structure. Changes in forest biomass density are brought about by natural succession; human activities such as silviculture, harvesting, and degradation; and natural impacts by wildfire and climate change. Biomass density is also a useful variable for comparing structural and functional attributes of forest ecosystems across a wide range of environmental conditions.
Biomass of forests is also very relevant for issues related to global change. For example, the role of tropical forests in global biogeochemical cycles, especially the carbon cycle and its relation to the greenhouse effect, has heightened interest in estimating the biomass density of tropical forests. The biomass of forests provides estimates of the carbon pools in forest vegetation because about 50% of it is carbon. Consequently, biomass represents the potential amount of carbon, that can be added to the atmosphere as carbon dioxide when the forest is cleared and/or burned. Attempts to estimate the biomass density of tropical forests have been made by the scientific community for use in models that assess the contribution of tropical deforestation and biomass burning to the increase in atmospheric carbon dioxide and other trace gases (Brown et al. 1989, Crutzen et al. 1991, Hall and Uhlig 1991, Houghton et al. 1983).
Global interest in climate change led to the establishment of the UN Framework Convention on Climate Change (UN FCCC) at the 1992 UN Conference on Environment and Development (UNCED). Over 130 nations have ratified this convention which means that these nations need to make national greenhouse gas emission inventories. Changes in the cover, use, and management of forests produce sources and sinks of carbon dioxide to and from the biosphere. To estimate the magnitude of these sources and sinks requires reliable estimates of the biomass density of the forests undergoing change.
Biomass density estimates also provide the means for calculating the amount of carbon dioxide that can be removed from the atmosphere by regrowing forests or by plantations because they establish the rates of biomass production and the upper bounds for carbon sequestering. This issue is receiving more attention of late as countries look to forests as a means of mitigating greenhouse gas emissions, particularly carbon dioxide, a major greenhouse gas and the one fixed during photosynthesis. Practices such as sustainable forest management, slowing deforestation, and low-impact-logging decrease emissions or conserve carbon dioxide. Other practices such as plantation establishment or other tree planting programs on previously non-forested land sequester carbon dioxide (Brown et al. 1996). Furthermore, biomass density estimates of forests are extremely relevant for studying other global biogeochemical cycles, such as nitrogen, because the amount of other nutrient elements in forests is also related to the quantity of biomass present.
Another issue related to forest biomass has emerged since the 1980s. In addition to loss of forest area, forest degradation, resulting in biomass density loss, is known to be occurring (Brown et al. 1994, FAO 1993, 1995). Much of this biomass degradation appears to be unrecorded, thus it is in addition to that accounted for by sanctioned harvesting. An example is the illegal logging practices occurring in many forests of tropical Asia (Callister 1992). Clearly, this process of biomass density reduction has implications for the global carbon cycle, other biogeochemical cycles, and biodiversity. Biomass degradation is due to many factors mostly related to social, economic, and political factors.
Estimates of the biomass density for many of the world's forests have been made. For example, under the International Biological Program (IBP) biomass density estimates were made for many intensive study sites covering most of the major forest regions of the world (summarized in Reichle et al. 1981). A detailed summary of biomass density studies in tropical forests, from lowland to montane and from wet to very dry zones, was made by Brown and Lugo (1982). A later study by Olson et al. (1983) produced a global map of the biomass density of all ecosystem types, including disturbed and undisturbed forests, at a 0.50 x 0.50 grid-scale of resolution.
All of the above summaries of biomass density were based on ecological studies creating several problems with their use for global-scale analyses. Ecological studies are generally designed to characterize local forest structure and the study sites are usually not truly randomly located nor represent the population of interest (Brown and Lugo 1992). These types of studies are suitable for studying local forests but not for making inferences about larger populations (Brown et al. 1989). Furthermore, the total area covered by these studies is a very small fraction of the total forest area (e.g., less than 0.00001% for tropical forests; Brown and Lugo 1984).
A further problem with using biomass data from ecological studies for national to global analyses is the inherent bias of ecologists to adjust placement of plots based on the notion of what a mature forest should look like, i.e., one with many large diameter trees (Brown and Lugo 1992). The effect of adjusting plot placement to include large diameter trees is to overestimate biomass density of the forests because biomass per tree increases geometrically with increasing diameter. The result of this bias is to yield high biomass density estimates for forests (Brown et al. 1989). Thus data from ecological studies must be used with caution as they may not represent the biomass density of the forest over large areas.
Biomass density estimates for tropical forests have been made by the FAO (1993) based on the FAO FORIS (Forest Resources Information System), a computerized data base. Biomass estimates were computed from estimates of volume over bark (VOB, inventoried volume to a minimum tree diameter of 10 cm) often measured in forest inventories. On the positive side, VOB data from forest inventories are based on a large number of plots, generally collected from large sample areas using a planned sampling design from the population of interest. On the negative side, very few national or subnational inventories that report VOB have been done in the tropics. The compilation of the VOB data base by the FAO required much educated guesswork to produce estimates on a tropic-wide basis. This approach is, therefore, of unknown reliability and any errors in VOB estimates were compounded during the conversion of these data to biomass density values. Clearly, new efforts to estimate biomass density more directly from forest inventory data will provide more reliable data for national to global assessments of the quantity of forest resources.