Chapter 2. Wood volume and woody biomass

ABSTRACT

Wood volume and above-ground woody biomass in forests were estimated for each country in 2000. Changes of these parameters during the 1990s were estimated at the global level. Information to support estimates of forest volume and woody biomass were not satisfactorily available for many countries, particularly in the tropics. This meant that assumptions and extrapolations had to be used. The year 2000 estimate for the global volume of forests was 386 billion cubic meters and the estimate for worldwide above-ground woody biomass was 422 billion tonnes. Results showed that the wood volume increased by 2 percent during the 1990s, largely because of increment in temperate and boreal forests. At the same time the above-ground woody biomass decreased by about 1.5 percent. A simultaneous increase of volume and decrease of woody biomass was possible because tropical forests were lost that contained considerably more biomass in relation to stem volume, compared to boreal forests where gains were recorded.

INTRODUCTION

Wood volume and woody biomass levels are important indicators of the potential of forests to provide wood and to sequester carbon. Wood is needed as a construction material, for pulp and paper manufacture, for fuel and energy, and for a wide variety of other uses. Because living forests trap and hold large amounts of carbon in their woody biomass they have also been indentified as potentially important regulators of the world's climate. Conversely, forests also may be a source of emissions when forests are burned or when wood from trees and other organic matter decomposes. releasing carbon dioxide back into the atmosphere.

The role of forests as major terrestrial sinks (and sources) of carbon dioxide has received significant additional attention since the adoption of the 1997 Kyoto Protocol to the United Nations Framework Convention on Climate Change (UNFCCC). Data on the carbon content in forest ecosystems has been estimated by the Intergovernmental Panel on Climate Change (IPCC 2000) (Table 2-1) but is far from complete and uncertainty is large. Determination of the amount of carbon in the woody biomass was not possible without a number of assumptions and uncertainties. National forest inventories, which are carried out in many countries, can be an important source of data and information about net productivity and biomass, but these often use different inventory methodologies and are not widely available or aggregated at the regional or global levels (GTOS 2000, 2001).

Volume and biomass statistics were among the most important parameters for FRA 2000. Statistics were compiled from country information sources following standard terms and definitions. For FRA 2000, volume is defined as the "stem volume of all living trees more than 10 cm diameter at breast height (or above buttresses if these are higher), over bark measured from stump to top of bole" (FAO 1998a).[1] (The definition excludes branches.) This term is referred to as "volume over bark" (VOB). "Above-ground woody biomass" was estimated for the assessment, defined as "The above ground mass of the woody part (stem, bark, branches, twigs) of trees, alive or dead, shrubs and bushes, excluding stumps and roots, foliage, flowers and seeds" (FAO 1998a). While total woody biomass would provide a more comprehensive measure of a forest ecosystem's capacity to sequester carbon, the algorithms needed to convert forest volume data to total woody biomass are lacking for much of the world's forests.

Volume and biomass data were available for most of the industrialized countries. Many of these countries also had statistics on growing stock, increment, felling and natural losses. However, reliable national-level data on volume and biomass in developing countries were not widely available. In those countries, most of which were in the tropics, volume estimates had to be based on local inventories or on inventories that only covered certain aspects of the forests, such as the commercial timber volume, or that were limited to only a few species (see e.g. IBAMA 1997; Malleux 1975). Throughout the world, inventories rarely employed the same standards, terms and definitions applied by FAO for volume measurements.

Table 2-1. Global carbon stocks in vegetation and top 1 m of soils

Biome	Area	Global carbon stocks (Gt C)
	million km2	Vegetation	Soils	Total
Tropical forests	17.6	212	216	428
Temperate forests	10.4	59	100	159
Boreal forests	13.7	88	471	559
Tropical savannahs	22.5	66	264	330
Temperate grasslands	12.5	9	295	304
Deserts and semideserts	45.5	8	191	199
Tundra	9.5	6	121	127
Wetlands	3.5	15	225	240
Croplands	16.0	3	128	131
Total	151.2	466	2 011	2 477

Source: IPCC (2000). Note that definitions used may differ from those in FRA 2000.

Global studies encountered during the assessment (mentioned in FAO 1997) include those by Reichle (1981), Brown and Lugo (1982) and Olson et al. (1983). However, they were not appropriate for FRA 2000 since the study sites were often not representative of the population of interest (Brown and Lugo 1992). Consequently, their results could not be successfully extrapolated to the global level.

METHODS

Volume per hectare

For the developing countries, estimations of volume by hectare were based on inventory reports containing volume data for the various national forest types. In cases where the reported minimum diameter at breast height (DBH) was larger or smaller than 10 cm, data were adjusted. Stem volume of missing DBH classes was estimated either through regression equations established between DBH classes and the corresponding volume (when data were sufficient) or by using a volume expansion factor (VEF) (FAO 1997; FAO 1998b). The VEF was used in situations where the volume per hectare was reported for DBH larger than the threshold of 10 cm and regression analysis could not be applied.

Various VEFs had to be used to match the wide range of volume data coming from the inventory reports. Differences in data composition were frequently due to the range of species and the type of forest being inventoried. For example, the minimum DBH in the inventories ranged from 5 cm to more than 50 cm (CIRAD 1991; Hammermaster and Saunders undated). Timber producing countries in humid tropical areas often estimated only volume for DBH classes larger than 30 or 40 cm. Conversely, in dry regions of Africa, a minimum DBH of 7 to 10 cm was used (Chakanga and Selanniemi 1999; CIRAD 1991; Saket et al. 1999). Volume data from most Asian countries were reported for a minimum DBH of 10 cm, and some countries from the humid tropical regions (Indonesia, Bangladesh, Brunei) reported minimum DBH of 20 to 50 cm. Volume data that included stems and branches required additional adjustments for the biomass calculations. In such cases, the volume of branches was excluded by using the ratio of 46 percent branches to 53 percent stem found by Saket (1994).

In many countries, only local inventories were available, which frequently focused on high-volume forests of interest for exploitation. In these areas, additional data adjustments had to be made since direct extrapolation from high-volume forests to all forests in a country would lead to overestimations. For a small number of countries where information on volume was not available, estimates were made using collateral information including the global ecological zone and forest cover maps combined with data from neighbouring countries that have similar ecological and socio-economic conditions. Thus, the volume per hectare for all national forest types could be estimated.

Table 2-2. Wood density applied for tropical tree species (tonnes of oven-dry biomass per cubic metre green volume)

Tropical region	Mean	Common range
Africa	0.56	0.50-0.79
America	0.60	0.50-0.69
Asia	0.57	0.40-0.69

Source: FAO (1997).

Biomass per hectare

For developing countries, biomass per hectare was calculated for each national forest type based on the volume statistics (VOB per hectare) and information on wood density (Table 2-2), and by expanding the volume to take into account the biomass of other above-ground components as follows (See also FAO 1998b).

Total forest biomass (t/ha) = VOB * WD * BEF, where:

VOB = volume over bark (m³ per hectare),

WD = volume-weighted average wood density (tonnes of oven dry biomass per cubic metre green volume),

BEF = biomass expansion factor (ratio of above-ground oven-dry biomass of trees to oven-dry biomass of inventoried volume).

Total volume and biomass

Total volume and biomass for each developing country were obtained by multipying the estimated volume and biomass per hectare with the forest area for each national forest type, and then adding the results for the various forest types into national totals. This means that the area distribution of forest types was an important component of the total volume and biomass estimates. The FRA 2000 documentation of national forest types, their areas and correspondence with global classes was therefore essential (see Chapter 1).

Industrialized countries reported total volume and biomass as documented in UNECE/FAO (2000).

Changes 1990-2000

Changes in forest volume and biomass occur in two different ways. First, areas that are transformed into forests (through afforestation or natural expansion) or deforested represent changes to the overall stock of forest volume and biomass. Second, the balance between increment, natural losses and fellings affects the volume and biomass per hectare within the forest. Seen over the long term, the latter can be used to indicate degradation (decreasing volume per hectare) or improvement (increasing volume per hectare) of the forests.

Volume and biomass changes resulting from forest area changes were estimated by country by multiplying the net forest area change 1990-2000 (see Chapter 1) with the average standing volume and biomass per unit area for the country as a whole. The results by country were added for the tropics and non-tropics as a whole.

Changes within the forests could only be estimated for industrialized countries (UNECE/FAO 2000), representing temperate and boreal forests, about 40 percent of the world forest area. For the remaining area, no comprehensive data for change estimates were available.

Table 2-3. Forest volume and above-ground biomass by region

Region	Forest area	Volume		Biomass
		by area	total	by area	total
	million ha	m3/ha	Gm3	t/ha	Gt
Africa	650	72	46	109	71
Asia	548	63	35	82	45
Oceania	198	55	11	64	13
Europe	1 039	112	116	59	61
North and Central America	549	123	67	95	52
South America	886	125	111	203	180
Total	3 869	100	386	109	422

Source. Appendix 3, Table 7.

RESULTS

Wood volume 2000

The global volume of growing stock was estimated at 386 billion cubic metres in 2000. The regions with the largest volume were Europe (including the Russian Federation) with 30 percent (116 billion cubic metres) and South America with 29 percent (111 billion cubic metres) (Table 2-3). Oceania shows the lowest growing stock with 11 billion cubic metres or 3 percent of the global volume. Estimates for each country are found in Appendix 3, Table 7.

Woody biomass 2000

The global estimate for above-ground woody biomass was 422 billion tonnes. The region with the largest quantity of biomass was South America with 43 percent of the world total or 180 billion tonnes. Brazil alone accounted for 27 percent of the world's above-ground woody biomass. Africa had the second largest quantity with 17 percent of the world total, or 71 billion tonnes. The other regions together accounted for 40 percent of the global above-ground biomass (Figure 2-1, Figure 2-2, Figure 2-3). Estimates for each country are found in Appendix 3, Table 7.

Changes 1990-2000

Changes for the 1990s, related to the transformation of areas to and from forests, were estimated at -9 billion cubic metres, corresponding to -16 billion tonnes of woody biomass. The losses were mainly in the tropics, whereas the non-tropics had an increase of volume and biomass (Table 2-4).

Changes within the forests were only known for industrialized countries, which reported an aggregated increase of 18 billion cubic metres of wood for the 1990s, or just over 1 m³ per hectare per year, corresponding to 9 billion tonnes of woody biomass. These numbers represent the changes in temperate and boreal forests, about 40 percent of the total forest area (Table 2-4) (UNECE/FAO 2000).

Total changes for the 1990s, i.e. the sum of area-related changes and known within-forest changes, amounted to a volume increase of 9 billion cubic metres, or 2 percent. This corresponds to a decrease of 7 billion tonnes of woody biomass, or 1.5 percent (Table 2-4). An increase of volume and at the same time a decrease of woody biomass is possible because tropical forests contain considerably more biomass in relation to stem volume than boreal forests.

Figure 2-2. Distribution of above-ground woody biomass among regions

DISCUSSION

Problems of comparability of national data and reliability of aggregated results arose mainly because of differences in the national systems of nomenclature (i.e. measurement rules and definitions) and differences in the reference period(s). Differences in definitions and measurement rules were made comparable by harmonizing and standardizing the national nomenclature and data sets. Data from developing countries are highly variable in terms of quality and spatial, thematic and temporal resolution. Results of the assessments for the temperate/ boreal countries were more complete as, in addition to the growing stock, they generally included a comprehensive analysis of increment, natural losses, felling and removals.

One component of forest volume and biomass change was the transformation of areas to and from forests. The average stocking level was used to estimate these flows. This is a simplification, as the gained forest area will only over a longer period develop into well-stocked forests. Furthermore, and on the other hand, forests that are converted to other land uses may already to some extent have been degraded to lower levels of volume and biomass. Finally, the conversion of forests will not generally result in a completely treeless landscape. Without supporting data, it was reasonably assumed that the areas in transition involve forests at an average stocking level.

Forest volume and biomass stocks also changed within the forest as a balance of increment, natural losses and fellings. These factors were quantified only for industrialized countries. For remaining areas it was not possible to support assumptions on the changes during the 1990s. On the one hand, degradation occurs, for example in tropical forests, and reduces stocking levels. On the other, net increment occurs for example in large secondary forest formations.

Table 2-4. Changes in volume and above-ground woody biomass 1990-2000 for the tropics and non-tropics

Domain	Changes 1990-2000 as a result of forest area change			Changes 1990-2000 within the forest		Total change 1990-2000		Totals, 2000
	Area	Volume	Biomass	Volume	Biomass	Volume	Biomass	Volume	Biomass
	million ha	Gm3	Gt	Gm3	Gt	Gm3	Gt	Gm3	Gt
Tropics	-123	-12	-18	n.a.1	n.a.1	-12	-18	179	282
Non-tropics	+29	+3	+2	+182	+92	+21	+11	207	140
Total	-94	-9	-16	+18	+9	+9	-7	386	422

¹⁾ No data existed for estimating the balance of increment, losses and fellings within tropical forests.
²⁾ Refers to balance of increment, losses and fellings in industrialized countries as reported in UNECE/FAO (2000); changes within other non-tropical forests are not known.

The overall change estimates are thus not complete, as the within-forest development is not known for 60 percent of the forest area, including all tropical forests. The balance of increment, natural losses and fellings could not be estimated for these areas, and sufficient expert knowledge did not exist to make reasonable assumptions. It is likely that changes in both directions were significant, but no reliable knowledge existed to judge the relative magnitude of positive and negative changes. At the same time, the increase of volume in temperate and boreal forests was well documented and large enough to affect the overall balance of volume and biomass worldwide.

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