The process of review started with the search and the collection of the original sources (forest inventories, statistics, country reports, etc) used to develop the previous FRA 2000 estimates, in order to obtain as much information as possible on the different document used as source information. In some cases new and updated information was used to improve country estimates.
The search was carried out mainly in the FRA documentation room with the help of most of the officers that developed the earlier statistics. For some countries, information was also searched and downloaded through the internet. This allowed compilation of country briefs where the main features of the source documents together with the calculations and the methodology used are briefly described. The country brief lay out (see Appendix 2) was designed with the attempt to collect, describe and analyze the information source, following common criteria. Each country brief is composed of two parts, description of the source and calculations.
For those countries for which information on forest volume and biomass is available, a high degree of heterogeneity of the type of source documents was detected.
In an attempt to harmonize transformation of the national data sets into estimates according to the global definitions, the following grouping was performed:
• Forest inventory: includes both partial and national forest inventories based on field sampling and forest type mapping which results have either been directly taken from National Reports (available in the FRA documentation room) or from the country submissions to FAO, as compiled by national experts and supplied to FRA for the Global Forest Resources Assessment 2000.
• General forest assessment; include various general forestry documents or statistical yearbooks where volume data were provided without being referenced, or other secondary sources reporting data from previous forest inventories without further explanations of the methodology used to estimate the volume (minimum DBH, over bark or under bark, etc) or of the extent of volume provided (commercial, total, including or excluding branches, etc) or even the forest type where such volume comes from.
• Biomass study: includes all those forest assessment based on the evaluation of the woody biomass for multiple purposes (carbon estimates, wood energy, etc).
For all those countries where no reliable information could be identified, a fourth category was created:
• Expert estimates: includes all those cases where an expert opinion had to be used in order to develop estimates on volume and biomass, through comparison or extrapolations (based on known values of forest volume and biomass) from countries with similar social and ecological conditions.
The source documents were analyzed, commented and evaluated in terms of reliability of information. Three reliability levels were used:
-1 (high): assigned to partial or national forest inventories based on field sampling complemented by thematic mapping;
-2 (medium): assigned to forest inventories (essentially biomass assessment), mainly based on remote sensing;
-3 (low): secondary sources, general assessments, statistics, expert estimates.
Whenever it was possible the country brief was completed with additional information related to the type of forest/woodland inventoried (natural forest, plantations, others wooded lands) and inventoried species (all species, only commercial species).
After collection and review of the source documents and compilation of the first introductive part of the country brief, data were analyzed and processed.
In order to obtain standardized and comparable volumes between all countries, the calculations were aimed to obtain estimate of the mean forest volume (VOB 10), defined by FAO as:
“the stem volume of all living trees with more than 10 cm diameter at breast height (or above buttresses if these are higher), over bark measured from stump to top of bole, excluding branches” (FAO, 1998).
The procedures adopted for the calculations of the volume (and biomass) were mainly based on biomass estimation methodologies (using existing data) outlined by the FAO Forestry Paper 134 (FAO, 1997).
As already indicated a high degree of heterogeneity of the national baseline information was detected. National forest inventories have rarely employed the same standards terms and definitions applied by FAO for volume measurements. Adjustments and assumptions were often done to bring national information to the global standards used by FRA 2000.
In most countries, national estimates had to be based on local small scale inventories or on inventories that had covered only certain aspects of the forests, such as commercial timber volume (e.g. Gambia and Papua New Guinea).
In the cases where the reported minimum diameter was larger than 10 cm - as is the case of forest inventories carried out in tropical humid countries for commercial purposes using minimum DBH usually larger than 25-30 cm, data had to be adjusted.
Stem volume of missing DBH classes was estimated using a volume expansion factor (VEF), defined as the “ratio of inventoried volume for all trees with a minimum diameter (DBH) of 10 cm and above to inventoried volume for all trees with a minimum diameter of 25-30 cm and above” (FAO, 1997). The following formula was used to estimate the VEF:
VEF = Exp {1.300-0.209 * Ln (VOB x)} for VOB x ≥ 250 m³/ha → VEF=1.13 |
Where VOB x is the inventoried volume measured at a minimum DBH (x) larger than 10 cm.
The VOB 10 was then calculated by multiplying the VEF by the inventoried volume (VOB x):
VOB 10 = VOB x * VEF |
Conversely in dry regions of Africa (as for example in Morocco, Mali and Tunisia) volume of a minimum DBH of 5 or 7 cm was often reported. In these cases adjustments were made based on expert opinion to decide whether the contribution of these smaller diameter classes was negligible or not and to decide how the adjustments were to be made. Additional adjustments were necessary when the provided volume included both stem and branch volumes. In these cases the volume of branches was excluded by using percentages based on specific case study (Saket, 1994).
Quite often, especially when the provided data were coming from statistics or secondary sources (as is the case of Chile, Argentina, Venezuela etc) and no further information on the method used to estimate the volume was given, assumptions on whether or not the provided volume was corresponding to the VOB 10, as earlier defined, were made and adjustments were introduced when deemed necessary.
In many countries (for example Central Africa Republic and Kenya), local inventories frequently focused on only highly stocked forests of interest for exploitation. Adjustment of the national data was made since direct extrapolation from highly stocked forests to all forests types in a country generally leads to overestimations of mean volume.
Finally for a small number of countries (e.g. Suriname, Guyana, Liberia, and Libya) where information on volume was not available, estimates were based on expert opinion through either auxiliary information or based on comparison with data from neighbouring countries with similar social and ecological conditions.
Due to discrepancy in the terminology and between the classification systems, in some countries (e.g. Ghana, the Gambia and Ethiopia) it was necessary to reclassify the national forest types, using FRA definitions, in order to make sure that the volume of only the forest component is included.
The main steps of the calculations and brief explanations of the methodology used to estimate volume and biomass are provided in the last section of each country brief.
Primary data on woody biomass are scarce, particularly in the tropics, and estimates were derived from volume data and refer to the above ground biomass. FAO defines the woody biomass as:
“the mass of woody part (stem, bark, branches, twigs) of trees, alive or dead, shrubs and bushes, excluding stumps and roots, foliage, flowers and seeds” (FAO, 1998).
However, for practical reason the calculations were based on the definition of biomass as given by the FAO forestry Paper 134 stating that:
“biomass is the total amount of aboveground organic matter present in trees including leaves, twigs, branches, main bole and bark”.
The biomass of these components generally accounts for the greatest fraction of total living biomass in a forest and does not pose too many logistical problems in its estimation. Consequently, biomass is defined as the total amount of aboveground living organic matter in trees expressed as oven-dry tons per hectare.
Biomass could be calculated from the VOB 10 per hectare by first estimating the biomass of the inventoried volume and then expanding this value to take into account the biomass of the other aboveground components, using a biomass expansion factor (BEF) defined as “the ratio of aboveground oven-dry biomass of trees to oven-dry biomass of inventoried volume”(FAO, 1997).
Biomass of the inventoried volume was calculated multiplying the volume per hectare by the wood density (WD) defined as the oven-dry mass per unit of green volume.
BV = VOB 10 * WD |
Where:
BV is the woody biomass of inventoried volume
WD is the wood density
Wood density applied for tropical tree species varied depending on the region as highlighted in the Table 1.
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 |
Mean values of wood density as indicated in the table above were used, with the exceptions of some very dry countries of Africa and Middle East where a wood density equal to 0.8 was used, to consider the high density of wood growing in these specific areas (Saket, 1994).
Biomass of inventoried volume was then multiplied by the biomass expansion factor in order to include leaves, twigs, and branches, commonly not measured by the forest inventories. BEF was calculated through the following formula:
BEF= Exp {3.213-0.506* Ln (BV)} for BV ≥ 190 t/ha → BEF=1.74 |
Total biomass as earlier defined, was calculated multiplying the biomass of the inventoried volume by the BEF:
Bio= VOB 10*WD*BEF |
Exceptions to these calculations were the national studies of biomass assessment carried out in few central American countries (e.g. Belize, Costa Rica, Guatemala) and for some of the African Region (e.g. Angola, Tanzania, Botswana). In these cases, biomass data were taken straight from the source document and volumes were calculated on the other way round, starting from the biomass divided by the wood density and the BEF. This latter could not be calculated directly from the formula above, since volume data were unknown and for this reason it was extrapolated from a trend as showed by Figure 1.
Figure 1: Trend of biomass expansion factor.
Total volume and biomass for each developing country were obtained by multiplying the mean volume and mean biomass per hectare as resulting from the calculations by the total forest area given by the Global Forest Resources Assessment 2000 (FAO, 2000).
Forest resources assessment of industrialized countries was carried out by the UNECE as a contribution of the Forest Resources Assessment 2000 and results were provided in the UNECE 2000 Main report for Europe, CIS, North America, Australia, Japan and New Zealand.
The report provides a country by country presentation of the data complete by the calculations and the necessary assumptions.
Volume is defined by TBFRA 2000 2000 as:
“the abovestump volume of living trees measured overbark to the three tops. It includes all trees with diameter over zero cm at breast height (DBH) and large branches”.
Differences with the FRA definition of volume were considered negligible and results for industrialized countries were thus not review but only converted to the format displayed by the FRA Main Report and no country brief was compiled for these countries.
Mean volume per hectare were obtained dividing the total growing stock (as given by Table 34, column 4) by the total forest area (from Table 1, column 7) of UNECE Report. Multiplying these averages by the total forest area 2000 as given by the FRA 2000 Main Report it was possible to calculate the total growing stock for the year 2000.
Biomass is defined as:
“the dry mass of the woody parts wood, bark, branches, twigs, stumps and roots, of all trees alive and dead as well as the dry mass of all shrubs and bushes on their forest and other wooded land”.
As stated by the TBFRA 2000 2000 Main Report complete biomass data were available for 45 countries but missing for 10: for these countries the missing biomass figures were calculated with the help of conversion factors calculated both for coniferous forests and for broadleaved forest.
Although the definition given by TBFRA 2000 slightly differs from the one adopted during this review, the TBFRA 2000 2000 Final Report provided also the total abovestump woody biomass of forest excluding stump, roots and other wooded land components (Table 40, column 5 of the TBFRA 2000 2000 Main Report).
These biomass data divided by the total forest area (from Table 1 column 7 of the TBFRA 2000 2000 Main Report) allowed calculating the mean biomass per hectare. The mean biomass per hectare multiplied by the total forest area 2000 as given by the FRA 2000 Main Report gave the total woody biomass for the year 2000.
The figure showing the total amount of woody biomass (as defined by TBFRA 2000 2000) is given in Appendix 3.
