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Quantifying forest energy - Inventory methods to determine biomass

K. Kuusela and A. Nyyssönen

K. KUUSELA is a member of the Finnish Forest Research Institute, and A. NYYSSÖNEN is a Professor of Forestry at the University of Helsinki. This article is derived from an FAO paper written for the United Nations Conference on New and Renewable sources of Energy, Nairobi, August 1981.

FOREST INVENTORY IN COLOMBIA for fuelwood and charcoal, another way of measuring the forest

In current forest inventories, growing-stock volume means different things in different forest zones. In temperate and boreal zones, for example, trees are inventoried by measuring the stem from ground to top, and then assessing either the total stem volume or the stem volume above a certain minimum diameter. Specifications commonly vary depending upon whether saw timber, pulpwood or logging residues are being considered. The use of wood as fuel is rather limited here; in some areas it is almost non-existent.

In tropical and sub-tropical zones (including monsoonal climates) trees often have a large proportion of their volume in branch wood. Forest inventories here consider only the volume of clear bole of the commercial tree species. Consumption of fuelwood in these zones, however, is much greater than consumption of industrial wood.

During the past decade, especially in the temperate zones, there has been a marked shift away from considering the forest merely as a production system for stemwood to a realization that the total organic resource is important (Hitchcock and McDonnell, 1979). Since only about 55 percent of the green weight of the complete tree may be in the stem, the rest in the branches, foliage and the stump-root system (Young, 1979), reliable biomass data could help in the realization of the forest as a large-scale energy supplier. The greatest actual need for biomass inventories is probably in tropical and subtropical regions, where the scarcity of fuelwood is most acute.

There is a need to adapt methodologies for forest inventories which allow for both the assessment of the total forest biomass and for potential fuelwood. The most natural and effective way to begin the forest biomass inventories for fuelwood is to coordinate the collection of biomass data with that of other, more conventional forest resource data. From our experience with volume estimation, we can use the same basic principles of sampling design as well as current knowledge of statistical sampling theory and methods to construct and test cost-effective designs for biomass inventories (Ware, 1979). Attention should also be given to the increment and drain flows of the growing stock.

A number of terms are commonly used in biomass inventory (cf. Clark, 1979):

Forest biomass is the total volume of living organisms of all species at a given time and can be divided into three main groups: trees, shrubs and other vegetation.

Complete tree consists of all component parts of the tree including roots, the stump, stems, branches and foliage.

Stump and roots refers to the stump, the height of which is dictated by local practice, and all roots. For practical purposes roots smaller than a specified minimum diameter are often excluded.

Tree above stump means all components of the tree except stump and roots. (In forest biomass inventories, measurements are often made of the tree above the stump instead of the complete tree).

Stem is the trunk of the tree from stump to tip, excluding branches and foliage.

Merchantable stem or bole is the trunk of the tree from stump to a specified minimum diameter.

Stem topwood is the portion of stem from a specified top diameter to the tip of stem; this is often the main component of logging residues.

Branches refer to all limbs and twigs excluding foliage.

Foliage comprises needles, leaves, buds, flowers and fruits.

Biomass measurements are made using units of volume, green weight and dry weight. In all but dry weight, the moisture content should be indicated. Weight per unit volume (specific gravity) can be called density; dry weight in kilograms per cubic metre is called the basic density of wood.

Traditional inventory techniques developed for temperate and boreal forests are aimed at industrial needs only. For the fuelwood and charcoal requirements of developing countries the new biomass inventories are more useful. They take account of branches which make up a large proportion of the tree species of tropical forests.

Conventional inventories

Information required of a forest inventory can be divided into two broad groups: area information and area-related tree information. The former refers to the size of the area occupied by forest and other land use classes as well as to such factors as site quality, forest type, stand size, structure and condition within forest land. Area-related tree information describes quantitatively the volume of growing stock and its composition by species, diameter, quality classes and other factors.

Remote sensing and ground measurements are used jointly to collect data for both information groups. Due to the large areas and great number of individual trees concerned, the utilization of sampling methods is indispensable. Sampling layouts are well established, although they are in need of continuous development. Multi-phase sampling is commonly used in stem volume estimation. The first phase is the measurement of basic diameter (often diameter at breast height, or dbh) on all standing trees in the sample. The second phase is a more complete measurement of sample trees; these form a sub-sample of the first phase. In addition to dbh, such things as height of stem or clear bole and some upper diameters are measured. In the third phase, a sub-sample of sample trees may be taken for still more detailed observations.

Finally, a sub-sample of the third phase trees may be felled ("destructive sampling") and measured in detail in order to be able to calculate the unit volumes of these trees. In some cases a measurement of several diameters, the heights of these diameters and other parameters is made on standing trees. However, instead of these direct methods, it is much more common to utilize unit volumes derived from stem volume functions or tables. The functions are based on a number of sample trees which should represent, without bias, trees in the area to be inventoried. Independent variables in the functions include such stem dimensions as dbh, height, and diameter at a specified upper height. In addition to the stem volume, the functions often indicate the bark percentage as well as the amount of timber products and topwood.

Biomass inventories

In biomass inventories the principal object is to measure either the complete tree or the entire tree above the stump. Measurement procedures are much more complicated than in conventional inventories, where the main object is stem volume estimation. However, a biomass inventory may begin from the same point as a conventional forest inventory. Consequently, derivation of information about forest area and its division into classes is equally important. In addition, tree enumeration may be carried out on the sample plots in very much the same way, although additional work is required in order to include small-size trees, shrubs and possibly other biomass.

In the northeastern United States, where the volume plot is usually a variable radius plot and all trees 11.8 cm dbh and larger are tallied by species and product, Tryon and Edson (1979) recommend a procedure in which biomass data on trees below commercial size be collected on .008 ha, fixed-radius plots, and all stems between 1.59 cm dbh and 11.5 cm dbh, inclusive, be sampled. Trees and shrubs are tallied by species and dbh to the nearest 2.56 cm class. All trees and shrubs 1.28 cm dbh and smaller are recorded by species and height class to a minimum height of 15.38 cm. Finally, a .0004 ha plot (1.14 m radius) is examined for regeneration. The stems 1.59 cm dbh and smaller are counted by species and 30.5 cm height classes. For other areas, of course, the limits and sizes should be adapted accordingly.

Conventional measurement of timber is carried out mainly by volume units, but the use of weight is increasing. For biomass inventory, weight measurements are mandatory. The first step in weight measurements could be the determination of the green and dry weights of the stem. For this, it would be expedient to utilize functions in which basic density is the dependent variable and independent variables consist of stem dimensions, climatic region, tree age or rate of growth.

Basic density estimates can be based on borer core measurements of sample trees. Another method is to take the dry weight of all parts of the complete tree other than the stem and express it in relation to the dry weight of the stem, utilizing functions which have suitable tree characteristics as independent variables (cf., e.g. Kuusela and Hakkila, 1979). An alternative procedure is to prepare functions which give the tree weight directly with a few stem dimensions as independent variables. According to the information provided by Dr Harold Young of the University of Maine, a great proponent of biomass inventories, three local functions can be formed which give the weight of various tree sizes with either diameter or height alone as independent variables.

Irrespective of the methods used to compute the functions, the general procedure to estimate the weight of each tree involves "destructive sampling" based on variations of the following (cf. Hitchcock and McDonnell, 1979):

1. Fell the tree and separate materials according to components of the complete tree.
2. Divide and weigh components by sections.
3. Take sub-samples of each component.
4. Determine the volume of sub-samples by water immersion or other measurements (optional).
5. Oven-dry and weigh the sub-samples.
6. Determine the total oven-dry weight of each section.
7. Apply green-weight and dry-weight density factors for each component.
8. Add the weights of the components to the weight of the complete tree.

The green weight of the complete tree and its components can be derived either in this way or through sampling. Deriving moisture content and dry weight usually requires laboratory work. Dry weight is, in general, about one half of the green weight.

Methods for estimating the weight and volume of shrubs and other vegetation involve the same principles as those described for trees. Rather specialized studies are often needed for the purpose. Independent variables of dry weight functions in this case may include, for instance, the height and density of vegetation.

Estimation of biomass flows

Forest biomass at any date of inventory represents growing stock. Increment and drain, both determined per unit of time, change the quantity of the biomass. Drain from the growing stock and its components can be divided into removal (harvesting by man for usable products), residues (mainly logging residues) and mortality (comprising those parts of the growing stock which die naturally).

A portion of the mortality, especially stems, stumps and roots of coniferous tree species in the boreal vegetation zone, may remain as dead stock in the forest a number of years, where it may be of importance as a resource for fuel. It should therefore be estimated in some areas as part of biomass. Mortality increases fuel resources, removal and decay decrease them.

In temperate zone inventories using temporary sample plots the increment estimates usually concern the stem volume only. If the increment of other components of tree biomass needs to be estimated, the stem proportion of the increment may be used as a guideline in order to get an approximate result. The drain is estimated mostly by removal, transport and consumption statistics. Stump measurements may also be utilized to estimate the drain.

More reliable estimates of the increment and drain of different components of the forest biomass require measurement of sampling units on successive occasions. There exist well-established methods to estimate changes in stem volume of growing stock as a function of time, utilizing repeated measurement on permanent sample plots. Estimation of the flows in total forest biomass through remeasurement is technically possible but obviously rather complicated and expensive. Another possibility is to carry out special investigations of increment and drain to develop regression functions in which probable changes are explained by variables observable in forest inventories.

The cost

Only scattered information is available concerning the costs of forest biomass inventories. It has been demonstrated that in some inventories in the northeastern United States the biomass inventory and estimates do not add more than 10 percent to the total cost of the whole inventory (Tryon and Edson, 1979). This figure may warrant the inclusion of the measurement of variables needed for biomass survey into forest inventories even in the tropics.

However, the cost depends very much on the number of sample trees selected for "destructive sampling," i.e. trees harvested and weighed to determine total and tree component biomass. The cost of destructive biomass sampling in the United States ranges from $100 to $300 per tree depending upon tree size. Due to such high sampling costs, over 80 percent of the total costs of a biomass inventory are associated with destructive sampling (Hitchcock, 1979). Dr Young has claimed, in conversation, that 30 trees per species selected through purposive sampling are sufficient to formulate a regression function, and that two men will be able to measure the variables during a period of six weeks.

Because tree biomass seems to be more, rather than less, variable than the merchantable bole volume, the biomass functions based on relatively few sample trees can only give tentative results (Curia, 1979). For instance, it has been found that the weight of the tree crown is highly variable, and that approximately 10 trees per 5.13 cm class are needed to estimate average excurrent crown weights within ± 12 percent; and that 17 trees are required to estimate the average deliquescent crown weights within ±15 percent (Clark, 1979).

It is a common experience in conventional forest inventories that mixed tropical forests are more difficult than temperate forests in making sampling designs. This is obviously the case also for forest biomass inventories. In addition, in mixed tropical forests the clear bole of merchantable tree species comprises only a small percentage of the complete tree and a still smaller percentage of the forest biomass. Therefore, the stem volume does not have the same value as a key-characteristic as it does in temperate forests. One has also to remember that forests and other wooded areas in the tropics vary considerably. The needs and pressure for biomass inventories on marginal woodlands in the tropics are commonly greater than in better forests. Further, constraints with regard to trained staff and special equipment must also be taken into account.

It is necessary to expand the role of forest biomass inventories in order to find out the potential resources for fuel. There is also an urgent need to find feasible methods for the assessment of the quantity of other woody components of the tree stock than that of the merchantable stem. Although quite an extensive literature is already available (cf. Hitchcock and McDonnell, 1979), considerable research is still needed to meet the changing requirements of forest inventories.


CLARK III, A. 1979, Suggested procedures for measuring tree biomass and reporting free prediction equations. Proc. For. Inventory Workshop, SAF-IUFRO. Ft. Collins, Colorado: 615-628.

CUNIA, T. 1979, On sampling trees for biomass tables construction: some statistical comments. Proc. For. Inventory Workshop, SAF-IUFRO. Ft. Collins, Colorado: 643-664.

HITCHCOCK III, H.C. 1979, Converting traditional CFI data into biomass values: a case study. Proc. For. Inventory Workshop, SAF-IUFRO. Ft. Collins, Colorado: 596-614.

HITCHCOCK III, H.C. & J.P. MCDONNELL. 1979, Biomass measurement: a synthesis of the literature. Proc. For. Inventory Workshop, SAF-IUFRO. Ft. Collins, Colorado: 544-595.

KUUSELA, K. & P. HAKKILA. 1979, Steps towards biomass estimates in the Finnish National Forest Inventory. ECE Timber Committee, FAO European Forestry Commission, loins Working Party on Forest Economics and Statistics, Ad hoc meeting on Forest Resource Assessment. TIM/EFC/WP.2/AC.2/R.7. 9 p.

TRYON, T.C. & D. EDSON. 1979, Forest biomass inventories in the northeastern United States - state of the art. Proc. For. Inventory Workshop, SAF-IUFRO. Ft. Collins, Colorado: 730-735.

WARE, K.D. 1979, Statistical aspects in sampling for biomass inventory. Proc. For. Inventory Workshop, SAF-IUFRO. Ft. Collins, Colorado: 745-756.

YOUNG, H.E. 1979, Forest biomass as a renewable source of energy: inventory productivity and availability. UNITAR Conference on long-term energy resources, Montreal. 18 p.

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