And Stock Difference Approaches

Of the two approaches, the "Stock-Difference" approach may be more suitable for estimating carbon stock differences for carbon mitigation as well as land conservation and development projects because of the following reasons:

• "Gain-Loss" approach requires estimation of rates of growth and losses of carbon pools, which can be obtained through "Stock-Difference" approach.

• It is difficult to estimate the losses due to extraction, fire, decay, burning and other causes in the project area.

• "Gain-Loss" approach requires apportioning the annual transfer of biomass into litter, deadwood and soil carbon pools, which requires significant additional effort.

• In the "Stock-Difference" approach, it is easier to account for changes in the stocks of all the relevant pools to obtain the per-hectare change although the frequency of measurement is different for different pools.

IPCC (2006) concludes that the "Gain-Loss" method is the default method, to be used when limited measured data are available. Further, the "Stock-Difference" method is suggested for greater accuracy. Accordingly, this handbook focuses on the "Stock-Difference" method.

9.2 Methodological Options for Estimating Carbon Pools

The project developer or manger and greenhouse gas inventory compiler will have to decide on the method to be adopted for carbon inventory of different pools at different phases. This section provides a generic description of various methods and their applicability to inventory of different pools for land-based projects and national greenhouse gas inventory. A list of methods for different carbon pools and their applicability is given in Table 9.1.

Table 9.1 Methodological options for estimating carbon pools (with details provided in the chapters)

Carbon pool Methods

Suitability for carbon inventory of land-use systems

Harvest method

- Not suitable, not often permitted, leads to

disturbance of forest and even carbon

emissions, expensive

Carbon flux measurements

- Not suitable, expensive, requires skilled staff

s s a

Satellite/remote sensing

- May not be suitable for multiple land-use


systems and project activities

bi d

- Not suitable for small projects


- Practical methods still evolving



- Suitable for projections

ev o

- Requires basic input parameters to be obtained


using other methods

Plotless method

- Suitable, but less suitable for periodic monitoring

and dense vegetation

Plot method

- Most suitable, cost-effective, commonly adopted

and familiar

Root extraction and weight

- Expensive and not suitable

d ns


- Requires uprooting of trees or grass and disturbs

2 !

the soil

S x

Root to shoot ratio

- Most commonly adopted

lo el

or conversion factor

- Requires above-ground biomass estimate


Biomass equations

- Requires input data on tree parameters, girth,


Litter trap

- Not always suitable in village or forest conditions,

2 J

large effort needed

1 É ►J T3

Stock measurement

- Feasible, commonly adopted

Diffuse reflectance

- Not suitable, expensive and requires skilled staff,

n o


has future potential

b r ca


- Suitable for projections


- Requires input data from other methods

Field sampling and

- Most suitable, commonly adopted and

laboratory estimation

familiar method

Deadwood: standing deadwood biomass could be estimated using plot or plotless method. Fallen deadwood biomass could be estimated along with litter biomass.

Deadwood: standing deadwood biomass could be estimated using plot or plotless method. Fallen deadwood biomass could be estimated along with litter biomass.

9.3 Methods for Estimating Above-Ground Biomass

Harvest method The principle involves measuring the weight of tree and non-tree plant biomass in selected sample plots at a given point in time. It involves harvesting all the trees in the sample plots and measuring the weight of different components such as tree trunk, branches and leaves. Harvest method for non-tree biomass (shrubs, herbs, climbers and grass) also requires harvesting of non-woody biomass and of woody biomass, if any, from sample plots. The method gives the most accurate estimate of woody and non-woody biomass stock at the time of harvest and involves the following steps:

• Select the land-use category or project activity strata (see Section 10.3 for strata definition), sampling method and locate sample plots (based on methods described in Chapter 10).

• Harvest tree and non-tree biomass separately and, if necessary, separate the tree biomass into its components (trunk, branches and leaves) in each sample plot.

• Measure the fresh weight of tree and non-tree biomass and components and estimate the dry weight.

• Extrapolate dry biomass from the sample plot to per hectare values for tree and non-tree biomass.

• It may not be feasible to repeat the harvesting approach periodically because it destroys the vegetation in selected plots and new plots may have to be harvested each time. The harvest method is generally not regarded as suitable because it is expensive and destructive. Further, local land regulations and/or project design may not permit harvesting of tree or non-tree biomass. Finally, harvesting may lead to carbon emissions and loss of or disturbance to biodiversity. The method is not cost-effective, especially if large trees need to be harvested and weighed; however, it is suitable or can be adopted in following situations:

- For estimation of non-tree annual biomass production such as grasses, herbs and even shrubs

- For developing location- and species-specific allometric equations

- For short-rotation commercial plantations, where plots are harvested every five to 10 years or so

- For data on biomass stock or growth rates by component (trunk, branches, leaves)

Carbon flux measurement (Eddy covariance) method A range of methods exist for measuring and estimating the flux of CO2 from vegetation cover over a land surface over different spatial scales. The method involves installing a chamber to enclose a small area or a particular component of an ecosystem (e.g. soil, stems, leaves). Changes in the concentration of CO2 within the chamber or the difference between the concentrations in incoming and outgoing air are used to calculate the CO2 flux. Methods exist for measuring the flux of CO2 for an entire ecosystem (less than 1 km2) without enclosures (Noble et al. 2000). The technique most commonly used is the "Eddy correlation/covariance technique", wherein measurements are continuous and semi-automatic (often at hourly intervals). The net flux of CO2 entering or leaving the ecosystem integrated over an area typically of the order of 20 ha determines the overall net carbon exchange at a stand level.

Traditionally, the net ecosystem carbon exchange over multiple years has been estimated by quantifying temporal changes in biomass (Clark et al. 2001) and soil carbon (Amundson et al. 1998; Lal et al. 2001). Earlier, the method was employed to study CO2 exchange of crops under ideal conditions during short field campaigns. The Eddy covariance method has emerged as an important tool for evaluating fluxes of CO2 between terrestrial ecosystems and the atmosphere over the course of 1 year or more. This method is being applied in a nearly continuous mode to study CO2 and water vapour exchange worldwide (Baldocchi 2003) and can be used to quantify how CO2 exchange rates of whole ecosystems respond to environmental perturbations. The method provides an estimate of mass and energy exchange between vegetation surfaces and the atmosphere and allows direct and non-destructive measurement of net exchange of CO2 comprising its uptake via photosynthesis and loss through respiration, evaporation and sensible heat.

The reasons for this method to emerge as an alternative way to assess ecosystem carbon exchange (Running et al. 1999; Canadell et al. 2000; Geider et al. 2001) include the following:

• It is a scale-appropriate method and can assess net CO2 exchange of a whole ecosystem.

• It produces a direct measure of net CO2 exchange across the canopy-atmosphere interface.

• The area sampled with this method, called the flux footprint, can extend from 100 m to several kilometres in length (Schmid 1994): in other words, it can spread over 10-100 ha, with near-continuous measurements possible.

• It can measure CO2 exchange across a spectrum of timescales ranging from hours to years (Wofsy et al. 1993; Baldocchi et al. 2001).

• Data generated by this method provide key inputs to calibrate and validate canopy-and regional-scale carbon balance models.

The limitations of this method include the following:

• It is not feasible in areas that comprise different land-use systems, or landscapes with a mosaic of multiple land-use systems.

• It is expensive to establish.

• It is applicable only over flat terrains.

• It requires stable environmental conditions (wind, temperature, humidity and CO2).

The technique is generally not regarded suitable for typical carbon mitigation or forest, plantation, grassland and cropland development projects because it is costly and needs highly trained staff.

Satellite or remote sensing method Remote sensing involves several techniques such as aerial photography, optical parameters and radar, which can be effectively used to track land-use changes in a project area. Further, remote sensing techniques provide an alternative to traditional methods for estimation, monitoring and verification of changes in areas under different land-use systems as well as in biomass production and growth rates (see Chapter 14 for more information on remote sensing). Remote sensing techniques provide spatially explicit information and enable repeated monitoring even in remote locations. The basic approach to applying remote sensing is to understand the relationship between the parameters of a forest stand (e.g. diameter at breast height (DBH), tree height, crown cover, basal area, and even biomass stock) and their spectral representation, depending on the characteristics of the study area and the sensor data used. Interpretation of remote sensing imagery therefore requires ground truthing and field measurements.

Remote sensing techniques for estimating biomass stocks are still evolving and are yet to be applied to land-based projects extensively; they are not currently suitable as the only method, but rather as a supplement to other methods, for land-based projects or for national greenhouse gas inventories because of limitations such as: (i) high cost, particularly for small-scale projects; (ii) technical and institutional capacity needed at the project level; and (iii) non-suitability to projects such as watersheds, village ecosystems or agroforestry, which involve a mosaic of small parcels (of a few hectares) of different land-use systems. Remote sensing techniques, when further developed with higher resolution, may become cost-effective for forest biomass inventory and national GHG inventory, particularly when the inventories have to be prepared periodically.

Modelling of carbon stock changes Models are available for projecting carbon stocks in biomass and growth rates of above-ground biomass of different commercial plantations as well as forest types. These models can be used to supplement field methods such as plot and plotless methods, where indicators of carbon stocks are measured and estimated. Models could be used to project changes in carbon stocks in biomass in forests and plantations. These growth models estimate biomass (kilograms/tree or tonnes/hectare) as a function of tree parameters such as DBH (in metres) and height (in metres). The biomass could be expressed in terms of volume (cubic metre) or weight (kilograms/tree). The volume could be converted to weight using the density of the tree species or woody biomass. The methods for developing biomass estimation equations are given in Chapter 17.

Such biomass estimation functions are normally available for specific tree species (also in Chapter 17) but not for many native or non-commercial tree species, stands of mixed species, natural forests, non-tree vegetation such as shrubs and grasslands or agroforestry systems. Separate equations are available for a whole tree and merchantable volume. The most popular ones estimate merchantable tree woody biomass. The application of models is limited because of the following shortcomings:

• Models of biomass stock or rate of growth may not be available for multiple land-use systems and project activities common to land-based projects.

• Equations developed for mature trees cannot be used for young trees and vice versa.

• Even when available, the models may not be applicable to local situations, as equations developed for one location may not always be appropriate to other locations because of variation in varieties of a given tree species, vegetation and tree density.

• Developing the equations may require many trees of different sizes to be harvested and weighed and relating the weights to parameters such as DBH and height.

Despite these limitations, biomass equations, whenever available, provide the most suitable and sometimes the only approach to estimating biomass stock. Height and DBH, the parameters required for biomass equations, can be easily estimated from field methods. Such equations are not only a rapid method of estimating biomass stocks but also one that estimates standard error and the coefficient of determination.

Plotless method Plotless method involves measuring tree density and diameter (DBH) along a series of parallel sample lines (MacDicken 1997) and comprises the following steps:

• Select the land-use category or project activity strata, and locate sample plots (Chapter 10).

• Establish a series of parallel sample lines in each stratum.

• Locate sample points every 10 m along the sample line.

• At each sample point, divide the area into four quarters.

• Record the species name, DBH and height of the tree along with distance between the sample point and each tree or shrub.

• Ensure at least 100 measurements per stratum.

Using the data on the distance between the sample point and trees along the sample line, mean distance between trees in the plot can be estimated. Density of trees per hectare can then be calculated, either for each species or covering all the trees, using the estimated mean distance between trees. Above-ground biomass of trees can thus be calculated using DBH and height of trees and biomass equations.

The plotless method is more suitable for land-use systems with sparse tree density such as savannah and grasslands. This method is particularly useful for single period estimation, when large areas need to be covered in a short time and with limited personnel. The plotless method may not be suitable for periodic revisits and for measuring and monitoring changes in biomass or carbon stock of tree and non-tree vegetation in systems that involve multiple uses of land.

Plot method The principle of the plot method is to estimate the volume or weight of tree and non-tree biomass in a set of sample plots using the measured values of various indicator parameters such as DBH and height of tree. There are several variants of the plot method, namely quadrats (square or rectangular), circular plots and transects (long rectangular plots). The broad approach involves the following procedure:

• Select a land-use category or project activity; stratify and lay sample plots.

• Lay separate sample plots for trees, shrubs and ground-layer vegetation (herbs).

• Vary the size and number of sample plots depending on the type and size of the project and diversity of vegetation (which is discussed in Chapter 10).

• Record the species name, height and DBH for each tree or shrub.

• Estimate above-ground tree biomass per tree and per hectare using height and DBH data using different approaches, namely:

° Biomass estimation equation ° Harvest method within the plots

° Calculating the volume of each tree using DBH, height and tree form data and then converting the volume to weight using wood density

• Estimate the biomass for non-tree vegetation such as shrubs and grasses by adopting the harvest method.

The plot method is the most commonly adopted method for assessing above-ground biomass of tree and non-tree vegetation. Application of the plot method is described in detail in Chapters 10-13. The merits of plot method include the following:

• Applicable to forests, plantations, grassland, shelterbelt and agroforestry systems

• Applicable equally to one-time measurement of biomass or long-term periodic monitoring through the "permanent plot" method

• Can be adopted by any team with minimal resources and technical capability

• Cost-effective

• Suitable for both sparse and dense vegetation

• Applicable to large or small patches of forest, plantation or grassland

• Suitable for both monoculture and diverse vegetation

• Suitable for both mature forests and young regenerating forests or plantations

9.4 Estimation of Below-Ground Biomass or Root Biomass

Below-ground or root biomass is necessary for natural forests, areas under natural regeneration, protected area and agroforestry systems. Root biomass is likely to be important for afforestation, reforestation, watershed and grassland reclamation projects. Methods for estimating below-ground tree biomass include the following:

(i) Root extraction and weight measurement

(ii) Default root to shoot ratio

(iii) Biomass equations

Root extraction and weight measurement method The method of root extraction and weight measurement involves measuring the quantity of root biomass present in a given volume of soil extracted from a known depth, which is normally 30 cm because most fine roots are confined to this shallow depth. The method involves the following steps (MacDicken 1997):

• Using a core sampler to remove a known volume of soil from a selected depth

• Washing the core soil sample and extracting the roots by separating the soil

• Measuring the fresh and dry weight of the root biomass for the selected core volume

• Extrapolating the root biomass from the core volume to unit area, such as 1 ha, for a given depth, say 30 cm

Measuring root biomass is complex, time consuming and expensive (Cairns et al. 1997). Root biomass is not measured in most land-based projects, and alternative methods such as default root to shoot ratio and biomass equations are adopted. The method is described further in Chapter 11.

Default root to shoot ratio Root biomass is normally within a small range of proportion of above-ground biomass. A review by Cairns et al. (1997) covering more than 160 studies from tropical, temperate and boreal forests estimated a mean root to shoot ratio of 0.26 with a range of 0.18-0.3. Thus, it may be practical to use a mean default value of 0.26 for estimating the root biomass in most forestry projects. Biomass equations Regression equations have been developed linking root biomass to above-ground biomass. Cairns et al. (1997) have developed a set of equations for tropical, temperate and boreal forest types. Refer to the equations given in Chapter 11. These equations provide reliable estimates with high coefficient of determination.

Root biomass of non-tree vegetation in land-use systems such as grassland, cropland and savannah can be estimated by measurement. The broad method is identical to that of root extraction using a core sampler, described for tree roots. The detailed steps are described in Chapter 11. It is very important to make an expert judgment as to when root biomass measurement is required. Root biomass is normally estimated if grassland or degraded forest land has been converted to cropland or even to managed grassland involving disturbance to topsoil (Chapter 4).

9.5 Estimation of Litter and Deadwood Biomass

Litter and deadwood biomass could be estimated using the two methods, namely (i) production measurement and (ii) stock change. Deadwood consists of standing and fallen deadwood.

Annual litter and fallen deadwood production (tonnes/hectare/year) Estimating annual litter and fallen deadwood production is a very complex and lengthy procedure, involving the following steps:

• Selection of the land-use category or project activity, stratification and location of sample points

• Installation of a large number of rectangular or circular litter traps on the floor of forests or plantations and protecting them from damage for many years

• Monthly collection and weighing of litter collected in the litter traps in sample plots and estimation of dry weights

• Extrapolation and computation of annual litter production per hectare

Default values are available from literature for some forest and plantation types. Default values could be used since litter accounts for a small fraction of total biomass (<10%) and it is too complex to install and maintain the litter traps, apart from the cost and technical effort involved.

Stock change method Litter and fallen deadwood can be estimated by measuring the stock in sample plots at two points in time and calculating the difference. The sample plots selected for tree or shrub measurements can be used for the estimation, adopting the following procedure:

• Select the sample plots used for tree and shrub biomass estimation.

• Collect and weigh all the fallen litter and deadwood from the sample plots.

• Estimate the dry weight of litter and deadwood.

• Repeat the measurement at two time periods.

• Estimate the difference between the two measurements.

• Extrapolate litter and deadwood stock from sample plots to per hectare values.

Biomass of standing deadwood could be estimated using the plot method described in Section 9.3. The procedure described for above-ground tree biomass in Section 9.3 (details in Chapter 11) is applicable to estimating biomass of standing dead-wood and involves measuring DBH and height of standing dead trees from plots or quadrats selected for tree or shrub measurement. Estimation of standing deadwood requires very little additional effort, since it can be measured along with the measurements of tree and shrub biomass.

9.6 Estimation of Soil Organic Carbon

Soil organic carbon is a critical carbon pool for majority of land-use categories and afforestation, reforestation, land reclamation, grassland management, shelterbelt and agroforestry projects. Soil carbon pool is particularly important for projects involving savannah, cropland, grassland and rangeland. Soil carbon stock is the highest in the upper soil profile (0-15 cm), which should be sampled most intensively (Richter et al. 1999). Soil organic carbon is routinely estimated for all forestry, grassland and cropland conservation and development projects by any of the following methods, namely diffuse reflectance spectroscopy, modelling and wet digestion or titrimetric determination.

Diffuse reflectance spectroscopy Diffuse reflectance spectroscopy (DRS) is a technology for characterization of the composition of materials based on the interaction of visible-infrared light (electromagnetic energy) with matter. Soil samples are illuminated with an artificial light source and the reflected light diffusing from the sample is measured. Reflectance readings are taken with a spectroradiometer, and each wavelength band is expressed relative to the average of the reference readings for each property of soil. The method has the potential to increase efficiencies and reduce costs in large-area applications (soil survey, watershed management and soil quality indicators).

The main merits of DRS are its repeatability and speed compared to conventional soil analyses; a single operator can comfortably scan several hundred samples a day. The repeatability among laboratories is expected to be greater with DRS than with conventional methods of analysis. The speed of analysis and the ability to estimate many soil properties from a single robust measurement are major advantages of DRS, especially for analysing soil properties that are time consuming to measure by conventional methods.

The main limitations of DRS are the need to build calibration libraries for a given population of soils for all the soil properties of interest and the complexity of the data analysis this involves. The method requires specialized equipment, a well-equipped laboratory and trained staff. The technology could be increasingly used in a wide range of soil studies and surveys, and spectrometers are likely to become standard equipment in soil laboratories.

Modelling of soil carbon Soil carbon dynamic models include CENTURY (CENTURY 1992) and RothC (Coleman and Jenkinson 1995).

CENTURY simulates long-term dynamics of carbon, nitrogen and phosphorus for different plant soil systems. The model simulates the flow of carbon, nitrogen, and phosphorus through the plant litter and different inorganic and organic pools in the soil. The advantage is that the model can be applied to forest, grassland, savannah, and cropping systems or projects at the plot, project, regional and national level. The limitations include data needed such as precipitation, maximum and minimum air temperature, lignin, nitrogen, phosphorus and sulphur in the plant material, texture of the soil, initial contents of total carbon, nitrogen and phosphorus in the soil, amounts of agricultural inputs used during the management cycle, etc.

RothC computes changes in organic carbon content in tonnes/hectare. RothC requires input of a large number of variables such as weather data (monthly mean temperature, total monthly precipitation and open pan evaporation), soil data (percentage of clay in soil and soil depth), land management data related to carbon simulation and monthly inputs of organic matter to soil. The limitation of the model is that the data needed for diverse land-use systems are hard to come by.

Simple regression models for predicting soil carbon accumulation rates or stock changes have not evolved as much as those for, say, above-ground biomass. Such models are specific to soil, vegetation or species and will have limited application. Wet digestion or titrimetric determination (Walkley and Black method) The method of wet digestion or titrimetric determination involves rapid titration procedure for the estimation of organic carbon content of the soil (Kalara and Maynard 1991). Organic matter is oxidized with a mixture of K2Cr2O7 and H2SO4. Unused K2Cr2O7 is back-titrated with FeSO4. The dilution heat of concentrated H2SO4 with K2Cr2O7 is the sole source of heat. The soil is digested by the heat of dilution of H2SO4 and thus organic carbon in the soil is oxidized to CO2. Among the various methods, wet digestion (Walkley and Black) is the most commonly adopted and cost-effective method, which involves the following procedure:

• Selection of the land-use category or project activity strata, sampling method and location of sample plots

• Collection of soil samples at two depths (0-15 cm and 15-30 cm) from each stratum

• Estimation of bulk density

• Estimation of organic matter or carbon content in the soil sample in the laboratory using the Walkley and Black method

• Calculation of carbon stock in tonnes of carbon/hectare using organic matter content, bulk density and depth of soil

9.7 Conclusions

This chapter provided an overview of different methods available for estimating the stocks and growth rates of different carbon pools. Carbon inventory of different carbon pools requires adoption of different field, laboratory and modelling techniques. Multiple methods are available for different pools. The selection of a suitable method for a given pool depends on the land-use category or project type, size of the project area, accuracy needed, cost-effectiveness, infrastructure and technical capacity available. The details of these methods are described in later chapters. The focus of this handbook in the remaining chapters is on adopting the carbon "Stock-Difference" method using "permanent plot" technique for carbon inventory.

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