Grass Biomass Production Above the Ground

Estimating grass biomass production (tonnes per hectare per year) is important for grassland development projects in assessing the impact of a management practice on grass production. Grass biomass is expressed as dry tonnes per hectare per year. The method of estimation is described in Chapter 10. The calculation procedure for grass and herb biomass is as follows:

Step 1: Obtain the fresh weights of grass or herbs harvested monthly from the sample plots during field studies and estimate the dry weight of samples

Step 2: Extrapolate the dry weight of grass or herb estimated on monthly basis for the sample plots area to a per-hectare value Step 3: Tabulate or plot on a graph the grass or herb production estimated as tonnes of dry biomass per hectare for each month of the year Step 4: The highest value among the monthly values is considered as the grass or herb productivity for that area expressed as dry tonnes per hectare per year.

17.3 Below-Ground Biomass

Methods for measuring root biomass are described in Chapter 11. The methods of excavation of roots and monoliths of soil are not practical in most situations because of high cost and the difficulty in uprooting or digging inside a forest or plantation or an agroforestry plot. Therefore, the two most common and feasible approaches are

(i) Standard root to shoot ratios (Table 17.6)

(ii) Allometric equations (Table 17.7).

(i) Root to shoot ratio Using root to shoot ratios to estimate root biomass involves the following steps:

Step 1: Estimate the above-ground biomass, using one of the methods described in Section 17.2, in terms of tonnes of dry biomass per hectare Step 2: Select the appropriate root to shoot ratio from the literature. 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 0.18-0.30. Thus, for most projects, a root to shoot ratio of 0.26 could be used

Step 3: Calculate the root biomass using the data on above-ground biomass and the root to shoot ratio selected with the following formula:

Root biomass (in dry t/ha) = 0.26 x above-ground biomass

(ii) Allometric equations Biomass equations have been developed to estimate root biomass using data on above-ground biomass. Examples of biomass equations are given in Table 17. 7. The method involves

• Estimating the above-ground biomass using one of the methods described in Section 17.2

• Selecting the appropriate biomass equation

• Substituting the above-ground biomass value in the equation to obtain root biomass in tonnes of dry root biomass per hectare

Table 17.6 Ratio (R) of below-ground biomass to above-ground biomass (tonnes of root dry matter: tonnes of shoot dry matter). (From IPCC 2006, compiled from various sources.)


Ecological zone

Above-ground biomass (t/ha)



Tropical rain forest



Tropical moist deciduous forest


0.20 (0.009-0.25)


0.24 (0.22-0.33)

Tropical dry forest


0.56 (0.28-0.68)


0.28 (0.27-0.28)

Tropical mountain systems

0.27 (0.27-0.28)


Subtropical humid forest


0.20 (0.09-0.25)


0.24 (0.22-0.33)

Subtropical dry forest


0.56 (0.28-0.68)


0.28 (0.27-0.28)


Temperate oceanic forest, temperate continental forest, temperate mountain systems

Conifers <50

0.40 (0.21-1.06)

Conifers 50-150

0.29 (0.24-0.50)

Conifers >150

0.20 (0.12-0.49)

Eucalyptus spp. >50

0.44 (0.29-0.81)

Eucalyptus spp. 50-150

0.28 (0.15-0.81)

Other broad-leaved forest <75

0.46 (0.12-0.93)

Other broad-leaved forest 75-150

0.23 (0.13-0.37)

Other broad-leaved forest >150

0.24 (0.17-0.44)


Boreal coniferous forest, Boreal tundra woodland, Boreal mountain systems


0.39 (0.23-0.96)


0.24 (0.15-0.37)

Table 17.7 Regression equations for estimating root biomass of forests. (From Cairns et al. 1997.)

Conditions and independent variables

Equation Y = root biomass (t)

Sample size


All forests, AGB


Exp[-1.085 + 0.9256*LN (AGB)]



All forests, AGB and age (years)


Exp[-1.3267 + 0.8877*LN(AGB) + 0.1045*LN(AGE)]



Tropical forests, AGB


Exp[-1.0587 + 0.8836*LN(AGB)]



Temperate forests, AGB


Exp[-1.0587 + 0.8836*LN(AGB) + 0.2840]



Boreal forests, AGB


Exp[-1.0587 + 0.8836*LN(AGB) + 0.1874]



LN = natural log, Exp = "e to the power of', AGB = above-ground Biomass (t); R2 = coefficient of determination

LN = natural log, Exp = "e to the power of', AGB = above-ground Biomass (t); R2 = coefficient of determination

Estimating root biomass for non-tree vegetation For non-tree vegetation such as shrubs, herbs and grasses, it is not possible to calculate below-ground biomass using above-ground biomass data. Therefore, on-site measurement using soil core method or pit method is used for the purpose, as described in Chapter 11. The root biomass for non-tree plants could be calculated as follows:

• Obtain the dry weight of root biomass by the core sample method along with the volume of the core (depth and height) of the core sampler

• Calculate the dry weight of root biomass for the volume of the core sampler calculated using the diameter and depth of the core (volume of the core = p r2 x H)

• Extrapolate the root biomass from the volume of the soil on per hectare basis for the depth, usually 30 cm depth (where 30 cm depth = 3,000 m3) of soil per hectare

17.4 Calculation of Deadwood and Litter Biomass

Deadwood and litter are unlikely to be the key carbon pools for majority of land-based projects and greenhouse gas inventories. These two pools may not exist at all or exist only in insignificant quantities for grassland and agroforestry projects or even plantation forestry projects. Calculation of deadwood and litter biomass may be of significance only for forestry projects. The methods for monitoring deadwood and litter biomass are described in Chapter 12. This section describes the procedure for calculating the biomass from these two pools.

17.4.1 Deadwood

Deadwood biomass includes standing deadwood and fallen deadwood. It is important to distinguish between deadwood and litter based on the size of the wood. Further, it may not be possible to identify the species of the deadwood, especially of fallen deadwood in multispecies forests. In monoculture plantations, the dead trees could be assumed to be of the same species as the living ones that make up the plantation.

(i) Standing deadwood

Step 1: Obtain the values of DBH and height of the standing dead trees from field measurements. The dead trees may or may not have a crown (Chapter 12) Step 2: Calculation of the weight of the standing deadwood

• Assume that the relation between biomass and DBH or height is the same in dead and living trees

• Adopt the biomass equation method using the DBH and, if necessary, the height using the procedure and steps given in Section 17.1.5

• Estimate the standing deadwood in dry kg or tonnes per sample plot and total the weight for all the sample plots

• Extrapolate the value from sample plot area to per hectare area

(ii) Fallen deadwood

• Fallen deadwood could be the whole tree, only the main stem or only large branches or a combination of these forms

• Fallen whole tree Adopt the method described for standing deadwood, which is same method as the biomass equation method (Section 17.1.5)

° Estimate the dry biomass of fallen deadwood per sample plot and total the weight for all the sample plots ° Extrapolate the value to per hectare

• Fallen stem Estimate the volume of the fallen stems and calculate their volume and dry weight using the steps given in Section 17.1.2

° Estimate the volume of each stem using DBH and height ° Estimate the weight of the stems from the wood density and volume of the stem

° Calculate the weight of all the stems in the sample plots and aggregate it for the total sample area ° Extrapolate the weight of total fallen stems to per hectare (dry tonnes)

• Fallen dead branches Biomass of fallen dead branches could be estimated as follows

° From the weight Weigh all the fallen branches (excluding the litter) using a field (spring) balance, estimate the dry weight by taking a sample and drying in the oven and extrapolate the weight of the fallen branches to per hectare from the sampled area

° From the volume If the branches are too large for weighing with a field balance, estimate the volume using the method described for fallen stems in Section 17.1.2

(iii) Total deadwood

Total deadwood biomass is the sum of standing deadwood and fallen deadwood estimated using the following equation and expressed in tonnes of dry matter per hectare.

Total deadwood = standing deadwood + (fallen whole tree + fallen stem + fallen dead branches)

17.4.2 Litter

Two methods are used in estimating litter biomass, namely annual production and stock change (Chapter 12). Calculation procedures for these two methods are as follows.

(i) Annual production The method described in Chapter 12 will provide monthly production figures for woody and non-woody litter from the sample plots

Step 1: Obtain the monthly values of fresh and dry weight of litter from all the sample plots (in kg)

Step 2: Aggregate the monthly weights of dry litter from all the sample plots for all the 12 months

Step 3: Extrapolate the dry litter weight from sample area to per hectare value and express it as dry litter production in tonnes per hectare per year

° It is possible to estimate woody litter and non-woody litter production per hectare separately by repeating the above method for both the components of litter

(ii) Stock change This method of stock change requires estimation of litter stock at two points in time. The field method for measuring litter stock at a given point in time is described in Chapter 12.

Step 1: Obtain the fresh and dry weight of the litter stock in sample plots for the two periods (t2 and t1) Step 2: Add the sample dry weight of all plots and extrapolate to tonnes of dry matter per hectare

Step 3: Calculate the annual litter stock change (in tonnes per hectare per year) using the following formula:

Annual litter stock change = (litter stock at period t2 - litter stock at period

17.5 Soil Organic Carbon

Estimation of soil carbon density (tC/ha) involves estimation of bulk density of the soil and soil organic matter content. The method for estimating these two parameters is described in Chapter 13. This section summarizes the steps involved in calculating soil carbon density

Step 1: Select the land-use category, project activity and stratum

Step 2: Conduct field and laboratory studies and estimate the bulk density and soil organic matter or carbon content (Chapter 13).

Bulk density Estimate bulk density by using tube core or clod method for undisturbed soil and the following formula:

Bulk density (g/cc) = (weight of soil with tin - weight of empty tin)/volume of the tin or weight of soil clod/volume of the soil clod

Soil carbon density (tC/ha) The content of organic carbon in soil estimated in percentage terms needs to be converted to tonnes per hectare using bulk density, depth of the soil and area (10,000 m2).

SOC (t/ha) = [soil mass in 0 - 30 cm layer x SOC concentration (%)] / 100

Soil mass (t/ha) = [area (10,000 m2/ha) x depth (0.3 m) x bulk density (t/m3)]

17.6 Formulae and Calculations for Estimating Different Carbon Pools

The main goal of carbon inventory is to estimate carbon stocks and changes in selected land-use categories, project activities and strata. Estimating changes in carbon stock requires carbon stock values at two points in time and estimating total stocks requires estimates of changes in all the relevant carbon pools for a given land-use system. The key issues to be considered in carbon inventory calculations are as follows, which are also summarized in Table 17.8:

• The goal of carbon inventory calculation

• Type of inventory output required for the project or the programme o

Table 17.8 Features of carbon (C) inventory calculations required for different projects and programmes

Type of carbon stock

Carbon pools to be

Frequency of

Project or programme

Objective of carbon inventory

estimates needed




Land-based mitigation

Estimation of additional

Total C stock change

All 5 pools

3-5 years

C stock-difference


carbon sequestered over baseline

between two points in time

Commercial roundwood

Calculation of commercial

Total commercial

Largely above-ground

At the end of

C stock-difference

production projects

merchantable biomass production

biomass production


rotation period

Community forestry

Calculation of fuelwood

Total biomass


Annual biomass

C gain-loss

production: trunk +




twigs + branches

Land reclamation or

Improvement in soil fertility

Increase in soil

Soil organic carbon

Periodically, say

C stock-difference

grassland development

organic carbon

once in 3-5 years

Greenhouse gas inventory

Calculation of carbon

Changes in total

All pools

Inventory year

C stock-difference


emissions and removals

C stocks

or C gain-loss

2 Ui

• Carbon pools to be calculated and reported

• Frequency of calculation and reporting

• Approach or method to be adopted

(i) Goals The goal of a carbon inventory is to answer the questions relevant to the specific land-use category, vegetation type and objectives of the project or programme. Some examples of programmes and projects that require carbon inventory are

• Climate change mitigation

• Roundwood (fuelwood, timber) production

• Greenhouse gas inventory of land-use categories

• Costs and benefits analyses

(ii) Types of inventory output The types of carbon inventory estimates or outputs required are

• Tonnes of carbon sequestered in biomass and soil in mitigation projects

• Tonnes of carbon dioxide emissions reduced (from avoided deforestation)

• Tonnes of commercial biomass or traditional fuelwood produced

• Estimates of emissions or removals of carbon dioxide in tonnes from land-use categories

(iii) Carbon pools The carbon pools to be calculated and reported depend on the type of project and its objective. Projects can vary from reporting only above-ground biomass to reporting that from all the carbon pools. Some examples are

• Climate mitigation projects: all the carbon pools

• Roundwood production programmes: above-ground biomass, particularly commercial timber (main trunk)

• Community forestry: above-ground biomass, deadwood and litter

• Greenhouse gas emissions estimation: all the five carbon pools, depending on the key category analysis

(iv) Frequency Estimates of carbon stocks and changes are required at various phases of project cycle, namely

• Project proposal preparation

• Project implementation

• Project monitoring

• End of the project period or end of rotation period

• Annual changes in stocks

Further, carbon inventory estimates are required at different frequencies depending on the objectives of the carbon inventory projects or programmes. The reporting could be annual, periodical or only at the end of the project.

(v) Approach or method The approach to or methods of calculation includes the steps to be adopted based on the indicative parameters measured or observed in the field or laboratory. The calculation procedure is presented as a series of equations based on IPCC (2003, 2006) and involves estimating the changes in stocks of above-ground and below-ground biomass, deadwood, litter and soil carbon. Although five carbon pools are considered, given the resource constraints, there is a need to identify the key carbon pools for a given project or a programme or inventory.

Making a carbon inventory requires estimation of carbon stocks at two points in time or carbon gain and loss for a given year. These two methods are described using a series of equations from IPCC (2003, 2006) in Chapter 9.

In the carbon "Stock-Difference" method, carbon stock changes are calculated per year per hectare and then multiplied by the total area under each stratum to obtain the total stock changes in each pool. These values are finally aggregated. In the carbon "Gain-Loss" method, the gains and losses of different carbon pools in a given year or over a period of years are calculated.

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  • Selina
    How to calculate carbon stores in grass biomass?
    11 months ago
  • aleena hamilton
    How we estimate grass biomass?
    3 months ago

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