Below-ground biomass is defined as the entire biomass of all live roots, although fine roots less than 2 mm in diameter are often excluded because these cannot easily be distinguished empirically from soil organic matter. Below-ground biomass is an important carbon pool for many vegetation types and land-use systems and accounts for about 20% (Santantonio et al. 1997) to 26% (Cairns et al. 1997) of the total biomass. Below-ground biomass accumulation is linked to the dynamics of above-ground biomass. The greatest proportion of root biomass occurs in the top 30 cm of the soil surface (Bohm 1979; Jackson et al. 1996). Revegetation of degraded land leads to continual accumulation of below-ground biomass whereas any disturbance to topsoil leads to loss of below-ground biomass.
Since below-ground biomass could account for 20-26% of the total biomass, it is important to estimate this pool for most carbon mitigation as well as other land-based projects. Estimation of stock changes in below-ground biomass is also necessary for greenhouse gas inventory at national level for different land-use categories such as forest lands, cropland and grassland. This chapter presents methods of estimating and monitoring below-ground biomass.
11.1 Below-Ground Biomass: Features and Broad Methods
Methods for measuring and monitoring above-ground biomass are relatively well established, in regular use and cost-effective; however, those for below-ground biomass are less developed and less frequently used in the field. Further, the methods for below-ground biomass for different land-use systems are not standardized (IPCC 2006). Live and dead roots are generally not distinguished and hence root biomass is reported as total of live and dead roots. The methods for estimating and monitoring below-ground biomass are listed below:
1. Excavation of roots
2. Monolith for deep roots
3. Soil core or pit for non-tree vegetation
4. Root to shoot ratio
The choice of method depends on site conditions, vegetation type and the accuracy required, but in most carbon inventory projects root to shoot ratio and allometric equations are the most commonly used as will be explained in this chapter. Data on below-ground biomass are required for estimating and projecting total change in carbon stock for the following:
• Baseline scenario land-use systems
• Project scenario land-use systems
Such estimates are also required during project development and monitoring phases of a project cycle.
Project development phase Below-ground biomass is estimated and projected during the project development phase largely based on the default values of root to shoot ratio or allometric equations for tree biomass. Default values are also used for non-tree vegetation.
Project monitoring phase In project monitoring phase too, because of the large human effort and cost involved, below-ground biomass is normally estimated using the default root to shoot values or allometric equations. However, if suitable allometric equations or values of root to shoot ratio are not available for the location and the species, the below-ground biomass has to be measured physically, although it is a destructive method (Sections 11.2 and 11.3).
If the below-ground biomass of trees is considered an important pool in a project, it may be necessary to measure the biomass. It may be possible to measure the below-ground biomass especially if there are only a few tree species in the project area or if the project area consists of a monoculture plantation. Since the measurement method is quite complex, requires large human effort and destroys the trees, it may be adopted only when no root biomass equations suitable for the species or the project location are available. This method is mostly used in tree-based systems and involves selecting the plots, excavating all roots, measuring their fresh weight, converting it to dry weight and extrapolating to per tree or per unit area (per hectare). The following steps could be adopted:
Step 1: Select and stratify land-use category or project activity (refer to Chapter 10). Step 2: Select and locate plots in each stratum:
° Use the plots selected for shrub measurements within the tree quadrats (Chapter 10).
° Normally, select eight to 10 shrub plots for each stratum. ° If the aim is to estimate the root biomass of trees, select all the trees in the plots and number them for excavation. ° If the aim is to estimate root biomass on area basis, excavate the whole plot.
Step 3: Assemble the required material:
° Tape for measuring DBH and height
° Balance for weighing the shrub biomass
° Rope and pegs for marking plots
° GPS for locating sample plot boundaries
Step 4: Measure the trees, shrubs and herbs:
° Adopt the methods described in Chapter 10. ° Record DBH, height, species and other parameters for trees. ° Harvest and measure the weight of the shrub and herb biomass.
Step 5: Harvest all the trees to the ground level and number them. Step 6: Weigh each tree thus harvested:
° Record the weight of the tree along with its DBH and height against its number.
Step 7: Excavate the roots of trees and other non-tree plants separately by digging and loosening the soil:
° Surface soil around the tree stem is excavated
- If the aim is to estimate the root biomass of individual trees; in that case, mark the area around the trees for excavation until bulk of the roots belonging to that tree are included (the plot boundary need not be a restriction).
- If the aim is to estimate the root biomass for the plot area, roots going beyond the plot boundary are excluded, although this may be difficult in practice.
° Since root biomass is concentrated in the top 30-50 cm layer, a minimum depth of 30 cm is necessary; for root biomass beyond that depth, refer to the monolith method (Section 11.3). ° Separate the roots of trees and non-tree biomass, although this may not be feasible in situations with mixed tree species and a mix of tree, shrub and herb plants. ° The soil along with roots may have to be washed in a sieve (mesh size of 2.5 or 5 mm) to separate the roots from the soil. ° If feasible, store the roots of each tree in a separate cloth bag after recording the number, DBH and weight of the tree.
Step 8: Measure the fresh weight of roots:
° For estimating root biomass of an area, it is desirable to estimate the weight of root biomass for each tree separately and pool them later. ° Pool all the non-tree biomass separately for each shrub plot. ° Take samples of root biomass (about 0.5 kg each) for each of the dominant tree species and for non-tree biomass separately for drying. ° Take fresh weight of samples for each tree species separately.
Step 9: Estimate the dry weight of the fresh root biomass by drying in the oven to a constant weight:
° Estimate dry weight separately for each dominant species and for the pooled non-tree biomass.
Step 10: Extrapolate the dry weight of the biomass to per hectare and stratum level:
° First, extrapolate according to individual tree and species at the plot level.
° Second, extrapolate for the whole plot.
° Third, extrapolate to per hectare.
° Finally, extrapolate to the stratum area from which the samples are taken.
The main disadvantages of the method are the large human effort, high cost, destruction of trees and disturbance of soil, leading to loss of carbon from the plots. However, if the measurement method is adopted, use the information collected to develop regression equations. The following relationships can be developed:
• Above-ground biomass equation based on DBH and height values of the trees
• Below-ground biomass equation using DBH and height of the trees
• Below-ground biomass equation using above-ground biomass values
Parameters to be recorded for estimating root biomass are shown below (apart from information on name of location, land-use category, project activity, tree quadrat number, GPS reading, date of recording and name of investigator, as given in Chapter 10 for tree quadrats).
Tree Tree DBH Height Fresh weight Fresh weight Dry weight of Dry weight species number (cm) (m) of shoot of root shoot biomass of root biomass (kg) biomass (kg) (kg) biomass (kg)
The monolith method is adopted for estimating root biomass below the depth of 30 cm and is mostly used in non-tree based land-use systems, such as grassland. The procedure involves cutting a monolith that is a large block of soil from a plot, separating the roots and weighing them. This method is used to obtain quantitative estimates of root biomass (FAO 2004). The broad steps in using the monolith method (Weaver and Darland 1949) are as follows:
Step 1: Select and stratify the land-use category or project activity. Step 2: Dig a deep pit at the sample site (1 x 1 x 1 m). Step 3: Make the wall of the pit smooth and vertical.
Step 4: Take a long shallow wooden or steel box (30 cm wide and 8 cm deep) without a top and lacking one of the four sides.
Step 5: Hold the box against any of the smoothened walls with its bottom away from the wall and the open side facing the sky but flush with the top of the pit. Step 6: Press or hammer the box against the wall so that the three edges of the box leave three clear dents or grooves, two vertical and one horizontal, on the wall of the pit.
Step 7: Remove the box, take a knife and deepen the three grooves up to 10 cm, making a column of soil 10 cm broad. Step 8: Fit the box around the column holding it in place and cut the column free from the other side with a knife or a spade and free the monolith from the surrounding soil.
Step 9 Wash the column of soil under running water to separate the roots embedded in the column. Step 10: Measure the fresh weight and estimate the dry weight of the root biomass. Step 11: Calculate the volume of the soil monolith extracted. Step 12: Estimate the root biomass in grams in the volume of soil monolith extracted and extrapolate to per hectare to the depth of the monolith.
Among the disadvantages of the method are the facts that some roots may be lost during washing, that it is time-consuming and expensive and that excavating large monoliths often requires expensive machinery (MacDicken 1997). Further, the method can only be used with satisfying results in easily workable soils (Majdi 1996); it is not applicable to most land-use categories or project activities.
It is not feasible to estimate the root to shoot ratio or develop a root biomass equation linking above-ground biomass to root biomass in the case of non-tree vegetation such as herbs, shrubs and grasses; with such plants, the soil core method is the one most commonly used for the purpose. The broad steps in estimating the stock of below-ground biomass at a given point in time for a given land-use system by the soil core method are as follows:
Step 1: Select and stratify the land-use category and project activity. Step 2: Select sampling plots for root biomass:
° Select the plots by following the procedure given for selecting shrub plots in Chapter 10. ° Select 8-10 shrub plots for each stratum and mark the centre of each plot for sampling root biomass.
Step 3: Assemble the material required:
° A soil core sampler (5-10 cm in diameter and 30 cm deep), metallic sieve for washing roots, balance for weighing
Step 4: Drive or insert the core sampler into the ground at the selected point and remove the soil along with roots.
Step 5: Separate the roots from soil by placing the soil samples on a sieve (mesh size of 2.5 or 5 mm) and wash the roots under running water.
Step 6: Collect all the roots and weigh them.
Step 7: Estimate the dry weight by oven-drying a sample of roots at 70°C to a constant weight (for at least 8 h).
Step 8: Estimate the dry weight of the roots for the volume of the core sampler (calculated from its diameter and depth) for all the sample plots.
Step 9: Extrapolate the root biomass to per plot and hectare, using dry weight of the root samples collected to the depth of the core.
There is a relationship between biomass in shoot and roots for a tree of a given species as well as for a given forest or plantation type. It is possible to estimate the biomass in roots based on data on above-ground biomass. Given the limitations of measuring below-ground biomass in the field, it is desirable to use indirect methods to obtain the value of below-ground biomass. A comparative review by Cairns et al. (1997) includes more than 160 studies covering native tropical, temperate and boreal forests that reported both below-ground biomass and above-ground biomass. The below-ground (root) biomass to average above-ground (shoot) biomass ratio developed based on these studies was 0.26 with a range of 0.18-0.30. The ratios did not vary significantly with latitude (tropical, temperate or boreal), soil texture (fine, medium or coarse) or tree type (angiosperms or gymnosperms).
Specific root to shoot ratios could also be developed for the project location and species, if necessary, by adopting the root excavation method described in Section 11.2. The key steps are as follows (for details, refer to Section 11.2):
Step 1: Select the tree species for root biomass measurement from the strata and sample plots for measurement. Select trees of different DBH or height; the selection need not be confined to the sample plots. Step 2: Harvest a selected number of trees for excavation; about 30 trees of different girth sizes may be adequate. Step 3: Excavate the roots up to 30-50 cm depth, clean the roots of soil and take their fresh weight.
Step 4: Measure the tree DBH, height and shoot (above-ground biomass) weight
(refer to Section 11.2). Step 5: Measure the fresh weight and estimate dry weight of the root biomass excavated.
Step 6: Estimate the weight of shoot biomass using the DBH (and height)-based biomass equations (Refer to Chapter 17). Step 7: Calculate the root to shoot biomass ratio from the estimated weights.
The development of root to shoot ratio involves a large human effort and cost. The root to shoot (R) ratios for tropical forests (Mokany et al. 2006) are as follows:
• Tropical moist deciduous forest: R = 0.20 (0.09-25) for forests with above-ground biomass less than 125 t/ha
• Tropical dry forest: R = 0.28 (0.27-0.28) for forests with above-ground biomass greater than 20 t/ha
In addition to root to shoot ratio, allometric equations have been developed linking above-ground biomass to below-ground biomass. Such equations could be developed for an individual tree species or for a plantation or a forest type. Allometric equations for broad forest types, as developed by Cairns et al. (1997), are illustrated in Chapter 17.
It is important to note that allometric equations are developed based on observations from native forests and thus may not be applicable to plantations or other vegetation types. However, in the absence of allometric equations specific to location, forest type and species, the broader tropical or temperate or boreal equations could be used. For example, root biomass (Y) for tropical forests (dry tonnes/hectare) could be calculated using the following equation (Cairns et al. 1997).
where LN = natural logarithm, AGB = above-ground biomass (dry tonnes/hectare).
Estimation of below-ground biomass using the allometric function requires the estimated value of above-ground biomass using methods given in Chapters 10 and 17 and of below-ground biomass using methods given in Sections 11.2-11.5. These equations provide below-ground biomass values in tonnes of dry biomass per hectare. The method described for developing root to shoot ratio in Section 11.5 could be adopted for developing the allometric equations.
If region- and species-specific root to shoot ratios are available, these could be used for carbon inventory; if not, the general equations developed by Cairn et al. (1997) may be used.
Below-ground biomass accumulates over decades and centuries. The below-ground biomass stock is linked to the above-ground biomass stock. Changes in stocks of above-ground biomass will be monitored over long periods in all projects using the methods given in Chapter 10. Periodical harvesting and extraction of below-ground biomass for estimation is not feasible in most situations. Thus, the practical approach is to monitor and estimate the above-ground biomass stock and use the root to shoot ratio or allometric equations to calculate the below-ground biomass stock. Thus, there is no need for long-term monitoring of below-ground biomass stocks.
In forests and plantations, below-ground biomass could account for about a quarter of the total biomass. In grassland and cropland, bulk of the below-ground biomass is part of the annual cycle. Estimation of below-ground biomass is important largely for tree-based land-use categories or projects such as forests and plantations, especially for cropland, grassland and degraded forest land converted to forests or plantations, where root biomass accumulates. Measurement and estimation of root biomass is complex and expensive and leads to loss of vegetation and disturbance to topsoil. Therefore, measurement should be resorted to only if specifically needed. Below-ground biomass is estimated for most carbon mitigation projects and national GHG inventory using root to shoot ratio or allometric equations, which require estimates of above-ground biomass. There is high correlation between above-ground biomass and below-ground biomass, and the root to shoot ratio varies within a narrow range. Thus, the default root to shoot ratios or allometric equations could be used in carbon inventory programmes.
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