10.2 Harvest and Estimation Methods

Video Presentation

Learning Guide

Plant biomass is most often measured by directly harvesting or estimating the amount of plant material in quadrats (or plots).  With harvesting methods, we remove and weigh plant material in quadrats.  With estimation, observers carefully train to become skilled at estimating the weight of plant material in the quadrat.  For both approaches, the resulting values obtained on a quadrat-basis are usually converted to larger scales, and reported as kg/ha or lbs/ac.

Harvest Method

The basic idea of the harvest method is simply to remove and weigh all the biomass occurring in quadrats and use that information to determine how much biomass occurs across the landscape, site, or pasture. This is sometimes referred to as direct or destructive sampling of biomass. Harvesting is usually preferred for research applications because it is a very objective measure of biomass.

Advantages of Harvesting

• Harvesting is the most accurate way to measure biomass.
• Does not require extensive training for observers.

Limitations of Harvesting

• Very time consuming and labor intensive.
• Sampling date can have dramatic impact on results.
• Current year’s growth can be difficult to separate from previous years’ growth.
• Harvest methods are destructive to the vegetation, so not suitable for rare or endangered plants.
• It is difficult to directly measure biomass of trees and shrubs.

Harvest Method to Determine Biomass

The basic steps are as follows:

1. Select the appropriate size quadrat for the type of vegetation being sampled.
2. Place the quadrat along a transect line or in a location determined by the selected sampling protocol.
3. Sample enough quadrats to capture the variation in biomass across the landscape of interest. The number of quadrats that need to be sampled increases as the spatial variability of the vegetation increases.
4. Use clippers to remove the aboveground material of plants that are rooted within the plot. Clipping height, inclusion or exclusion of dead plant material, or sampling certain plant species should be done in accordance with your sampling protocol. Clip to ground level for most production estimates. Grass shears, sheep shears, power grass shears, sickles, and hand lawnmowers equipped with grass catchers can all be used to accomplish clipping.  However, it can be difficult to separate plants by species when using power tools.
5. Put the plant material into a bag (Figure 1a). If all of the harvested material is going to be taken from the field to be dried and weighed, be sure to clearly mark the bags with information about the species and quadrat.
6. We often weigh the bags of clipped material in the field using spring scales (Figure 1b). These field weights, called “fresh weight” or “green weight” are recorded in the field.  Usually a subset of these samples are brought back to the lab to be air-dried or oven-dried, and reweighed so that the fresh weights can be adjusted to a dry-weight basis.  Be sure to subtract the weight of the bag when weighing all samples, and clearly label all bags being returned to the lab.

Figure 1. Observers harvest biomass along a transect, putting the material into paper bags (a).  Harvested samples are weighed and recorded in the field (b).

Calculating and reporting Biomass Data

The weight of clipped plant material includes water inside the plant (within and between cells) and water on leaves and stems such as dew and precipitation. Therefore, the weight of freshly harvested plant material is highly variable and depends on recent weather, atmospheric conditions, and the water status of the plant. For more consistent interpretation of production, biomass is expressed on a dry-weight basis.  Freshly harvested material is usually air-dried or oven-dried in order to determine the dry weight of plant materials.

Recommended Drying Procedure:

1. Samples should be dried within 24 hours of clipping to prevent the production of mold.
2. Place samples (in paper bags) in a forced-air oven at 140-160°F (60-70°C) for 24-48 hours (Figure 2a).
3. To determine if a sample is dry, a few bags can be removed from the oven, weighed and then returned to the oven. A few hours (4-8 hours) later, the bags can be removed again and reweighed (Figure 2b). Samples are dry when no changes in weight occur between reweighing. This is called “drying to a constant weight”.

Figure 2. Vegetation samples in paper bags being dried to a constant weight in a forced-air oven (a), and samples are weighed and reweighed for “drying to a constant weight” (b).

4. Once a sample is dried, it must be stored in a dry place until it is weighed or it will absorb atmospheric moisture and gain weight.
5. If an oven is not available, and if samples are collected in a very dry environment (where molding is unlikely), the samples can be placed in a warm, dry place to dry-out over several days to reach an “air-dried” weight that can be used to compare production. If the plants have a high moisture content, you can carefully open the bags so that exposure to air will promote drying.  This needs to be done in a location where the bags will not be at risk of being tipped over by wind coming through an open window, or upset by marauding cats!

Converting to Fresh Weights to Dry Weights:

Once the dry weight of a sample is known, you can calculate the percent dry weight (%DW) as:

% Dry Weight = (Dry Wt / Fresh Wt) *100

Where the “Dry Wt” is the weight of a sample after oven drying and “Fresh Wt” is the weight of a sample recorded in the field. Calculate %DW for each of the samples that were both weighed in the field and dried and reweighed.  Then calculate an average %DW.  These calculations must be done on a species-by-species basis.

The DW% value is then used to convert recorded fresh weights from the field to dry weights.  Use dry weights to report biomass estimates (lbs/acre or kg/ha).

If it is extremely difficult of impractical to transport samples from the field for drying, you can use “book values” to convert fresh weights to dry weights.  These book values are provided as generic estimates of “typical” plant moisture contents, and so this approach should only be used if no drying options are available.

Estimation Methods

Estimation is the most rapid method for determining biomass of a quadrat. The method involves weighing representative units of a plant species, and training observers to recognize these “weight units” within a quadrat.

Advantages of Estimation

• Rapid technique that allows for larger sample sizes.
• Not destructive of vegetation so suitable for rare or endangered species.

Limitations of Estimation

• Estimation is a subjective approach, which influences accuracy.
• Extensive training and skill required.
• Current year’s growth can be difficult to separate from previous years’ growth.

Training and Calibrating

It takes a lot of time to clip the number of plots needed to get an accurate assessment of biomass. However, with a little training most field technicians can become skilled in estimating the amount of biomass in a plot, particularly for small herbaceous plants.

To accurately estimate the amount of biomass in a plot, the observer must spend time training. The training procedure entails weighing representative units of a plant and establishing an “eye” for what 5-, 10-, or 15-grams etc., of forage looks like. Estimating biomass is both visual and tactile.  Good estimates generally require looking at the plant or plot and then “feeling” it to assess density.

What is a Weight Unit?

Weight units can consist of a part of a plant (such as an individual grass tiller), an entire plant, or even a group of plants. Weight units should be established for each plant species individually.

The size of the weight unit selected will vary with plant size and weight (Figure 3). For example, a single tiller of smooth brome grass (Bromus inermis) may be a weight unit at 30g, while an entire plant of red grama (Bouteloua trifida) may only constitute 5g. Other factors that affect weight to size ratio are the length and thickness of stems, ratio of leaves to stems, and relative compactness or foliar density of species.

Figure 3. Examples of weight units from Chapter 4 of the National Range and Pasture HandbookLinks to an external site..

How to Choose a Weight Unit for a Species:

1. Determine a weight unit that is appropriate for the species (e.g. grams, lbs, kg). Then determine the categories within those units that will be estimated. For example, when the weight unit is in grams, observers often estimate biomass to the nearest 5g, so that categories are 5-, 10-, 15-, 20g and so on.
2. Visually select a portion of a plant, an entire plant, or a group of plants that approximates this weight.
3. Harvest and weigh the selected portion to determine actual weight.
4. Repeat the process until the weight unit can be estimated with reasonable accuracy.

Steps to Estimate Biomass

1. Select a quadrat appropriate for the type of vegetation being sampled.
2. Place the quadrat along a transect line or in a location determined by the selected sampling protocol.
3. Estimate the biomass of each species by counting the number of weight units of that species in the quadrat. You may need to “visually conglomerate” or “add” portions of plant units that are dispersed in the quadrat. It is okay to estimate partial units, such as 2.5 or 3.75 plants units.
4. Record the number of plant units observed in each quadrat for each species in the quadrat.
5. Estimate a sufficient number of quadrats to capture the variation in biomass across the landscape of interest. The more variation in the vegetation — the more plots that will need to be estimated.
6. Periodically recalibrate to verify your accuracy throughout the day by harvesting and weighing identified weight units.
7. Collect samples of each species to take back to the lab and dry to determine percent dry weight.
8. Calculate the biomass of each species by multiplying the average dry weight of a plant unit by the number of plant units recorded.

Estimating Biomass of Shrubs

A slight modification of this method is particularly well suited for shrubs. The observer simply clips a unit of the plant and carries it along to each plant instead of learning to recognize a specific unit of weight:

• A small unit of a plant, such as an average sized branch (Figure 4), is designated as the reference unit and clipped from the plant.

Figure 4. Examples of a reference unit for estimating biomass of shrubs; big sagebrush (Artemisia tridentata), antelope bitterbrush (Purshia tridentata), and rabbitbrush (Chrysothamnus spp.), respectively.

• A reference unit should be 10-20% of the foliage weight of the average plant.
• The reference unit is then held up against plants for which biomass estimates are required. The number of reference units in these plants is recorded.
• The weight of current season’s growth or total mass of the reference unit is then determined and a sample is kept for drying so results can be reported on a dry matter basis.
• The weight of estimated plants equals the number of reference units multiplies by the average dry weight of the reference unit.
• The techniques works well for some shrubs, but is not well suited for compact, dense, or un-segmented growth forms.

This approach can be sued for trees as well, although it becomes difficult for taller trees.  An alternative approach to biomass estimation for trees is to estimate the weight of plant material in a tree of a certain height, and then use the tree as the weight unit.

The accuracy of the observer’s estimate depends on:

• The experience of the observer. Well trained technicians with a good deal of field experience can estimate the amount of forage in a plot with little error.
• The alertness of the observer. Accurate estimation requires significant concentration. Accuracy often decreases at the end of the day when observers are tired, hot, or hungry.
• The vegetation type. Some plant types are simply easier to estimate than others. For example, bunchgrasses are often easier to estimate than sod-forming grasses.

Calculating and Reporting Estimation Data

Biomass data is calculated and reported in the same way, whether plots were clipped or estimated. Plots are generally clipped and weighed or estimated in grams. Then, drying procedures convert field weights to dry weights in grams/plot. However, to be useful for reporting information, biomass should be expressed in lbs/acre or kg/hectare.  Here are a few useful conversions taken from the National Range and Pasture Handbook (Chapter 4 page 4-6). To convert grams per plot to pounds per acre:

• 0.96 ft² plots = multiply grams by 100
• 1.92 ft² plots = multiply grams by 50
• 2.4 ft² plots = multiply grams by 40
• 4.8 ft² plots = multiply grams by 20
• 9.6 ft² plots = multiply grams by 10

To convert grams per plot to kilograms per hectare, use the following conversions:

• 0.25 m² plots = multiply grams by 40
• 1 m² plots = multiply grams by 10
• 10 m² plots = multiply grams by 1