10.1 Biomass and Production Overview

Video Presentation

Learning Guide

Biomass is the total weight of all living organisms, both plants and animals, for a given area at a given time.  Although it is difficult and time-consuming to measure, biomass is an extremely important and informative attribute of rangeland vegetation, because it provides information about the functioning of ecological processes as well as management effectiveness.  It is important to note that the terms “biomass”, “production”, and “yield” are often used interchangeably by rangeland scientists and managers to describe the amount of plant material produced on an area basis.  In this module we will generally use the term biomass when we are discussing the approaches to measuring the weight of plant material.  Biomass and production are expressed in units of weight per unit area, most often as kg/ha, lbs/acre, or g/m².

Why Measure Biomass?

Biomass data are easy to interpret, and biomass is generally an easy attribute to visualize, which makes it one of the most commonly measured attributes in vegetation monitoring programs.  Biomass is also regarded as an important indicator of ecological and management processes in vegetation communities.

Ecological Indicators

• Biomass reflects the amount of resources used by different species within a site. For example, Figure 1 depicts the relative amount of biomass produced by different species at two different points in time. We can see a shift in resource use reflected as an increase in biomass of the exotic invasive species spotted knapweed (Centuarea stoebe).

Figure 1.  The two charts demonstrate a) bluebunch wheatgrass (Pseudoroegneria spicata) community recently invaded by spotted knapweed, and b) altered community dominance following expansion of spotted knapweed.

• Many scientists believe that the relative production of different plant species is the best measure of these species’ role in the ecosystem. Therefore, some measure of production is often used in dominance studies.
• Measures of standing crop also reflect the amount of energy stored in the vegetation, and the potential for a site to produce certain types and amounts of vegetation. This relates directly to site potential.
• The amount of biomass and residual biomass also strongly influence the hydrologic properties of the site including infiltration, runoff, and erosion.
• Biomass of grasses and woody plants constitutes potential fuels that can be measured to assess the risk of wildfire. Consequently, an estimate of biomass is a required variable in most fire-behavior and risk assessment models.

Management Indicators –

• Biomass is a valuable tool to assess range condition and health. The relative production of different species is used to determine similarity to historic and desired plant communities.
• Biomass can be used to determine how many grazing animals an ecosystem can support. Forage inventories measure the availability of biomass available for herbivores, particularly livestock, to consume. Biomass can also be monitored throughout the grazing season to make necessary adjustments to stocking rates based on forage reserves and residual biomass.
• Some measures of production may also be necessary to assess the value of a site for wildlife habitat. For example, the amount of herbage affects the value of a site as cover for upland game birds.
• It is necessary to estimate the amount of herbaceous and woody biomass when assessing the feasibility and potential behavior of prescribed fires.

Terms Related to Biomass

A number of terms relate to plant biomass. While some of these terms may be used interchangeably in casual conversation, it is important to be able to distinguish between them because our decisions about which plant parts are measured may vary depending on the term that we use.  Being clear on terminology helps to avoid confusion when communicating about what was measured, and also ensures that the data we collect is aligned with our purpose for measuring plant biomass.

• Biomass: Total amount of living plants and animals above and below ground in an area at a given time.
• Phytomass: Total amount of plant biomass in an area, including attached dead material, both above and belowground at a given time.
• Primary Production: Total amount of organic material produced within a given period by vegetation. Primary production is a measure of the conversion of solar energy to chemical energy through the process of photosynthesis.
• Standing Crop: Total amount of above-ground plant material in a unit area at a given time. This term may be modified with the adjectives “dead” or “live” to more accurately define the specific nature of the biomass.
• Peak Standing Crop: Maximum amount of standing crop observed during a given year. Due to differing plant phenology, this maximum value of standing crop may not occur concurrently for all species on a site.
• Current Year’s Growth: The above-ground plant biomass produced during the previous 12 months. This term is sometimes used interchangeably with “annual production”.
• Residue or Residual Biomass: Weight of vegetation remaining after grazing or harvest is completed.
• Herbage: Aboveground material of any herbaceous plants.
• Browse: The part of leaf and twig growth of shrubs, woody vines, and trees that is available for animal consumption.
• Forage: Herbage and browse that is available and acceptable as food for grazing and browsing animals, or that may be harvested for feeding purposes.
• Yield: The quantity of a product in a given space and/or time. This term is used in reference to harvest, or that the material that has been produced could be harvested either mechanically or through consumption.

A NOTE OF CAUTION!!!!!  Productivity and production are very different terms, but they are sometimes confused because they sound similar.  Productivity is not synonymous with production!!!  Productivity is the rate of plant production per unit area, whereas production refers to the weight of biomass produced, NOT the rate at which biomass is produced.

Advantages of Estimating Biomass

• Most approaches to measuring biomass are straight-forward, easy to interpret, and can be objectively measured.
• Biomass can be directly measured with little training.
• Biomass can be measured for all types of vegetation, meaning that comparisons can be made among different communities or ecosystems.
• Biomass can be directly measured and therefore, the accuracy of estimation techniques can be verified.

Disadvantages of Estimating Biomass

• Collecting biomass data can be very time consuming and labor intensive.
• There are many methods to directly measure aboveground biomass of herbaceous plants, but it is difficult to directly measure or estimate biomass of shrubs and trees.
• Due to variability in the vegetation and in observer estimates, a large sample size is usually required to detect differences between sites or years.
• Phytomass is rarely measured in rangeland studies because it is very difficult (and usually impractical) to measure belowground biomass.
• Peak standing crop may be difficult to measure in ecosystems with a large variety of species because each species will generally reach peak growth at different points in time. This issue is compounded when both warm- and cool-season vegetation grows in the same area.
• Current year’s growth can be difficult to separate from previous years’ growth.
• Assessing biomass of rare plants can be difficult since some level of destructive removal of plant material is required.
• Grazing by wild and domestic herbivores removes herbage, affecting the standing crop available for measurement. Therefore, exclosures are usually necessary to get an accurate measure of this attribute. Additionally, smaller herbivores (e.g., rodents and insects) can remove substantial proportions of the standing crop, and cannot be easily excluded by traditional exclosures or cages.
• Seasonal and annual climatic fluctuations affect biomass; therefore, it is not a suitable measure for long-term trend studies that compare data taken in different years (Figure 2).

Figure 2.  Repeat photos from a site in Central Idaho illustrate substantial year to year variation in biomass production. This study focused on production of crested wheatgrass (Links to an external site.Links to an external site.Links to an external site.Links to an external site.  (Sanders et al 1992).

Ground Rules and Decisions

Clear establishment of ground rules prior to data collection will improve the consistency of sampling and aid in the interpretation of resulting data. The following points should be considered when determining sampling protocols:

1. Sampling date – Biomass changes through a growing season, so the date when samples are harvested or estimated can have a significant impact on the resulting estimates. Production information can be collected for a specific period, such as the month of June, or to quantify regrowth following a grazing event. Ideally, peak standing crop, should be measured at the end of the growing season.

   However, this can present challenges when both cool- and warm-season plants are growing in the same area. For example, grassland regions in the Central Great Plains may have nearly equal proportions of cool-season and warm-season grasses. However, the cool-season grasses reach peak standing crop in June, while the warm season grasses will not reach peak standing crop until July or August. When should peak standing crop be measured in these situations? As a compromise, peak standing crop is often measured in early July, after the cool season grasses are finished growing, but before a significant loss of senesced plant tissue.

2. How to determine dry-weight – Fresh weights of clipped plant materials can be highly variable depending on the water content of the plant; water can account for up to 70% of plant weight and can vary greatly with species, plant phenology, season, and atmospheric conditions. Consequently, for an accurate measurement of plant biomass, we need to report biomass on a dry weight basis.  Usually this is achieved by either air-drying or oven-drying clipped materials, and comparing the fresh and dried weights to determine a correction factor to account for plant moisture.  If samples cannot be retrieved from the field, fresh weights can be converted to dry weights using the conversion rates available from the National Range and Pasture Handbook, although some of the accuracy of the harvest method will be lost.  A detailed explanation of the procedure to determine dry weight values is provided in Lesson 2.

3. Separation of live and dead material – If the attribute of interest is the current year’s growth, then only live plant material is of importance. However, if standing crop is the attribute of interest, both live and dead plant material will be assessed for biomass.

Figure 3. A mixture of live and dead plant material from purple threeawn (Aristida purpurea) clipped on Montana rangelands.

4. Which plants or plant parts to measure – On what plants or plant parts will biomass be assessed? Biomass can be measured on: all plants (herbaceous and shrubs), herbaceous plants only, or just selected species of interest, which depends on your reasons for estimating biomass. Will biomass be assessed on all plants rooted in the plot or those that occur within or above the perimeter of the quadrat? In theory, biomass should be estimated in the volume of space that occurs within the vertical projection above the perimeter. However,  it is generally easier, and much less subject to non-sampling error, to harvest or estimate biomass of plants that are rooted in the plot, regardless of whether the plant materials extend beyond the perimeter of the plot.
5. Taxonomic level of separation – Will biomass measurements be made separately for individual species, can species be grouped by functional group (e.g., cool season grasses, annual forbs, etc.), or are we measuring biomass of all plants, regardless of species? We need to know the purpose behind collecting the data to answer these questions. In many applications, biomass is reported on a species basis.  For example, it is necessary to estimate production by species to calculate a similarity index for range condition.  Sometimes we may group species into groups based on forage value when gathering data for a forage inventory.  Species separation is very labor intensive and time consuming, so knowing how the data will be used is an important consideration when deciding on the sampling protocol.
6. Height of clipping and estimation – Clipping and estimating is conducted at the level of the soil surface to get an estimate of total production, or at 1” from ground level to get estimate of available forage (Figure 4). This height better represents available forage for livestock, particularly cattle, who do not graze to the ground surface. Again, knowing how the information will be used once it is gathered will help you determine the appropriate height for clipping or estimating.

Figure 4. a) Clipping vegetation to ground level, and b) clipping vegetation to 1” stubble height.

Overview of Methods to Sample Biomass

There are a number of different ways to sample biomass either directly or indirectly. Direct methods include vegetation harvest, estimation, and double sampling. Indirect methods involve using a related parameter, such as canopy volume or dimensions, to estimate biomass production.