Principles of Vegetation Measurement & Assessment
and Ecological Monitoring & Analysis

 

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Why Biomass?

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measuring Biomass

Why Measure Biomass or Production?

Biomass is a plant attribute that is time consuming and difficult to measure or estimate, but easy to interpret. Biomass is regarded as an important indicator of ecological and management processes in the vegetation.

Ecological Indicators --


Assessing biomass on rangeland of Hawaii

Photo by Ron Nichols courtesy of NRCS Photo Gallery http://photogallery.nrcs.usda.gov

  • Plants that dominate a site, in terms of biomass, are a reflection of the plants that are controlling the nutrient, water, and solar resources on the site.  Therefore, biomass is often measured to assess the ecological status of a site.

  • Measures of standing crop also reflect the amount of energy stored in the vegetation, which can indicate the potential productivity at the site. Therefore, estimates of biomass are used in assessing rangeland condition.

  • Many scientists believe that the relative production of different plant species is the best measure of these speciesí role in the ecosystem. Therefore, some measure of production is often used in dominance studies.

  • Estimates of biomass and residual biomass also strongly influence the hydrologic properties of the site including infiltration, runoff, and erosion.

  • Biomass of both grasses and woody plants constitute potential fuels that can be measured to assess the risk of wildfire.

Management Indicators --

  • Phytomass or standing crop are most often measured to set stocking rates or assess an ecosystemís capability to support grazing animals. In fact, the term "forage availability" is a way to describe biomass as an amount to be used for grazing animals. Biomass can also be monitored throughout the grazing season to make necessary adjustments to stocking rates. 

  • 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 measure herbage when assessing the feasibility and potential behavior of a prescribed fire.


Prescribed Burning in Grassland. Photo courtesy of the National Interagency Fire Center's Image Portal

Advantages of Estimating Biomass

  • Most biomass attributes are strait-forward, easy to interpret, and can be objectively measured.

  • Biomass can be directly measured with little training, although, it is time consuming.

  • Biomass can be measured for all types of vegetation and therefore comparisons can be made among different communities or ecosystems.

  • Biomass can be easily measured and therefore the accuracy of estimation techniques can be easily tested. In contrast, cover is easy to estimate, but direct measures of cover are very difficult to make and therefore the accuracy of cover estimates are seldom examined.

  • Biomass is considered a good measure of plant dominance on a site because it reflects the amount of sunlight, water and minerals a plant is able to capture and turn into plant mass.

Disadvantages of Estimating Biomass

  • Collecting biomass data can be very time and labor consuming. Cover, frequency and density are generally more quickly estimated.

  • There are many methods to directly measure biomass of herbaceous plants, but, it is difficult to estimate biomass of shrubs and trees.

  • In many grassland and shrubland areas, the variability between quadrats and the accuracy of estimating production within individual quadrats necessitates that many quadrats be sampled to detect differences between sites or years.

  • Biomass and Gross Primary Production are rarely measured in rangeland studies because it is very difficult (and usually impractical) to measure below ground biomass.

  • Peak standing crop may be difficult to measure in ecosystems with a large variety of species because each species will generally reach itís peak phytomass at a different time of year. For example, grassland regions in the Central Great Plains may have about equal proportions of cool-season and warm-season grasses. However, the cool-season grasses will peak out in June while the warm-season grasses will not reach peak biomass until July or August. When should peak standing crop be measured in these situations? As a compromise, peak standing crop is often measured at the end of the growing season.

  • When measuring annual production, current yearís growth can be difficult to separate from previous yearís growth.

  • Not good for assessing rare plant populations because destructive removal of forage is usually required.

  • Standing crop can also be altered by herbivore utilization. Therefore, exclosures are usually necessary to measure this attribute. Additionally, up to 25% of the phytomass can be removed by insects or rodents that cannot be easily excluded from study areas.


Students clipping vegetation to estimate biomass in central ID. Photo by K.Launchbaugh


Workshop on estimating biomass. Photo from: www.tx.nrcs.usda.gov/news/


Monet Prairie Farm Rehabilitation Area, Saskatchewan, Canada. Photo by Mae Elsinger

 

  • Seasonal and annual climatic fluctuations affect biomass, therefore, production is not a suitable measure for long-term trend studies that compare data taken in different years. Density, frequency, and basal cover are less susceptible to yearly variation created by climatic fluctuations.

 

Repeat photos from a site in Central Idaho illustrates immense year to year variation in biomass production. This study focused on production of crested wheatgrass (Agropyron cristatum)  (Sanders, K., N. Rimbey, and L.A. Sharp. 1992. Variability of crested wheatgrass production over 35 years. Rangelands. 14(3):153-168.) --Link to original publication

 

1957 - Annual precipitation was 10.2 inches.
 

Crested wheatgrass production = 846 lbs/acre.

 

1960 - Annual precipitation was 6.8 inches.

Crested wheatgrass production = 186 lbs/acre.


1971 - Annual precipitation was 16.2 inches.

Crested wheatgrass production = 1,090 lbs/acre.


1974 - Annual precipitation was 8.1 inches.

Crested wheatgrass production = 324 lbs/acre.

Summary Questions

  1. Why is biomass not often used to for determining trend?

  2. Why is biomass a good estimate of what plants are dominating a site?

  3. When would it be better to measure cover than biomass?

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