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
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
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
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
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
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:
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
from a site in Central Idaho illustrates immense year to year variation
in biomass production. This study focused on production of crested
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
Crested wheatgrass production = 846 lbs/acre.
1960 - Annual precipitation was 6.8
Crested wheatgrass production = 186 lbs/acre.
1971 - Annual precipitation was 16.2
Crested wheatgrass production = 1,090 lbs/acre.
1974 - Annual precipitation was 8.1
Crested wheatgrass production = 324 lbs/acre.