Principles of Vegetation Measurement & Assessment
and Ecological Monitoring & Analysis


Veg Sampling
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Indirect Estimates of Biomass

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measuring Biomass

All Indirect Estimates are based on Double Sampling

There are dozens of attributes or parameters that could be measured for plants that are related to biomass.  For example, the canopy dimensions of a shrub would be related to biomass -- the larger the shrub, the greater its mass.

Height of trees is related to biomass and can be used to estimate biomass.

This examples are from  Chapter 4 of the
National Range and Pasture Handbook

Crown Area - The crown area of a plant is often highly correlated to current seasonís growth biomass. Therefore, crown area can be used in double sampling techniques.

  • The widest dimension of the plant is recorded as dimension 1 (D1).
  • The dimension perpendicular to DI is dimension 2 (D2).
  • Crown area = π*D1*D2
  • A sub-sample of the measured plants are then clipped and weighed.
  • A regression analysis is completed, as outlined in Lesson 7_4 with crown area as the x-variable and clipped weight as the y-value.

Crown Volume or Dimension Analysis - The crown volume of a plant is also often highly correlated to current seasonís growth biomass. Therefore, the 3-dimensional volume of a plant can be used in a double sampling technique.

The dimensions measured to estimate crown volume depends on the 3-dimensional shape that best describes the plant (e.g., inverted cone, half spheroid, sphere). Most shapes require the measurement of several diameter and height dimensions.

Step 1 - Look at the plant in its natural state:
Step 2 - Envision a geometric shape that describes the shape of the plant:
Step 3 - Take appropriate Measurement of the shape:
Step 4 - Calculate Volume (v) of the plant:
Step 5 - Clip and weigh biomass from a sub-sample of the measured plants Step 6 - Develop a relationship between volume and biomass with regression analysis. Then, predict weight of all measured plants based on volume.

Basal Diameter or Stem Diameter - For many shrubs a measure of basal diameter (or diameter of the central stem) is highly correlated with browse or leaf biomass and can be used in a double sampling technique.

Twig Measurements - The number of basal stems or the average diameter of basal stems is also highly correlated to browse biomass in many shrubs. In a double sampling method, the number or diameter of basal stems is measured for many shrubs and a sub-sample of these shrubs are clipped and weighed.

The key to all indirect techniques is that biomass can be estimated if there is a way to establish the relationship between the parameter being measured and actual biomass. Thus, a double sampling procedure is needed where several plants are measured and then clipped and weighed.

Relationship Between Sward Height and Biomass

A long time ago, people studying grasslands realized that plant height is strongly related to biomass.  This is especially true for grasslands made of sod-forming grasses creating dense stands called swards.  Thus, an array of devises was born to consistently estimate the height of the sward.  These are called Rising Plate Meters, Falling Plate Meters, Sward Boards, and other names.  But, the principle is the same -- 1) find a way to consistently estimate height, 2) clip a few plots, 3) develop a relationship between height and weight, and, then 4) estimate weight based on height:

Height meters can be very effective in dense stands of grass. However, they often do not work well in more arid landscapes dominated by a mix of bunchgrasses and shrubs.

Capacitance Meters

In 1976, Black and colleagues (J. British Grassland Society. 24:168-172) developed a device with finger-like electrodes that when placed over a plot measure the amount of electronic capacitance in the plot. Electronic capacitance is related to biomass because capacitance is directly affected by height and density of forage in a plot. Capacitance meters are generally observed to work well in grasslands and shrublands.

The major disadvantage of the capacitance meter is that they are heavy and difficult to transport across rough landscapes.  Capacitance can also be affected by plant moisture and ambient conditions.


Carpenter, L.H., O.C. Wallmo, and M.J. Morris. 1973 Effect of woody stems on estimating herbage weights with a capacitance meter. 26:151-152.

** Link to complete article

Spectral and Remote-Sensing Options

The era of digital technologies and remote sensing have spawned a series of option for measuring spectral attributes of plants and landscapes and then relating these measures to biomass through double sampling. In this case the estimate of actual biomass on-the-ground is known as "ground truthing."

The most frequently used remotely sensed measure of vegetation known to be related to biomass is a Normalized Difference Vegetation Index or "NDVI." This normalized difference is the ratio of the difference between near-infrared and red wavelengths to the sum of the 2.

These graphs illustrate that NDVI is fairly closely related to Live Biomass and Total Biomass. But, it is not well related to Dead Biomass.

(Thoma, D.P., D.W. Bailey, D.S. Long, G.A. Nielsen, M.P. Henry, M.C. Breneman, and C. Montagne. 2002. Short-term monitoring of rangeland forage conditions with AVHRR imagery. Journal of Range Management 55:383-389.)

There are several newer remotely sensed attributes that are currently being examined for relationships with biomass.  For example, the Moderate Resolution Imaging Spectroradiometer or MODIS was launched and started offering image products in early 2000. This satellite offers products that approximate leaf area index and thus have been successfully related to biomass.

The bottom line ... if some attribute of a plant can be measured and shown to be related to biomass it can be used in an indirect measure of biomass.  Indirect measurement techniques can be developed from leaf to plant to landscape.

Summary Questions

  1. List several plant characteristics that could be measured as "indirect" estimates of plant biomass?

  2. What might be the advantages for using remotely sensed data for estimating biomass as compared to measurements accomplished "on-the-ground"?

  3. What does the term "double-sampling" mean?

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