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