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by Tim Ellis, Kristen Feher, Wayne Murray, Keryn Paul, Jim Brophy, Kris Jacobsen, Vijay Koul, Peter Leppert, and John Smith
April 2008
RIRDC Publication No 08/042 RIRDC Project No CSL-22A
Executive Summary
What the report is about
This report investigates
two electrical methods for the measurement of roots, which have previously
shown promise for the development of rapid non-destructive techniques for
use by researchers or plant breeders. We show that: 1) low frequency (kHz
range) capacitance measurements can be indicative of the mass and length
of the root systems of eucalypts; and 2) roots can transmit detectable
high frequency (MHz range) signals over short distances (~ 1 metre). The
use of the latter, however, appears limited by the relative electromagnetic
properties of soil and roots.
Intended audience
This report is relevant
to forestry, agroforestry and plant breeding researchers, in projects where
root mass and root location are measured.
Background
The first method of low
frequency (low MHz range) capacitance measurement, has been known for several
decades to provide an easy comparison of root system ‘size’, and is understood
to be directly related to root mass and root length. This method has traditionally
been applied to herbaceous crop and pasture species; here we evaluate the
method on plantation eucalypts as there was interest in measuring below
ground forest biomass to determine carbon sequestration.
The second method described here involves the use of high frequency (low MHz range), whereby roots tend to transmit signals along their length. These signals have been shown to be remotely detectable and thereby potentially useful for the non-destructive measurement of the spatial location of roots. For example, because some water use characteristics of plants are related to rooting depth, a rapid estimate of maximum depth is useful to water balance modellers and plant breeders selecting high water use cultivars that could reduce deep drainage. A second need for such a method often arises in agroforestry research, where it can be useful to measure the lateral extent of roots from trees which compete with adjacent crops and pastures.
Aims
Methods
To address Aim (1), we made
capacitance measurements on plantation eucalypts prior to their destructive
sampling for the measurement of below ground biomass (undertaken by another
project funded by DAFF). To Address Aim (2), we undertook 2 laboratory
experiments to a) measure the attenuation (signal loss) along a length
of root embedded in sand; and b) measured the dielectric properties of
sand, loam and root materials for use with antenna theory to allow interpretation
of (a) and an understanding of the likely limits to signal propagation
in roots.
Key results
Low Frequency Capacitance
- we provide the first published evidence known to us, that low frequency
capacitance C measurements can be related to root mass and, possibly, to
root length of plantation eucalypts. Trees at 3 mature plantation sites
produced similar relationships between C and root mass. The one younger
site showed a different relationship between C and root mass, but the results
from the 4 sites produced a relatively unified relationship between C and
root length, although the latter was inferred and not directly measured.
This is consistent with root capacitance theory. It is likely that the
younger site had a greater proportion of fine (<2mm diameter) roots
and therefore a greater total root length.
High Frequency Measurement of Root Location - signals can be transmitted from plant stems to roots, which tend to act as underground antennas, emitting electromagnetic fields which can be detected remotely. However, it appears that the relative dielectric properties of roots and soils are such that signals will travel only relatively short distances (2 to 3 m) along large diameter (10 to 20 mm) roots, before becoming too small to detect.
Implications
These results are relevant
to researchers of plant root systems and we have identified some theoretical
and practical limits of electrical measurement. In this preliminary project,
however, we were unable to develop the methods to a point that they are
likely to be directly useful to the agricultural and forest industries.
Recommendations
Our recommendations relate
to root research and geophysical research:
 
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