Non-Destructive Estimation of Sapwood and Heartwoo
Non-Destructive Estimation of Sapwood and Heartwoo
Non-Destructive Estimation of Sapwood and Heartwoo
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Bieker, D. & Rust, S. 2010. Non-destructive estimation of sapwood and heartwood width in
Scots pine (Pinus sylvestris L.). Silva Fennica 44(2): 267–273.
Accurate estimates of the water conducting sapwood area are necessary to scale sapflow
measurements to tree and stand level transpiration. We tested a non-destructive method,
electric resistivity tomography (ERT), to estimate the area of conductive sapwood in 9 Pinus
sylvestris L. trees in lower Saxony, Germany. Tomograms were compared to cross-sections
stained with benzidine after harvesting. All tomograms displayed a distinct pattern of low
resistivity at the stem perimeter and high resistivity in the stem centre with a steep increase
in resistivity in between, assumed to indicate the transition from sapwood to heartwood. The
tomograms showed a sapwood width 2 cm smaller than the staining method. This indicates
that staining methods overestimate the amount of active sapwood because when heartwood is
formed, moisture content decreases before extractive contents reach levels visible by staining.
The ERT method is a new powerful method for the non-destructive estimation of sapwood
and heartwood width.
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Silva Fennica 44(2), 2010 research articles
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Bieker and Rust Non-Destructive Estimation of Sapwood and Heartwood Width in Scots Pine (Pinus sylvestris L.)
by hand at constant height (30 cm) above ground. tree spaced at 45°-intervals were estimated from
A low frequency current (8 1/3 Hz) was applied the tomograms.
and measured in a dipol-dipol-configuration (Rey- All tests were computed with the R statistical
nolds 1997). The software DC2dTree (Günther et programming language (v2.9.2., R Development
al. 2006) was used for tomographic inversion. Core 2009). Linear models were analyzed with
The entire process including the calculation is the robust package (v0.3-4, Wang et al. 2008).
completed within 15 minutes per tree.
12
10
4 6 8 10 12
Sapwood width, cm
(electrical resistivity tomography)
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Silva Fennica 44(2), 2010 research articles
20 4 Discussion
15
The sapwood width measured with the staining
method was about 28% ± 3% higher than that
Frequency
10
measured by ERT. This is even more than the dif-
ference between computer tomography and stain-
5
ing of 17% found by Rust (1999). Therefore it is
likely that ERT-results display the physiological
0 active sapwood, whose width is over-estimated
by staining. But the sharp boundary between low
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
and high electric resistivity is not just an effect of
Ratio linear estimate/tomogram the higher water content in the sapwood. Other
Fig. 3. Frequency distribution of the ratios of sapwood parameters related to heartwood formation like
area calculated from single linear estimates of sap- resin content (Tattar and Blanchard 1974) and
wood width and area estimates from tomograms. pit closure (Du 1991) increase electric resistivity
as well.
The strong differences in wood moisture con-
tent between sapwood and heartwood that can be
The sapwood area calculated from the mean found in pine and other conifers allow the accurate
of 8 individual radial measurements in the tomo- estimation of the different zones. However, most
grams did not differ significantly from the mean central European hardwoods do not have such
sapwood area estimated from the tomograms strong contrasts between sapwood and heartwood,
(p = 0.23). There was, however, a large variation which might limit the possibilities of ERT.
(Fig. 3): estimates from one linear measurement The location of the pith of the trees corre-
ranged from 43 per cent to 170 per cent of sap- sponded closely to the spot of relatively low elec-
wood area as estimated by tomography. tric resistivity in the tomograms (Figs. 1 and 4),
The inner zone of high resistivity is not homo- confirming results of Shigo and Shigo (1974),
geneous, though. In the centre of most of the who found a good correlation between relative
tested trees the there is an area of relatively low low electric resistivity and the pith of a stem.
resistivity. This area coincides with the pith of Even the decentral pith in tree no. 6, which
the trees (Fig. 4). was placed on a steep hillside, was depicted in
Fig. 4. Tomogram and stem cross section with decentral pith of Pinus sylvestris L.
270
Bieker and Rust Non-Destructive Estimation of Sapwood and Heartwood Width in Scots Pine (Pinus sylvestris L.)
the tomogram (Fig. 4) which indicates that ERT within areas of similar wood moisture content
is able to show reaction wood. (Cermak et al. 2004). But with non-destructive
The growth rings next to the pith are made of measurements of sapwood area, sapflow sensors
juvenile wood, whose anatomical and physical can be placed in parts of the stem that are most
properties differ considerably from the properties suitable. Installing sensors partly in heartwood
of mature wood: specific gravity is lower, mois- can be avoided and representative points of meas-
ture content higher, fibres are shorter, cellulose urement can be selected.
content is considerably lower (Knigge and Schulz Thus there are many applications for a fast
1966), lignin content (Zobel and Sprague 1998) and non-destructive method for the detection of
and earlywood percentage are higher than in the sapwood area in experimental plant physiology,
surrounding mature wood. e.g.:
A variation of electrolyte concentration from – Guide the positioning of sapflow sensors (Nadezh-
the stem centre to the heartwood/sapwood border dina et al. 2002)
might also explain the relative low electric resis- – Scaling of sapflow density or velocity to tree and
tivities around the pith. Helmisaari and Siltala stand level
(1989) found increasing concentrations of Ca, Mg, – Calculation of specific hydraulic conductance
Mn and Zn towards the pith of Pinus sylvestris L. (Rust 1999, Cruiziat et al. 2002)
while concentrations of K and P decreased. Bieker – Monitoring of heartwood formation
and Rust (2010) could show that increasing con- – Monitoring of heartwood/sapwood relationships
centrations of K and Mg decrease electric resis-
tivities in the heartwood of Quercus robur while
wood moisture content remained constant.
Further investigations are needed to clarify the Acknowledgements
factors causing the relative low electric resis-
tivities around the pith of Pinus sylvestris L. and The authors acknowledge the constructive com-
other conifers. The variation of wood moisture ments of three anonymous reviewers.
content in stem cross-sections of different species
and the influence of electrolytes that could be
found for oak trees indicate that tomograms have
to be interpreted carefully and species specific References
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