Tension wood occurrence in three cultivars of Populus × euramericana. Part I: Inter-clonal and intra-tree variability of tension wood
Ann. For. Sci.
Tension wood occurrence in three cultivars of Populus × euramericana. Part I: Inter-clonal and intra-tree variability of tension wood
Miguel Angel BADIA 0
Thiéry CONSTANT 0
Frédéric MOTHE 0
Gérard NEPVEU 0
0 LERFOB, UMR INRA-ENGREF, Wood Quality Research Team, Research Centre of Nancy , 54280 Champenoux , France
- The main wood quality problem affecting poplar trees is tension wood occurrence associated to end-splits after felling, distortions of products, woolly wood, etc. The objective of this study was to estimate the occurrence of tension wood by using the external shape of the stem and the 3D distribution of annual growth rings. This paper is the first of two papers and presents the inter-clonal and intra-tree tension wood variability. A clear clonal-effect was observed on tension wood area percentage (I-MC>I214>Luisa Avanzo). The clone with the best tree morphology (I-MC) showed the highest percentage of tension wood (15.3%). Concerning the intra-tree variability, tension wood extent increased significantly at the tree base (< 30-50 cm). In the radial direction of the stem, the percentage of tension wood showed an U-shape distribution. The first peak, close to the pith, corresponds to the “establishment growth period” during the open growth and the second peak, close to the bark corresponds to the “competition growth period”.
The choice of cultivars in a plantation management depends
mainly on the environmental conditions, but also on their
resistance to pathogens, growth rate, tree form and of course wood
quality. One of the most important wood properties for the
traditional use of poplar wood is the low percentage of tension
wood. However, due to the complexity of tension wood
measurements, this defect is not always considered in the clonal
selection process. Different methods of macroscopic detection
are reported in the literature: staining [
11, 18, 20
and tangential shrinkage [
], naked eye [
3, 23, 28, 29
However, all these techniques allowed the authors to observe
a strong inter-clonal effect on tension wood variability. Sacré
and Leclercq showed that tension wood proportion varies from
7.1 to 40% according to the clone [
The complexity of techniques for tension wood
identification have often led the authors to restrict them to only one
position within the stem in order to compare several clones without
considering the intra-tree variability. Researchers interested in
the internal distribution of tension wood through the whole tree,
and who consider the active function of reaction wood in the
straight part of the tree [
], have tried to connect the
longitudinal and radial distribution of reaction wood with the tree
form (lean, curves, eccentricity, etc.) [
1, 7, 9, 13, 34, 37
the architectural structure (effect of lateral branch
development, etc.) [
10, 15, 25
]. In both types of studies, the main
difficulty encountered in the analysis of mature trees was that the
past of the trees was unknown: only the current form can be
observed, but the history responsible for the final form is not
accessible. The internal distribution of reaction wood can vary
considerably according to this history. However, beyond the
behaviour of each specimen in response to its individual
environment certain constraints will be found in all specimens
leading to the same response. Thus we can observe a structured
pattern in the reaction wood distribution in response to
controlled environmental factors such as silviculture.
In this study, we observed the tension wood variability
between three of the most planted cultivars in Valley of Cinca
(Spain): I214, Luisa Avanzo and I-MC, as well as the intra-tree
variability. We present here the distribution of tension wood in
nine stems of poplar rapidly grown under plantation management.
2. MATERIALS AND METHODS
Nine poplar trees were sampled in Valle del Cinca (Spain). A
preliminary inventory was carried out to define the most representative
typology of tree shape. A rather good homogeneity of the growth
conditions (wind direction, soil, exposure, watering by flooding) was
ensured by selecting the plots close to each other, and by selecting the
individuals of the same cultivar in the same plantation. The plantation
density was 277 stems per hectare (6 m spacing). The dendrometrical
characteristics of these nine trees are illustrated in Table I.
The tree shape of the nine trees was digitised using an experimental
method developed during a Ph.D. work [
]. In order to connect the
occurrence of tension wood (TW) to the standing trunk shape it was
necessary to find a method to calculate the tree shape, the log shape,
the discs and the ring properties in the same system of coordinate. In
this case, a chain of devices combining laser distancers and image
analysis systems was used [
]. The result was a precise location of the
characteristics measured on each disc in the standing tree. Using this
information, the local lean per disc or per growth ring could be
calculated as the angle between the vertical axis and a line joining the centre
of gravity of the disc below and above the disc studied [
]. Then the
mean local lean per tree, was calculated as the average of the different
local lean per disc analysed in the stem. Figure 1 shows the three levels
of shape analysis: (A) shape of the standing tree, (B) location of each
disc and wood properties inside the tree, (C) disc level with growth
rings and TW (dark grey).
The macroscopic identification of TW was done by the naked eye
3, 23, 28, 29
]. It was performed by using the highest reflection of some
areas under a low-angle natural light (Fig. 2). These areas with a shiny
and silky appearance were composed of gelatinous fibres (TW). A
microscopic validation by 80 thin sections (15 µm thick) double
stained with safranine and astra blue, verified the relevancy of results
. Figure 1B shows the TW distribution inside the stem and a clear
longitudinal continuity along several discs.
The height of the analysed stems was between 15–17.5 m,
including a “trunk zone” which corresponds to the pruned length, up to 8m
and the “crown zone” to the non-pruned length, and the total number
of discs and growth rings were 298 and 3801, respectively. Table II
sums up for each tree the number of growth units, discs, growth rings,
stem height and the position of the first green branch and dead branch
(pruned height). For each growth unit we sampled several discs, to
calculate the proportion of TW per growth unit and per growth ring. The
results presented in this paper were generally confirmed by statistical
analysis (SAS® GLM procedure) and the comparisons of means were
based on the Student-Newman-Keuls (SNK) test. Nevertheless, the
authors draw the attention of the reader to the risk of generalisation
of statistical results based on such a small number of trees.
3.1. Inter-clonal variability of tension wood
Figure 3 shows in a bar chart the mean values per cultivar
and for each of the nine trees analysed. The TW percentage
varied considerably between the different cultivars. Luisa Avanzo
and I214 presented a small proportion of TW, 7.6% and 8.2%
respectively, while it was much higher in I-MC cultivars
(15.3%). These substantial differences between cultivars
corroborate the observations carried out by other authors: Sacré
] observed in some 3-year-old stems, very strong
differences between I214 (16%) and Robusta (40%); Leclercq [
classified ten other cultivars between 7 and 16 years of age, in
three levels: 8%, 15% and 40%.
The TW percentage obtained in our study for the I214
cultivars was weaker than that reported by Sacré [
difference can be explained by the age difference between the two
samples (3 and 16 years respectively). A comparison at equal
age (3 years) showed that I214 cultivars of our sampling had
an average proportion of TW equal to that observed by Sacré
(16.4% versus 16% in Sacré sampling). Concerning Luisa
Avanzo and I-MC cultivars, there is no data in the literature on
TW percentage. Considering our data, the differences between
the average TW percentages were statistically significant
(pvalue > 0.0001) for the clone I-MC. LA and I214 cannot be
distinguished except for some of the first growth units.
I-MC cultivars had the narrowest crown area (46.5 m2) and
the highest TW content. By contrast, Luiza Avanzo exhibited
a low TW percentage and large crown areas (69.5 m2). The I214
cultivar was between these two extremes. Because of the low
number of trees involved in our study, a negative correlation
between crown areas and TW percentage could not be
established, nevertheless these results are in agreement with the
observations made on the beech by Ferrand [
] and the
European Project “Stress in Beech” [
]. However, they are in
disagreement with Berlyn [
], who found a weak but positive
relationship between crown projection area and tension wood
percentage in 84 Populus deltoides (Correlation Coefficient,
r = 0.19).
The mean local lean (Fig. 3B) shows that the straighter
cultivar was I-MC (3.4°), followed by Luisa Avanzo (4.1°) and
I214 (5.8°). This classification does not have a general
significance, considering that the strategy used for selecting the
trees was based on shape index. Compared to TW percentage
(Fig. 3A), the cultivars with the best tree morphology (I-MC)
presented the highest percentage of TW (15.3%). This result
underlines the importance of knowing the wood quality for
selection. In this case, morphological factors are not the best
criterion to take into account in poplar plantations in order to
produce wood with lower values of TW. Similar results have
often been observed by other authors in the genus Populus [
] Isebrands and Bensend [
] put forth the assumption
that the cambium of this species would be particularly sensitive
to the stimuli involved in the formation of TW.
3.2. Intra-tree variability of tension wood
3.2.1. Longitudinal distribution
Figure 4 presents the longitudinal distribution of TW. One
tree per cultivar and per shape-index are illustrated: straight
I214, leaning Luisa Avanzo and flexuous I-MC. In addition to
TW distribution, a line was added to indicate the height position
for each branch measured in the field. In this way, we can see
the different behaviour of TW distribution between “trunk
zone” (pruned stem) and “crown zone”. In the “crown zone”,
the occurrence of TW was quite irregular. In the “trunk zone”,
TW percentage was rather continuous and regular except in the
bottom of the stem. In seven of the nine trees studied, the
percentage of TW at the bottom of the stem (below 50 cm) was
higher than in the discs located just above. The same
characteristic was observed by Washusen et al.  in 10–
11 year-old Eucalyptus globulus Labill. and by Jourez [
Delavault et al. [
] in young stems of poplar and Eperua
falcata respectively. In six out of nine trees, if the first 50 cm
were not considered, the proportion of TW tended to increase
slightly with the height. In the litterature both situations have
been reported, TW increasing with the height in: poplar [
] or beech  and TW decreasing with height [
]. The different results from the literature would be explained
by the position of the samples analysed by the authors. In fact,
according to the position of the discs sampled, closer to the butt
portion of the tree or to the top, the trend could be reversed.
3.2.2. Radial distribution
Figure 5 illustrates the distribution of TW percentage observed
in each growth ring according to their radial position. The data
were divided per cultivar (I214, Luisa Avanzo and I-MC) and
per stem zone according to the longitudinal position of the discs
(“trunk zone” or “crown zone”). The plots coming from the
“trunk zone” and from any cultivar showed a clear bimodal
distribution (two peaks). On the other hand, the discs sampled in
the “crown zone” did not follow a bimodal distribution. In fact,
this bimodality indicates that the phenomenon of TW
appearance is not homogenous during the tree growth and the
observations result from two “overlapping” distributions. One is
composed by the first growth rings that are close to the pith and
the other one by the growth rings formed the last years of the
tree life, i.e. close to the bark.
In the first distribution, the high TW percentage decreases
strongly with the tree growth. This reduction in TW occurrence
could be linked to the increase of stiffness due to larger
diameter. This stage was called “establishment growth period”. The
proportion of TW reaches a minimum, which is, for the three
cultivars, situated in a similar radius between 90 and 100 mm.
Field observations showed that until this diameter (between 8
and 10 years) the trees were in open growth with no competition
for light. The second peak corresponds to the moment when
crowns begin to touch and interact, i.e. to the canopy closure.
If it is considered that following the canopy closure the
phototropic response of the trees forces them to increase their surface
exposed to the light, and thus to change the position of the
branches and even of the stem, this process must be inevitably
accompanied by a production of TW. This second growth stage
was called “competition growth period”. Coming back to the
“crown zone”, a radial decrease of TW percentage is visible
close to the pith especially for I214 and LA. The reasons of the
decrease had the same origin than in the “trunk zone”. The
minimum value seems to be closest to the pith than in the trunk zone
as it could be expected according to the hypothesis of the
coincidence with canopy closure. The second peaks are less visible.
Several reasons could be put forward: the local distribution of
tension wood could be disturbed by branches at this level, and
there was not enough radial increment recording through TW
the change in competition.
This kind of radial reaction wood distribution has been
reported in several poplars and coniferous, and called “waves
of reaction wood” [
8, 10, 15, 25, 27, 33
]. The authors consider
that part of the first wave corresponds to the reorientation of
the stem and the second wave, elaborated in mature age, to the
stabilisation of the tree. In addition, these waves are explained
also by a thinning effect or by the death of the apical meristem.
However, the effet of the canopy closure in the production of
TW has never been observed before.
3.2.3. 2D Tension Wood map
Figure 6 shows in a 2D map the radial (growth rings) and
longitudinal (growth units) distribution of TW for the nine
sampled trees. The longitudinal data was limited in order to
homogenize the nine pictures, we mapped for all trees up to the sixth
growth unit. The average of TW percentage was represented
in a grey scale where each step was 2.5%.
We can notice the longitudinal increase in TW between the
“trunk zone” (up to the fourth growth unit for Luisa Avanzo
and I-MC and fifth growth unit for I214) and the “crown zone”,
especially for the growth rings close to the pith. An ANOVA
was carried on this set of data for testing the effects of cultivar,
shape, and age from the pith for each growth unit. The statistical
analysis showed that the shape effect on TW percentage was
very significant for the first growth units (GU ≤ 4, p-value <
0.0001) and the degree of significance decreased towards the
crown (p-value = 0.0209 for GU5, p-value = 0.5591 for GU6).
A variation of TW distribution was also observed in the radial
direction. We found that the bimodal distribution of TW was
clearer in the first growth units than in the crown. Regarding
the basal effect observed in Figure 4, it is not visible in Figure 6
because it is hidden by the average of several discs per growth
The main characteristics of I-MC cultivar were: strong
apical dominance, fast growth and very good morphological
character. However, it had a proportion of TW (15.3%) higher than
the other clones which can result in many technological
problems during the industrial wood processing [
]. Luisa Avanzo
and I214 produced only 7.6% and 8.2% of TW respectively, but
the main disadvantage of Luisa Avanzo is the huge quantity of
wetwood (up to 40%, [
]) and the flexuosity for I214.
Thus the choice of the cultivar to be planted cannot be only
based on morphological characters and fast growth, since wood
properties, like TW, associated to each cultivar are essential.
We have seen that the crown projection area could be a good
inter-cultivar indicator of TW occurrence. Cultivars with
narrow crowns correspond to the trees with highest values of TW
and the widest crowns correspond to cultivars with the lowest
percentages of TW.
The spatial distribution of TW showed that it was located
predominantly below 30–50 cm. If this basal part is eliminated,
the production of TW increased slightly towards tree height.
This longitudinal distribution suggests that TW content estimated
through sampled cores taken below 50 cm in the stem lead to
overestimate the real proportion of TW in the whole stem.
In the pruned stem (up to 8 m), the radial distribution of the
percentage of TW (calculated per growth ring) presented a clear
bimodal structure. During the first peak, “establishment growth
period”, the trees were in open growth and the high proportion
of TW was probably due to a high sensitivity to environmental
stimuli as wind or non uniform crown development added to a
slight stiffness of the stem. During the first stages of the tree
growth, TW occurrence decreased strongly until the canopy
closure, which appeared when the trees were between 18 and
20 cm in diameter. After this event, the phototropic response
of the trees was inevitably accompanied by a major production
of TW, a stage called “competition growth period”.
The effect of canopy closure on the distribution of TW shows
the relevance for the selection process of the plantation density,
which must be in accordance with the growth rate and
phototropic response of the different cultivars available.
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