Shrinkage of the gelatinous layer of poplar and beech tension wood

TL;DR: In this article, the authors showed that longitudinal shrinkage is much greater in the gelatinous layer than in other layers of beech and poplar tension wood than in normal wood, due to mechanical interactions of cell wall layers.

Abstract: Macroscopic longitudinal shrinkage of beech and poplar tension wood is higher than in normal wood. This shrinkage is the result of mechanical interactions of cell wall layers. SEM observation of cut, dried surfaces showed that longitudinal shrinkage is much greater in the gelatinous layer than in other layers. AFM topographic images of the same cells, both in water and in air-dry conditions, confirm this result. Measurements on sections indicate around 4.7% longitudinal shrinkage for the G layer.

Summary

Longitudinal shrinkage in wood

• Like all other wood properties, hygroexpantion presents a very important anisotropy.
• The knowledge of the wood cell structure, as a multi-layer fibre composite, allows the modelling of the longitudinal shrinkage.
• A high local level of growth stress is always related to presence of tension wood (Trénard & Guéneau 1975; Sassus 1994).
• Finally a last superficial planning is done manually with a brand new razor blade in order to produce a nice transverse surface, the sample being always kept in moist condition.

Scanning electron microscopy

• Both in poplar and beech, one cell is observed with two angle of view, at first perpendicular to the surface and then tilted 70° from that direction (Fig. 8).
• The x coordinate is given directly by the first image while the y coordinate can be calculated with equation 1 using both images (Fig. 7).
• These topographic profiles allow measurements of differential shrinkage between cell wall layers.
• Thin sections Several poplar cells were observed after drying.
• Mean measurements of differential restraint between G layer and compound middle lamella (CML) are 1.99 µm for a face, 1.83 µm for the other and 3.82 µm for the sum of faces.

Atomic force microscopy

• The profile in water (Fig. 11 A') shows that there is already a small retract of Glayer before drying.
• The profile in water after 2 hours in 80°C water (Fig. 11 B') shows very few additional retract of G-layer before drying.
• The profile in air-dry conditions (Fig. 11 C') confirms the presence of a more important shrinkage in G-layer than in other layers.
• After the blade moving, due to recovery of these different stress states, softer and thinner layers lay above stiffer ones like the G-layer.
• Again, it can be argued that this separation allows a more complete release of growth stress in the G-layer, so that, this further differential shrinkage is another expression of growth stress only.

Figures

Fig. 6: AFM topographic images of same cells in water (A, A') and in air-dry condition (B, B'). Scale bars: 10 µm.

Fig. 7: Principle for calculation of topographic profiles

Fig. 8: SEM images and topographic profiles for one cell of beech (A, C, E) and poplar (B, D, F). A & B: normal to surface images; C & D: 70° oblique images. Scale bars: A & C: 2 µm, B: 10 µm, D: 5 µm. (CML: compound middle lamella).

Fig. 11: measurements on AFM topographic profiles of same poplar cells in water (A), in water after 2 hours in 80°C hot water (B), in air dry condition (C) and in water after drying (D). Vertical distance between S2 layer and G-layer: W: white cursors, G: grey cursors, B: black cursors.

Fig. 9: SEM images of both faces of poplar section with different scales and view angles. Some cells on the edge (arrow) permit to locate all other. A & C: face a, B, E & F: face b. Measurements are made on images like F (Scale bars: 20 µm).

Fig. 3: Growth stress measurement on standing beech tree, on 8 angular positions of trunk periphery. I: tree with regular low levels of growth stress, II: tree with a zone (Z) of very high tensile growth stress.

Fig. 10: differential restraints (µm) and total shrinkage (%) of G-layer for 14 cells.

Fig. 5: SEM images of poplar: A massive bloc, B thin section; scale bars: 100 µm.

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Shrinkage of the Gelatinous Layer of Poplar and Beech
Tension Wood
Bruno Clair, Bernard Thibaut
To cite this version:
Bruno Clair, Bernard Thibaut. Shrinkage of the Gelatinous Layer of Poplar and Beech Tension Wood.
IAWA Journal, Brill publishers, 2001, 22, pp.121-131. �10.1163/22941932-90000273�. �hal-00004542�

SHRINKAGE OF THE GELATINOUS LAYER OF
POPLAR AND BEECH TENSION WOOD
by
Bruno Clair & Bernard Thibaut
LMGC – Bois, Université Montpellier II, CC 081, Place E. Bataillon,
34095 Montpellier, France (e-mail: clair@lmgc.univ-montp2.fr).
Published in IAWA Journal, Vol. 22 (2), 2001: 121–131
SUMMARY
Macroscopic longitudinal shrinkage in beech or poplar tension wood
is higher than in normal wood. This shrinkage is the result of cell
walls layers mechanical interactions. In order to complete the basic
data with a view to modelling the cell wall, we are interested in
shrinkage differences between cell wall layers and especially of G-
layer in poplar and beech. Wood samples in green condition are cut
with a razor blade, and then dried before observation. SEM
observation shows longitudinal shrinkage much more important in
gelatinous layer than in other layers. AFM topographic images of
same cells, both in water and in air-dry conditions, confirm this result.
Measurements on thin sections allow quantitative results around 4.7 %
longitudinal shrinkage for G-layer.
Key words: cell wall, gelatinous layer, shrinkage, tension wood.
INTRODUCTION
Longitudinal shrinkage in wood
Like all other wood properties, hygroexpantion presents a very important
anisotropy. Between green condition and ovendry condition, shrinkage ranges from
0.05 % to 0.3 % in longitudinal direction, 3 % to 6 % in radial direction and from
6 % to 12 % in tangential one (Skaar 1988). According to these values, the
hygroexpension in axial direction is not apparently a problem for the user. However,
two cases exist when longitudinal shrinkage starts to be more important: in reaction
wood (tension wood of angiosperms and compression wood of gymnosperms) and
juvenile wood (Skaar 1988). In these two types of wood, axial shrinkage can reach
1 % or more (Nepveu 1994). For these woods, shrinkage value cannot be considered
as negligible, because wood beams have generally their longer distances in axial
direction. These important differences can be explained by the wood fibre structure.

2 IAWA Journal,
From wood fibre structure to shrinkage modelling
The knowledge of the wood cell structure, as a multi-layer fibre composite, allows
the modelling of the longitudinal shrinkage.
One of the first models, which is still a reference, is the Barber and Meylan 's
one (1964) refined by Barber (1968). This model considers that the cell wall is
reduced to S
2
layer. S
2
layer is described like an amorphous hygroscopic matrix in
which are imbedded parallel crystalline microfibrils which act to restrain
hygroexpention in the direction parallel to their axes (Fig. 1) (Cave 1972a). Thus,
microfibril angle is the determinant factor of longitudinal shrinkage. Low angle of
microfibril in relation to axial direction induces low axial shrinkage (like in normal
wood) and high angle allows a higher shrinkage (like in juvenile or compression
wood). Later, other models integrating other components properties (cellulose,
hemicellulose and lignin), changes in matrix behaviour during drying and
introducing the different cell wall layers have been proposed to refine this first
theory (Barrett et al. 1972; Cave 1972b, 1978; Sassus 1998; Gril et al. 1999;
Yamamoto 1999).
Matrix Microfibrils Woody mater
Fig. 1: schematic representation of the "reinforced matrix" (Sassus 1998)
These models give a good understanding of macroscopic axial shrinkage for
different values of microfibril angle, for normal, compression and juvenile wood.
However, they cannot explain the behaviour of tension wood with gelatinous
layer. In fact, in G layer, microfibril angle is very low or nil (Chaffey 2000), even
when macroscopic longitudinal shrinkage is high (Clarke 1937; Chow 1946; Sassus
1998). Norberg and Meier (1966) had isolated portion of G layer and said that they
do not show high longitudinal shrinkage. The G layer is generally loosened from S
2
layer and this latter one is very thin in tension wood. So these authors and Boyd
(1977) assume that in that case, longitudinal shrinkage is produced by S
1
layer, G
layer being unable to prevent it.
MATERIAL AND METHODS
One poplar (Populus cv I4551) and one beech (Fagus sylvatica L.), were chosen for
this study. These species are known to have characteristic tension wood with G layer
and a high macroscopic axial shrinkage.
Populus cv I4551
During the growing period, a young one year old poplar tree in a container is tilted
35° from the vertical. At the end of that period, the stem has nearly regained its
verticality by producing tension wood on the upper side (Fig. 2). Wood sample
taken from this tension wood zone have characteristic anatomical features
presenting a large amount of fibre with G layer and very thin S
2
layer (Fig. 4 A).

Clair G layer shrinkage 3
35°
Fig. 2: Recovery of the verticality of a poplar stem after the container have been
tilted 35°. Tension wood is produced on the upper side.
Fagus Sylvatica (L.)
A 150 years old tree was chosen after measurement of peripheral growth stresses at
breast height level on the standing tree, on eight positions around the trunk. This tree
was typical of a strongly dissymmetrical distribution of growth stresses (Fig. 3). A
high local level of growth stress is always related to presence of tension wood
(Trénard & Guéneau 1975; Sassus 1994). Wood sample were taken around the
highest values of growth stress (Z position on Fig. 3). In spite of large G layer in the
fibre cell wall, S
2
layer remains thicker than in poplar wood (Fig. 4 B).
0
50
100
150
200
250
0 45 90 135 180 225 270 315
angular position of trunk periphery (in degree)
DRLM (µm)
selected beech typical low stressed beech
II
I
Z
Fig. 3: Growth stress measurement on standing beech tree, on 8 angular positions of
trunk periphery. I: tree with regular low levels of growth stress, II: tree with a zone
(Z) of very high tensile growth stress.
Fig. 4: SEM observation of poplar (A) and beech (B) with gelatinous layer (G)
(also indicated S
2
layer) (Scale bar: 20 µm)
Tension
Wood
A
B
G
S
2

4 IAWA Journal,
Wood samples were stored in green condition before further processing into small
blocks or thin sections.
Massive blocks
Wood sticks (2 cm in longitudinal direction, section 5 x 5 mm²)
are cut up by splitting in order to guarantee a good axial direction. Sticks were then
cut to obtain 5 mm size cubes. Finally a last superficial planning is done manually
with a brand new razor blade in order to produce a nice transverse surface, the
sample being always kept in moist condition.
Thin sections
Transverse sections, 80 µm thick, were cut under water drop with a
microtome equipped with disposable razor blade. These sections were glued on the
edge with fibre direction parallel to support, in order to allow observations on
transverse sections on both sides of the sample.
Scanning electron microscopy
Massive blocks or thin sections are dehydrated with absolute ethanol, passed to
critical point and coated (300 Å of platinum) before observation. Thus, observations
are made in oven dry condition with a Cambridge S360 Scan Electron Microscope
(Fig. 5).
The tilting of receptor allows to obtain images of a same object for different view
angles.
Fig. 5: SEM images of poplar: A massive bloc, B thin section; scale bars: 100 µm.
Atomic force microscopy
Smaller massive blocks (500 x 500 x 500 µm
3
), prepared the same way as before,
are observed in their transversal section in water and in air-dry condition. Four states
are studied: green condition, green condition after 2 hours in 80°C water, air-dry
conditions, wet conditions after air-drying. Atomic Force Microscope (Dimension
3100, Nanoscope IIIa, Digital Instruments) was used to obtain topographic images
of a 50 x 50 µm² area (around 10 cells). The same cells are observed successively in
these conditions (Fig. 6).
A B