Contribution Of Gelatinous Layer To Tension Wood Behaviour In Poplar: Growth Stresses And Wood Material Properties

In this study,tension wood behaviours from microscopic to macroscopic levels were observed and measured compared to normal wood.Poplar(Populus cv.I4551 from France and Populus deltoids cv.I-69/55 from China) was chosen as plant material,because poplar is known to have a characteristic tension wood with G-layer and to produce a high longitudinal stress.Poplar is also a very important fast growth plantation tree in the world.Longitudinal growth stress was regarded as indication of tension wood severity and growth strain(GS) was used to indicate the growth stress level.Two methods were used in this study to measure the GS according to the different purposes:- for the microscopic study,the strain gauge method giving a direct estimate of the GS;- for the macroscopic study,the single hole drilling method providing with a "growth stress indicator"(GSI) approximately proportional to the GS.This study advances the discussion on the origin of growth stresses generation and reveals the growth stresses at cell wall and tissue levels.The measurements were performed on embedded samples sectioned with glass knife and diamond knife,which allows to avoid the uncontrolled swelling and detachment of G-layer during sample preparation with conventional sectioning method.From normal wood to different severities of tension wood,defined by measured growth strain,wood properties were measured including anatomical,physical,mechanical and chemical properties,as well as the properties of rotary cutting veneer including woolly area proportion and distortion,which are the two most important problems for poplar veneer utilizations.The relationships between these properties and growth stress level are established.Due to its high importance for wood utilizations,the transverse drying shrinkage of tension wood was studied in particular compared to that of normal wood at cell wall,tissue and massive wood level.A shrinking model is proposed for tension wood and normal wood. From this study,following conclusions can be drawn:- This study further confirmed the observations of Clair et al.(2005b;2005a) that G-layer observed with the conventional method of sectioning is in a swollen state and detached from S₂ layer,due to cutting end-effect artefacts.In living trees or massive tension wood in both wet and dry conditions,G-layer is well adhered with S₂ layer and not swollen.Thus for studying the real state and properties of G-layer,conventional sectioning method is not appropriate and embedded sample method,taking into account the distance of the sectioning location to the border,is propositional.- The adherence between G-layer and S₂ layer are not as strong as that among other layers,i.e.among compound middle lamella,primary layer,S₁,S₂ and S₃ layer in normal wood.But this adherence is strong enough not to be altered by the high transverse and longitudinal shrinkage of G-layer after drying of tension wood.It confirms the contribution of G-layer to the physical and mechanical behaviour of tension wood.- For the severe tension wood compared to mild tension wood as defined by the GSI measured at macroscopic level,higher longitudinal tensile stress was deduced in G-layer.- The highest growth stress values were located in the upper sides of the inclined trunks. Other growth stress values mostly distributed between those of upper and lower sides. Most of wood characteristics exhibited similar distributions around the inclined trunks, their correlation with GSI being either positive or negative.- In the samples examined,no G-fibres were observed for a GS up to 610μεwhile their surface ratio amounted to 50%or more from 816με,suggesting a hypothetical threshold for G-fibres occurrence between these two GS values.Almost 100%of the fibres contained G-fibres above another hypothetical GS threshold between 1540 and 193.5με.- In the samples examined,more G-fibres per unit of tissue area and thicker G-layer accompany higher longitudinal growth stress(proportional to GS) in tension wood with G-fibres.It suggests that these factors contribute to the growth stress generation and hereinto G-layer plays the most important role in high growth stress generation,which supposedly can be explained by the hypothesis that the tensile stress of microfibrils governs the longitudinal tensile stress in tension wood(Bamber 1978;Okuyama et al. 1986;Bamber 1987;Clair et al.2006a).- The thickness of normal wood cell wall was notably lower than that of the tension wood cell wall including G-layer but markedly larger than that of the other layers excluding G-layer.In tension wood the thickness of G-layer and other layers increased and decreased,respectively,with the increase of growth stress.Negative and positive correlations were found for cell diameter and fibre length,respectively,against growth stress level.Fibre and vessel proportion increased and decreased,respectively,with the increase of growth stress.- The drying shrinkage,measured as a relative thickness decrease,was significantly higher for G-layer than other layers.There were no significant correlation between GS and G-layer or all layers shrinkage,but a negative one was observed with other layers shrinkage.- The absence of relationship between G-layer drying shrinkage and GS tend to prove that G-layer would be nor qualitatively different from low GS to high GS and would confirms the ideas that just the quantity of G-layer would be the driving force of growth strain,i.e.growth stress.- In G-fibre,lumen size increased during drying and this increase was positively related with GS,but in normal wood fibre lumen size decreased during drying in our observations.These findings suggest that G-layer shrank outwards(i.e.its internal perimeter increases),so that its shrinkage weakly affected the total cell shrinkage and the mesoscopic shrinkage was controlled by the other layers shrinkage which shrank inwards(i.e.its external perimeter decreases).- At both tissue and macroscopic levels,based on the observations on sections and massive wood,respectively,tangential shrinkage was notably higher than radial one both in normal and tension wood.The shrinkages in tangential and longitudinal directions were negatively and positively,respectively,correlated with GS/GSI,while no clear relation was found between radial shrinkage and GS/GSI.- GSI was significantly but weakly correlated positively with basic density and negatively with FSP.With the increase of GSI,MOE and specific MOE increased, compressive strength and specific compressive strength decreased.From normal wood to tension wood the colour of wood became less reddish and less yellow,and darker specimens tend to approach the b^* axis.- The cellulose content rose to a maximum in the upper sides where highest GSI values were measured,and the opposite for the lignin content.- Severe veneer distortion and woolly phenomena happened in the veneers located in the upper sides of inclined trunks.In the lower sides the distortion and woolly phenomena were negligible.Positive correlations were found for them with growth stress level.