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Tree Physiology Advance Access originally published online on September 28, 2009
Tree Physiology 2009 29(11):1419-1431; doi:10.1093/treephys/tpp077
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© The Author 2009
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses?by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Shrinkage processes in standard-size Norway spruce wood specimens with different vulnerability to cavitation

Sabine Rosner1,2, Bo Karlsson3, Johannes Konnerth4 and Christian Hansmann5

1 Department of Integrative Biology, Institute of Botany, University of Natural Resources and Applied Life Sciences, BOKU Vienna, Gregor Mendel-Str. 33, A-1180 Vienna, Austria
2 Corresponding author (sabine.rosner{at}boku.ac.at)
3 The Forestry Research Institute of Sweden (Skogforsk), Ekebo, S-26890 Svalöv, Sweden
4 Department of Material Sciences and Process Technology, Institute of Wood Science, University of Natural Resources and Applied Life Sciences, BOKU Vienna, Gregor Mendel-Str. 33, A-1180 Vienna, Austria
5 Competence Centre for Wood Composites and Wood Chemistry, St.-Peter-Str. 25, A-4021 Linz, Austria


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Funding
 References
 
The aim of this study was to observe the radial shrinkage of Norway spruce [Picea abies (L. Karst.)] trunkwood specimens with different hydraulic vulnerability to cavitation from the fully saturated state until the overall shrinkage reaches a stable value, and to relate wood shrinkage and recovery from shrinkage to cavitations of the water column inside the tracheids. Radial shrinkage processes in standard-size sapwood specimens (6 mm x 6 mm x 100 mm; radial, tangential and longitudinal) obtained at different positions within the trunk, representing different ages of the cambium, were compared. Cavitation events were assessed by acoustic emission (AE) testing, hydraulic vulnerability by the AE feature analysis and shrinkage was calculated from the changes in contact pressure between the 150 kHz AE transducer and the wood specimen. Two shrinkage processes were observed in both juvenile (annual rings 1 and 2) and mature wood (annual rings 17–19), the first one termed tension shrinkage and the second one cell wall shrinkage process, which started when most of the tracheids reached relative water contents below fiber saturation. Maximum tension shrinkage coincided with high-energy AEs, and the periods of shrinkage recovery could be traced to tension release due to cavitation. Juvenile wood, which was less sensitive to cavitation, had lower earlywood tracheid diameters and was less prone to deformation due to tensile strain than mature wood, showed a lower cell wall shrinkage, and thus total shrinkage. Earlywood lumen diameters and maximum tension shrinkage were strongly positively related to each other, meaning that bigger tracheids are more prone to deformation at the same water tension than the smaller tracheids.

Keywords: acoustic emission testing, conduit reinforcement, tensile strain, wood shrinkage

Received February 17, 2009; Accepted August 12, 2009


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Funding
 References
 
Cell wall shrinkage starts when a conduit reaches moisture contents below the fiber saturation point. Fiber saturation is reached when the cell lumina no longer contain free water, but cell walls are fully saturated with liquid (Stamm 1971, Skaar 1988, Berry and Roderick 2005). However, before most of the tracheids reach fiber saturation, volumetric changes can be observed. This shrinkage (hereafter termed tension shrinkage) is a consequence of adhesive forces between the water molecules and the inner conduit walls when xylem water of trees is transported under tension (Kozlowski and Winget 1964, Kozlowski 1972, Irvine and Grace 1997, Cochard et al. 2001, Offenthaler et al. 2001, Perré 2007). Structural demands on the conduit walls to withstand implosion when water is transported under tension are becoming increasingly better known (Innes 1995, Hacke and Sperry 2001, Hacke et al. 2001, Hunter 2001, Pitterman et al. 2006). There is, however, no experimental study reported in which wood shrinkage above (tension shrinkage) and below fiber saturation (cell wall shrinkage) was assessed on wood with different vulnerability to cavitation.

According to the cohesion theory, the continuous water column in the soil–plant continuum is subjected to fluctuating hydrostatic pressures with a negative sign (i.e., tensions). This water column extends from the small-diameter channels in leaf parenchyma cell walls to the much wider xylem conduits and from there via root parenchyma walls to the soil pores (e.g., refer to Zimmermann 1983). Tensile stresses inside the conduits induce diurnal stem diameter changes that are strongly related to the sap flow rate and xylem water potential (Neher 1993, Herzog et al. 1996, Cochard 2001, Offenthaler et al. 2001, Conejero et al. 2007). The diurnal stem diameter changes are therefore attributed not only to bark tissues but also to sapwood (Offenthaler et al. 2001, Perämäki et al. 2001, Ueda and Shibata 2001, 2002, Zweifel et al. 2000, 2001, Gall et al. 2002, Alméras et al. 2006). Tension shrinkage induced by tensile stress can also be observed on debarked dehydrating wood boles (Irvine and Grace 1997) and on wood beams (Rosner 2007). The relationship between tensile strain and changes in stem water potential is supposed to be influenced by the elastic properties of the swelling tissues (Neher 1993, Irvine and Grace 1997, Alméras and Gril 2007, Alméras 2008).

Tension shrinkage should be reversed not only by an increase in water potential (Irvine and Grace 1997) but also by the tension release after cavitation (Offenthaler et al. 2001, Hölttä et al. 2002, 2005). Although the origin of acoustic emission (AE) in the high-frequency range (> 15 kHz) in live stems is still unclear (Zweifel and Zeugin 2008), the bulk of AEs of dehydrating wood is supposed to be induced by the tension release in the conduit lumen when liquid water at negative pressure is replaced by water vapor very near to vacuum pressure (Sandford and Grace 1985, Tyree and Sperry 1989a, 1989b). Recently, Rosner et al. (2006) found experimental evidence for the theoretical postulate that dehydrating wood with bigger conduits emits stronger acoustic signals (Milburn 1973, Sandford and Grace 1985, Ritman and Milburn 1988, 1991). AE feature analysis was found to be a reliable method to quantify the loss in hydraulic conductivity, and thus vulnerability to cavitation, because waveform characteristics can provide information about the structural properties of wood. The number of AEs and the number of tracheids are strongly linearly related in Norway spruce sapwood. A high number of AEs from a given volume are associated with a lower mean AE energy, which is a waveform feature that is derived from signal amplitude and duration (Rosner et al. 2006).

Within a conifer tree trunk, a higher number of tracheids per given volume imply smaller lumen diameters. An increase in lumen diameter with age will, however, not go along with a 1:1 increase in wall thickness (Hannrup et al. 2004, Pitterman et al. 2006, Sperry et al. 2006), resulting in lower double wall to lumen diameter (span) ratios. According to Hacke et al. (2001), tensile stresses in a water filled conduit increase with decreasing double wall to span ratio, based on the assumption that both mechanical strength and stiffness increase with wood density (Pitterman et al. 2006, Sperry et al. 2006, Domec et al. 2009). Secondary xylem (wood) of different species has evolved to resist implosion under the negative tension that can be reached in live stems during water transport (Raven 1977). Primary xylem of needles is, however, prone to reversible cell wall collapse under natural field conditions (Cochard et al. 2004, Brodribb and Holbrook 2005). Irreversible implosion of wood conduits due to tension forces has been observed under artificial lumber drying conditions (e.g., Booker 1994, Innes 1995, Hunter 2001).

Safety from cavitation is not necessarily causally linked to safety from implosion, because the two phenomena are localized at different wall regions. A lower vulnerability to cavitation implies, however, the need for a more safe cell design with thicker walls to resist implosion, because the cell walls have to withstand higher negative xylem pressures before cavitation occurs (Hacke and Sperry 2001, Hacke et al. 2001). If, as hypothesized, tracheids with a big lumen diameter are more prone to deformation, then more elastic energy has to be stored in bigger conduits under the same tension. The energy released after cavitation should therefore increase with increasing conduit diameter (Milburn 1973, Sandford and Grace 1985, Ritman and Milburn 1988, 1991). Using a testing method introduced by Rosner (2007), AEs and wood shrinkage processes can be simultaneously measured on the same wood surface area. Tensile strain and possible regeneration stages due to cavitations can be pinpointed to cavitations by counting AEs and to the corresponding loss in hydraulic conductivity by analyzing the course of the cumulative AE energy (Rosner et al. 2006). In Norway spruce trunkwood, 50% of the cumulative AE energy is a much better estimate for 50% loss of hydraulic conductivity than the cumulated AE percentage per se.

The reversibility of tension shrinkage due to cavitation should, however, be overlaid by cell wall shrinkage when the first tracheids reach fiber saturation. The contraction of the conduit cell wall is controlled by the mechanisms described in the reinforced matrix hypothesis (Barber and Meylan 1964): crystalline microfibrils are embedded in an amorphous matrix. When the matrix loses water, it tends to contract isotropically but in resisting this change the microfibrils deform the matrix. Cell wall shrinkage processes at moisture contents below fiber saturation might also be of biological relevance, when one thinks about stress generation during heartwood formation (Bernhart 1965, Berry and Roderick 2005) or in and around the gas-filled voids reported from sapwood and heartwood of live stems (Gartner et al. 2004).

This study is the first attempt to relate cavitations and the corresponding loss in hydraulic conductivity assessed by AE testing to shrinkage and shrinkage recovery processes in wood specimens with different wood structure and hydraulic vulnerability. Shrinkage processes of hydraulically less vulnerable juvenile and more vulnerable mature Norway spruce trunkwood (Rosner et al. 2006) were compared. An increase in hydraulic safety against cavitation should go along with a higher safety against implosion (Hacke and Sperry 2001, Hacke et al. 2001). It was, therefore, hypothesized that wood less prone to implosion should show a lower tensile strain when the water columns are under the same tension. We based possible structure–function relationships on the mechanically weakest part of wood, on earlywood (Müller et al. 2003), because we expected stronger relationships between tensile strain and tracheid dimensions (Kozlowski 1972, Caspari and Sachsse 1990, Cherubini et al. 1997, Grabner et al. 2006). If we hypothesize that earlywood tracheids with bigger lumen diameters and lower wall to span ratios are more prone to deformation under the same tension, big lumen tracheids should store more elastic energy, which is suddenly released when cavitation occurs.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Funding
 References
 
Plant material, sample harvesting and storage
Wood samples were obtained from 25-year-old Norway spruces [Picea abies (L.) Karst.] of a clonal trial in Vissefjärda (Southern Sweden, 56°54', longitude 15°53'; 60 m a.s.l.). Four ramets, in each case, of three fast-growing clones were selected. Clones showed no significant differences in tree height (10.7 ± 0.4 m, n = 12) and diameter at breast height (123 ± 7 mm, n = 12).

Trees were harvested during a wet period in June 2004. Wood samples, 0.2–0.3 m in length, were taken immediately after felling at the tree top (second internode) and at 1.3 m height from the ground. The samples were thus taken from the same individuals but at different heights, representing different ages of the cambium. During transport to the laboratory, the samples were kept wet in plastic bags containing some fresh water. Wood bole segments were debarked and split along the grain. An outer sapwood zone of 20 mm was separated from the split samples, and the specimens were stored frozen at –18 °C for a few weeks until further preparation steps (Mayr et al. 2003, Rosner et al. 2006).

Preparation of standard beams
Wood samples were thawed in fresh water for at least 3 h. Outer sapwood samples with a transverse surface of about 9 x 9 mm were isolated by splitting the wood along the grain with a chisel. Tangential and radial faces of the beams were planed on a sliding microtome. The samples were shortened on a band saw, and the sample ends were re-cut using a razor blade. During all these steps, the wood samples were kept wet. They were then soaked in distilled water under vacuum for 48 h to refill embolized tracheids and afterward stored at 4 °C in degassed water containing 0.01 vol.% Micropur (Katadyn Products Inc., Switzerland) to prevent microbial growth. The final standard shape of the samples was 6 mm tangential, 6 mm radial and 100 mm longitudinal. Standard beams were produced from samples taken at 1 m from the ground containing annual rings 17–19 (mature wood) and at the tree top comprising annual rings 1 and 2 (juvenile wood).

AE and shrinkage testing
AEs were monitored using the µDiSPTM Digital AE system from Physical Acoustics Corporation (Princeton Jct, PA). Preamplifiers (40 dB) were used in connection with resonant 150 kHz R15C transducers over a standard frequency range of 50–200 kHz. Data in a frequency range between 100 and 1 MHz were recorded with a detection threshold of 30 dB (0 dB = 1 µV input). The detection threshold was chosen as the peak amplitude of signals produced by waving the AE transducer in air, plus 10 dB. Extraction of features such as the peak amplitude (dB), the AE duration (µs) and the relative AE energy (pVs) of each AE signal was carried out using AE Win® software (Physical Acoustics Corporation). Relative AE energy (also referred to as ‘PAC-Energy’) is defined as the area of the rectified voltage signal over the duration of the AE signal.

AE transducers were positioned on the tangential face of fully saturated standard beams using an acrylic resin clamp (Figure 1). Silicone paste (Wacker, Burghausen, Germany) served as a coupling agent. The sample was positioned on an acrylic resin plate fixed upon the compression spring. The compression spring below the sample was used to minimize the decrease in contact pressure during the wood shrinkage processes. Wood shrinkage was assessed by a load cell (DMS, Type 8416-5500, range 0–500 N; amplification with an inline amplifier for DMS, Type 9235; Burster, Gernsbach, Germany) between the AE transducer and the screw of the acrylic resin clamp. The empty clamp assemblage was kept so deep in water such that water coverage of the sample was guaranteed to avoid evaporative loss during mounting. The sample was then positioned on the sample holder, and the contact pressure between transducer and wood was set to 30 N (Jackson and Grace 1996, Beall 2002). When the applied pressure reached a stable value, which was achieved after < 20 min (Cheng et al. 2004), superficial water was quickly removed from the clamp and the wood specimen, and the recording of AEs and coupling pressure was started. Water loss of the standard beams at ambient temperature (25 °C, 30% r.h.) was quantified every 10 min by placing the whole clamp assemblage on a balance (resolution 10–3 g, Sartorius, Göttingen, Germany). AE and shrinkage testing were done until the contact pressure reached a constant value, which took about 24 h. Wood shrinkage caused a decrease in contact pressure of up to 4 N. The loss of contact pressure within this small range had neither influence on total AE nor on AE energy, as tested in pre-trials. Contact pressure and compression spring also had no influence on the course of radial shrinkage as verified in post-trials (Figure 2) using a video extensometer (ME46, Messphysik, Fürstenfeld, Austria) equipped with a b/w high-resolution CCD-videocamera (OS-65D, Mintron Enterprise Co., Ltd., Taipei, Taiwan). Absolute shrinkage (µm) could therefore be calculated by relating the total radial shrinkage (digital gauge, accuracy 1 µm, Mitutoyo Corporation, Japan) to the total coupling pressure decrease using the following equation:


Formula 1

(1)
where the absolute shrinkage at time t is the difference between the radial dimension at the beginning of dehydration (Dsat) and the radial dimension at time t (Dt). Dend is the radial dimension at the end of dehydration, Psat is the contact pressure at full saturation, Pt at time t and Pend at the end of dehydration. Cumulative radial shrinkage (given in negative percent values) was calculated for 10-min time steps. The shrinkage rate/10 min is the percent shrinkage from one-to-the-next 10-min time step.


Figure 1
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  		Figure 1. Acrylic resin clamp assemblage for combined AE and shrinkage testing. The AE transducer (B) was positioned on the tangential face of a fully saturated standard beam [6 mm (radial) x 6 mm (tangential) x 100 mm (longitudinal)]. The wood specimen (C) was placed on an acrylic resin plate (20 x 20 mm) fixed upon the compression spring (D). Wood shrinkage was assessed by means of a load cell (A) between the AE transducer (B) and the screw of the acrylic resin clamp.

 

Figure 2
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  		Figure 2. Time course of the cumulative radial shrinkage of mature wood standard beams [6 mm (radial) x 6 mm (tangential) x 100 mm (longitudinal)] assessed by video-extensometer. Above are seven photographs of a 1 mm longitudinal window of the surface of a rectangular cut to the tangential wood direction (almost radial). The same 1 mm longitudinal window is shown at different dehydration stages that are indicated by different letters. The whole radial shrinkage course assessed by video-extensometer is given in the figure below. Letters shown in this graph correspond to time steps (A–G) when photographs of the same 1 mm longitudinal window were taken. The reference bar represents 1 mm.

 
Dry mass of the wood beams was obtained by drying the wood samples at 103 °C to constant weight to calculate the relative water loss (R) and the basic density (volume in the green state/dry mass). Shrinkage rate and cumulative radial shrinkage/10 min were referenced to the nearest 5% R step and thereafter by cubic power functions to 0.5 MPa steps, as described below. Analysis of contact pressure and AE data filtering was done using Vallen VisualAETM software (Vallen Systeme GmbH, Munich, Germany). AE signals that passed the detection threshold only once or twice were interpreted by their waveform as signals that originated from the system itself or from the background noise and were excluded from the analysis. A mean AE energy value was calculated for 1- and 10-min time steps. Relative cumulative AE energy data were obtained by relating the stepwise cumulative mean AE energy values/10 min to the sum of all mean AE energy values/10 min. Relative water loss during dehydration of the standard beams and AE signals were related to each other in 10 min time steps. Clustered AE data and mean AE energy/10 min were referenced to the nearest 5% R and to 0.5 MPa steps to allow statistical analysis. The relationship between R and positive pressure applied was assessed on a parallel sample set (see the next chapter). Vulnerability curves based on the relative cumulative AE energies/10 min related to relative water loss or calculated positive pressure were fitted by cubic functions (Rosner et al. 2006).

We detected 0.6–1.7 AEs x 10–6 in a single specimen; even in small samples, it was therefore impossible to relate single AEs and their waveform characteristics to elastic deformation. To allow statistical analysis, we used the parameter ‘maximum mean AE energy/min’, corresponding to the maximum value of the mean AE energy calculated for 1-min time steps during a dehydration cycle.

Calculation of positive pressure for AE and shrinkage tests based on relative water loss
Relative water loss induced by positive pressure was determined on a parallel sample set (Rosner et al. 2006). After soaking and weighing, air pressure was applied to the lateral sides of the samples, while the transverse ends protruded from a double-ended pressure chamber (PMS Instruments Co., Corvallis, OR), to induce cavitation. Following pressure treatment, the samples were weighed again. Initially, the pressure chamber was pressurized to 0.5 MPa, and the pressure was subsequently increased in steps of 0.5 MPa until > 95% loss of hydraulic conductivity was reached (Rosner et al. 2006). Juvenile wood samples were therefore pressurized up to 7 MPa (n = 6) and mature samples only up to 5 MPa (n = 12). Cubic functions calculated for juvenile [Eq. (2), r2 = 0.99, P < 0.001] and mature wood samples [Eq. (3), r2 = 0.99, P < 0.001] were used to relate R (%) of AE and shrinkage testing specimens to positive pressure applied [P (MPa)]:


Formula 2

(2)


Formula 3

(3)

Anatomical investigations
Wall and lumen diameters were measured using a Leica DM4000M microscope interfaced with a digital camera and Leica image analysis software (Leica Microsystems Wetzlar GmbH, Germany). Earlywood tracheid dimensions were measured on microtomed transverse cut faces. The wall reinforcement parameter (t/b)2 was calculated from radial lumen diameters (b) and thickness of the tangential double cell wall (t) (Hacke et al. 2001) of the earlywood formed early in the growing season (first 10 cell rows), because stronger relationships between tensile strain and tracheid dimensions were expected (Kozlowski 1972, Caspari and Sachsse 1990, Cherubini et al. 1997, Müller et al. 2003, Grabner et al. 2006).

Specimen number and statistics
AE feature extraction and shrinkage assessment were done for 12 juvenile and 12 mature standard beams, originating from four ramets of three different clones. Relative water loss induced by positive pressure was assessed on a parallel sample set, on 6 juvenile and 12 mature wood beams. Mean values of anatomical parameters were derived from 40 single measurements per standard beam, respectively. Statistical analysis was carried out using SPSS® 11.0. The values are given as mean ± standard error (SE). Mean values were tested for significance with the t test for independent samples, after analysis of variance and checking for normal distribution. Associations between two variables were examined using linear or nonlinear regression analysis. Differences between mean values and relationships were accepted as significant if P was < 0.05.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Funding
 References
 
Vulnerability to cavitation assessed by AE feature analysis
Juvenile wood was less vulnerable to cavitation than mature wood. Fifty percent of cumulative AE energy/10 min was reached after 51.4 ± 2.9% R in juvenile wood and after 41.0 ± 1.9% R in mature wood (P < 0.01, Figure 3B), which corresponded to 4.1 ± 0.1 and 2.4 ± 0.1 MPa, as derived from Eqs. (2) and (3), respectively (P < 0.001, Figure 3F). Fifty percent of cumulative AE was reached in juvenile and mature wood at Rs corresponding to 5.8 ± 0.2 and 4.6 ± 0.3 MPa, respectively (Figure 3E).


Figure 3
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  		Figure 3. Courses of the cumulative relative AE rate (A, E and I), relative cumulative AE Energy (B, F and J), mean AE Energy/10 min (C, G and K) and cumulative radial shrinkage (D, H and L) at different stages of relative water loss (R), positive pressure applied [calculated from Eqs. (2) and (3)] and time. Error bars show 1 SE. Dotted lines and open circles represent mature wood, closed circles and solid lines juvenile wood (n = 12, respectively).

 
Mean AE energy/10 min was significantly higher in mature than in juvenile wood throughout almost the whole dehydration period (Figure 3C). Maximum differences in the mean AE energy/10 min were reached at R corresponding to 2 MPa (P < 0.001, Figure 3G).

Course of cumulative AEs, AE energy and shrinkage
Tension shrinkage started right from the beginning of dehydration, even before any AEs were detected, and temporarily stabilized when the first AEs were detected (Figure 4A–C). Thereafter, cumulative shrinkage reached a steeper slope at 10.8 ± 1.3% R in mature wood and at 21.4 ± 1.7% R in juvenile wood (P < 0.001, Figure 4F), corresponding to 1.3 ± 0.1 and 2.7 ± 0.1 MPa overpressure, respectively. Tension shrinkage rate started to decrease (became less negative) shortly after very high AE energies were measured (Figure 4D and E), in mature wood at 24.5 ± 1.3% R and in juvenile wood at 29.7 ± 1.3% R (P < 0.01), which corresponded to 2.0 ± 0.1 and 3.1 ± 0.1 MPa, respectively. The decrease in the shrinkage rate led to a measurable decrease in the cumulative tension shrinkage at 54.0 ± 1.9% R in mature wood and at 58.9 ± 2.5% R in juvenile wood (Figure 3D), corresponding to 3.0 ± 0.1 and 5.0 ± 0.3 MPa overpressure, respectively (P < 0.001, Figure 3H). At maximum cumulative tension shrinkage, only about 35% of the cumulative AEs were measured, but in mature wood it had already reached 65.1 ± 2.0% and in juvenile wood 54.4 ± 2.0% of the cumulative AE energy (P < 0.05, Figure 3A–D). Maximum cumulative tension shrinkage was > 3 times higher in mature than in juvenile wood (Table 1). Tension shrinkage was reversible only to some extent, amounting to 19.6 ± 5.1% in juvenile wood and 32.9 ± 3.1% in mature wood (P < 0.05).


Figure 4
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  		Figure 4. Courses of the mean AE energy/10 min (A and D), radial shrinkage rate/10 min (B and E) and cumulative radial shrinkage (C and F) at 10-min time steps and different stages of relative water loss (R). Error bars show 1 SE. Dotted lines and open circles represent mature wood, closed circles and solid lines juvenile wood (n = 12, respectively).

 

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  		Table 1. Maximum cumulative tension shrinkage (shrink max), radial lumen diameters (b) and thickness of tangential double cell walls (t) of earlywood tracheids, the ratio between the tangential double cell wall and the radial span (t/b)2, the total number of acoustic emissions (AEs), maximum mean AE energy/min, of juvenile (J) and mature wood (M) specimens. Mean values were tested for significant differences by Student’s t test.

 
Cell wall shrinkage started after the shrinkage recovery peak, in juvenile wood at 80.2 ± 1.6% R and in mature wood at 81.5 ± 1.4% R (Figure 3D), corresponding to moisture contents of 35.5 ± 2.0% and 37.4 ± 1.9%, respectively. At this stage, > 80% of cumulative AE (Figure 3I) and AE energy (Figure 3J), but very weak AEs (Figure 3K) were detected in both juvenile and mature wood.

Mature wood showed almost a two-times higher total shrinkage than juvenile wood (–3.9 ± 0.1%, –2.3 ± 0.2%, P < 0.001, Figure 3L).

Relationships between AE-, shrinkage- and structural parameters
Radial earlywood tracheid lumen diameters and tangential double cell walls were significantly smaller, whereas the parameter (t/b)2 was significantly higher in juvenile than in mature wood specimens (Table 1). Juvenile wood beams emitted much more AEs than mature wood beams, and maximum AE energy/min was lower in juvenile than in mature wood beams (Table 1). The total number of AEs was strongly negatively related to radial earlywood tracheid diameters (r2 = 0.83, P < 0.001) and maximum cumulative tension shrinkage (r2 = 0.76, P < 0.001) by quadratic functions. Maximum tension shrinkage increased with increasing lumen diameters (Figure 5B), but decreased with increasing conduit wall reinforcement (Figure 5D). Maximum AE energy/min was negatively related to the total number of AEs (r2 = 0.88, P < 0.001) and to wall reinforcement (Figure 5C) and positively to maximum tension shrinkage (r2 = 0.77, P < 0.001) and radial lumen diameters in earlywood (Figure 5A).


Figure 5
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  		Figure 5. Maximum mean AE energy/min and maximum cumulative tension shrinkage related by linear (A) and quadratic power functions (B–D) to radial lumen diameter and conduit wall reinforcement (t/b)2. Closed circles represent juvenile wood, open symbols mature wood.

 
Basic density had similar values in juvenile and mature wood (0.39 ± 0.01 g cm3), and was therefore not related to total AEs, AE energy/min and tension or total shrinkage across cambial age. In mature wood samples, density was, however, positively related to total AEs (r2 = 0.37, P < 0.05) and negatively to maximum AE energy/min (r2 = 0.36, P < 0.05). Juvenile as well as mature wood with a higher density showed a lower maximum tension shrinkage (r2 = 0.64, P < 0.001; r2 = 0.32, P = 0.05, respectively), but no such relationship was found across the cambial age. An increase in the total shrinkage with decreasing basic density was found only in juvenile wood (r2 = 0.34, P < 0.05).


   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Funding
 References
 
Tension shrinkage, cavitations and tension recovery
The bulk of acoustic signals (AEs) in the high-frequency range emitted from dehydrating wood are supposed to result from cavitations (Tyree et al. 1984, Kawamoto and Williams 2002). Before cavitation, dehydration stress generates tensile strains inside the most vulnerable tracheids (Kozlowski 1972, Irvine and Grace 1997, Offenthaler et al. 2001). Further dehydration stress induces cavitations, which should release tensile stresses inside the tracheids (Offenthaler et al. 2001). In live Pinus sylvestris L. stems, the rate of cumulative AEs increases, however, almost simultaneously with periods of diurnal stem diameter decrease (Hölttä et al. 2005). Tension release in live stems can either take place as a result of water release from embolized conduits, or because of a decrease in transpiration (Hölttä et al. 2005). Cavitation itself can prevent further cavitation in other conduits at a short time scale due to the tension release in the transpiration stream (Hölttä et al. 2009). In dehydrating sapwood specimens, tension shrinkage can only be reversed by a local tension stress release after cavitation, when water is drawn out of the conduit into the surrounding wood. We observed a transient stabilization of the shrinkage rate due to cavitation of a very small percentage of tracheids already at the beginning of dehydration. Thereafter, the cumulative shrinkage increased when the AE rate reached a steeper slope, as observed in live stems (Hölttä et al. 2005), but started to recover when the AE rate showed a further increase.

The course of the mean AE energy gives additional information about the cavitation of hydraulically relevant tracheids (Rosner et al. 2006): shortly after the AE energies reach maximum values, the spruce wood sample reaches 50% of the cumulative AE energy, which coincides well with the 50% loss of hydraulic conductivity. Mean AE energy decreases thereafter, although attenuation of AE in wood decreases at high moisture losses (Tyree and Sperry 1989b, Rosner, unpublished data). It is supposed that the high-energy AEs result from cavitations of more vulnerable, large diameter, tracheids (Rosner et al. 2006). Within a species, vulnerability of a tracheid to embolism is a direct function of its diameter and pit pore characteristics (Lo Gullo and Salleo 1991, Hacke and Sperry 2001, Mayr et al. 2002); therefore, earlywood tracheids are supposed to be the first to cavitate in live stems (Tyree and Sperry 1989a). Low-energy AEs measured at very low water losses in our standard-size specimens did not necessarily represent cavitations of the most vulnerable tracheids but of the tracheids that are near the specimen surface (Rosner et al. 2006). AE energy showed a rapid increase when the shrinkage rate continuously reached a steeper slope. We therefore assume that the periods of rapid tension shrinkage coincided with the cavitation of the most vulnerable, presumably high-diameter tracheids.

In mature, live Norway spruce trunks, Offenthaler et al. (2001) observed an increase in the rate of cumulative tension shrinkage when water potentials dropped below the cavitation threshold of –1.9 MPa, although they expected a decrease in shrinkage due to tension release induced by cavitation. The authors explained this discrepancy with the development of radial gradients in the water potential together with the limited measuring depth of the stem psychrometer attached to the xylem surface. In our small mature Norway spruce wood specimens, tension shrinkage rate started already to decrease at 2 MPa, when only 10% of the total AE (cavitations), but already very high mean AE energies, had been measured. Cumulative recovery from tension shrinkage started, however, at higher relative water losses than one would expect (above 50% R). Tension recovery of the cavitated wood parts was masked by simultaneous tension shrinkage in other, less vulnerable, wood parts that still contained free water. When cumulative shrinkage started to decrease, only about 35% of cumulative cavitations had been measured. Although more than 50% of hydraulic conductivity was already lost by then (Rosner et al. 2006), the bulk of cavitations followed thereafter. Tension shrinkage and recovery from the shrinkage occurred therefore at different locations at the same time in the numerous conduits within a standard-size wood specimen. Simultaneous shrinkage and recovery within a radial window might be the reason why there is almost no time lag between periods of stem diameter decrease and increase in cumulative cavitations in live stems (Offenthaler et al. 2001, Hölttä et al. 2005).

During late dehydration stages, at moisture contents below 35–37%, the reversibility of tension shrinkage was markedly overlaid by cell wall shrinkage. This moisture content range corresponds exactly to the fiber saturation point given for spruce species (Bernhart 1965), which is about 10% higher for never-dried than for re-soaked wood (Stamm 1971). The fiber saturation point refers to the moisture content of a cell and not to a piece of wood, and it may therefore not be the same throughout a whole wood specimen (Skaar 1988). During late dehydration stages, when the drier shell of the specimen had already reached water contents below saturation, cell wall contraction could therefore additionally mask recovery from tension. The partial reversibility of tension shrinkage indicates, however, that the first shrinkage process was not induced by a shell–core effect, meaning that the shrinkage at moderate water losses in standard-size wood specimens was not entirely induced by cell wall shrinkage of the shell at water contents below fiber saturation, but by tensile stresses due to water tension. Reversibility of tension shrinkage was also not due to cell wall rewetting of the outer parts of the specimen by movement of free water from the inner wood parts, because Tarmian et al. (2009) recently showed that moisture contents near the sample surface of Norway spruce wood do not increase during a dehydration cycle.

Wood structure, hydraulic vulnerability and tension shrinkage
Kozlowski and Winget (1964) supposed that the differences in diurnal tensile strain between species and within tree trunks are traceable to the differences in internal water stress, wood structure, effective radius measured, bark moisture, dendrograph pressure and variations in the patterns of upward water transport in the stem. In this study, juvenile Norway spruce wood was less vulnerable to cavitation than mature wood, because 50% of the cumulated AE energy was reached at lower water potentials (Rosner et al. 2006). A lower vulnerability to cavitation implies a safer design for resisting implosion, because the cell walls have to withstand a higher tensile strain before cavitation occurs (Hacke et al. 2001, Domec et al. 2009). Accordingly, hydraulically less safe mature wood under the same tension was more prone to deformation than juvenile wood.

The mechanisms leading to ultrasound production due to cavitation in wood are not yet well understood (Kawamoto and Williams 2002, Zweifel and Zeugin 2008). It is supposed that more elastic energy is stored in bigger conduits when under tension (Milburn 1973, Sandford and Grace 1985, Ritman and Milburn 1988, 1991, Rosner et al. 2006), which is suddenly released when the cell wall relaxes due to cavitation. Accordingly, maximum mean AE energy/min had higher values in specimens with bigger radial lumen diameters of the first formed earlywood cell rows and with maximum cumulative tension shrinkage. These results suggest that bigger conduits show a higher deformation under tensile stress and are thus more susceptible to implosion. Especially the thin-walled, low-density earlywood and the poorly developed latewood of fast-growing trees were found to be prone to implosion and internal checking under tensile strain (Kozlowski 1972, Caspari and Sachsse 1990, Booker 1994, Cherubini et al. 1997, Rozenberg et al. 2002, Ball et al. 2005, Grabner et al. 2006). In radial compression tests, Norway spruce wood also ‘fails’ in the weakest tracheid layers, in the earlywood cells at the ring border or in the first 3–10 earlywood cell rows from the beginning of a growth ring (Müller et al. 2003). The reinforcement against collapse due to tension forces increases when the thickness of the double cell wall (t) increases relatively to its span (b) (Hacke et al. 2001). The increase in earlywood lumen diameter with cambial age did not go along with a 1:1 increase in wall thickness (Table 1). The (t/b)2 ratio was therefore much higher in juvenile than in mature earlywood. The age-dependent decrease of the (t/b)2 ratio reveals that juvenile trunkwood from the tree top is designed to resist implosion under high tensions (Domec et al. 2009). Innes (1995) and Hunter (2001) developed models in which susceptibility to deformation under tension was negatively related to wall strength, to cell wall thickness and thus to wood density. Shrinkage from the green state to 17% moisture content was negatively related to wood density in Tasmanian mountain ash (Eucalyptus regnans F. Muell.) (Chafe and Ilic 1992). In our specimens, tension shrinkage was not related to basic density across cambial age, but increased in samples with a lower basic density within an age class. The reason why overall wood density was not negatively related to tension shrinkage across cambial age might be the increase in the latewood content with the cambial age. Juvenile conifer wood shows less lumen diameter variations within an annual ring, whereas mature wood is a composite of wide lumen earlywood and high-density latewood (Brändström 2001, Domec et al. 2009). Mature wood was more prone to elastic deformation under tension because of its highly conductive big-diameter earlywood tracheids. Due to changing hydraulic and mechanical demands within a tree trunk with biomass allocation (e.g., Mencuccini et al. 1997, Hacke and Sperry 2001, Pitterman et al. 2006, Sperry et al. 2006), we suppose, however, that bending strength and stiffness as well as compression stiffness along the grain were higher in mature than in juvenile wood.

Shrinkage at moisture content below fiber saturation
Transverse cell wall shrinkage is reported to decrease with decreasing wood density (Stamm 1971, Skaar 1988, Chafe and Ilic 1992, Pang and Herritsch 2005) and increasing microfibril angle of the S2 layer of the secondary tracheid cell wall (Cockrell 1974, Yamamoto et al. 2001). Microfibril angles of juvenile Norway spruce wood are very high (Lindström et al. 1998, Gorisek and Torelli 1999) and might be one reason for its lower total shrinkage.

The total number of AEs (cavitations) was much higher in juvenile than in mature sapwood specimens (Table 1). Similar basic density values in juvenile and mature specimens may thus indicate in juvenile wood a higher number of tracheids/volume (Rosner et al. 2006) with smaller lumen diameters but also thinner cell walls (Hannrup et al. 2004). Accordingly, cell wall thickness was significantly lower in juvenile than in mature wood. The lower cell wall shrinkage (and thus total shrinkage) in juvenile wood might therefore be attributable to thinner tracheid walls (Perrè and Huber 2007). Wood density itself had an impact on total shrinkage in juvenile wood only, where it increased with decreasing density. At first sight, this finding is inconsistent with the results published by other authors, who found a positive relationship between density and transverse shrinkage (e.g., Stamm 1971, Skaar 1988, Chafe and Ilic 1992, Pang and Herritsch 2005). The inverse relationship between total shrinkage and density in juvenile Norway spruce wood might, however, be due to the complex density structure of the first annual rings, where structures resembling compression wood are present (Mayr and Cochard 2003, Mayr et al. 2005, Rosner et al. 2007). Recently, Perrè and Huber (2007) found that compression wood exhibits an inverse anisotropy ratio (radial shrinkage was higher than tangential shrinkage) and a much lower transverse shrinkage than latewood. To avoid internal wood checking, a lower cell wall shrinkage associated with higher (t/b)2 ratios (> 0.03) and wood density should be advantageous especially at the alpine timber line, where water contents of Norway spruce wood may be close to fiber saturation, because up to 100% native embolism during winter time is reported by Mayr et al. (2006).

Consequences for dendrometer measurements in live trunks
The results presented in this study give experimental proof that the relationship between changes in stem water potential and tensile strains measured at the surface of a tree trunk is strongly influenced by wood structure as supposed by many authors (Kozlowski and Winget 1964, Neher 1993, Irvine and Grace 1997, Offenthaler et al. 2001, Alméras and Gril 2007, Alméras 2008). Mature wood with bigger earlywood tracheids was more susceptible to deformation at moderate negative xylem pressure than juvenile wood. We suppose that more elastic energy was stored in tracheids with big lumen diameters when under tension, which was released when cavitation took place, because higher mean AE energies/min were measured in mature wood. The physical mechanisms behind this relationship should be the topic of further studies. The structure–function relationships presented here are based on the mechanically weakest part of wood, on earlywood. Wood is, however, a composite material, and multi-cellular models are necessary to understand the relationship between water potential and diurnal stem diameter changes in single individuals of the same species (Alméras 2008). We assume that AE feature analysis could be a useful tool to find out more about the structure–function relationships of tension shrinkage in live stems because of its high information potential on the structural characteristics of the conduits cavitated.

We performed laboratory experiments on small standard-size wood samples; nevertheless, our results on shrinkage may help to avoid misinterpretations of dendrometer measurements in live trunks. Recovery from tensile strain due to cavitation in live stems is seldom reported in the literature (Offenthaler et al. 2001, Hölttä et al. 2005). We suppose that recovery processes at moderate drought stress are masked by ongoing shrinkage in other, less vulnerable, wood parts. The rate of cumulative AE (cavitations) increases, therefore, almost simultaneously with periods of diurnal stem diameter decrease (Hölttä et al. 2005). Severe drought stress should however lead to a measurable tension shrinkage recovery due to tension release by cavitation. Ueda and Shibata (2002) compared the daily maxima (shrinkage during the day) and minima (swelling at night) of stem diameter in Chamaecyparis obtusa Siebold & Zucc. during a period of drought stress. Whereas diurnal changes were similar in the two trees investigated at the beginning of the drought stress, maxima and minima decreased in the trunk of the tree with reduced hydraulic conductance. We conclude from our results on standard-size samples that tension shrinkage recovery due to cavitation might underestimate actual drought stress in live trunks.


   Funding
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Funding
References
 
The authors thank the Austrian Science Fund (FWF, Project T304-B16) for financial support.


   Acknowledgements
 
The authors thank Andrea Klein and Gudmund Ahlberg for technical assistance out in the field and Joachim Sell for technical advice and helpful discussions. Hanno Richter is thanked for a critical reading of the manuscript and for linguistic corrections.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Funding
 References
 

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