Differences in intra-tree variation in spiral grain angle for radiata pine
© Watt et al.; licensee Springer. 2013
Received: 13 February 2013
Accepted: 29 August 2013
Published: 10 September 2013
Spiral grain angle (SGA) is an important factor affecting the distortion and utilisation of solid timber. Little research has investigated how SGA varies at a fine scale within trees and whether this fine-scale three-dimensional variation is similar between genotypes. The objectives of this research were to (i) characterise three-dimensional variation in SGA within stems and clones, and (ii) understand how intra-stem variation in SGA varies between genotypes.
Detailed measurements of SGA were taken from 12 radiata pine (Pinus radiata D. Don) clones. Analyses were undertaken to characterise variation in three dimensions and determine if this variation significantly differed between genotypes.
Spiral grain varied significantly with distance from the pith, showing a sharp initial increase with distance from the pith, followed by a gradual decline. Values of SGA increased significantly with height up the stem, reaching a maximum at ca. 5 m. Circumferential variation in SGA showed no significant trend. There was significant variation in mean SGA between clones; however, the within tree patterns in SGA did not significantly vary between clones.
If further research confirms the uniformity of within tree patterns in SGA between clones this may greatly simplify efforts to model three-dimensional variation in SGA.
One of the factors that has the greatest influence on the utilisation of wood from a wide range of species, including radiata pine (Pinus radiata D. Don), is lack of dimensional stability caused by spiral grain (Johansson et al. 1994). Spiral grain angle (SGA) is defined as the orientation of fibres (tracheids) with reference to the longitudinal axis of the tree stem. The relatively high values of SGA found in radiata pine (Ormarsson and Cown 2005), particularly in corewood, can cause twist in dry timber, distortion in plywood sheets, surfacing problems during machining (Tsehaye and Walker 1995; Ekevad 2005) and are a major cause of drying degrade in radiata pine and other conifers (Haslett and McConchie 1986; Haslett et al. 1991; Danborg 1994). Cown et al. (1995) reported that twist in radiata pine is responsible for at least 90% of the instability of affected radiata pine timber produced in fast-grown plantations in New Zealand. Similarly, Sorensson and Lausberg (1996) demonstrated that a decrease in spiral grain in juvenile wood of 3 degrees was sufficient to reduce rejection rates for twist from 12% to 5%, and would save the New Zealand forest industry about $58 million per annum.
In order to understand whether selective tree breeding can be used to reduce spiral grain angle, research has been undertaken to quantify the heritability of this trait in radiata pine (Burdon and Low 1992; Jayawickrama 2001; Harris 1989; Gapare et al. 2007; Lindstrom et al. 2004). These studies found that the narrow-sense heritability of SGA in radiata pine ranged from moderate to high, which was consistent with research on other conifers such as Sitka spruce (Picea sitchensis (Bong.) Carr.) (Hansen and Roulund 1998) and Norway spruce (Picea abies Karst.) (Costa e Silva et al. 2000). A weak negative correlation between stem diameter and spiral grain has also been found indicating there may be some trade off in growth when reducing spiral grain angle through selective breeding (Jayawickrama 2001).
Characterisation of intra-tree variation in spiral grain angle has received considerably less attention in the literature than its genetic variability. In radiata pine, SGA generally declines from the innermost rings to about the 15th ring from the pith (Cown et al. 1991; Xin et al. 1996), while in the longitudinal direction there is generally an increase in SGA with height in the lower stem (Cown et al. 1991; Tsehaye and Walker 1995). Although circumferential variation in SGA has been noted in a single radiata pine stem (Cown et al. 2010) the authors are unaware of any research that has investigated if there is a consistent circumferential influence on SGA for this species between trees. Similarly, little research has investigated how the magnitude of intra-stem variation in SGA differs between genotypes.
Recent advances in measurement techniques (Brännström et al. 2008; Ekevad 2004) now mean that it is possible to assess the three-dimensional pattern of SGA within a stem. This paper reports on a study in which detailed measurements of SGA were made on 12 selected genotypes of radiata pine, aged seven years old. The objectives of this study were to characterise three-dimensional within-stem variation in SGA and identify whether intra-stem variation in SGA varied between genotypes. Given the relatively small number of clones used in this study the objective of this work was not to describe fully the genetic effect but rather understand how intra-tree patterns might vary from genotype to genotype within a current breeding programme.
Data were collected between July 2008 and March 2009 from a radiata pine clonal variety trial at Esk Forest, Hawke’s Bay, New Zealand (latitude 39.274 S, longitude 176.789 E, elevation 180 m). The trial was established in 2001 as an incomplete block, single tree plot design, and consists of 42 plots each containing 48 trees planted at a 4 x 4 m spacing (nominally 625 stems ha-1). The site is in its second rotation of radiata pine. The trial has been pruned in three lifts, occurring between 2005 and 2007, to a height of around 6 m. Hydrated sodium calcium borate hydroxide (Ulexite) was applied in 2005 at a rate of 6 kg ha-1 in response to a foliar boron deficiency but no other applications of fertiliser were made. Over the trial period, this site has had a mean annual rainfall of 1500 mm and a mean annual temperature of 13°C.
Twelve clones were selected (Mike Carson, pers. comm.) to span the range of available breeding values for diameter (at breast height), height, wood density, acoustic velocity, dothistroma resistance, branching, malformation and straightness. Selected clones also covered the range in outerwood densities and stress wave velocities (as assessed using a standing tree instrument). For each clone, two ramets were randomly selected giving a total of 24 trees for measurements.
The mean (and range in brackets) for diameter at breast height, height and crown mass of the selected trees were respectively 20.95 cm (17.6 – 24.3 cm), 13.25 m (11.7 – 16.5 m) and 66.2 kg (29 – 146.2 kg).
Each selected tree was felled then cut into sequential, parallel 30-mm discs from the butt to a top diameter of approximately 50 mm. A single disc was retained for spiral grain measurements from each annual growth unit meaning that discs were taken at approximately 1.8 m intervals up each stem from a height of about 0.3 m to a height of 7.6 m – 12.0 m depending on the height of the tree, i.e. from 5 to 8 (average 6.6) discs per tree. These discs were selected to avoid grain deviation around branches. The discs were kiln dried under conditions of 60°C dry bulb/56°C wet bulb to a moisture content of between 18 and 25%. They were then further conditioned at 25°C and 65% relative humidity for 2–3 weeks, resulting in an equilibrium moisture content of approximately 10%. Four equally-spaced, 25-mm-wide, radial strip samples (in the N, E, S and W directions) were cut from each disc.
Spiral grain was measured at 5 mm intervals along each radial strip by exploiting the so-called “T2 effect” (McGunnigle 2009; Marschner et al. 2005; Matthews and Soest 1984). Due to the cellular nature of wood, light specularly reflected from a wooden surface is scattered in amounts that differ with fibre direction. By measuring the relative reflected intensities over a range of directions, the grain orientation vector (usually expressed as a pair of surface and dive angles) can be estimated. Careful surface preparation is critical to the success of this technique, since it relies on the microscopic texture and any preparation that alters the nature of this surface (e.g. sawing) will influence the results (Shen et al. 2000; Eastin and Johnson 1993). For this study, surfaces were first sawn then hand planed with a carefully honed blade.
A red laser (635 nm, 5 mm spot size) was projected at 90° to the longitudinal-radial surface of the radial strips. The laser was moved from pith to bark along each radial sample in 5 mm steps. The raw surface (i.e. deviation from vertical in the longitudinal-radial plane) and dive (i.e. deviation from vertical in the longitudinal-tangential plane) angles were obtained from the specular reflection peaks. A peak-fitting algorithm was used to find peak locations based on the intersection of the maximum slopes. Analyses were carried out on grain values in relation to distance from the pith – not annual rings as in most previous studies.
Previous experience indicates that tilt correction significantly reduces the observed variation in spiral grain (Cown et al. 2010). Consequently, tilt adjusted values of surface and dive angle were also calculated, where surface angles from adjacent radii were used to adjust the measured dive angles. The tilt adjustment was carried out using the radii averages. The surface angle averages for the N and S radii were adjusted (by rotating the disc mathematically) to make the N and S radii equal and opposite in sign with the corresponding adjustment of measured dive angles in the E and W radii. Similarly E and W radii were used to adjust the N and S dive angles.
The most common convention is to consider the left-hand, or “S” pattern (as viewed from the bark side) as a positive angle and right-hand “Z” pattern as negative (Harris, 1989) and this was adopted for this study.
Data were analysed using R (R Development Core Team 2011) and SAS Version 9.2 (SAS-Institute-Inc. 2000). An initial exploratory analysis was conducted using graphical procedures. These were used to examine the raw data, the probability distribution of SGA, and to produce box plots showing within- and between-tree variation in SGA. Means, standard deviations, and skewness and kurtosis parameters were calculated for each tree, and compared between clones using a one-way analysis of variance (ANOVA).
where, i = tree (1,…,24)
j = sample within tree
SGA ij = grain angle of the ijth sample
h ij = height within stem of the ijth sample
d ij = distance from pith of the ijth sample
In this model, a i , b i and c i were assumed to vary between tree and to be independent identically distributed normal variables with means α, β and γ respectively, and with an unstructured covariance matrix, while e ij is a randomly distributed error term. When fitting this model, the independent variables were standardised by subtracting their approximate mean values. Thus, 50 mm was subtracted from distance from pith, and 3 m subtracted from height within the stem. The effect of this approach was to make the random intercept term correspond roughly to a random mean for each tree. The model was used to test whether grain angle varied significantly in either the radial or longitudinal directions within a typical stem (by testing whether β or γ differed significantly from zero). Nonlinear and interaction effects were also modelled by adding and testing the significance of quadratic and interaction terms to Model (1).
To test for differences in grain angle between circumferential direction (N, S, E, W), mean grain angle was calculated by direction and tree, and a two-way ANOVA fitted with terms for tree and direction.
Distributional statistics of spiral grain angle
Coefficient of skewness
Coefficient of kurtosis
Estimates of parameters for the random coefficient model
t value (23 d.f.)
Distance from pith (mm)
Spiral grain angle is a complex wood property and comprises two distinct components that include the grain direction and magnitude of the gradient. The positive (or left handed) mean value of 3.29° found here broadly agrees with the range found in other studies on radiata pine, where values range from 2.9° to 4.7° (Tsehaye and Walker 1995; Cown et al. 1991; Gapare et al. 2007).Values of SGA found in radiata pine regularly exceed mean values of 2° found for Pinus taeda (Zobel et al. 1968), and 1.4° to 2.7° for Picea abies (Costa e Silva et al. 2000), but are relatively similar to mean values of 3.6° for Araucaria cunninghamii (Eisemann et al. 1990) and 5° for Picea sitchensis (Hansen and Roulund 1998). It is worth noting that, for wood utilisation, the mean value is not as important as the variation in SGA as twist results from differential SGA within the same board. The intra-tree variation in SGA was found to be very high, which agrees with results from previous research on the within tree distribution of SGA (Cown et al. 1991).
Consistent with the findings of this study, the high variation in SGA within trees has been shown previously to exhibit certain patterns, but these patterns are known to differ between tree species (Harris 1989; Aebischer and Denne 1996; Bäckström and Johansson 2006; Bannan 1966; Danborg 1994; Elliott 1985; Fujimoto et al. 2006). In trees sampled from natural forests of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf.) Sarg), the highest values of SGA were found near the outside of the tree, where visible spiral grain causes timber degrade (McBride 1967). In contrast, highest values of SGA in radiata pine usually occur within the juvenile core, declining with distance, or ring number, from the pith (Cown et al. 1991; Tsehaye and Walker 1995).
Little systematic variation in SGA has been reported previously in the longitudinal direction (Cown et al. 1991; Tsehaye and Walker 1995) although a general small increase with height has been noted. In the current study, a significant increase in SGA was observed to a height of 5 m before it stabilised. These variations between studies may be attributable to the young age of the material studied here, which primarily shows longitudinal variation within the juvenile core, rather than variation over mature stems. This suggests that boards cut from the first log within the juvenile core may be subject to less twist, which agrees with previous sawing studies (Haslett et al. 1991). The age of the samples was too young to detect the normal transition with age from positive to negative values, which occurs after about 15 years (Cown et al., 1991). The results of the present study extend previous research by showing no consistent significant circumferential variation in SGA.
The major advance of this research is that it shows that three-dimensional within-tree variation in SGA pattern does not differ significantly among the clones tested. Further research is needed to confirm this result across a broader range of clones and sites. If found to be correct, this result will greatly simplify any further efforts to model within-tree variation in SGA as clonal effects can be treated as a simple offset in the model intercept. Given the young age of the material studied and the relatively low number of clones used within this study, further research should be undertaken to confirm this result. A tool recently developed (Riddell et al. 2012) will allow more detailed examination of within-tree patterns and eliminate problems associated with sample alignment (with respect to stem axis).
The radial variation in SGA in radiata pine observed in this study was consistent with results from other studies in this and other conifer species. There was no consistent significant circumferential variation in SGA and no significant differences in within-tree patterns in SGA between the clones studied. Differences in SGA between clones were limited to differences in mean values. This uniformity of within-tree patterns of SGA between clones may greatly simply efforts to model three-dimensional variation in SGA.
We gratefully acknowledge the access to material and assistance provided by Mike Carson, Judy Hayes (Forest Genetics) and by Brian Garnett (PanPac). The work reported herein was funded by Future Forests Research Ltd.
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