Trees (1993) 7:131-136

�9 Spfinge~Veflag 1993

Strength and structure of stems from fast grown Pinus radiata

Geoffrey M. Downes 1., Erwin P. J. Beckers 2, Nigel D. Turvey 1.*, and Hans Porada 3

1 University of Melbourne, Forestry Section, Creswick, 3363, Victoria, Australia 2 Wageningen Agricultural University, Wageningen, The Netherlands 3 New South Wales Forestry Commission, Tumut, N. S. W., Australia

Received November 11, 1991/July 21, 1992

Summary. Segments of living stems from 2-year-old, fast growing P i n u s r a d i a t a , established on a fertile ex-pasture, were examined in terms of their bending strength parallel to the grain. Stem anatomical and structural variables were examined to identify variables that could explain variation in bending strength. Half of the seedlings were physically supported to minimise the confounding effects of compres- sion wood production in response to environmental stresses. Differences between seedlings and cuttings were examined. Variation in microfibril angle and lignin content was sufficient to account for 81% of the variation in bend- ing strength. A positive correlation between both of these variables and elasticity was evident. Few differences were found to be caused by the support treatment or between seedlings and cuttings. The support treatment resulted in signifcantly fewer branches in the top most whorl, while cuttings had less branches in this whorl than seedlings.

Key words: P i n u s r a d i a t a - Stem strength - Elasticity - Lignin - Microfibril angle

Introduction

Recently the biomechanics of living wood has been ex- amined in terms of the strength characteristics of the tissue and its design (McMahon and Kronauer 1973; McMahon 1975). Cannell and Morgan (1987) studied the strength characteristics of living stems and branches and found bending strengths (Young's modulus) to be lower than those found for green wood, usually assumed to be similar

* Present address: CSIRO Division of Forest Products, Bayview Avenue, Clayton, 3168, Victoria, Australia ** Present address: Shell Companies in Indonesia, P. O. Box 2634, Jakarta, Indonesia

Correspondence to: G. Downes

in magnitude. Most associations between wood structure and strength have been examined in dried, mature wood specimens owing to the importance of these relationships to timber quality. Recently the importance of wood strength in the growth of trees has become apparent in fast grown softwoods (Turvey 1984; Pederick et al. 1984; Car- lyle et al. 1989; Downes and Turvey 1990 a), with the onset of bending moments occurring in stems following high rates of stem elongation during warm, wet periods (N. Turvey, unpublished data).

The relationships that exist in the current apical growth of young pines have been examined in recent years within the context of the Toorour syndrome (Carlyle et al. 1989; G. Downes, unpublished data). The underlying objective of this programme has been to define the structural/anatomi- cal aberrations that lead to persistent deformation, with a view to developing an early screening test for seed sources of unknown susceptibility. Previous studies have pointed towards associations between strength and structure using glasshouse grown material of known propensity to deform (Downes and Turvey 1990b, 1992). Relationships between stem form and stem lean have been indicated, as have relationships between bending strength and pith diameter, microfibril angle, and stem density.

The present paper details an experiment conducted on field grown material in which the current growth of half of the trees was physically supported over the course of 1 year. The major objective of the study was to model variation in bending strength of current growth from ana- tomical and structural data. Additionally trees from several different sources were investigated to determine whether genetic influences on bending strength and stem anatomy could be identified. Ideally the study would have included 2 to 3-year-old trees from families of known susceptibility to deform. Unfortunately access to this type of material was not available. The best available site consisted of trees growing within a trial established by the New South Wales Forest Commission at Green Hills state forest near Tumut, N. S. W. as part of a larger project examining seedlings and cuttings of known genetic origin across several sites at various locations. The families used were unknown with

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r e s p e c t o t t h e i r r e s i s t a n c e to T o o r o u r s y n d r o m e . H o w e v e r t he f o u r c h o s e n a l l o w e d th i s i n v e s t i g a t i o n to c o m p a r e s eed - l i n g s a g a i n s t c u t t i n g s , as w e l l as to e x a m i n e t w o f a m i l i e s o f c u t t i n g s u s e d e x t e n s i v e l y in o t h e r t r ia l s o n o t h e r si tes. T h i s s i te w a s a n e x - p a s t u r e s i t e e x p e c t e d to p r o d u c e r a p i d e l o n - g a t i o n r a t e s a n d t h e r e f o r e to h a v e t he p o t e n t i a l to p r o d u c e s t e m d e f o r m a t i o n s y m p t o m s .

Materials and methods

The experiment was located in compartment 856, Green Hills State Forest, near Tumut, N. S. W. The site was on a N. E. aspect with a moderate (10%) slope at an altitude of 465 m. The soil was derived from decomposed Green Hills granodiorite, a silurian S-type granitoid, with a depth to impeding layer exceeding 2 m. Some seasonal water logging was evident due to a slowly permeable subsoil. The plantation consisted of 2-year-old trees from 9 genetic sources established in 25 tree plots within 4 blocks. Buffer rows were planted with the same genetic stock as the plots they surrounded. The material used in this study was chosen from the buffer rows owing to the need to destructively sample the leaders. The 10 tallest trees from each of 4 genetic sources were chosen resulting in a maximum of 40 trees. Only trees from the 2 blocks which had received additional fertiliser were used; trees from each block were evenly divided. Two of the sources were from seed orchard origin (Green Hills seed orchard and Saxtons seed orchard), while the other 2 were grown from cuttings (1181 and 1164).

Of the above it was suspected that seedlings would be more suscepti- ble to deformation than cuttings due to previous investigations (Bail and Pederick 1989; Whiteman et al. 1990). Based on current form it was also suspected that cuttings from 1164 were more susceptible to deformation symptoms than 1181. In order to minimase the bending of stems prior to harvest, and hence the production of compression wood 20 of the 40 trees were physically supported for 12 months prior to harvest. Because it was expected that the site would generate stems that would be susceptible to deformation as defined by the Toorour syndrome, it was necessary to prevent bending in order to investigate the characteristics of the stem tissue that were causal for deformity, not resulting from it. Five trees from each source were selected randomly from the 10 available for staking. The staking treatment consisted of 70 mm diameter tubes of plastic mesh supporting the upper metre of growth. The tubes were held in place by rigid wire fasteners attached to a pole placed beside each tree. Tubes were raised periodically according to leader growth and holes cut in the mesh to accommodate branch growth. Tree heights were moni- tored at intervals over the year prior to harvest. At harvest the trees were photographed and the top 1.5 metres of stem removed. Several of the trees exhibited lammas growth and in these the most dominant leader was harvested.

The leader was dissected the day following harvest and a length taken and stored on ice prior to determining the modulus of elasticity. The stem segment taken was selected from the clear portion of the stem immediate- ly above the first whorl. The modulus of elasticity (E) was determined from a 3 point bending test using an Instron model 1185, and the ratio of stem diameter (minus bark) to span was held constant at 15. Spans ranged from 71 mm to 194 mm and were loaded at 10 mm/min using a blunt cylindrical loading point of 10 mm diameter. E values were determined from the linear region of the force-deflection curves according to the following formula.

4PL 3 E (GPa.) =

P = force (newtons) L = span ~, = deflection for given load d = diameter of stem segment

Tissue from the centre point of this span was then used to measure variation in the anatomical characteristics. Foliage material was dried at 70 ~ C, weighed, and nitrogen and phosphorous concentrations deter-

mined colorimetrically from sulphuric acid/hydrogen peroxide digests. Nitrogen was determined according to the method of Crooke and Simp- son (1971) and phosphorus by the method of Grigg (1977).

Following the 3 point bending test the stem was fixed in 2% formal- dehyde prior to dissection. Bulk density of the wood, pith diameter, wood annulus, cell wall area to toal area (percentage wall area), the average area occupied by individual tracheid lumens (average lumen area), mi- crofibril angle, bark thickness and lignin content were determined. Bulk density was determined on a stem cross-section; volume was calculated by mercury immersion of saturated stem pieces which were then dried at 80 ~ C for 48 h. Cell wall areas and average lumen area were determined on 15 gm transverse stem sections prepared using an American Optical sliding microtome and stained with 1% methyl violet. Each stem section was examined using an MD 20 image analysis system in conjunction with an Olympus BHS-2 microscope using an IF 550 filter. In these sections the frequency with which rays intersected a given length of cambium was determined for each seedling and expressed as rays per millimetre of cambium. Microfibril angles (MfA) were determined on macerated half fibres according to the method of Leney (1981). Twenty five tracheids were measured from each stem segment. Wood from the stem was extracted by refluxing in a 2 : 1 ethanol/benzene solution for 6 h. Lignin contents of extracted stem wood were determined by dissolv- ing wood tissue in acetyl bromide (Johnson et al. 1961; van Zyl 1978) and relating the absorbance to that obtained from ground wood meal of known Klason lignin content.

Prior to statistical analysis the frequency distribution of each of the measured variables was checked for normality. Of the variables de- scribed in Table 1, E, bark thickness and foliar N and P were transformed by natural log. The square root of branch number was used. Transforma- tions of these variables were used in all cited statistical analyses. Multiple regression analysis was used to find the combination of variables that explained as much of the variation in the modulus of elasticity as possi- ble. Prior to running the model, correlations of each of the independent variables with the dependent variable was examined and outliers remov- ed. Principal component analysis (PCA) was then used to identify factors driving changes in E. Analysis of variance was used to determine whether any change in the structural variables could be associated with families or tree types (cuttings vs. seedlings).

Results

Tree growth

O v e r the 11 m o n t h s p r i o r to t he h a r v e s t o f the l e ade r s in O c t o b e r 1990, t r ees i n c r e a s e d in h e i g h t f r o m a n a v e r a g e o f 1.46 m to 2 .69 m ; v i r t ua l l y d o u b l i n g t h e i r he igh t . T h e s tak- ing t r e a t m e n t w a s s u c c e s s f u l fo r the m a j o r i t y o f t r ees , h o w e v e r s e v e r a l l e a d e r s w e r e d a m a g e d d u r i n g t he p e r i o d i c a d j u s t m e n t s o f t he cages , or b y the c a g e i t se l f . S i m i l a r l y t w o o f t he l e a d e r s e x h i b i t e d s t e m l e s i o n s ( o n e in e a c h o f t he s u p p o r t e d a n d u n s u p p o r t e d t r e a t m e n t s ) w h i c h r e s u l t e d in t he c o l l a p s e o f t he l e a d e r s p r i o r to h a r v e s t . T h e s t a k e d t r e a t m e n t a p p e a r e d to h a v e l i t t l e e f f e c t o n t he g r o w t h o f t he l e a d e r s a n d t h e i r s u b s e q u e n t s t r e n g t h a n d a n a t o m i c a l c h a r a c t e r i s t i c s ( T a b l e 1). T h e o n l y n e a r s i g n i f i c a n t e f f e c t w a s in t he n u m b e r o f b r a n c h e s in t he t op w h o r l n e a r e s t t he s t e m apex , w i t h s t a k e d t rees h a v i n g f e w e r b r a n c h e s in th i s w h o r l t h a n u n s t a k e d t rees . S i m i l a r l y l i t t le d i f f e r e n c e w a s o b s e r v e d in the g r o w t h o f the t r ees f r o m the d i f f e r e n t t y p e s ( s e e d l i n g s vs. cu t t i ngs ; T a b l e 2) a g a i n w i t h t he e x c e p t i o n o f b r a n c h n u m b e r . T r e e s g r o w n f r o m c u t t i n g s h a d s ign i f i - c a n t l y less b r a n c h e s in t he t op w h o r l t h a n t r ees g r o w n f r o m s e e d l i n g s . T h i s r e s u l t w a s r e f e l c t e d in t he d a t a a n a l y s e d u s i n g t ree o r i g i n as the i n d e p e n d e n t v a r i a b l e ( T a b l e 3), w i t h s e e d l i n g s f r o m G r e e n Hi l l s s e e d o r c h a r d h a v i n g m o r e

Table 1. Comparison of variable means between stem support treat- ments. SE shown in parentheses

Variable Unstaked Staked P

1. Final height (m) 268.0 (8.7) 271.0 (9.9) 2. Height Increment (m) 121.0 (5.1) 128.0 (7.6) 3. No. branches in top whorl 9.1 (0.8) 7.0 (0.7) 4. E(GPa) 0.59 (0.06) 0.46 (0.05) 5. R (GPa) 0.016 (0.001) 0.014 (0.001) 6. Wood density (g. cm -3) 0.254 (0.008) 0.267 (0.016) 7. Microfibril angle (degrees) 43.79 (0.90) 42.16 (1.33) 8. Lignin content (%) 29.0 (0.7) 28.2 (0.6) 9. Cell wall area (%) 32.10 (0.89) 30.97 (0.85)

10. Average luman area (gm 2) 298 (11) 315 (14) 11. Rays (no./mm cambium) 4.90 (0.29) 4.80 (0.36) 12. FoliarN(%) 1.83 (0.05) 1.74 (0.08) 13. FoliarP(%) 0.16 (0.01) 0.16 (0.01)

NS NS 0.061 NS NS NS NS NS NS NS NS NS NS

Table 2. Comparison of variable means between tree types. SE shown in parentheses

Variable Cutting Seed P

1. Finalheight (cm) 273.0 (8.2) 269.0 (10.1) NS 2. Height Increment (cm) 126.0 (5.0) 122.3 (7.3) NS 3. No. branches in top whorl. 6.6 (0.7) 9.6 (0.8) 0.005 4. E(GPa) 0.50 (0.07) 0.55 (0.05) NS 5. R (GPa) 0.014 (0.001) 0.016 (0.001) NS 6. Wood density (g.cm -3) 0.248 (0.011) 0.270 (0.012) NS 7. Microfibrilangle (degrees) 42.80 (1.21) 43.300(1.03) NS 8. Lignin content (%) 27.4 (0.7) 29.5 (0.5) 0.033 9. Cell wall area (%) 31.9 (1.0) 31.4 (0.8) NS

10. Average lumen area (gm 2) 299 (15) 308 (11) NS 11. Rays (no./mm cambium) 4.75 (0.35) 4.94 (0.30) NS 12. FoliarN(%) 1.76 (0.07) 1.78 (0.08) NS 13. FoliarP(%) 0.17 (0.02) 0.16 (0.01) NS

branches than trees from the 2 cuttings families. No differ- ences in foliar N and P content were evident between staked and unstaked trees or between seedlings and cut- tings, with the exception of trees from 1164 which had significantly higher foliar P than the trees from the other 3 sources.

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The support treatment had little effect on the strength or anatomy of the leaders as can be seen from Table 1. Simi- larly little difference was observed between the tree types (Table 2) with cuttings having similar characteristics to seedlings, apart from the latter having higher lignin con- tents. In these respects the differences between the sampled stem material was minimal and for the purposes of further analyses were largely considered as a single population.

Stem strength

Bending strength values of the stem, as defined by the E, were low with the population mean being around 0.53 GPa. In order to determine the factors which ex- plained most of the variability in the bending strength, a multiple regression was used. Two factors, microfibril angle and lignin content explained 81.1% of the variation in E.

E = e( -5.983+0.063 x1+0-089 x2)

Where: X1 = average net microfibril angle, and X2 = lignin content

Microfibril angle and lignin content were not correlated with each other. Regression analysis also indicated that much of the variation in microfibril angle (43%) could be explained by variation in bark thickness suggesting a pos- sible relationship between these variables. Variation in bark thickness and foliar nitrogen explained 47.5% of the variation in microfibril angle. Three variables, bark thick- ness, wood density and wood radius explained 58.7% of the variation in the lignin content in the data set.

The data set was broken up into populations defined by treatment (staked and unstaked) and tree type (cuttings or seedlings). Within these populations the same two varia- bles, lignin content and microfibril angle were able to explain most of the variation in E. In cuttings they ex- plained 80.2% of the variation whereas in seedlings they explained 66.5%. Similarly in the unstaked trees variation in microfibril angle alone was sufficient to explain 74.3% of the variation with increasing microfibril angles leading to greater bending strength. In the staked trees the two variables explained 79.8% of the variation in E.

Table 3. Comparison of variable means between tree sources. Different letters indicate means that are significantly different at the 95% level of confidence. SE shown in parentheses

Variable GSHO SAXSO 1164 1181 P

1. Finalheight (m) 263.0 (14.7) 277.0 (14.4) 274.0 (10.3) 272.0 (12.8) NS 2. Height Increment (m) 111.9 (9.0) 134.0 (10.7) 128.0 (16.4) 125.0 (7.8) NS 3. No. branches in top whorl. 10.0a (1.3) 9.lab (0.8) 6.9b (1.2) 6.5b (0.8) 0.061 4. E(GPa) 0.60 (0.90) 0.51 (0.07) 0.44 (0.06) 0.57 (0.10) NS 5. R (GPa) 0.016 (0.002) 0.016 (0.002) 0.012 (0.001) 0.015 (0.002) NS 6. Wood density (g .cm -3) 0.267 (0.023) 0.276 (0.009) 0.237 (0.0012) 0.257 (0.016) NS 7- Mirofibril angle (degrees) 43.6 (1.2) 42.9 (1.8) 41.9 (1.9) 43.5 (1.7) NS 8. Lignincontent % 29.2 (0.9) 29.8 (0.6) 27.6 (1.0) 27.9 (1.0) NS 9. Cell wallarea (%) 30.4 (1.0) 32.4 (1.2) 30.1 (1.8) 33.2 (1.0) NS

10. Average lumen are (gm z) 315 (17) 301 (13) 334 (23) 280 (17) NS 11. Rays (no./mm cambium) 4.81 (0.87) 5.08 (0.56) 4.95 (1.92) 4.59 (0.32) NS 12. FoliarN(%) 1.62 (0.07) 1.96 (0.113) 1.81 (0.11) 1.73 (0.08) NS 13. Foliar P (%) 0.15b (0.014) 0.16b (0.014) 0.22a (0.025) 0.13b (0.01) 0.011

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Table 4. Oblique solution primary pattern matrix for variables measured in stems. A vafimax transformation was used

Variable Factor 1 Factor 2 Factor 3 Factor 4 Factor 5

E 1.029 0.119 -0.01 0.027 0.148 R 1.015 0.004 0.016 0.064 0.128 Stem diameter -0.159 0.714 0.247 0.149 0.172 Bark Thickness -0.682 0.312 0.092 0.166 0.014 Pith Diameter -0.459 0.385 0.465 0.314 0.065 Wood annulus 0.356 0.852 -0.193 -0.169 0.244 Wood density 0.389 0.012 0.363 0.744 -0.430 % Wall area 0.269 0.354 0.048 0.075 -0.904 Average Lumen area 0.045 0.386 -0.144 -0.159 0.599 Ray frequency 0.302 -0.038 0.238 0.074 0.899 MfA 0.785 -0.135 0.163 -0.147 -0.001 Lignin 0.872 0.342 -0.001 0.208 -0.007 Branch Number 0.003 -0.873 -0.038 0.005 0.357 Height increment 0.067 0.073 0.749 0.200 0.077 Foliar N 0.312 0.088 0.239 -0.745 -0.074 Foliar P -0.047 -0.116 0.749 -0.231 0.013

The modulus of rupture (R) provides a measure of the total amount of force that a stem or beam is able to absorb, prior to failure. Multiple regression showed that variation in 3 variables was sufficient to explain 83.1% of the varia- tion in R. Two of these variables were microfibril angle and lignin content as in the E regression. R was explained by the following equation:

R = e( -7.232+0.036 x1+0.062 x24).~ x3)

Where: X1 = Average net microfibril angle, and X2 = Lignin content X3 = Pith diameter

PCA allowed the relationships between measured vari- ables to be examined in multi-dimensional space. It then computed factors which associated variables whose varia- tion is orientated similarly within that space. Consequently the factors can be thought of as underlying processes which control or effect the variation in related variables which are significanty correlated with it. This analysis identified 5 factors driving changes in the measured variables. The relative weights with which the variables load on these factors are shown in Table 4. The result essentially sup- ports the multiple regression with E loading most heavily on the same factor as MfA and lignin content as did R. Bark thickness also loaded on this factor, while pith diameter loaded relatively evenly on this factor and on factor 3 with smaller loadings on factors 2 and 4. Factor 2 linked varia- tion in stem diameter (under bark), wood radius and branch number with branch number decreasing in inverse propor- tion to stem diameter and wood radius. This reflected the adverse influence branch number had on stem growth, with the competition of the branches in the top whorl for photo- synthate. Factor 3 linked foliar P with the magnitude of the height increment put on in the year prior to harvest as well as pith diameter. Factor 4 linked wood density with foliar N, while factor 5 linked percentage wall area, average lumen area and ray frequency.

Discussion

The growth data indicated few differences between the trees apart from the significantly greater number of branches in the top whorl of seedlings as opposed to cut- tings. The result is comparable to that found in other stud- ies (Fielding 1970; Whiteman et al. 1990). Birk (1990) similarly points to the heavy branching commonly as- sociated with the stem deformation prevalent on ex-pas- ture, and this is often associated with lammas growth and lack of apical dominance. An interesting result is the reduc- tion in branch number in the staked trees over the unstaked trees. This suggests that reducing the movement of the leader in response to external forces, may assist in main- taining the apical control over branch formation and reduce branch production. Therefore breeding strategies aimed at developing trees with stronger leaders may result in fewer branches. It was disappointing that no significant deforma- tion symptoms were observed in the leaders of the sampled trees prior to harvest, or in the remaining trees following harvest.

Two structural factors were found to explain a large proportion of the variation in the bending strength of the leaders; these being cellulose microfibril angle and lignin content. The correlations were strong and cannot be ade- quately explained in terms of other variables. The associa- tion of bark thickness with both MfA and lignin content is interesting but does not indicate that bark thickness is the more important in determining the strength of these trees. MfA has often been associated with stem strength in in- verse relationship. This study indicates a strong positive relationship between MfA and bending strength. In a pre- viously reported investigation using glasshouse grown ma- terial (Downes and Turvey 1990b) a similar weak correla- tion was evident, however the correlation explained only approximately 20% of the variation in stem lean. In a further study using glasshouse grown material (Downes and Turvey 1992) bending strength was used in addition to stem lean and no such relationship was found.

Panshin and de Zeeuw (1980) stated that increased mi- crofibril angles were associated with reduced bending strength in clearwood specimens. Similarly Mark and Gil- lis (1973), using individual fibres, report a correlation be- tween microfibril angle in the $2 and fibre modulus. How- ever they state that "The curves demonstrate that the axial stiffness of fibers with large angles (>25~ is largely insen- sitive to the properties of the cellulosic reinforcement, and very dependent on the properties of the matrix; conversely, the stiffness of small angle fibres (<10 ~ is largely insensi- tive to matrix properties, but extremely dependent upon the properties of cellulose." The microfibril angles of the trees in the study reported here ranged from 35 ~ to 52 ~ , well above the 25 ~ threshold identified by Mark and Gillis (1973). This suggests that the contribution of the microfi- bril angle to the axial stiffness of the fibres and hence the leader was less important than the properties of the matrix components such as lignin.

Most studies of bending strength have examined clear wood specimens from mature wood. The structural defi- ciencies of juvenile wood, with its large microfibril angles, low density and high lignin content, are well known.

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Schniewind and Gammon (1986) investigated the strength related properties of juvenile wood from P i n u s m u r i c a t a . In their study they found large microfibril angles up to 32.2 ~ (45.5 ~ in one radiate pine) and attributed the low strength values to large microfibril angles. In general how- ever the angles from their study were less than those re- ported here. Similarly they used clear wood specimens of square cross-section rather than the very young leader tissue used here, and did not report regressions that showed clear associations between bending strength and microfi- bill angle. Nevertheless, the trends they report are in con- trast to those reported here.

In the past considerable attention has been given to the interactive role between microfibril angle and lignin con- tent in compression wood in producing sufficient resis- tance to compression and generating a corrective force against the compressive strain (Boyd 1972, 1973; Boyd and Foster 1974). These and other studies have indicated that microfibril angles in severe compression wood are commonly around 45 ~ and that the high lignin contents in compression wood were important as a mechanism of pro- ducing counter forces. The lignification of the tracheid wall generates the stress which is then directed radially and longitudinally in proportions dictated by the microfibril angle. The similarity of these earlier findings with those reported here suggest the hypothesis that in young tissues in leaders, which are exposed to environmental stresses that induce bending, the main contribution to the axial strength is one of resisting compression rather than re- sisting tension. Thus the woody cylinder surrounding the pith consists of tissue that is more effective in resisting compression, similar to compression wood, rather than resisting tension as in mature wood further down the stem. Given this it would be of interest to examine the modulus of elasticity in compression of the stem sections.

Cannell and Morgan (1987) examined the elasticity of living sections of stems and branches of four species grow- ing in Scotland. They comment that the few studies that have examined the bending strength of living timbers have shown that the elasticities are generally below that of green timber, such as used by Schniewind and Gammon (1986). The data obtained from their study gave elasticity values for branches approaching a similar magnitude to those reported here. However the regression fitted to their data would suggest that tissue with a density similar to that found here would have negative elasticity values.

The modulus of rupture is potentially of as great, if not greater importance, than the modulus of elasticity to deter- mining the resistance of stems to deformation symptoms typical of the Toorour syndrome. Essentially R gives a measure of the total amount of force that a stem can absorb prior to failure. In the data given here it is evident that the factors that dictate E are largely the same as those that dictate R. However the contribution of pith diameter to the multiple regression suggests that larger piths tend to reduce the R. Most of the previous work in this area has indicated that pith diameter increased with increasing elongation rates (Downes and Turvey 1990 b, G. Downes unpublished data) and reduced stem strength.

The principal component analysis is a technique com- monly used in ecological studies to identify possible rela-

tionships or associations between populations or variables. In this experiment PCA was used to identify factors which drive the variation in the data set as a whole. The analysis supported the multiple regression data, but in addition gave more information concerning the inter-relationships be- tween variables that was not apparent from the regression. Factor 1 explained most of the variability in the variables that contributed to stem strength, or were correlated with variables explaining stem strength. It is difficult to specu- late on what this factor might be, however its association with non-structural anatomical variables such as bark thickness and pith diameter as well as structural variables such as lignin content and microfibril angle suggest that it might be a process regulating cell division and growth.

Conclusions

Clear correlations existed between E and MfA and lignin content. The latter 2 variables together explained 81% of the variation in the former. However the positive correla- tion between elasticity and MfA is in apparent contradic- tion to current theory, and can possibly be explained in terms of the very large microfibril angles found in the leader tissues. The role of the MfA may be similar to that in compression wood where high angles result in more of the force generated through lignification being directed axially. R was similarly explained by MfA and lignin con- tent with the additional contribution of pith diameter. It was evident that changes in structural or anatomical characteristics could not be associated with the genetic origin of the trees, or with the staking treatment, apart from the branch number produced by the current growth.

Acknowledgements. This study was supported by contributions from both private and government growers of Pinus radiata and members of the forest industry in Australia.

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