Variation of ring width and specific gravity within trees from unthinned Sitka spruce spacing trial in Clocaenog, North Wales

Transcription

1 Variation of ring width and specific gravity within trees from unthinned Sitka spruce spacing trial in Clocaenog, North Wales H. L. SIMPSON AND M. P. DENNE School of Agricultural and Forest Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, Wales Summary The aim of this work was to quantify patterns of change in ring width and specific gravity within trees of Picea sitchensis with ring number across juvenile and mature wood, in relation to height in tree, original spacing, and crown dimensions. Five trees were sampled from each of three plots on a 52-year-old unthinned spacing trial at Clocaenog, North Wales. The ring width of wood produced in early decades of the plantation was most strongly correlated with original spacing, while that produced in later decades was more strongly correlated with branch diameters of the upper crown. In later decades, trees originally at the widest spacing had higher specific gravity than those originally at closer spacing, presumably associated with self-thinning of the narrower-spaced unthinned plots. Differences in amount of juvenile wood between spacing plots were less marked than those reported from the same plots when harvested 7 years earlier; it is suggested that as trees surviving after self-thinning are likely to be those with a competitive advantage from an early stage, differences between plots in amounts of juvenile wood may become less apparent in trees harvested later in the rotation than in those sampled earlier. Underlying patterns of variation in ring width and specific gravity across the tree were found to be modified by a progressive drift with height in the tree. Linear regressions between specific gravity and ring width also varied in a systematic way; the intercept and slope of these regressions tended to increase with ring number from the pith, and with height in tree at a specified ring number. Equations are given as a basis for quantifying these trends, but more data are needed from other sites to determine the extent to which these equations represent trends inherent to cambial and apical ageing, as distinct from influence of changing environment around the trees. n ro uc ion shorter rotation crops. This plantation timber is As the natural forests of the world are depleted now appearing on the market, and concern they are being replaced by plantations that are about differences in quality compared with designed for the more complete utilization of that from natural forests has stimulated many C lraritntt of Ounercd Foraten, 1997 ForcniT, VoL 70, No. 1, 1997

2 32 FORESTRY investigations into the influence of silvicultural practices on wood structure and properties (Brazier, 1970; Denne, 1979; Savill and Sandels, 1983; Brazier et al., 1985; Mitchell and Denne, 1995, 1997). Systematic within-tree trends in wood structure and properties have been identified (Duff and Nolan, 1953; Richardson, 1961; Mitchell and Denne, 1997) and shown to be influenced by silvicultural techniques such as spacing and thinning (Brazier, 1970; Denne, 1979). To make practical use of that information, enabling forest managers to produce timber with properties desired for particular end-uses, the extent of that silvicultural control needs to be quantified. The present work aims towards such quantification of within-tree variation in wood production and specific gravity of Sitka spruce from an unthinned spacing trial. Data on ring width and specific gravity have been collected systematically throughout five trees from plots at three original spacing distances. In some of the same trees, variations in specific gravity have been analysed in relation to tracheid dimensions by Mitchell and Denne (1997); in the present paper the emphasis is on more detailed analyses of trends in ring width and specific gravity with cambial ageing and changing growing conditions through a rotation. Material and methods Selection of samples Trees were sampled from a Sitka spruce (Picea sitchensis (Bong.) Carr.) spacing trial, planted by the Forestry Commission in 1936 at Clocaenog, North Wales. The three plots sampled had been planted at 1.4, 1.8 and 2.4 m spacing, and had not been thinned. The trees were harvested between March and July 1988, and by that time there was evidence of heavy self-thinning in the 1.4 and 1.8 m stands (where there were many dead, small diameter trees) but relatively little self-thinning in the 2.4 m stand. Five trees were selected from each of the three plots (a total of 15 trees) taking trees of average diameter for each plot and rejecting any trees with obvious double leaders, uneven crowns or other major irregularities in the stem. The following measurements were taken after felling: total tree height to the nearest 10 cm, crown length (which was defined as distance from tree tip to lowest live branch, a live branch being defined as one with a minimum of 30 per cent live leaf) to the nearest 10 cm, and number of whorls with live branches. The diameter in centimetres of the three largest branches of each branch whorl were measured normal to the main axis of the branch at 1 cm from the junction with the main stem. The following parameters were calculated from those branch dimensions: total branch diameter (in centimetres) of the three largest branches of (1) every live whorl, (2) each whorl down to whorl 6 (top crown only), (3) each whorl down to whorl 14 (approximately the position of maximum crown). For two trees, the leaf dry weight of every branch was measured in grams, and regressions between leaf dry weight and basal branch diameter of those trees were used to estimate total leaf dry weight for each of the remaining trees. Ring width and specific gravity Two discs were cut from the central portion of every fourth internode from the top of each tree (counting the 1987 leader as internode 1), each 1 cm thick. One was sectioned for ring width measurements, the other used for specific gravity determinations. Based on some preliminary studies, the following rings were sampled; every ring from the first to the 20th from the pith, then every fourth ring out to the bark. Measurements were taken from both sides of the pith and later averaged. Any ring present from the 1988 growth season was excluded from the analyses. From each internodal disc a strip 1 cm wide was cut across a diameter through the pith. Where possible the strip was taken from the NW-SE direction, avoiding any irregularities such as compression wood or distorted grain around knots. Ring width was measured to the nearest 0.01 millimetre using a calibrated graticule eyepiece. For specific gravity a block was cut approximately 5 mm in tangential width, 3 mm in axial depth and radially across the entire growth ring, from each ring to be measured. The specific gravity was determined by the maximum moisture content method using the following formula (Kollman and Cote, 1968):

3 RING WIDTH AND SPECIFIC GRAVITY WITHIN S1TKA SPRUCE SPACING TRIAL 33 Specific gravity of wood sample = oven dry weight of wood weight of displaced volume of water M o + M n - M o Where M n = weight of sample saturated with water, M = o weight of sample when oven dry, Gsc specific gravity of cell wall substance (taken to be 1.53). To obtain the wet weight the samples were immersed in distilled water and placed under vacuum until saturated, which was taken to be when the samples remained submerged. Before weighing the samples were surface dried using absorbent paper, maintaining as standard a procedure as possible. After oven drying at 105 C they were cooled over a desiccant and reweighed. Data analysis Data values obtained were plotted according to Duff and Nolan sequences 1 and 2 (Duff and Nolan, 1953) to show variation within growth sheaths with height in the tree (sequence 1 as shown in Figures 1 and 3) and variation across discs with ring number from pith to bark (sequence 2 as shown in Figures 2 and 4). To demonstrate changes in ring width and specific gravity with time, all sequence 1 data have been summarized by taking a 10-year average for each internode, as shown in Figures 1 and 3; for these averages internodes were numbered at the time each growth sheath was formed, hence in Figure 1 for example internode 4 was the fourth internode from the apex in each year of each decade. These data were then summarized further by calculating the overall mean ring width or specific gravity of all growth sheaths for each decade. These means were then used in linear regressions against the crown dimensions (as defined above) and original spacing, as shown in Tables 3 and 6. Maximum ring width, initial specific gravity, minimum specific gravity in juvenile wood (corewood), and mature wood (outerwood) specific gravity were also calculated for each decade and then used in regressions against crown dimensions and original spacing. The boundary between juvenile and mature wood was here defined by the inflection of the curve of decrease in ring width from the maximum close to the pith to the relatively constant width of the outer rings: in most discs this was close to the point where ring width decreased below 2 mm. Results Tree dimensions At harvest, the mean diameter at breast height (d.b.h.) of 50 living trees sampled at random in each plot was 23.4 cm at 2.4 m spacing, 21.0 cm at 1.8 m spacing, and 20.6 cm at 1.4 m spacing. Table 1 summarizes mean height, crown length and estimated total leaf dry weight of the five trees felled from each plot. Crown length and estimated leaf dry weight varied considerably between the trees sampled within each plot, even through the trees were selected to represent the mean d.b.h. from each plot. Though the differences in crown length between spacings were not significant, total leaf dry weight was significantly less (P^0.05) at the 1.4 m than at the 1.8 m and 2.4 m spacings. Variations in ring width with height in tree Figure 1 shows mean ring width for the five trees sampled at each spacing, each line showing variation in ring width down the growth sheaths for each decade. The relatively slow earlier growth is likely to be associated with 'heather check', from which this plantation is reputed to have suffered until after the war years. It should be noted that breast height was between internodes 40 and 44 on these 52-yearold trees, indicating a very slow rate of height growth for the first decade ( ) after planting. For that reason data from the decade have not been included in this paper.

4 34 FORESTRY Internode number from apex ao Intemode number from apex Intemode number from apex Figure 1. Variation in ring width down growth sheaths, showing changes in ring width profile with decade in time. Internode number is as at time of growth sheath formation. Mean of five trees from each of the following original spacing distances from the Clocaenog Sitlca spruce spacing trial: (a) 1.4 m, (b) 1.8 m, (c) 2.4 m Decade: , , A , For the decade ring width increased to a maximum about 5-10 internodes below the apex, maintaining a more or less constant width in lower internodes, at all spacings. In later decades ring width increased to a maximum between internodes 5 and 10, then decreased to a minimum in internodes below the crown (Figure 1).

5 RING WIDTH AND SPECIFIC GRAVITY WITHIN SITKA SPRUCE SPACING TRIAL 35 Table 1: Mean tree height, crown length and estimated total leaf dry weight of 52-year-old Sitka spruce trees sampled from Clocaenog spacing trial. Mean of five trees per sample Spacing (m) Total height (m) Crown length (m) Estimated total leaf dry weight (g) a (2.37) a (0.78) a (1.06) 5.80 a 3927 a (2.33) (2006) 6.84 a 6057 b (1.90) (1327) 7.88 a 6034 b (2.06) (1879) Differing letters indicate differences significant between spacings at Values in parentheses are standard deviations. Table 2: Overall mean ring widths and maximum ring widths of the growth sheaths for each decade at three initial spacings of the Clocaenog Sitka spruce spacing trial. Mean of five trees for each spacing a) Overall mean ring width in mm (means of all internodes) Initial spacing 1.4 m 1.8 m 2.4 m Decade 1948/ a 4.12 b 4.47 b 1958/ a 4.28 b 4.39 b 1968/ a 2.59 a 2.46 a 1978/ a 1.69 a 1.47 a b) Maximum ring widths of growth sheaths in mm (mean of internodes 6-8) Initial spacing 1.4 m 1.8 m 2.4 m Decade 1948/ a 4.68 b 4.95 b 1958/ a 5.92 a 6.62 b 1968/ a 4.93 a 5.18 a 1978/ a 3.32 a 3.07 a Differing letters indicate differences significant between spacings at PS0.05 Table 2 shows overall mean ring width (the ing in and (Table 2), but in calculated mean of all internodes down the later decades there were no significant differgrowth sheaths for each decade) and maximum ences in mean or maximum ring width between ring width for each growth sheath (mean of spacings. To quantify the changes in the relainternodes 6-8). At each original spacing, mean tionship between ring width and spacing with and maximum ring width increased from time, linear regressions of ring widths with orig to , then decreased until har- inal spacing and/or crown parameters are vest. shown for each decade in Table 3. Regressions Both mean and maximum ring width were calculated for all the crown dimensions increased significantly with wider original spac- measured, but the crown parameter components

6 36 FORESTRY Table 3: Linear regressions between mean ring width for each decade and original spacing and/or crown dimensions at harvest, in Sitka spruce from the Clocaenog spacing trial Decade Regression equation R / / / /87 = sp*" = no. live wh + = sp»" no. live wh n - = sp + = no. live wh nj - = sp* no. live wh nl - = sp + = no. live wh* = upper br.d."'- = sp n « no. live wh" = sp = no. live wh" 1 - = upper br.d.*" = sp upper br.d.*" (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) = ring width in mm (mean of all intcrnodes in growth sheath), no. live wh = number of whorls of live branches, sp = original spacing in m, upper br.d. = total diam 3 largest branches from each of top 14 branch whorls in cm. * significant at PS0.1; * significant at P 0.05; ** significant at PS0.01; *** significant at P 0.001; n.s. not significant at PS0.1. of regressions shown in Table 3 include only those giving the highest values for coefficient of determination (R 2 ). In interpreting these regressions it should be remembered that while the spacing was the original spacing of the unthinned plantation, the crown parameters were not measured until harvest in Some crown parameters (such as the mean branch diameter) were likely to have been influenced to some extent by original as well as current spacing, while others (such as the diameter of branches of the upper crown) were more likely to be influenced by current space than by initial spacing. As shown in Table 3, the mean ring widths of early years ( and ) were closely correlated with original spacing; spacing alone accounted for 67 per cent of the variation in mean ring width (equation 1 in Table 3). In simple linear regressions for those decades, crown parameters were far less significant (equation 2) or did not reach significance at P50.1 (equation 5). Multiple regressions tested by including crown parameters improved the value of R 2 only slightly over that of spacing alone (equation 3, 6). The influence of original spacing on ring width became progressively weaker with time (compare equations 1, 4, 7, 11 in Table 3), while the influence of crown parameters became stronger. By the number of live whorls accounted for 54 per cent of the variation in mean ring width (equation 8); paradoxically this was a negative correlation. In the final decade ( ) mean ring width was more strongly correlated with branch diameter of the upper half of the crown (equation 13) than with original spacing (equation 11) or number of live whorls (equation 12). Multiple regressions suggested that for the last decade ( ) mean ring width was negatively correlated with original spacing (equation 14). Comparable trends were noted for linear regressions of maximum ring width of the growth sheath with original spacing and crown dimensions (data not given here, but presented in Simpson, 1993).

7 RING WIDTH AND SPECIFIC GRAVITY WITHIN SITKA SPRUCE SPACING TRIAL 37 Table 4: Analysis of variation in ring width from pith to bark with intemode number from tree apex. Sitka spruce form Clocaenog spacing trial, means of all trees from all spacings Internode number from tree apex a) Width of second ring from pith (mm) b) Maximum ring width (mm) c) Ring number from pith having maximum width d) Number of rings of juvenile wood* J * Here defined as number of rings from pith before ring width decreased below 2 mm, which approximated to the inflection point of the curve of decrease from maximum ring width outwards from the pith. Variation in ring width with ring number from pith Figure 2 shows variation in ring width with ring number from the pith of discs taken at (or slightly above) breast height. At that height, trees at the 1.4 m spacing had a lower maximum ring width. Comparison of discs from different internodes showed a progressive change in the shape of the curves of variation in ring width from pith to bark with height in tree (Table 4); similar trends were noted in trees from each spacing. Initial ring width (as shown by the second ring from the pith, Table 4a) increased from the tip (internode 8) down to internode 24 then decreased in lower internodes. Maximum ring width increased down the tree to internode 28, then decreased in lower internodes (Table 4b), the ring number with maximum width being reached further from the pith in lower internodes (Table 4c). The number of juvenile wood (corewood) rings increased progressively down the tree (Table 4d) Ring number from pith Figure 2. Influence of initial spacing on variation in ring width with ring number from the pith. Data from disc cut from 40th internode from apex (approximately breast height), mean of five trees from each original spacing of Sitka spruce from the Clocaenog spacing trial. Spacing: 1.4, 1.8, A 2.4 m.

8 38 FORESTRY Variation in specific gravity with height in tree Figure 3 shows mean specific gravity for the five trees sampled at each spacing, showing variation in specific gravity down growth sheaths for each decade. In comparable parts of the growth sheath at each spacing, specific gravity tended to decrease from to , then increase to For the decade specific gravity decreased from apex to base in each Intemode number from apex Internode number from apex Internode number from apex Figure 3. Variation in specific gravity down growth sheaths, showing changes with decade in time. Internode number is as at time of growth sheath formation. Mean of five trees from each of the original spacing distances of Sitka spruce from the Clocaenog spacing trial: (a) 1.4 m, (b) 1.8 m, (c) 2.4 m. Decade: , , A ,

9 RING WIDTH AND SPECIFIC GRAVITY WITHIN SITKA SPRUCE SPACING TRIAL 39 spacing (Figure 3), but in subsequent decades cific gravity of comparable parts of the growth specific gravity decreased from the leader to a sheath (Table 5b d). In later decades ( minimum between internodes 8 and 12, then and ) overall mean, initial and outerincreased to a maximum at about internode 20, wood specific gravity were significantly greater with specific gravity fluctuating about that max- in the 2.4 m trees than in those at 1.4 m (Table imum value in lower internodes. 5a, b, d), but the influence of spacing was not Table 5 shows data from the curves of specific significant for minimum juvenile wood specific gravity along growth sheaths, which were gravity (Table 5c). Averaging over all trees, speanalysed in the following way: (a) mean specific cific gravity decreased from at the top of gravity (mean of all internodes in growth the tree (internodes 1 and 2, Table 5b) to a mean sheath), (b) initial specific gravity (mean of minimum of in mid-crown (internodes internodes 1 and 2 at the top of the tree for each 8 12, Table 5c), then increased (at a mean rate year), (c) minimum juvenile wood (corewood) calculated to be per internode) to an specific gravity, (d) outerwood specific gravity, average value of below the crown (intern- In earlier decades ( and ) there odes 20-28, Table 5d). was no consistent influence of spacing on over- Table 6 shows multiple regressions between all mean specific gravity (Table 5a), or on spe- mean specific gravity (for each decade) and Table 5: Variation of specific gravity down growth sheaths for each decade at the three initial spacings of the Clocaenog Sitka spruce spacing trial. Means of five trees at each spacing Initial spacing 1.4 m 1.8 m 2.4 m a) Overall mean specific gravity (mean of all internodes) Decade 1948/ a b a 1958/ / a a a a a b 1978/ a a b Mean of all spacings/decades b) Initial specific gravity (mean of internodes 1 and 2) Decade 1948/ a a a 1958/ a a a 1968/ a ab b 1978/ a ab b Mean of all spacings/decades c) Minimum specific gravity of juvenile wood (internodes 8 12) Decade 1948/ a b ab 1958/ a a 0363 a 1968/ a a a 1978/ a a a Mean of all spacings/decades d) Mean specific gravity of outerwood (internodes 20-28, below crown) Decade 1958/ a a a 1968/ a a b 1978/ ab a b Mean of all spacings/decades after Differing letters indicate differences significant between spacings at

10 40 FORESTRY Table 6: Linear regressions of mean specific gravity of growth sheaths for each decade on original spacing or crown dimensions at harvest. Mean specific gravity of all intemodes within growth sheaths Decade Regression equation R /57 s.g. = sp"'- s.g. = sp"-' mean br + s.g. = /57"-'- s.g. = sp n /57 n ' mean br /67 s.g. = sp"'- s.g. = sp n mean br n s.g. = /67"- 1 - s.g. = sp / mean br" J /77 s.g. = sp + s.g. = sp"' cr.l" s.g. = /77 + s.g. = sp"' /77 n cr.l"'- 1978/87 s.g. = sp» s.g. = sp cr.l"-'- s.g. = /87"'- s.g. = sp /87" cr.l* (1) 0.22 (2) (3) 0.25 (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) s.g. = specific gravity, mean br. = mean diameter all live branches at branch base (cm), = ring width mean for decade given (mm), cr.l = crown length (m), sp = original spacing (m). * significant at P50.1; * significant at PS0.05; ** significant at P^O.01; n.s. not significant at P^O.l. original spacing and/or crown parameters. As in Comparable trends were noted in equations Table 3, Table 6 concentrates on the most sig- with all the component parts of the specific nificant correlations calculated for each decade. gravity curves that were analysed (Simpson, Original spacing accounted for a higher propor- 1993). In most cases the correlation between tion of the variation in mean specific gravity in specific gravity and spacing was positive and the later decades ( and ) than in ear- closeness of that correlation tended to increase Her ones (Table 6 equations 1, 5, 9, 13); this is over the decades. Also, as with mean specific contrary to the trend in ring width regressions, gravity, regression coefficients of the initial and where the closeness of the correlations with mature wood specific gravity with crown paraspacing decreased with time. Unexpectedly, meters tended to be negative in the earlier years, mean specific gravity increased with wider orig- positive in later decades, inal spacing throughout all growth sheaths, but only in the final decade ( ) was this sic ,..,..,, r n^-n nc i i c variation in specific gravity from pith to bark nihcant at PS0.05. Inclusion of crown parame- f i b J i r ters improved the R 2 over the relationship with Figure 4 shows the pattern of variation in spespacing alone (equations 2, 6, 10, 14) correla- cific gravity from pith to bark for typical interntions with crown parameters being negative in odes at the three original spacings. As with ring earlier years, but positive in succeeding decades width there was a progressive drift in curve (though not reaching significance at P50.05). shape with height in the tree, from the base up Adding mean ring width for the decade to to at least internode 28 (Figure 4, Table 7). Inispacing and crown parameters improved the tial specific gravity (mean of the first two amount of the variation accounted for up to 64 growth rings from the pith) increased towards per cent in (16). the base of the tree (Table 7a). Specific gravity

11 RING WIDTH AND SPECIFIC GRAVITY WITHIN SITKA SPRUCE SPACING TRIAL (a) 0 7 (b) , I Ring number from pith Ring number from pith (d) Ring number from ptth i 0.55 b I 0.5 1^0.45 ' Ring number from pith Figure 4. Influence of initial spacing on variation in specific gravity with ring number from the pith. Mean of five trees from each original spacing of Sitka spruce from the Clocaenog spacing trial: (a) internode 28, (b) internode 36, (c) internode 44, (d) internode 48. Spacing: 1.4 m, 1.8 m, A 2.4 m. decreased outwards over the first few rings from the pith (Figure 4) at a slower rate (Table 7f) to a higher minimum (Table 7b) in internodes towards the base of the trunk. Lower internodes also reached minimum specific gravity further from the pith (Table 7d), then increased more slowly (Table 7g) to a lower maximum (Table 7c) again further from the pith (Table 7e). For each internode there was little or no influence of original spacing on specific gravity in the first few rings from the pith (Figure 4). Following the initial decline to minimum specific gravity, the subsequent increase in specific gravity began earliest and reached a higher maximum in trees at the widest original spacing (Figure 4). Trees at 1.8 m original spacing tended to have the lowest minimum specific gravity in juvenile wood, and the slowest rate of subsequent increase up to the lowest maximum specific gravity in mature wood (Figure 4). Variation in specific gravity with ring width Comparison of Figures 2 and 4 suggests that the specific gravity curves appear as the mirror image of those for ring width. However, the regression equations given in Table 8 show that the relationship between specific gravity and ring width changed with ring number from the pith, and with height in the tree (i.e. between internodes). Thus for comparable ring numbers, the closeness of the correlations between ring width and specific gravity increased up the tree, R 2 values being particularly poor for internodes lower in the tree (Table 8). Although caution is needed in attempting to interpret regression equations with low correlation coefficients, there do appear to be consistent trends in the data shown in Table 8. Thus the intercepts and slopes of the regressions both tended to increase up the tree for each selected ring. For comparable

12 42 FORESTRY Table 7: Variation in specific gravity (s.g.) from pith to bark with internode number of Sitka spruce trees from Clocaenog spacing trial. Mean of 15 trees from all spacings combined Internode number from tree apex a) s.g. of rings 1 and 2 b)s.g. minimum in juvenile wood* c) s.g. maximum in mature wood d) Ring number of minimum s.g. e) Ring number of maximum s.g. f) Rate of decrease in s.g. per ring, from ring 2 to minimum s.g. g) Rate of increase in s.g. per ring, from minimum to maximum s.g * Here defined as number of rings from pith before ring width decreased below 2 mm, which approximated to the inflection point of the curve of decrease from maximum ring width outwards from the pith. internodes, R 2 value, intercept and slopes all increased with ring number from the pith. All the regressions shown in Table 8 were recalculated using I/ring width as an independent variable (as suggested by Olesen, 1976), but these gave no further improvement of the R 2 value. Discussion The major objective of this project was to quantify the basic patterns of change in ring width and specific gravity within the tree, with ring number outwards from pith towards bark (as the cambium aged), and with height in the tree Table 8: Influence of ring number from pith and height in tree on linear regressions between specific gravity and ring width of growth rings. Data combined from all trees sampled from the Clocaenog Sitka spruce spacing trial Ring number from pith Internodes from apex Regression equation R 2 P s.g. = s.g. = s.g. = s.g. = s.g. = s.g. = s.g. = s.g. = s.g. = s.g. = s.g. = » n.s. n.s. * n.s. n.s. n.s.» * n.s. s.g. = specific gravity, = ring width. n.s. not significant at P50.05; " significant at P^O.01; *** significant at P

13 RING WIDTH AND SPECIFIC GRAVITY WITHIN SITKA SPRUCE SPACING TRIAL 43 (with maturation of the shoot apex), under the influence of a range of original spacing. The data shown in Tables 3 8 are intended to form a preliminary baseline towards a more comprehensive model of inherent and environmental control of wood production and quality. The types of curves found are closely comparable to those illustrated elsewhere (Duff and Nolan, 1953; Richardson, 1981); the present task was to quantify those trends. Both ring width and specific gravity have here been shown to vary progressively in shape with time (between growth sheaths) and with height in tree (between internodes), but it remains to be determined how representative these data trends are of the underlying trends with cambial ageing and apical maturation, for they will also reflect progressive changes in growing conditions on the particular site sampled. Mitchell and Denne (1997) analysed between-site differences in specific gravity from pith to bark, and those appear to be analogous to the trends described here with height in the tree. Thus for example the differences in rate of decrease in specific gravity outward from the pith, which were shown by Mitchell and Denne (1997) to vary with growth rate between sites, resemble the differences with height in the tree in the present analyses; this suggests that at least part of the variation in specific gravity with height should be attributed to variation in growing conditions rather than to maturation of the shoot apex. The regression equations shown in Tables 3 and 6 were designed to indicate some external parameters that could be used as predictors of ring width or specific gravity without the need for destructive sampling. The significance levels of these equations do indicate some potential for using parameters visible at the end of the rotation to predict specific gravity throughout the rotation. However, since many of their R 2 values were very low, the involvement of environmental factors and ageing processes on these trends in specific gravity needs to be clarified by further systematic within-tree analyses of Sitka spruce grown on a wider range of sites. It is a common observation that conifers at wide spacing tend to have wider growth rings, with lower specific gravity, than those at narrower spacing. Earlier investigations of the strength properties of Sitka spruce (including samples from the same Clocaenog spacing trial) concluded that planting distances of over 2 m would reduce the yield of timber acceptable for construction (Brazier et al., 1985; Brazier and Mobbs, 1993), associated with a wider core of juvenile wood. By the time the present trees were harvested (7 years later) it was obvious that there had been heavy self-thinning in the plots originally spaced at 1.4 m and 1.8 m. The trees that survive after self-thinning in an unthinned plantation are likely to be those which had a competitive advantage from an early stage; this may account for the relatively minor influence of original spacing on ring width and specific gravity in juvenile wood of the trees sampled at the end of the rotation (Tables 2, 5). Comparably, Mitchell and Denne (1995) found the juvenile wood structure of trees from an oceanically thinned Sitka spruce plot to be very similar to that from trees in an adjoining conventionally thinned plot, when harvested towards the end of the rotation; in that case it seems likely that influence of the early wide respacing on wood structure was outweighed by the tendency to thin to the more dominant trees in the conventionally thinned plot. In contrast Petty et al. (1990) found that in plots where differences in planting distance were maintained by thinning until the end of the rotation, the rings were wider and of lower density at wider spacing throughout discs from pith to bark. In the trees that had been harvested earlier from the Clocaenog spacing trial, Brazier and Mobbs (1993) found a decrease in ring width of outer rings of the second length logs with increase in original spacing, and attributed this to 'the persistence of a larger number of dominant trees competing for available space at this height (8 m) compared with that at closer spacings'. In the present trees the lower log also had narrower outer rings at wider original spacing, probably associated with increased competition between those trees by the time they were sampled. Presumably ring width would continue to decrease in these outer rings (and specific gravity to increase) until eventually this plot also selfthinned. These examples show the importance of considering the demography of individual final crop trees, as influenced by thinning or self-thinning, when investigating the consequences of silvicultural management on wood quality.

14 44 FORESTRY It is well established that the density of Sitka spruce tends to decrease with increase in ring width, even after cambial age has been taken into consideration (Brazier, 1970; Harvald and Olesen, 1987; Petty et al., 1990). The regression equations produced by Brazier (1970) and Petty et al. (1990) fall within the range of variation in slopes and intercepts shown in Table 8. Thus for rings from the pith (at breast height) Petty et al. (1990) found specific gravity = ring width, and for a comparable position Brazier (1970) calculated the regression as specific gravity = ring width (these equations have been converted to specific gravity from the original density units). However, the low values of R 2 that were obtained for many of these regressions in the present work (especially for the lower internodes) suggests that the relationship between ring width and specific gravity was influenced by other variables, such as the presence of reaction wood for example. That possibility, and the cause of within-tree variation in slope and intercept can be resolved only by considering the wood anatomy of these rings in more detail. of the lower log of the present trees appears to be less marked than reported in trees from the same site which had been sampled a few years earlier by Brazier and Mobbs (1993); a possible explanation put forward here is that trees surviving after self-thinning in an unthinned plantation are likely to be those that had a competitive advantage from an early stage. Thus, the longer the rotation in unthinned plantations, the more likely the plot is to self-thin to the more dominant trees, hence an earlier association between original spacing and ring width or specific gravity of the juvenile wood is likely to diminish with time since plots originally at closer spacing tend to self-thin earlier than those at wider spacing. The specific gravity of the outerwood rings of the trees originally at wider spacing was significantly higher than that of trees originally at narrower spacing, suggesting that the former were in greater competition at the time of felling; presumably that difference too would tend to diminish when eventually this plot also began to self-thin. Clearly it is crucial to consider the history of spacing around the individual sample trees when quantifying the influence of silvicultural management on wood quality. Conclusions The present data demonstrate how the relationship between ring width, specific gravity and original spacing changed with time in unthinned plots as the trees came into competition and became self-thinning. From linear regressions, the association between original spacing and ring width decreased over the decades, eventually becoming negative in the final decade, while that between the diameter of the upper branches and ring width increased over the decades. In contrast, the correlation between specific gravity and original spacing became closer and increasingly positive in later decades. Although the R 1 values were very low for many of these regressions, the consistent trends in regression coefficients with time suggests that, given data from a wider range of sites, it may be possible to predict within-tree variation in ring width and specific gravity from external parameters, without the need for destructive sampling. The influence of original spacing on the amount and specific gravity of the juvenile wood Acknowledgements We are grateful to the EC and to the Forestry Commission for their financial assistance towards work on this project, which was a part of EC contract MA1B.0032.UK 'The effect of modern forest practices on the wood quality of fast grown spruce'. We also thank Mrs Val Whitbrcad and Mr W. Morris Jones for their valuable technical assistance. References Brazier, J.D Timber improvement-ii. The effect of vigour on young-growth Sitka spruce. Forestry 43, Brazier, J.D., Hands, R. and Seal, D.T Structural wood yields from Sitka spruce: the effect of planting spacing. For. Br. Timber September, Brazier, J.D. and Mobbs, I.D The influence of planting distance on structural wood yields of unthinned Sitka spruce. Forestry 66, Denne, M.P Wood structure and production within the trunk and branches of Picea sitchensis in relation to canopy formation. Can. J. For. Res. 9,

15 RING WIDTH AND SPECIFIC GRAVITY WITHIN SITKA SPRUCE SPACING TRIAL 45 Duff, G.H. and Nolan, N.J Growth and morphogenesis in the Canadian forest species. 1. The controls of cambial and apical activity in Picea resinosa Ait. Can. ]. Bot. 15, Harvald, C. and Olesen, P.O The variation of the basic density within the juvenile wood of Sitka spruce. Scand. ]. For. Res. 2, Kollman, F. and Cote, W.A Principles of Wood Science and Technology. Volume I Solid Wood. Allen and Unwin, London. Olesen, P.O The interrelation between basic density and ring width of Norway spruce. Forstl. Forsogsvaes Dan. 34, Mitchell, M.D. and Denne, M.P Oceanic forestry: whatever happened to the wood quality? Q. ]. For. 89, Mitchell, M.D. and Denne, M.P Variation in density of Picea sitchensis in relation to within-rree trends in tracheid diameter and wall thickness. Forestry 70, Petty, J.A., Macmillan, D.C. and Steward, CM Variation of density and growth ring width in stems of Sitka and Norway spruce. Forestry 63, Richardson, S.D A biological basis for sampling in studies of Wood Properties. Tappi 33, Savill, P.S. and Sandels, A.J The influence of early respacing on the wood density of Sitka spruce. Forestry 56, Simpson, H.L Silvicultural influences on production and properties of juvenile wood in Sitka spruce. MPhil Thesis. University of Wales, Bangor. Received 17 July 1995