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Effect of nursing mixtures on stem form, crown size, branching habit and wood properties of Sitka spruce (Picea sitchensis (Bong.) Carr.)
Forest Ecology and Management 122 (1999) 113±124
Effect of nursing mixtures on stem form, crown size, branching habit and wood properties of Sitka spruce (Picea sitchensis (Bong.) Carr.) A.D. Cameron*, B.A. Watson Department of Forestry, University of Aberdeen, 581 King St., Aberdeen AB24 5UA, UK
Abstract This study investigated the effects of Alaskan lodgepole pine (Pinus contorta Loud.) and hybrid larch (Larix eurolepis Henry) nurses, planted in triplet mixture with Sitka spruce (Picea sitchensis (Bong.) Carr.), on growth rate, stem form, branching habit and wood properties of the spruce. These mixtures were compared with pure stands of Sitka spruce which had been regularly and periodically fertilised with nitrogen. Hybrid larch promoted diameter increments in Sitka spruce greater than those achieved by the other treatments over the last 12 years of the experiment (current age 30 years). Growth of regularly fertilised pure Sitka spruce was not signi®cantly greater than that of periodically fertilised pure spruce, suggesting that regular applications of nitrogen fertiliser did not result in increased stem growth. The greatest increase in growth of the larch-nursed spruce occurred during the period immediately following canopy closure, thus demonstrating the failure of the larch to compete with the spruce crowns. This period of high growth was associated with a high branch, cross-sectional area and deep knots within the wood on the lower part of the spruce stems in comparison with other treatments. While each treatment was associated with a similar number of branches, lodgepole pine-nursed spruce had more small branches (0±10 mm diameter) and fewer big branches (>20 mm diameter) compared with other treatments, highlighting the capacity of lodgepole pine to control branch development of the spruce. Spruce trees nursed by larch had deeper and more imbalanced living crowns with longer lived branches in comparison with lodgepole pine-nursed spruce, and both the pure spruce treatments, suggesting that more juvenile wood may have formed within the stem. Larch-nursed spruce also had the highest stem taper and lodgepole pinenursed spruce the lowest. Basic wood density was not in¯uenced by treatments. Overall, the evidence from this study suggests that the use of larch as a nursing species on deep peats is inadvisable and that Alaskan lodgepole pine is better able to control the branching habit of Sitka spruce with the prospect of better quality sawlogs in the future, albeit with a lower average tree size. Periodically fertilised pure spruce appears to maintain a growth rate consistently above that of the lodgepole pine-nursed spruce but without many of the disadvantages linked with the regularly fertilised pure spruce, particularly the development of large branches and an irregular pattern of annual rings within the wood. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Picea sitchensis; Nursing mixtures; Branching habit; Wood properties
1. Introduction The advantages of establishing Sitka spruce (Picea sitchensis (Bong.) Carr.) on nitrogen de®cient sites by *Corresponding author. Tel: +44-1224-272673; fax: +44-1224272685; e-mail: [email protected]
planting it with `nurse' species, such as pine (Pinus spp. ) and larch (Larix spp. ), are well known in preventing the spruce suffering a prolonged period of induced nitrogen de®ciency caused by Calluna vulgaris L. (O'Carroll, 1978; Taylor, 1985). Since the 1970s it has become the most frequently used
0378-1127/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 0 3 6 - 5
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A.D. Cameron, B.A. Watson / Forest Ecology and Management 122 (1999) 113±124
management system in Britain for establishing spruce stands on un¯ushed peats. Although the nutritional aspects of nursing mixtures have been studied (Carlyle and Malcolm, 1986), little work has been undertaken on the effect of mixtures on stem form and wood properties of Sitka spruce. Watson and Cameron (1995) noted that trees on the edge of Sitka spruce blocks, adjacent to blocks of Japanese larch (Larix kaempferi (Lamb) Carr.), lodgepole pine (Pinus contorta Loud.) and Scots pine (Pinus sylvestris L.), had unbalanced or deeper living crowns than trees within the spruce block. Since size of the living crown is linked with juvenile wood content (Larson, 1962) in addition to other timber characteristics, these ®ndings may indicate that the wood properties of the spruce, in mixture with other species, are being negatively in¯uenced. Most of the planting of nursing mixtures since the 1970s has been as intimate mixtures, usually in the form of triplets. Thus, the main objective of the present study was to determine the effects of lodgepole pine and hybrid larch (Larix eurolepis Henry) nurses, in triplet mixture with Sitka spruce, on growth rate, stem form, branching habit and wood properties of the spruce. These mixtures were compared with pure stands of Sitka spruce which had been regularly and periodically fertilised with nitrogen. 2. Materials and methods 2.1. Description of study area The study site is located at Strathy Forest, North Scotland (National Grid Reference NC813555). The soil is an acidic deep peat (>2 m) with an underlying geology of Moine schist. The dominant vegetation over most of the site is Calluna vulgaris (L.) with smaller areas of Molinia caerulea (L.). The elevation is over 100 m and this site is highly exposed with a
mean annual rainfall of 975 mm. The site was cultivated by spaced furrow ploughing (1.8 m between rows and 45 cm deep). 2.2. Experimental design The experiment was laid out in 1965 by the Forestry Commission with the original aim of examining the economics of Sitka spruce in comparison with lodgepole pine on deep acid peats. Since the focus shifted in the 1970s towards nursing mixtures, the experiment was used to examine the nursing of Sitka spruce in comparison with pure spruce treatments. Four treatments were examined in the current study (Table 1) with each treatment replicated three times. The species used in the experiment were Sitka spruce (provenance QCI), lodgepole pine (provenance Skagway, Alaska) and hybrid larch (provenance East and Central Scotland). The species were laid out in a `triplet' pattern comprising alternate groups of three spruce and three nurse trees in rows. Adjacent rows of trees were planted to ensure that each triplet of spruce was next to triplets of nursing species. All treatments received P (phosphorous in the form of ground mineral phosphate) in Year 5, K (potassium in the form of KCl) in years 3, 4 and 23, and PK in Year 16. Nitrogen (N) was applied in the form of urea. 2.3. Collection of samples Five spruce trees were felled from the borders of assessment plots, using the middle tree from the triplet, and the north side of each stem marked and total height of each tree measured. Twenty-centimetre long sections were removed from the felled trees at 0.1, 0.5, and 0.9 tree height and two 50-cm sections removed at 0.3 tree height (within the main sawlog length) and 0.7 tree height (within the living
Table 1 Treatment descriptions and codes Treatment code
Treatment description
NAV
pure Sitka spruce regularly fertilised with N (years 5±13 (170 kg/ha/year) then year 16, 19, 22 and 25 (150 kg/ha)), vegetation untreated pure Sitka spruce periodically fertilised with N to prevent induced N deficiency only (year 2, 8, 13 (170 kg/ha), and 18, 23 (150 kg/ha), vegetation untreated Sitka spruce in a triplet arrangement with lodgepole pine, no N, vegetation untreated Sitka spruce in a triplet arrangement with hybrid larch, no N, vegetation untreated
NPV L H
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crown). A 10-cm thick basal disc was also taken. The samples were labelled and placed in a cool, wellventilated store to minimise shrinkage and splitting of the wood and bark. 2.4. Stem analysis Ten centimetre thick discs were removed from each of the stem sections and the upper surfaces ®nely sanded. Stem sections were passed under the microscope of a computer-linked digital positiometer on a N-to-S and E-to-W axis and the distances of the ring boundaries were recorded on computer ®les. Diameter increments derived from these data were used for volume estimation as shown below. The historical pattern of height growth was calculated by describing the top part of each annual stem pro®le as a cone. The height of the cone was calculated using similar triangles based on the stem taper of the previous stem section. Cone height was added to the height of the lower disc, thus giving an estimate of tree height. Analyses of tree-diameter increment were made on the disc at 0.1 tree height. Total volume (from the base to the tree tip) was calculated using Newton's formula for the lower sections and the top section calculated as a cone. The annual volume increment was calculated by subtracting total tree volume in the previous year from the total tree volume. Thus, annual volume increment and total volume were calculated over most of the life of the trees. 2.5. Crown development and branching habit Crown depth was measured on the felled trees as the distance from the tip of the tree to the lowest complete living whorl and crown imbalance measured as the distance between the lowest complete living whorl (with complete sets of living whorls above) and the lowest living branch. The number and diameter of all branches were recorded on the 50-cm long stem sections taken at 0.3 and 0.7 tree height. 2.6. Knotwood area and knot depth Cross-sectional discs, containing a complete whorl of branches, were removed from ca. 0.3 tree height. These were used to measure knot area and knot depth. Measuring knot development on stem whorls assesses
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the worst locations in the log for these defects and, thus, the weakest points in sawn timber. The disc was cut successively into 5-mm slices and a tracing made of each slice. From these tracings, depth and maximum cross-sectional area of individual knots were calculated. 2.7. Stem taper Taper was derived from the difference between diameters (cm) at either end of stem sections, cut from 0.1±0.5 tree height (approximate length of the butt log) and from 0.1±0.7 tree height (approximate length of the merchantable log), divided by the length of each section (m). 2.8. Estimated extent of juvenile core from crown characteristics The extent of juvenile core was made on each tree by counting the number of annual rings at the base of the living crown (i.e. the age of the lowest living whorl of branches). Juvenile wood is produced within the living crown and there is a strong association between live crown depth and proportion of juvenile core within the stem (e.g. Larson, 1962). 2.9. Wood density and compression wood content Basic density (oven dry weight/water swollen volume) was measured on small blocks of wood (2 cm 1 cm 5 annual rings) cut from the sample discs, in a series from the bark to the pith, taken at 0.1 tree height. Compression wood was assessed on thin discs, 3±6 mm thick, cut from the main sample discs. These were immersed in water for at least 24 h, then placed on a light table and areas of compression wood, showing up as red and opaque, were traced. The areas were coloured black, photocopied and assessed using a machine designed to measure leaf area. 2.10. Statistical analysis Analyses were performed using analysis of variance. Analyses of the number of branches by size category were carried out using 2-tests. The vertical bars in Figs. 1±6 are 0.5s.e. rather than 1s.e. to improve clarity of the graphs.
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Fig. 1. Mean tree height of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H). Vertical bars 0.5s.e.
Fig. 2. Annual height increment of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H). Vertical bars 0.5s.e.
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Fig. 3. Mean tree diameter at 0.1 tree height of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H). Vertical bars 0.5s.e.
Fig. 4. Annual diameter increment of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H). Vertical bars 0.5s.e.
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Fig. 5. Mean tree volume of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H). Vertical bars 0.5s.e.
Fig. 6. Annual volume increment of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H). Vertical bars 0.5s.e.
A.D. Cameron, B.A. Watson / Forest Ecology and Management 122 (1999) 113±124
3. Results 3.1. Height Heights of L spruce (nursed by lodgepole pine) were greater by the end of the study period than the other treatments (Fig. 1) although ANOVAs for each year showed no signi®cant treatment effects. Annual height increments were variable, with only a general decline noted from Year 23 to Year 30, and no signi®cant treatment effects were found (Fig. 2). 3.2. Diameter Diameters of trees measured at 0.1 tree height of L spruce were consistently less than the other treatments from around the age 15 onwards while mean diameters of the H (larch-nursed spruce) treatment were greater than the other treatments from Year 18 onwards (Fig. 3). Patterns of diameters of the NAV (regularly fertilised pure spruce) and NPV (periodically fertilised pure spruce) treatments were very similar. Individual ANOVAs for each year showed signi®cant differences between treatments from age 18 to age 30 (years 18±20 and 28±30, p < 0.05; years 21±27, p < 0.01) with consistent differences between L spruce and H spruce (years 18±21 and 27±30, p < 0.05; years 22±26, p < 0.01). L spruce had consistently lower mean tree diameters than the NAV and NPV treatments between years 26±30 (p < 0.05). Annual diameter increments of H spruce reached a peak between years 14 and 17 and were clearly greater than those of the other treatments (Fig. 4). Growth patterns of the NAV spruce were highly irregular while those of the NPV spruce were relatively uniform by comparison. ANOVAs for each year showed signi®-
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cant differences between treatments from 9 to 22 (years 9±14, p < 0.05: years 15±22, p < 0.01 or less). Annual diameter increments of the L spruce were greater than those of other treatments between years 26±30; however, these differences were not signi®cant. H spruce had consistently higher diameter increments between the years of 13 and 22 than L spruce (years 13±14, p < 0.05: years 15±22, p < 0.01 or less). During the period from age 15 to 17, H spruce had higher diameter increments than NPV spruce (p < 0.05). The erratic patterns of annual increments observed with NAV spruce made meaningful comparisons with other treatments dif®cult. 3.3. Volume Differences in tree volumes (Fig. 5) between treatments were only marginally signi®cant (p < 0.1) from age 23 onwards although mean values for H spruce were consistently higher and those for L spruce consistently lower than other treatments over this period. Annual volume increment (Fig. 6) was only signi®cantly different in year 19 (p < 0.05) when the NAV spruce declined sharply in this year in comparison with NPV spruce (p < 0.05). L spruce appeared to maintain a higher annual volume increment over the last four years in comparison with the other treatments but these differences were not signi®cant. 3.4. Branch number and cross-sectional area There were no signi®cant differences between treatments in the average number of branches measured on 50-cm long stem sections removed from 0.3 and 0.7 tree height (Table 2). There were, however,
Table 2 Mean number of branches and branch cross-sectional area (mm2) measured on 50-cm long stem sections at 0.3 and 0.7 tree height from Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV) and in nursing mixtures with lodgepole pine (L) and hybrid larch (H) Treatment
NAV NPV L H a
0.3 tree height
0.7 tree height
mean number of branches a
mean branch area a (mm2)
mean number of branches a
mean branch area a (mm2)
13.9 14.1 12.0 14.6
126 (18.4) 157 (9.8) 69 (6.9) 212 (38.4)
14.0 12.8 11.5 11.3
184 113 128 114
Standard errors are in parenthesis.
(0.37) (1.73) (0.72) (0.94)
(0.78) (1.28) (0.81) (0.74)
(22.1) (9.5) (25.2) (27.0)
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Table 3 Total number (N) and proportion (%) of branches of diameter 0±10, 11±20 and >20 mm measured on 50-cm long stem sections at 0.3 and 0.7 tree height from Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV) and in nursing mixtures with lodgepole pine (L) and hybrid larch (H) 0.3 tree height 0±10 (mm)
NAV NPV L H
0.7 tree height 11±20 (mm)
>20 (mm)
N
%
N
%
N
%
121 108 129 107
53.0 51.2 71.7 48.9
66 60 39 65
28.7 28.4 21.7 29.7
24 43 12 47
18.3 19.4 7.0 15.1
differences between treatments in mean branch area at both tree heights (p < 0.05). Mean branch area of L spruce was less at 0.3 tree height than in NAV spruce (p < 0.05), NPV spruce and H spruce (p < 0.01). H spruce had the greatest branch area (L spruce and NAV spruce, p < 0.01; NPV spruce, p < 0.05). At 0.7 treeheight, branchareaofNAV sprucewasgreater than that of the NPV spruce and H spruce (p < 0.05). A 2 analysis showed that the number of branches by size category, measured at 0.3 tree height, was not independent of treatments (p < 0.001) (Table 3). L spruce had more smaller branches (0±10 mm diameter) and fewer bigger branches (>20 mm diameter) than in other treatments. A further 2 analysis showed that the number of branches by size category, measured at 0.7 tree height, was independent of treatments. 3.5. Knot area and knot depth L spruce had the lowest mean knotwood area, measured on cross sections of branch whorls at 0.3 tree height, and H spruce and NPV spruce the Table 4 Mean knotwood area (% of cross section) and knot depth (mm) measured on cross sections of branch whorls from 0.3 tree height of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H) Treatment
Knotwood area a (%)
Knot depth a (mm)
NAV NPV L H
11.0 15.9 9.8 16.0
43.3 31.7 36.7 45.0
a
0±10 (mm)
(2.01) (3.24) (1.37) (2.57)
Standard errors are in parenthesis.
(2.66) (2.05) (4.24) (4.01)
N 100 103 98 93
11±20 (mm)
>20 (mm)
%
N
%
N
%
51.2 53.7 56.6 55.2
51 67 44 41
26.2 35.0 25.4 24.2
45 22 31 35
22.3 11.3 18.0 20.6
highest (Table 4). However, these differences were not signi®cant due to high variation and the problems of variable knot angle biasing the measurements. There was a signi®cant difference in knot depth (i.e. the length of visible knots), measured on cross sections of branch whorls, between treatments (p < 0.05). H spruce and NAV spruce had deeper knots than NPV spruce (p < 0.05). 3.6. Stem taper Stem taper, measured from 0.1±0.5 and 0.1± 0.7 tree height, was signi®cantly different between treatments (p < 0.05). Stem taper, measured from 0.1± 0.5 tree height, of H spruce was greater (p < 0.01) than in other treatments (Table 5). Stem taper of L spruce was less than that of NPV spruce (p < 0.01) when measured between 0.1 and 0.5 tree height, and less than that of all other treatments when measured to 0.7 tree height (p < 0.01). Table 5 Stem taper (cm/m), measured between 0.1 and 0.5 tree height (sawlog length) and between 0.1 and 0.7 tree height (merchantable timber length), of stems of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H) Treatment
NAV NPV L H a
Stem taper a 0.1±0.5 tree height (cm/m)
0.1±0.7 tree height (cm/m)
0.86 1.05 0.65 1.52
1.05 1.17 0.83 1.58
(0.056) (0.098) (0.072) (0.098)
Standard errors are in parenthesis.
(0.053) (0.093) (0.065) (0.066)
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Table 6 Relative crown depth (crown depth as a proportion of tree height), vertical crown imbalance ((height to first living whorl ÿ height to first living branch)/tree height) and number of annual rings at the base of the crown of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H) Treatment
Relative crown depth a
Relative vertical crown imbalance a
Number of annual rings at base of the crown a
NAV NPV L H
0.48 0.49 0.49 0.57
0.095 (0.021) 0.109 (0.021) 0.114 (0.017) 0.24 (0.030)
11.85 13.05 12.70 16.75
a
(0.022) (0.033) (0.020) (0.036)
(0.459) (0.634) (0.309) (0.651)
Standard errors are in parenthesis.
3.7. Relative crown depth, vertical crown imbalance and extent of juvenile core derived from crown characteristics There was a signi®cant difference in relative crown depth and vertical crown imbalance between treatments (p < 0.05). H spruce had a greater crown depth (p < 0.05) and vertical crown imbalance (p < 0.01) than that found in the other treatments (Table 6). A signi®cant difference was found between treatments (p < 0.05) in the number of annual rings, recorded on the stem at the height of the ®rst living whorl of branches (i.e. measure of the age of lowest branches in the crown). H spruce had a signi®cantly greater number of annual rings at the base of the crown in comparison with other treatments, indicating that these trees were retaining living branches for longer (p < 0.01). 3.8. Wood density and compression wood content There were no signi®cant differences in whole-disc density and whole-disc ring width between treatments (Table 7). There was a difference, however, in whole-
tree compression wood content between treatments (p < 0.05). Further analysis showed that H spruce had a smaller percentage of compression wood (p < 0.05) than NPV spruce. 4. Discussion Diameter increment of Sitka spruce nursed by hybrid larch (H treatment) was greater than those achieved by the other treatments over the last 12 years of the experiment and signi®cantly greater than the L treatment (lodgepole pine-nursed spruce) over the same period of time. Growth of the NAV treatment (regularly fertilised pure spruce) was not signi®cantly greater than that of the NPV treatment (periodically fertilised pure spruce) suggesting that regular applications of nitrogen fertiliser did not result in increased stem growth. Applying nitrogen fertiliser regularly to pure stands of spruce, however, resulted in an erratic pattern of growth whereas periodic fertiliser applications induced relatively short increases in annual diameter and volume increment of around three years before growth levels returned to pre-fertiliser levels,
Table 7 Mean whole-disc wood density (g/cm3), mean whole-disc ring width (mm) and mean whole-tree compression wood content (%) of Sitka spruce of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H) Treatment
Whole-disc density a (g/cm3)
Whole-disc ring width a (mm)
Whole-tree compression wood content a (%)
NAV NPV L H
0.406 0.416 0.395 0.411
3.32 2.78 2.81 3.19
17.83 19.14 16.66 14.41
a
Standard errors are in parenthesis.
(0.0094) (0.0129) (0.0092) (0.0122)
(0.227) (0.191) (0.191) (0.185)
(1.314) (1.706) (1.513) (1.513)
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and this is typical of the response period to nitrogen reported elsewhere (see, e.g. Dickson and Savill, 1974) on these site types. Patterns of annual diameter increments (Fig. 4) revealed that the greatest increase in growth of the H spruce occurred during the period immediately following canopy closure (years 14±17). The high level of growth of the H spruce during this period demonstrates the failure of the larch to compete with the spruce crowns. This is supported by the very large branch cross-sectional area found on the lower part of the spruce stems within this treatment (Table 2). Although the H spruce produced a similar number of branches to that of the other treatments, it promoted the development of more branches of greater diameter (Table 3). L spruce, on the other hand, had more small branches (0±10 mm diameter) and fewer large branches (>20 mm diameter) in comparison with other treatments. H spruce had deeper knots, thus a greater proportion of the stem cross section is in¯uenced by the presence of knots in comparison with other treatments. This treatment also had a few very large branches (6.4% of branches >30 mm diameter in comparison with no branches of this size in the L spruce and 0.9% in the NPV spruce) and it is the knots formed by these large branches that have a greater in¯uence on timber strength in comparison with small knots. NAV spruce also developed large branches (8.7% of branches >30 mm diameter) supporting the view of Malcolm (1975) who noted increased lateral growth of the crowns of Sitka spruce that had received frequent applications of N. H spruce also had deeper and more imbalanced living crowns with longer living branches (Table 6), in comparison with L, NAV and NPV treatments. Juvenile wood is formed in the region of the living crown and is associated with deleterious properties, such as low density, high late-wood percentage, low tracheid length, high spiral-grain angle and large micro®bril angle (Larson, 1962; Panshin and De Zeeuw, 1970) resulting in warping of sawn boards when seasoned (Massey and Reeb, 1989) and reduced strength (Richardson, 1976; Pearson, 1988). Thus, the larger the crown becomes, or the slower the rate of crown recession, the greater the proportion of juvenile wood. Limiting the proportion of juvenile wood is very important in Sitka spruce since its normal wood has strength properties close to the unacceptable limits for
some industrial uses (Savill and Evans, 1986). Generally, crown size is proportional to growing space (Bennett, 1960; Hall, 1965; Stiell, 1966) whereas crown form is related to nutrient uptake (Bennett, 1960; Hall, 1965). For example, Valinger (1993) found that fertilisation concentrated diameter growth within the upper crowns of Scots pine, as observed in the regularly fertilised spruce of the current study, whereas thinning promoted growth in the lower crown. It appears, therefore, that the larch/spruce mixture was reacting as though a thinning had taken place as the larch became suppressed. H spruce also had the highest stem taper in comparison with the other treatments L spruce the lowest. High taper in logs may lead to excessive wastage during conversion in the sawmill. A taper ratio of 0.83 cm/m is considered as the value beyond which excessive wastage may be expected (Hamilton, 1975). If the stands used in the current study were to be felled now, the L spruce appears to be the best treatment with only one-in-ten of the trees with a taper value >0.83 cm/m in the lower half of the stem, whereas around half of the spruce trees in the pure stands (NAV and NPV treatments) and none of the H spruce would meet this requirement. Heiskanen (1955) and Uusvaara (1974) found a strong relationship between taper and branchiness with an increase in taper with increasing crown vigour. This relationship was evident in the H spruce. The location of maximum stem diameter growth within a tree is usually near the base of the crown. Thus, as taper declines with age, trees with long vigorous crowns will be the last to achieve a cylindrical form (Larson, 1963). While there may be perhaps 15 or more years before the end of the rotation of the stands used in the present study, it seems unlikely that H spruce trees will develop stems with signi®cantly less taper in the future. Mean annual ring width of the H and NAV spruce was greater than that found in the other treatments. Mean ring width values, however, were not statistically different between treatments due to high withinstem variation. Basic density was not in¯uenced by treatment although this is not surprising, given the relatively slow growth rate of all the treatments within the experiment. H spruce, interestingly, had the lowest level of compression wood in comparison with the other treatments. The presence of compression wood is impor-
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tant as sawn boards containing normal and compression wood dry with a differential longitudinal shrinkage and may warp and check (Timell, 1986). Many environmental factors have been found to stimulate the formation of compression wood but one of the most frequently cited is wind (e.g. Nicholls, 1982). The highly tapered spruce stems in mixture with larch, therefore, are probably more stable and sway less, resulting in the formation of less compression wood within the stem. This view is supported by studies carried out on various species by Pillow et al. (1959) that linked a high level of compression wood in trees with slender stems in comparison with that found in tapered stems. While larch as a nurse species greatly enhanced the growth of Sitka spruce, it is linked with deleterious characteristics such as the development of deep and imbalanced crowns associated with tapered stems, large branches, deep knots and possibly a large juvenile core. Since only the `middle' trees of the triplets were used in this study (i.e. one spruce tree either side of the sample tree), the two `outer' trees will almost certainly have characteristics worse than those measured. Overall, the evidence from this study suggests that the use of larch as a nursing species on deep peats is inadvisable and that lodgepole pine is better able to control the branching habit of Sitka spruce with the prospect of better quality sawlogs in the future, albeit with a lower average tree size. This view is contrary to that held by others where the superior growth of larchnursed spruce is the favoured choice of mixture (e.g. Taylor, 1985). While a programme of pruning may reduce the problem of knot development in spruce nursed by larch, such an approach is likely to be prohibitively expensive on the marginal sites where nursing mixtures are usually planted. Periodic applications of fertiliser to stands of pure spruce appears to maintain a growth rate consistently above that of the lodgepole pine-nursed spruce but without many of the disadvantages linked with regular fertiliser applications to pure spruce, particularly the development of large branches and an irregular pattern of annual rings within the wood. The latter regime, however, was never intended to be an economically viable option. Assuming that the wood quality of lodgepole pinenursed Sitka spruce and periodically fertilised pure Sitka spruce is similar, the silvicultural approach adopted largely depends on economics. Sitka spruce
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within the Alaskan lodgepole pine mixture grows less quickly than the periodically fertilised pure spruce, thus yielding less volume in addition to loss of volume if the pine succumbs to `self-thinning'. The cost of this loss of volume, however, must be weighed against the cost of applying fertiliser, perhaps regularly depending on soil fertility, to pure stands of spruce. On deep peats, use of the lodgepole pine mixture probably carries fewer risks, whereas on better quality sites, investing in pure stands with periodic fertiliser applications if needed may give a better ®nancial return. Acknowledgements The authors wish to express their gratitude to Dr. Janet Dutch, Forestry Commission Research Agency for providing background information on the experimental site, and Mr. Bruce Reay, Forestry Department, University of Aberdeen, for preparing samples used for estimating knot area and analysing the data. Funding for this research was provided by the Scottish Forestry Trust. References Bennett, F.A., 1960. Spacing and early growth of planted slash pine. J. For. 58(12), 966±967. Carlyle, J.C., Malcolm, D.C., 1986. Nitrogen availability beneath pure spruce and mixed larch and spruce stands growing on a deep peat. Plant Soil 93, 105±112. Dickson, D.A., Savill, P.S., 1974. Early growth of Picea sitchensis (Bong.) Carr. on deep oligotrophic peat in Northern Ireland. Forestry 47, 57±88. Hall, G.S., 1965. Wood increment and crown distribution relationships in red pine. For. Sci. 11(4), 438±449. Hamilton, G.J., 1975. Forest Mensuration Handbook. F.C. Booklet 39, HMSO. Heiskanen, V., 1955. On the interdependence of annual ring width and sawlog quality. MetsaÈntukt Lait Julk 44, 5. Larson, P.R., 1962. A biological approach to wood quality. Tappi 45(6), 443±448. Larson, P.R., 1963. Development of forest trees. Forest Science Monograph 5. Malcolm, D.C., 1975. The influence of heather on silvicultural practice ± an appraisal. Scottish Forestry 29, 14±24. Massey, J.E., Reeb, J.A., 1989. A method for estimating juvenile content in boards. For. Prod. J. 39, 30±32. Nicholls, J.W.P., 1982. Wind action, leaning trees and compression wood in Pinus radiata D. Don.. Aus. For. Res. 12, 75±91.
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O'Carroll, N., 1978. The nursing of Sitka spruce. I. Japanese larch. Irish Forestry 35, 60±65. Panshin, A.J., De Zeeuw, C., 1970. Textbook of Wood Technology, third edn. McGraw±Hill, New York. Pearson, R.G., 1988. Compressive properties of clear and knotty pine juvenile wood. For. Prod. J. 38, 15±22. Pillow, M.Y., Schafer, E.R., Pew, J.C., 1959. Occurrence of compression wood in black spruce and its effect on properties of ground wood pulp. Pap. Trade J. 102, 28±36. Richardson, B.A., 1976. Wood in Construction. The Construction Press Ltd. Savill, P.S., Evans, J., 1986. Plantation Silviculture in Temperate Regions with Special Reference to the British Isles. Clarendon Press, Oxford.
Stiell, W.M., 1966. Red Pine Crown Development in Relation to Spacing. Dep. For. Can. Publ. 1145. Taylor, C.M.A., 1985. The return of nursing mixtures. Forestry Brit. Timber 14, 18±19. Timell, T.E., 1986. Compression Wood in Gymnosperms. Springer Verlag. Uusvaara, O., 1974. Wood quality in plantation grown Scots pine. Comm. Inst. For. Fenn. 80(2), 105. Valinger, E., 1993. Crown development of Scots pine trees following thinning and nitrogen fertilisation. Studia Forestalia Suecica 188. Watson, B.A., Cameron, A.D., 1995. Some effects of nursing species on stem form, branching habit and compression wood content of Sitka spruce. Scottish Forestry 49, 146±154.
Effect of nursing mixtures on stem form, crown size, branching habit and wood properties of Sitka spruce (Picea sitchensis (Bong.) Carr.) A.D. Cameron*, B.A. Watson Department of Forestry, University of Aberdeen, 581 King St., Aberdeen AB24 5UA, UK
Abstract This study investigated the effects of Alaskan lodgepole pine (Pinus contorta Loud.) and hybrid larch (Larix eurolepis Henry) nurses, planted in triplet mixture with Sitka spruce (Picea sitchensis (Bong.) Carr.), on growth rate, stem form, branching habit and wood properties of the spruce. These mixtures were compared with pure stands of Sitka spruce which had been regularly and periodically fertilised with nitrogen. Hybrid larch promoted diameter increments in Sitka spruce greater than those achieved by the other treatments over the last 12 years of the experiment (current age 30 years). Growth of regularly fertilised pure Sitka spruce was not signi®cantly greater than that of periodically fertilised pure spruce, suggesting that regular applications of nitrogen fertiliser did not result in increased stem growth. The greatest increase in growth of the larch-nursed spruce occurred during the period immediately following canopy closure, thus demonstrating the failure of the larch to compete with the spruce crowns. This period of high growth was associated with a high branch, cross-sectional area and deep knots within the wood on the lower part of the spruce stems in comparison with other treatments. While each treatment was associated with a similar number of branches, lodgepole pine-nursed spruce had more small branches (0±10 mm diameter) and fewer big branches (>20 mm diameter) compared with other treatments, highlighting the capacity of lodgepole pine to control branch development of the spruce. Spruce trees nursed by larch had deeper and more imbalanced living crowns with longer lived branches in comparison with lodgepole pine-nursed spruce, and both the pure spruce treatments, suggesting that more juvenile wood may have formed within the stem. Larch-nursed spruce also had the highest stem taper and lodgepole pinenursed spruce the lowest. Basic wood density was not in¯uenced by treatments. Overall, the evidence from this study suggests that the use of larch as a nursing species on deep peats is inadvisable and that Alaskan lodgepole pine is better able to control the branching habit of Sitka spruce with the prospect of better quality sawlogs in the future, albeit with a lower average tree size. Periodically fertilised pure spruce appears to maintain a growth rate consistently above that of the lodgepole pine-nursed spruce but without many of the disadvantages linked with the regularly fertilised pure spruce, particularly the development of large branches and an irregular pattern of annual rings within the wood. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Picea sitchensis; Nursing mixtures; Branching habit; Wood properties
1. Introduction The advantages of establishing Sitka spruce (Picea sitchensis (Bong.) Carr.) on nitrogen de®cient sites by *Corresponding author. Tel: +44-1224-272673; fax: +44-1224272685; e-mail: [email protected]
planting it with `nurse' species, such as pine (Pinus spp. ) and larch (Larix spp. ), are well known in preventing the spruce suffering a prolonged period of induced nitrogen de®ciency caused by Calluna vulgaris L. (O'Carroll, 1978; Taylor, 1985). Since the 1970s it has become the most frequently used
0378-1127/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 0 3 6 - 5
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management system in Britain for establishing spruce stands on un¯ushed peats. Although the nutritional aspects of nursing mixtures have been studied (Carlyle and Malcolm, 1986), little work has been undertaken on the effect of mixtures on stem form and wood properties of Sitka spruce. Watson and Cameron (1995) noted that trees on the edge of Sitka spruce blocks, adjacent to blocks of Japanese larch (Larix kaempferi (Lamb) Carr.), lodgepole pine (Pinus contorta Loud.) and Scots pine (Pinus sylvestris L.), had unbalanced or deeper living crowns than trees within the spruce block. Since size of the living crown is linked with juvenile wood content (Larson, 1962) in addition to other timber characteristics, these ®ndings may indicate that the wood properties of the spruce, in mixture with other species, are being negatively in¯uenced. Most of the planting of nursing mixtures since the 1970s has been as intimate mixtures, usually in the form of triplets. Thus, the main objective of the present study was to determine the effects of lodgepole pine and hybrid larch (Larix eurolepis Henry) nurses, in triplet mixture with Sitka spruce, on growth rate, stem form, branching habit and wood properties of the spruce. These mixtures were compared with pure stands of Sitka spruce which had been regularly and periodically fertilised with nitrogen. 2. Materials and methods 2.1. Description of study area The study site is located at Strathy Forest, North Scotland (National Grid Reference NC813555). The soil is an acidic deep peat (>2 m) with an underlying geology of Moine schist. The dominant vegetation over most of the site is Calluna vulgaris (L.) with smaller areas of Molinia caerulea (L.). The elevation is over 100 m and this site is highly exposed with a
mean annual rainfall of 975 mm. The site was cultivated by spaced furrow ploughing (1.8 m between rows and 45 cm deep). 2.2. Experimental design The experiment was laid out in 1965 by the Forestry Commission with the original aim of examining the economics of Sitka spruce in comparison with lodgepole pine on deep acid peats. Since the focus shifted in the 1970s towards nursing mixtures, the experiment was used to examine the nursing of Sitka spruce in comparison with pure spruce treatments. Four treatments were examined in the current study (Table 1) with each treatment replicated three times. The species used in the experiment were Sitka spruce (provenance QCI), lodgepole pine (provenance Skagway, Alaska) and hybrid larch (provenance East and Central Scotland). The species were laid out in a `triplet' pattern comprising alternate groups of three spruce and three nurse trees in rows. Adjacent rows of trees were planted to ensure that each triplet of spruce was next to triplets of nursing species. All treatments received P (phosphorous in the form of ground mineral phosphate) in Year 5, K (potassium in the form of KCl) in years 3, 4 and 23, and PK in Year 16. Nitrogen (N) was applied in the form of urea. 2.3. Collection of samples Five spruce trees were felled from the borders of assessment plots, using the middle tree from the triplet, and the north side of each stem marked and total height of each tree measured. Twenty-centimetre long sections were removed from the felled trees at 0.1, 0.5, and 0.9 tree height and two 50-cm sections removed at 0.3 tree height (within the main sawlog length) and 0.7 tree height (within the living
Table 1 Treatment descriptions and codes Treatment code
Treatment description
NAV
pure Sitka spruce regularly fertilised with N (years 5±13 (170 kg/ha/year) then year 16, 19, 22 and 25 (150 kg/ha)), vegetation untreated pure Sitka spruce periodically fertilised with N to prevent induced N deficiency only (year 2, 8, 13 (170 kg/ha), and 18, 23 (150 kg/ha), vegetation untreated Sitka spruce in a triplet arrangement with lodgepole pine, no N, vegetation untreated Sitka spruce in a triplet arrangement with hybrid larch, no N, vegetation untreated
NPV L H
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crown). A 10-cm thick basal disc was also taken. The samples were labelled and placed in a cool, wellventilated store to minimise shrinkage and splitting of the wood and bark. 2.4. Stem analysis Ten centimetre thick discs were removed from each of the stem sections and the upper surfaces ®nely sanded. Stem sections were passed under the microscope of a computer-linked digital positiometer on a N-to-S and E-to-W axis and the distances of the ring boundaries were recorded on computer ®les. Diameter increments derived from these data were used for volume estimation as shown below. The historical pattern of height growth was calculated by describing the top part of each annual stem pro®le as a cone. The height of the cone was calculated using similar triangles based on the stem taper of the previous stem section. Cone height was added to the height of the lower disc, thus giving an estimate of tree height. Analyses of tree-diameter increment were made on the disc at 0.1 tree height. Total volume (from the base to the tree tip) was calculated using Newton's formula for the lower sections and the top section calculated as a cone. The annual volume increment was calculated by subtracting total tree volume in the previous year from the total tree volume. Thus, annual volume increment and total volume were calculated over most of the life of the trees. 2.5. Crown development and branching habit Crown depth was measured on the felled trees as the distance from the tip of the tree to the lowest complete living whorl and crown imbalance measured as the distance between the lowest complete living whorl (with complete sets of living whorls above) and the lowest living branch. The number and diameter of all branches were recorded on the 50-cm long stem sections taken at 0.3 and 0.7 tree height. 2.6. Knotwood area and knot depth Cross-sectional discs, containing a complete whorl of branches, were removed from ca. 0.3 tree height. These were used to measure knot area and knot depth. Measuring knot development on stem whorls assesses
115
the worst locations in the log for these defects and, thus, the weakest points in sawn timber. The disc was cut successively into 5-mm slices and a tracing made of each slice. From these tracings, depth and maximum cross-sectional area of individual knots were calculated. 2.7. Stem taper Taper was derived from the difference between diameters (cm) at either end of stem sections, cut from 0.1±0.5 tree height (approximate length of the butt log) and from 0.1±0.7 tree height (approximate length of the merchantable log), divided by the length of each section (m). 2.8. Estimated extent of juvenile core from crown characteristics The extent of juvenile core was made on each tree by counting the number of annual rings at the base of the living crown (i.e. the age of the lowest living whorl of branches). Juvenile wood is produced within the living crown and there is a strong association between live crown depth and proportion of juvenile core within the stem (e.g. Larson, 1962). 2.9. Wood density and compression wood content Basic density (oven dry weight/water swollen volume) was measured on small blocks of wood (2 cm 1 cm 5 annual rings) cut from the sample discs, in a series from the bark to the pith, taken at 0.1 tree height. Compression wood was assessed on thin discs, 3±6 mm thick, cut from the main sample discs. These were immersed in water for at least 24 h, then placed on a light table and areas of compression wood, showing up as red and opaque, were traced. The areas were coloured black, photocopied and assessed using a machine designed to measure leaf area. 2.10. Statistical analysis Analyses were performed using analysis of variance. Analyses of the number of branches by size category were carried out using 2-tests. The vertical bars in Figs. 1±6 are 0.5s.e. rather than 1s.e. to improve clarity of the graphs.
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Fig. 1. Mean tree height of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H). Vertical bars 0.5s.e.
Fig. 2. Annual height increment of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H). Vertical bars 0.5s.e.
A.D. Cameron, B.A. Watson / Forest Ecology and Management 122 (1999) 113±124
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Fig. 3. Mean tree diameter at 0.1 tree height of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H). Vertical bars 0.5s.e.
Fig. 4. Annual diameter increment of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H). Vertical bars 0.5s.e.
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Fig. 5. Mean tree volume of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H). Vertical bars 0.5s.e.
Fig. 6. Annual volume increment of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H). Vertical bars 0.5s.e.
A.D. Cameron, B.A. Watson / Forest Ecology and Management 122 (1999) 113±124
3. Results 3.1. Height Heights of L spruce (nursed by lodgepole pine) were greater by the end of the study period than the other treatments (Fig. 1) although ANOVAs for each year showed no signi®cant treatment effects. Annual height increments were variable, with only a general decline noted from Year 23 to Year 30, and no signi®cant treatment effects were found (Fig. 2). 3.2. Diameter Diameters of trees measured at 0.1 tree height of L spruce were consistently less than the other treatments from around the age 15 onwards while mean diameters of the H (larch-nursed spruce) treatment were greater than the other treatments from Year 18 onwards (Fig. 3). Patterns of diameters of the NAV (regularly fertilised pure spruce) and NPV (periodically fertilised pure spruce) treatments were very similar. Individual ANOVAs for each year showed signi®cant differences between treatments from age 18 to age 30 (years 18±20 and 28±30, p < 0.05; years 21±27, p < 0.01) with consistent differences between L spruce and H spruce (years 18±21 and 27±30, p < 0.05; years 22±26, p < 0.01). L spruce had consistently lower mean tree diameters than the NAV and NPV treatments between years 26±30 (p < 0.05). Annual diameter increments of H spruce reached a peak between years 14 and 17 and were clearly greater than those of the other treatments (Fig. 4). Growth patterns of the NAV spruce were highly irregular while those of the NPV spruce were relatively uniform by comparison. ANOVAs for each year showed signi®-
119
cant differences between treatments from 9 to 22 (years 9±14, p < 0.05: years 15±22, p < 0.01 or less). Annual diameter increments of the L spruce were greater than those of other treatments between years 26±30; however, these differences were not signi®cant. H spruce had consistently higher diameter increments between the years of 13 and 22 than L spruce (years 13±14, p < 0.05: years 15±22, p < 0.01 or less). During the period from age 15 to 17, H spruce had higher diameter increments than NPV spruce (p < 0.05). The erratic patterns of annual increments observed with NAV spruce made meaningful comparisons with other treatments dif®cult. 3.3. Volume Differences in tree volumes (Fig. 5) between treatments were only marginally signi®cant (p < 0.1) from age 23 onwards although mean values for H spruce were consistently higher and those for L spruce consistently lower than other treatments over this period. Annual volume increment (Fig. 6) was only signi®cantly different in year 19 (p < 0.05) when the NAV spruce declined sharply in this year in comparison with NPV spruce (p < 0.05). L spruce appeared to maintain a higher annual volume increment over the last four years in comparison with the other treatments but these differences were not signi®cant. 3.4. Branch number and cross-sectional area There were no signi®cant differences between treatments in the average number of branches measured on 50-cm long stem sections removed from 0.3 and 0.7 tree height (Table 2). There were, however,
Table 2 Mean number of branches and branch cross-sectional area (mm2) measured on 50-cm long stem sections at 0.3 and 0.7 tree height from Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV) and in nursing mixtures with lodgepole pine (L) and hybrid larch (H) Treatment
NAV NPV L H a
0.3 tree height
0.7 tree height
mean number of branches a
mean branch area a (mm2)
mean number of branches a
mean branch area a (mm2)
13.9 14.1 12.0 14.6
126 (18.4) 157 (9.8) 69 (6.9) 212 (38.4)
14.0 12.8 11.5 11.3
184 113 128 114
Standard errors are in parenthesis.
(0.37) (1.73) (0.72) (0.94)
(0.78) (1.28) (0.81) (0.74)
(22.1) (9.5) (25.2) (27.0)
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Table 3 Total number (N) and proportion (%) of branches of diameter 0±10, 11±20 and >20 mm measured on 50-cm long stem sections at 0.3 and 0.7 tree height from Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV) and in nursing mixtures with lodgepole pine (L) and hybrid larch (H) 0.3 tree height 0±10 (mm)
NAV NPV L H
0.7 tree height 11±20 (mm)
>20 (mm)
N
%
N
%
N
%
121 108 129 107
53.0 51.2 71.7 48.9
66 60 39 65
28.7 28.4 21.7 29.7
24 43 12 47
18.3 19.4 7.0 15.1
differences between treatments in mean branch area at both tree heights (p < 0.05). Mean branch area of L spruce was less at 0.3 tree height than in NAV spruce (p < 0.05), NPV spruce and H spruce (p < 0.01). H spruce had the greatest branch area (L spruce and NAV spruce, p < 0.01; NPV spruce, p < 0.05). At 0.7 treeheight, branchareaofNAV sprucewasgreater than that of the NPV spruce and H spruce (p < 0.05). A 2 analysis showed that the number of branches by size category, measured at 0.3 tree height, was not independent of treatments (p < 0.001) (Table 3). L spruce had more smaller branches (0±10 mm diameter) and fewer bigger branches (>20 mm diameter) than in other treatments. A further 2 analysis showed that the number of branches by size category, measured at 0.7 tree height, was independent of treatments. 3.5. Knot area and knot depth L spruce had the lowest mean knotwood area, measured on cross sections of branch whorls at 0.3 tree height, and H spruce and NPV spruce the Table 4 Mean knotwood area (% of cross section) and knot depth (mm) measured on cross sections of branch whorls from 0.3 tree height of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H) Treatment
Knotwood area a (%)
Knot depth a (mm)
NAV NPV L H
11.0 15.9 9.8 16.0
43.3 31.7 36.7 45.0
a
0±10 (mm)
(2.01) (3.24) (1.37) (2.57)
Standard errors are in parenthesis.
(2.66) (2.05) (4.24) (4.01)
N 100 103 98 93
11±20 (mm)
>20 (mm)
%
N
%
N
%
51.2 53.7 56.6 55.2
51 67 44 41
26.2 35.0 25.4 24.2
45 22 31 35
22.3 11.3 18.0 20.6
highest (Table 4). However, these differences were not signi®cant due to high variation and the problems of variable knot angle biasing the measurements. There was a signi®cant difference in knot depth (i.e. the length of visible knots), measured on cross sections of branch whorls, between treatments (p < 0.05). H spruce and NAV spruce had deeper knots than NPV spruce (p < 0.05). 3.6. Stem taper Stem taper, measured from 0.1±0.5 and 0.1± 0.7 tree height, was signi®cantly different between treatments (p < 0.05). Stem taper, measured from 0.1± 0.5 tree height, of H spruce was greater (p < 0.01) than in other treatments (Table 5). Stem taper of L spruce was less than that of NPV spruce (p < 0.01) when measured between 0.1 and 0.5 tree height, and less than that of all other treatments when measured to 0.7 tree height (p < 0.01). Table 5 Stem taper (cm/m), measured between 0.1 and 0.5 tree height (sawlog length) and between 0.1 and 0.7 tree height (merchantable timber length), of stems of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H) Treatment
NAV NPV L H a
Stem taper a 0.1±0.5 tree height (cm/m)
0.1±0.7 tree height (cm/m)
0.86 1.05 0.65 1.52
1.05 1.17 0.83 1.58
(0.056) (0.098) (0.072) (0.098)
Standard errors are in parenthesis.
(0.053) (0.093) (0.065) (0.066)
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Table 6 Relative crown depth (crown depth as a proportion of tree height), vertical crown imbalance ((height to first living whorl ÿ height to first living branch)/tree height) and number of annual rings at the base of the crown of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H) Treatment
Relative crown depth a
Relative vertical crown imbalance a
Number of annual rings at base of the crown a
NAV NPV L H
0.48 0.49 0.49 0.57
0.095 (0.021) 0.109 (0.021) 0.114 (0.017) 0.24 (0.030)
11.85 13.05 12.70 16.75
a
(0.022) (0.033) (0.020) (0.036)
(0.459) (0.634) (0.309) (0.651)
Standard errors are in parenthesis.
3.7. Relative crown depth, vertical crown imbalance and extent of juvenile core derived from crown characteristics There was a signi®cant difference in relative crown depth and vertical crown imbalance between treatments (p < 0.05). H spruce had a greater crown depth (p < 0.05) and vertical crown imbalance (p < 0.01) than that found in the other treatments (Table 6). A signi®cant difference was found between treatments (p < 0.05) in the number of annual rings, recorded on the stem at the height of the ®rst living whorl of branches (i.e. measure of the age of lowest branches in the crown). H spruce had a signi®cantly greater number of annual rings at the base of the crown in comparison with other treatments, indicating that these trees were retaining living branches for longer (p < 0.01). 3.8. Wood density and compression wood content There were no signi®cant differences in whole-disc density and whole-disc ring width between treatments (Table 7). There was a difference, however, in whole-
tree compression wood content between treatments (p < 0.05). Further analysis showed that H spruce had a smaller percentage of compression wood (p < 0.05) than NPV spruce. 4. Discussion Diameter increment of Sitka spruce nursed by hybrid larch (H treatment) was greater than those achieved by the other treatments over the last 12 years of the experiment and signi®cantly greater than the L treatment (lodgepole pine-nursed spruce) over the same period of time. Growth of the NAV treatment (regularly fertilised pure spruce) was not signi®cantly greater than that of the NPV treatment (periodically fertilised pure spruce) suggesting that regular applications of nitrogen fertiliser did not result in increased stem growth. Applying nitrogen fertiliser regularly to pure stands of spruce, however, resulted in an erratic pattern of growth whereas periodic fertiliser applications induced relatively short increases in annual diameter and volume increment of around three years before growth levels returned to pre-fertiliser levels,
Table 7 Mean whole-disc wood density (g/cm3), mean whole-disc ring width (mm) and mean whole-tree compression wood content (%) of Sitka spruce of Sitka spruce in pure stands fertilised regularly (NAV) and periodically (NPV), and in nursing mixtures with lodgepole pine (L) and hybrid larch (H) Treatment
Whole-disc density a (g/cm3)
Whole-disc ring width a (mm)
Whole-tree compression wood content a (%)
NAV NPV L H
0.406 0.416 0.395 0.411
3.32 2.78 2.81 3.19
17.83 19.14 16.66 14.41
a
Standard errors are in parenthesis.
(0.0094) (0.0129) (0.0092) (0.0122)
(0.227) (0.191) (0.191) (0.185)
(1.314) (1.706) (1.513) (1.513)
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and this is typical of the response period to nitrogen reported elsewhere (see, e.g. Dickson and Savill, 1974) on these site types. Patterns of annual diameter increments (Fig. 4) revealed that the greatest increase in growth of the H spruce occurred during the period immediately following canopy closure (years 14±17). The high level of growth of the H spruce during this period demonstrates the failure of the larch to compete with the spruce crowns. This is supported by the very large branch cross-sectional area found on the lower part of the spruce stems within this treatment (Table 2). Although the H spruce produced a similar number of branches to that of the other treatments, it promoted the development of more branches of greater diameter (Table 3). L spruce, on the other hand, had more small branches (0±10 mm diameter) and fewer large branches (>20 mm diameter) in comparison with other treatments. H spruce had deeper knots, thus a greater proportion of the stem cross section is in¯uenced by the presence of knots in comparison with other treatments. This treatment also had a few very large branches (6.4% of branches >30 mm diameter in comparison with no branches of this size in the L spruce and 0.9% in the NPV spruce) and it is the knots formed by these large branches that have a greater in¯uence on timber strength in comparison with small knots. NAV spruce also developed large branches (8.7% of branches >30 mm diameter) supporting the view of Malcolm (1975) who noted increased lateral growth of the crowns of Sitka spruce that had received frequent applications of N. H spruce also had deeper and more imbalanced living crowns with longer living branches (Table 6), in comparison with L, NAV and NPV treatments. Juvenile wood is formed in the region of the living crown and is associated with deleterious properties, such as low density, high late-wood percentage, low tracheid length, high spiral-grain angle and large micro®bril angle (Larson, 1962; Panshin and De Zeeuw, 1970) resulting in warping of sawn boards when seasoned (Massey and Reeb, 1989) and reduced strength (Richardson, 1976; Pearson, 1988). Thus, the larger the crown becomes, or the slower the rate of crown recession, the greater the proportion of juvenile wood. Limiting the proportion of juvenile wood is very important in Sitka spruce since its normal wood has strength properties close to the unacceptable limits for
some industrial uses (Savill and Evans, 1986). Generally, crown size is proportional to growing space (Bennett, 1960; Hall, 1965; Stiell, 1966) whereas crown form is related to nutrient uptake (Bennett, 1960; Hall, 1965). For example, Valinger (1993) found that fertilisation concentrated diameter growth within the upper crowns of Scots pine, as observed in the regularly fertilised spruce of the current study, whereas thinning promoted growth in the lower crown. It appears, therefore, that the larch/spruce mixture was reacting as though a thinning had taken place as the larch became suppressed. H spruce also had the highest stem taper in comparison with the other treatments L spruce the lowest. High taper in logs may lead to excessive wastage during conversion in the sawmill. A taper ratio of 0.83 cm/m is considered as the value beyond which excessive wastage may be expected (Hamilton, 1975). If the stands used in the current study were to be felled now, the L spruce appears to be the best treatment with only one-in-ten of the trees with a taper value >0.83 cm/m in the lower half of the stem, whereas around half of the spruce trees in the pure stands (NAV and NPV treatments) and none of the H spruce would meet this requirement. Heiskanen (1955) and Uusvaara (1974) found a strong relationship between taper and branchiness with an increase in taper with increasing crown vigour. This relationship was evident in the H spruce. The location of maximum stem diameter growth within a tree is usually near the base of the crown. Thus, as taper declines with age, trees with long vigorous crowns will be the last to achieve a cylindrical form (Larson, 1963). While there may be perhaps 15 or more years before the end of the rotation of the stands used in the present study, it seems unlikely that H spruce trees will develop stems with signi®cantly less taper in the future. Mean annual ring width of the H and NAV spruce was greater than that found in the other treatments. Mean ring width values, however, were not statistically different between treatments due to high withinstem variation. Basic density was not in¯uenced by treatment although this is not surprising, given the relatively slow growth rate of all the treatments within the experiment. H spruce, interestingly, had the lowest level of compression wood in comparison with the other treatments. The presence of compression wood is impor-
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tant as sawn boards containing normal and compression wood dry with a differential longitudinal shrinkage and may warp and check (Timell, 1986). Many environmental factors have been found to stimulate the formation of compression wood but one of the most frequently cited is wind (e.g. Nicholls, 1982). The highly tapered spruce stems in mixture with larch, therefore, are probably more stable and sway less, resulting in the formation of less compression wood within the stem. This view is supported by studies carried out on various species by Pillow et al. (1959) that linked a high level of compression wood in trees with slender stems in comparison with that found in tapered stems. While larch as a nurse species greatly enhanced the growth of Sitka spruce, it is linked with deleterious characteristics such as the development of deep and imbalanced crowns associated with tapered stems, large branches, deep knots and possibly a large juvenile core. Since only the `middle' trees of the triplets were used in this study (i.e. one spruce tree either side of the sample tree), the two `outer' trees will almost certainly have characteristics worse than those measured. Overall, the evidence from this study suggests that the use of larch as a nursing species on deep peats is inadvisable and that lodgepole pine is better able to control the branching habit of Sitka spruce with the prospect of better quality sawlogs in the future, albeit with a lower average tree size. This view is contrary to that held by others where the superior growth of larchnursed spruce is the favoured choice of mixture (e.g. Taylor, 1985). While a programme of pruning may reduce the problem of knot development in spruce nursed by larch, such an approach is likely to be prohibitively expensive on the marginal sites where nursing mixtures are usually planted. Periodic applications of fertiliser to stands of pure spruce appears to maintain a growth rate consistently above that of the lodgepole pine-nursed spruce but without many of the disadvantages linked with regular fertiliser applications to pure spruce, particularly the development of large branches and an irregular pattern of annual rings within the wood. The latter regime, however, was never intended to be an economically viable option. Assuming that the wood quality of lodgepole pinenursed Sitka spruce and periodically fertilised pure Sitka spruce is similar, the silvicultural approach adopted largely depends on economics. Sitka spruce
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within the Alaskan lodgepole pine mixture grows less quickly than the periodically fertilised pure spruce, thus yielding less volume in addition to loss of volume if the pine succumbs to `self-thinning'. The cost of this loss of volume, however, must be weighed against the cost of applying fertiliser, perhaps regularly depending on soil fertility, to pure stands of spruce. On deep peats, use of the lodgepole pine mixture probably carries fewer risks, whereas on better quality sites, investing in pure stands with periodic fertiliser applications if needed may give a better ®nancial return. Acknowledgements The authors wish to express their gratitude to Dr. Janet Dutch, Forestry Commission Research Agency for providing background information on the experimental site, and Mr. Bruce Reay, Forestry Department, University of Aberdeen, for preparing samples used for estimating knot area and analysing the data. Funding for this research was provided by the Scottish Forestry Trust. References Bennett, F.A., 1960. Spacing and early growth of planted slash pine. J. For. 58(12), 966±967. Carlyle, J.C., Malcolm, D.C., 1986. Nitrogen availability beneath pure spruce and mixed larch and spruce stands growing on a deep peat. Plant Soil 93, 105±112. Dickson, D.A., Savill, P.S., 1974. Early growth of Picea sitchensis (Bong.) Carr. on deep oligotrophic peat in Northern Ireland. Forestry 47, 57±88. Hall, G.S., 1965. Wood increment and crown distribution relationships in red pine. For. Sci. 11(4), 438±449. Hamilton, G.J., 1975. Forest Mensuration Handbook. F.C. Booklet 39, HMSO. Heiskanen, V., 1955. On the interdependence of annual ring width and sawlog quality. MetsaÈntukt Lait Julk 44, 5. Larson, P.R., 1962. A biological approach to wood quality. Tappi 45(6), 443±448. Larson, P.R., 1963. Development of forest trees. Forest Science Monograph 5. Malcolm, D.C., 1975. The influence of heather on silvicultural practice ± an appraisal. Scottish Forestry 29, 14±24. Massey, J.E., Reeb, J.A., 1989. A method for estimating juvenile content in boards. For. Prod. J. 39, 30±32. Nicholls, J.W.P., 1982. Wind action, leaning trees and compression wood in Pinus radiata D. Don.. Aus. For. Res. 12, 75±91.
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O'Carroll, N., 1978. The nursing of Sitka spruce. I. Japanese larch. Irish Forestry 35, 60±65. Panshin, A.J., De Zeeuw, C., 1970. Textbook of Wood Technology, third edn. McGraw±Hill, New York. Pearson, R.G., 1988. Compressive properties of clear and knotty pine juvenile wood. For. Prod. J. 38, 15±22. Pillow, M.Y., Schafer, E.R., Pew, J.C., 1959. Occurrence of compression wood in black spruce and its effect on properties of ground wood pulp. Pap. Trade J. 102, 28±36. Richardson, B.A., 1976. Wood in Construction. The Construction Press Ltd. Savill, P.S., Evans, J., 1986. Plantation Silviculture in Temperate Regions with Special Reference to the British Isles. Clarendon Press, Oxford.
Stiell, W.M., 1966. Red Pine Crown Development in Relation to Spacing. Dep. For. Can. Publ. 1145. Taylor, C.M.A., 1985. The return of nursing mixtures. Forestry Brit. Timber 14, 18±19. Timell, T.E., 1986. Compression Wood in Gymnosperms. Springer Verlag. Uusvaara, O., 1974. Wood quality in plantation grown Scots pine. Comm. Inst. For. Fenn. 80(2), 105. Valinger, E., 1993. Crown development of Scots pine trees following thinning and nitrogen fertilisation. Studia Forestalia Suecica 188. Watson, B.A., Cameron, A.D., 1995. Some effects of nursing species on stem form, branching habit and compression wood content of Sitka spruce. Scottish Forestry 49, 146±154.