Introduction

Dimensions of fibres have been subject of investigationsfor more than a century, and the general pattern of vari-ations in fibre dimensions of conifers is well known (e.g.Panshin and de Zeeuw 1980; Zobel and van Buijtenen1989). Fibre length and diameter increase rapidly andnon-linearly during the first years of radial growth, andthereafter more gradually in mature wood. Within astem, the main factor causing variation in fibre proper-ties is ageing (maturation) of the cambium producingnew fibres. Olesen (1982) stated that the vascular cambi-um is subject to two types of maturation processes,namely (i) formation of the cambium from the apicalmeristem (cyclophysis) and (ii) processes which thecambium undergoes after its formation. According toOlesen’s investigations in Norway spruce, fibre widthchanges both in a radial direction and with height in thestem.

Helander (1933) found that trees growing in sites oflower fertility have longer fibres than trees on fertilesites. Differences in cell length, diameter, cell wall thick-ness and cell arrangement reflect the changes occurringin the cambium and the effect of environmental factors.Xylem development is influenced by external factorssuch as water availability, temperature, light and nutri-ents. Recent investigations have shown that increasing

availability of light, water and nutrients increased treegrowth and fibre diameter, but decreased fibre length(Lindström 1997; Dutilleul et al. 1998; Herman et al.1998). In addition to maturation processes, fibre proper-ties seem, therefore, to be related to the growth rate oftrees. Contradictory results have, however, been report-ed on the effects of growth rate on fibre dimensions (e.g.Zobel and van Buijtenen 1989; Bergqvist et al. 2000).

From a management point of view, more detailed in-formation is needed on the effects of silvicultural treat-ments on fibre properties. Forest management haschanged during the last decades and delayed thinningsare a problem, especially in young stands. At the sametime there is a general trend for more intensive silvicul-ture. For example, one of the main goals of Finland’s‘National Forest Programme 2010’ is to intensify silvi-cultural treatments. More intensive silvicultural prac-tices will increase the rate of tree growth, but will alsolead to changes in fibre and wood properties.

Characteristics of fibres largely determine the suit-ability of wood for further processing in the pulp andpaper industry, as well as in the sawmill (e.g. Tyrväinen1995). Fibre morphology and cell wall structure directlyinfluence fibre flexibility, plasticity and resistance toprocessing and, therefore, influence the strength andother physical and optical properties of the end-prod-

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Holzforschung / Vol. 56 / 2002 / No. 5© Copyright 2002 Walter de Gruyter · Berlin · New York

Holzforschung56 (2002) 449–460

Effect of Growth Rate on Fibre Characteristics in NorwaySpruce (Picea abies (L.) Karst.)By Harri Mäkinen1, Pekka Saranpää1 and Sune Linder2

1 Finnish Forest Research Institute,Vantaa, Finland2 Swedish University of Agricultural Sciences, Department for Production Ecology, Uppsala, Sweden

Summary

To study the effect of growth rate on fibre characteristics and their variations in Norway spruce,trees were sampled in a nutrient optimisation experiment in northern Sweden. Data was collectedfrom 24 trees (40 years old) from fertilised and control plots after 12 years of annual nutrient ap-plication, as well as from older trees outside the experimental area. Fibre length, fibre diameter,cell wall thickness, lumen diameter and cell wall percentage were measured from every third an-nual ring at breast height and at a height of 4 m. Fibre properties, as well as their standard devia-tion, were closely related to ring number and distance from the pith. Intra-ring variation of fibrecharacteristics was high compared to their variation between trees. Fertilisation reduced fibrelength and cell wall thickness, but increased fibre and lumen diameter in rings of the same age.The difference in fibre width, cell wall thickness and lumen diameter between fertilised and con-trol trees was less apparent, but a greater difference in fibre length was found between the treat-ments with regard to distance from the pith. There was a similar effect of fertilisation on fibreproperties in early- and latewood. The effect of enhanced growth rate was less pronounced at aheight of 4 m (near the pith) than at breast height (in older rings). It was demonstrated that it ispossible to model intra-tree variability of fibre characteristics using ring width and cambial age asindependent variables. Models presented are, however, limited by the relatively young age of thesample trees used.

Keywords

FertilisationFibre lengthFibre widthCell wall thickness

uct. Therefore, knowledge of the processes determiningfibre characteristics under different growing conditionsand silvicultural treatments is of economic interest. Un-til now, the pulp and paper industry has mainly exam-ined fibre properties and chemical composition of woodafter processing, and the biological aspects related towood formation have been taken into account to a less-er degree.

In addition to mean fibre characteristics, their varia-tion influences the quality of wood as raw material.Many paper properties are related to mean fibre charac-teristics, but knowing also their variation may lead to abetter control of the properties of the end-product.Studies on frequency distributions of fibre characteris-tics are, however, scarce. As an exception, Ollinmaa(1959) and Herman et al. (1998) reported on the varia-tion of fibre characteristics in relation to cambium age,growth rate and height within the tree.

The aim of the present study was to investigate theeffects of different growth rates obtained by fertilisationin a long-term nutrient-optimisation experiment on thefibre properties in young Norway spruce trees. The ex-periment was established to demonstrate the potentialyield of Norway spruce, under given climatic conditionsand non-limiting soil water, by optimising the nutrition-al status of the stand (Linder and Flower-Ellis 1992; Lin-der 1995). The yield of stemwood surpassed the bestyields obtained by conventional silvicultural means (cf.Bergh et al. 1999), and should therefore represent theextreme impact of fertilisation on fibre properties. Theeffect of treatment on the branch characteristics in thesame stands was presented in Mäkinen et al. (2001).

Materials and Methods

Site description and treatments

The study was performed in a long-term nutrient-optimisationexperiment at Flakaliden (64 °07’N; 19 °27’E; alt. 310 m a.s.l.)in northern Sweden. The principal aim of the experiment wasto demonstrate the potential yield of Norway spruce (Piceaabies (L.) Karst.), under given climatic conditions and non-lim-iting soil water, by optimising the nutritional status of the

stands, at the same time as leaching of nutrients to the ground-water was avoided (cf. Linder and Flower-Ellis 1992; Linder1995). The experiment was established in 1986 in a young Nor-way spruce stand planted in 1963 with four-year-old seedlingsof a local provenance. Before the initiation of the experiment,the site was classified according to Hägglund and Lundmark(1977) as fairly infertile (H100=17–19 m). The monthly meantemperature at the site varies from –8.7 °C in February to14.4 °C in July and mean annual precipitation is approximately600 mm, of which more than one-third falls as snow (Berghet al. 1999).

The treatments which began in 1987 included non-treatedcontrol plots, irrigated plots and two nutrient optimisationtreatments. Treatments were replicated four times in a ran-domised block design, and each replicate consisted of a doubleplot made up of two 50 × 50 m plots. Each plot contained a netplot (1000 m2) surrounded by a buffer zone. In the presentstudy, only control (C) and irrigated-fertilised (IL) plots wereincluded. In the IL treatment, all essential macro- and mi-cronutrients were supplied every second day during the grow-ing season (mid-June to mid-August) and water was suppliedto maintain a soil water potential above –100 kPa.The amountand composition of the nutrient addition was determined eachyear on the basis of nutrient analysis of foliage, the monitoringof nutrients in the soil water and predicted growth response.For further details regarding treatments, see Linder (1995).

When establishing the experiment in 1986, the stand densitywas ca. 2400 trees per hectare and no thinnings were donethereafter. At that time the mean height of the trees in C- andIL-plots was 2.8 m and 3.0 m, respectively, and diameter atbreast height 33.3 mm and 35.8 mm (Bergh et al. 1999).

Measurements

The 24 trees used in the present study were harvested in au-tumn 1998, after 12 years of treatment. On each replicatedplot, three trees were chosen as sample trees according to thediameter distribution of the trees in the stands, i.e. one treerepresenting the mean diameter and two trees larger than 1.5standard deviation added to the mean diameter. Suppressedtrees were not measured because they will be removed in thin-nings. Stem diameter at breast height, tree height, crownlength, maximum crown width and crown width perpendicularto the maximum width were recorded for each sample tree.Statistics of the sample trees are given in Table 1.

The sample trees were felled and 15 cm-thick stem discswere taken at breast height and at a height of 4 m. Along thesouth-north radius of each disc, two 3 cm-thick wedges were

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Table 1. Statistics of the sample trees from control (C) and irrigated-fertilised (IL) plots, respectively. Twelve trees per treatmentwere used in the analysis.Values within brackets represent plot means in autumn 1996 based on Bergh et al. (1999) and an invento-ry of four replicated plots in autumn 1998 (Linder, unpubl.)

Mean STD Min Max

Diameter at breast height (cm) C 9.4 (8.0) 1.6 (0.5) 6.6 11.7IL 14.8 (12.7) 2.8 (0.5) 10.8 17.9

Height (m) C 6.6 (6.3) 1.0 (0.3) 5.2 8.1IL 8.9 (8.7) 0.7 (0.8) 7.9 10.1

Annual radial increment1 (mm) C 2.0 (2.0) 0.5 (0.2) 0.93 3.46IL 4.1 (3.8) 1.3 (0.1) 1.34 6.73

Annual height increment1 (cm) C 28.7 (29.2) 10.4 (2.9) 7.0 63.0IL 42.6 (47.9) 16.0 (5.5) 5.0 79.0

1 Mean value for the treatment period 1987–1998

sawn through the discs. From the first wedge, annual radial in-crements were measured in two directions (south-north) basedon computer aided tree-ring measurement equipment includ-ing a stereomicroscope connected to a video camera.

At breast height, the sample trees had on average 21 annualrings at the time of the measurements. It was supposed that thelast 12 annual rings formed during the fertilisation period rep-resented mainly mature wood. At a height of 4 m on the stem,the sample trees had on average 10 annual rings, i.e. all ringswere formed after the initiation of the experiment and repre-sented juvenile wood.

Beginning from the pith, earlywood zones of every third an-nual ring of the south radius of the wedges, from breast heightand a height of 4 m, were split into small pieces and then mac-erated in glacial acetic acid and hydrogen peroxide solution(1:1, v/v) at 60 °C overnight (Franklin 1945). At breast height,latewood zones of the same rings of two fertilised and two con-trol trees were also macerated. Suspensions of washed fibreswere deposed on a microscope slide.The images (256 levels ona grey scale) were captured with a video camera (Cohu 4912;Cohu Inc., USA) attached to a light microscope (Olympus BH-2; Olympus Optical Co., Japan).The resolution of captured im-ages was 10.64 µm per pixel. The lengths of 50 unbroken fibreswere measured with the help of an image analysis system (Im-age-Pro Plus; Media Cybernetics, USA). Measurements of dif-ferent fibre properties on the individual trees at different stemheights are described in Table 2.

In the earlywood, fibre diameters of the same rings weremeasured from the suspension by the means of Kajaani Fiber-Lab (Kajaani Electronics Ltd, Kajaani, Finland). The numberof measured fibre diameters per growth ring ranged between784 and 3447, with an average of 2388. Since the FiberLab isnot able to distinguish cut, curled and broken fibres or rayparenchyma elements from unbroken fibres, fibre lengthsmeasured with the FiberLab were not used.

For samples at breast height of 7 fertilised and 7 controltrees, the same annual rings of the wedges were cut into 16 µmthick cross-sections on a cryo-microtome (–16 °C). The sec-tions were stained with a 1% solution of safranin, immersed ina graded ethanol series, embedded with Canada balsam andmounted with a medium. Three parallel images from early-wood and latewood of each sample were captured using theequipment described above. In order to avoid irregular fibresaround the boundary, the images were placed about five fibresaway in the radial direction from the annual ring boundary,both in early- and latewood.

Lumen diameter of an individual fibre was calculated as theaverage length of the diameters measured at two-degree-inter-vals joining two outline points of the lumen and passingthrough the centroid. They were measured from 7 fertilised

and 7 control trees using an objective lens with magnificationof 20. Resolution of the captured images was 0.54 µm per pixel.The number of lumen diameters measured per image was onaverage 79, i.e. 237 per annual ring.

Cell wall proportion from the whole image area was meas-ured with a magnification of 40 and resolution of 0.27 µm perpixel. Cell wall proportion was only measured on the early-wood images. In earlywood and latewood, the thickness ofdouble cell walls in the radial direction was measured with amagnification of 60 and resolution of 0.18 µm per pixel. Thenumber of cell wall widths measured per image was on average14, i.e. 42 per annual ring.

In order to compare fertilised and non-fertilised trees ofsimilar size, three older non-fertilised trees were sampled out-side the experimental area. Fibre properties of the older treeswere measured as described in Table 2.

Statistical analyses

All fibres of each annual ring were treated as individual obser-vations, rather than as mean dimensions only. This approachmade it possible to use all the information contained in the dataset. Observations in the data have a hierarchical structure(block, plot, tree, stem height, annual ring, fibre), i.e. the individ-ual observations belonging to the same batch are not independ-ent of each other. In the mixed models, the mutual correlationstructure of the dependent variable can be taken into accountby allowing the parameters to vary randomly around the fixedpopulation mean from one individual to another (e.g. Searleet al. 1992). Restricted maximum likelihood (REML) estima-tion in the MIXED procedure of SAS (SAS Institute, Inc. 1996)was used in estimating fixed and random parameters.

The treatment effects on fibre characteristics were tested byusing the model:

Ybpthri = µ + τIL + βxbpth + ub + ubpt + εbpthri (1)

where Ybpthri is a fibre characteristic (or its standard deviation)of fibre i in annual ring r assigned from block b, plot p, tree tand stem height h. The µ is the overall mean of the controlplots, τIL is the effect of treatment IL, β is the regression coeffi-cient, ub, ubp and ubpt are random effects for blocks, plots andtrees, respectively, and εbpthri is random error. The initial differ-ences between trees were removed by applying the arithmeticmean of each characteristic of the last annual ring formed be-fore the treatment as a covariate (xbpth). At a height of 4 m, allannual rings were formed after the initiation of the treatmentand no covariate was used. The data for three-year periods af-ter the treatment (1 – 3, 4 – 6, etc.) were pooled at each stemheight and the statistical tests were undertaken separately foreach period.

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Table 2. Number of sample trees used in the different measurements

Property Stem height Control Fertilised Old(m) non-fertilised

Fibre length, earlywood 1.3 12 12 3Fibre length, earlywood 4.0 12 12 –Fibre length, latewood 1.3 2 2 –Fibre diameter, earlywood 1.3 12 12 –Fibre diameter, earlywood 4.0 12 12 –Cell wall thickness, earlywood 1.3 7 7 2Cell wall thickness, latewood 1.3 7 7 –Lumen diameter, earlywood 1.3 7 7 2Lumen diameter, latewood 1.3 7 7 –Cell wall %, earlywood 1.3 7 7 2

To describe the profile of fibre characteristics with increas-ing age or distance from the pith, as well as to investigate ran-dom variation between blocks, plots and trees, the mixed mod-els considered were:

Ybpthri = β0 + β1zbpth + β2rbpthr + β3zbpth × rbpthr + β4τIL + ub + ubp + ubpt + βbrbpthr + βbprbpthr + βbptrbpthr + εbpthri (2)

where β0, β1, β2, β3 and β4 are fixed regression coefficients,while ub, ubp, ubpt, βb, βbp and βbpt are random regression coeffi-cients for blocks, plots and trees, respectively. The rbpthr is ringnumber (RN) or distance from the pith (DP), and zbpth is adummy variable describing the stem height of 4 m. The τIL isthe effect of treatment IL and always has a value 0 for the non-fertilised trees, as well as for the fertilised trees before the initi-ation of the treatment, but following the treatment fertilisedtrees are assigned a value of 1.

To evaluate the model performance, the following error sta-tistics were calculated: (i) mean error (E=∑(Ybpthri–Ybpthri)/n),(ii) mean absolute error (|E|=∑|Ybpthri–Ybpthri|/n) and (iii) meansquared error (E2=∑(Ybpthri–Ybpthri)2/n) where Ybpthri is a meas-ured observation, Ybpthri is a predicted observation and n is thenumber of observations.

Results

Fibre length

Fertilisation increased radial growth, at breast height,threefold in sample trees (Fig. 1). In the earlywood ofannual rings near the pith, fibre length was on average1.2 mm, but increased rapidly outwards (Fig. 2A). Therate of increase slowed down, however, at 25–35 mmfrom the pith (Fig. 2B). The increased growth rate re-duced fibre length at breast height, i.e. in mature wood.The fibres formed 10–12 years after the initiation of theexperiment were on average 17% shorter in fertilisedtrees than in control trees (Fig. 2). This difference wasstatistically significant over the whole treatment period(Table 3). Furthermore, the difference between the fer-tilised and control trees was even more apparent whenexamined in relation to the distance from the pith, in-stead of ring number from the pith (Fig. 2B). Fibrelength of the older non-fertilised trees from outside ofthe experiment area was rather similar to that of theyounger control trees within the experiment (Fig. 2B).The difference in fibre length observed between the fer-tilised and control trees was also apparent between thefertilised trees and the older trees, i.e. between trees ofsimilar size.

At a stem height of 4 m, i.e. in juvenile wood, the dif-ference in fibre length of earlywood between the fer-tilised and control trees was small when rings of thesame age were compared. The fibres of the last-formedannual rings of the control trees were, however, againlonger compared to the fertilised trees (Fig. 2C,D), butthe difference was not statistically significant (Table 3).

Within each annual ring, fibre length in earlywoodwas normally distributed (data not shown). Standarddeviation of fibre length increased from the pith out-wards, but the increase slowed down at 25–35 mm fromthe pith, i.e. at the same point as the increase in fibre

length (Fig. 2E-H).At stem heights of 1.3 m and 4 m, thedifference in standard deviation of fibre length betweenthe fertilised and control trees was mainly not statisti-cally significant (Table 3).

In earlywood, the relationship between fibre lengthand RN or DP was described by Eq. 2. Several alterna-tive transformations of RN and DP were tested, but thelogarithmic transformation was the best in terms of errorstatistics (E, |E| and E2) and consequently it was used(Table 4). The difference in fibre length between breastheight and a height of 4 m was statistically significant,but small and in opposite directions in models 1, 2 and 3(Table 4). Furthermore, at both heights, fibre length in-creased at the same rate from the pith outwards, i.e. β3

was not statistically significant. The dummy variable de-scribing the fertilisation treatment (τIL) was statisticallysignificant in all the models presented in Table 4. Its ef-fect was, however, in some models in an opposite direc-tion to what was found above, and it reduced the vari-ance components and residuals of the models slightly. Itseffect was also illogical and its importance low in severalof the models used for the other fibre properties de-scribed below.Therefore, it was not included in the equa-tions presented in Tables 4, 6 and 8 to 10.

In addition, variance components describing averagedifferences in fibre length between blocks and plots (ub

and ubp) were not statistically significant; nor were thedifferences between blocks or plots in the increasingrate of fibre length from the pith outwards (βb and βbp).Thus, significant random variation was only found be-tween individual trees (ubpt and βbpt,Table 4).They were,however, relatively small compared to the fixed parame-ter β2 and random variation εbpthri. Since no systematictrend in residuals of the models were found with respectto RN or DP (results not shown), the variation of fibrelength, not explained by the fixed part of Eq. 2, wasprobably mainly intra-ring variation between individualfibres.

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Fig. 1. Annual radial increments at breast height of the sam-ple trees from control plots (continuous line) and irrigated-fer-tilised plots (dashed line). The initiation of the treatment in1987 is indicated by a vertical line.

Latewood fibres were on average 11% longer thanearlywood fibres. The length ratio of latewood and ear-lywood fibres (fibre length in latewood / fibre length inearlywood) was constant from pith to bark. Further-more, the difference in fibre length between the fer-tilised and control trees was small in latewood (Fig. 3).The number of sample trees was, however, small withonly two fertilised and two control trees.

Fibre diameter

Fbre diameter in earlywood also increased from the pithoutwards but the increase slowed down towards thebark (Fig. 4). At breast height, i.e. in mature wood, thefertilisation significantly increased fibre diameter whenexamined with respect to ring number, and the last-formed fibres of the fertilised trees were on average12% wider (Fig. 4A, Table 5). However, the differencebetween the fertilised and control trees diminishedwhen it was examined with respect to distance from thepith (Fig. 4B). At a stem height of 4 m, i.e. in juvenilewood, the difference between the fertilised and controltrees was smaller, and it was only statistically significantin the annual rings formed 7–12 years after the initia-tion of the treatment (Table 5). Furthermore, fertilisa-tion increased standard deviation of fibre diameter atboth heights (Fig. 4,Table 5).

Logarithmic transformation of RN and DP was usedwhen their relationship to fibre diameter in earlywoodwas described by Eq. 2. As in the case of fibre length, asmall difference was found between breast height andthe height of 4 m (Table 6). The random coefficients de-scribing blocks and plots (ub, ubp, βb and βbp) were notstatistically significant.The random components for treelevel (ubpt and βbpt) were statistically significant, butrather small compared to the unexplained variation(εbpthri). Since no systematic trends were found in residu-als of the models with respect to RN or DP (results notshown), random variation was mainly intra-ring varia-tion between individual fibres.

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Fig. 2. Mean fibre length of the sample trees from control plots (continuous line) and irrigated-fertilised plots (dashed line) in ear-lywood at breast height (A, B) and at a height of 4 m (C, D) plotted against ring number and distance from the pith, as well as theirstandard deviations (E-H). For comparison, results from three older trees (thin line) of similar diameter as the fertilised trees areincluded (B).

Table 3. Tests on fibre length in earlywood at stem heights of1.3 m and 4 m in the fertilised trees as compared to controltrees.The numbers presented are values of variable τIL in Eq. 1.A negative value means that the fibres were shorter in the fer-tilised trees; p-values in parenthesis

Years since the initiation of fertilisationHeight 1–3 4–6 7–9 10–12

Fibre length1.3 m –0.200 –0.322 –0.273 –0.198

(0.013) (0.000) (0.011) (0.093)

4 m –0.117 0.073 0.039 –0.017(0.370) (0.495) (0.757) (0.904)

Standard deviation of fibre length1.3 m –0.003 –0.041 –0.079 –0.043

(0.916) (0.138) (0.024) (0.290)

4 m 0.045 –0.004 0.078 –0.025(0.235) (0.868) (0.034) (0.437)

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Fig. 3. Mean fibre length of the sample trees from control plots (continuous line) and irrigated-fertilised plots (dashed line) inlatewood at breast height plotted against ring number (A) and distance from the pith (B).

Fig. 4. Mean fibre diameter of the sample trees from control plots (continuous line) and irrigated-fertilised plots (dashed line) inearlywood at breast height (A, B) and at a height of 4 m (C, D) plotted against ring number and distance from the pith, as well astheir standard deviations (E-H).

Table 4. Fixed and random regression coefficients, their standard errors in parenthesis and error statistics of the models (Eq. 2) forfiber length in earlywood as a function of ring number (RN) or distance from the pith (DP)

Model 1 Model 2 Model 3

Intercept 0.514 (0.046) 0.273 (0.054) 0.438 (0.050)zbpth 0.154 (0.009) –0.168 (0.008) 0.083 (0.019)log(RN) 0.700 (0.031) 0.518 (0.034)log(DP) 0.520 (0.028) 0.144 (0.034)

Variance componentsubpt 0.047 (0.031) 0.065 (0.031) 0.051 (0.031)βbpt × log(RN) 0.023 (0.031)βbpt × log(DP) 0.018 (0.031) 0.013 (0.031)εbpthri 0.173 (0.031) 0.178 (0.031) 0.173 (0.031)

Error statisticsE 0.000 (0.031) 0.000 (0.031) 0.000 (0.031)|E| 0.324 (0.031) 0.328 (0.031) 0.324 (0.031)E2 0.173 (0.031) 0.178 (0.031) 0.173 (0.031)

Cell wall thickness, lumen diameter and cell wallproportion

At breast height,cell wall thickness and lumen diameter inearlywood, as well as in latewood, increased from the pithoutwards (Figs.5 and 6). In both earlywood and latewood,the cell walls of the fertilised trees were thinner after thetreatment initiation, but lumen diameters larger com-pared to the control trees when examined with respect toring number. In most cases the difference was statisticallysignificant (Table 7). Thus, the cell wall proportion of thefertilised trees was lower (Table 7,Fig.7).The difference incell wall thickness, lumen diameter and cell wall propor-tion between the fertilised and control trees was,however,smaller when examined with respect to distance from thepith. Cell wall thickness, lumen diameter and wall propor-tion of the older unfertilised trees were similar to those ofthe younger control trees (Figs.5,6 and 7).

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Table 5. Tests on fibre diameter in earlywood at stem heightsof 1.3 m and 4 m in the fertilised trees as compared to controltrees.The numbers presented are values of variable τIL in Eq. 1.A positive value means that the fibres were wider in the fer-tilised trees; p-values in parenthesis

Years since the initiation of fertilisation1–3 4–6 7–9 10–12

Fibre diameter1.3 m 1.710 3.370 4.324 4.744

(0.016) (0.000) (0.000) (0.000)

4 m 0.579 1.439 3.918 2.584(0.805) (0.357) (0.009) (0.000)

Standard deviation of fibre diameter1.3 m 2.704 4.590 4.283 4.522

(0.024) (0.002) (0.001) (0.000)

4 m 0.500 –0.490 3.387 2.459(0.855) (0.786) (0.021) (0.002)

Table 6. Fixed and random regression coefficients, their standard errors in parenthesis and error statistics of the models (Eq. 2) forfibre diameter in earlywood as a function of ring number (RN) or distance from the pith (DP)

Model 1 Model 2 Model 3

Intercept 23.418 (0.770) 21.551 (0.722) 21.619 (0.721)zbpth 3.297 (0.182) 0.815 (0.096) 0.935 (0.102)log(RN) 5.320 (0.354) 1.559 (0.369)log(DP) 3.107 (0.121) 2.936 (0.131)log(RN)*zbpth –0.226 (0.094)

Variance componentsubpt 13.093 (0.031) 11.757 (0.031) 11.714 (0.031)βbpt × log(RN) 2.710 (0.031)βbpt × log(DP) 4.053 (0.031) 2.389 (0.031)εbpthri 255.931 (0.031) 255.700 (0.031) 255.675 (0.031)

Error statisticsE 0.000 (0.031) 0.000 (0.031) 0.000 (0.031)|E| 9.755 (0.031) 9.739 (0.031) 3.735 (0.031)E2 255.931 (0.031) 255.931 (0.031) 255.675 (0.031)

Fig. 5. Mean cell wall thickness of the sample trees from control plots (continuous line) and irrigated-fertilised plots (dashed line)at breast height in earlywood (A, B) and in latewood (C, D) plotted against ring number and distance from the pith. For compari-son, results from two older trees (thin line) of similar diameter as the fertilised trees are included (B).

As with the other fibre properties described above, cellwall thickness, lumen diameter and wall proportion in ear-lywood and latewood were closely related to ring numberand distance from the pith (Tables 8 to 10). Distance fromthe pith better explained the variation in cell wall thick-ness, lumen diameter and wall proportion than ring num-ber,but random variation between the trees was high.

Relationship between fibre properties

Fibre properties were averaged for each individual an-nual ring, and correlations between them were calculat-ed irrespective of ring age and fertilisation treatment.Ascould be expected on the basis of the results above, meanfibre length was positively correlated with fibre diame-ter, cell wall thickness and lumen diameter, but negative-ly correlated with cell wall proportion (Table 11). Ac-cordingly, fibre diameter was positively correlated withcell wall thickness and lumen diameter, but negativelywith cell wall proportion. Correlations between fibreproperties were similar both in early- and latewood, ex-cluding the relationship between cell wall thickness and

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Fig. 6. Mean fibre lumen diameter of the sample trees from control plots (continuous line) and irrigated-fertilised plots (dashedline) at breast height in earlywood (A, B) and in latewood (C, D) plotted against ring number and distance from the pith. For com-parison, results from two older trees (thin line) of similar diameter as the fertilised trees are included (B).

Fig. 7. Mean cell wall proportion in the cross-sections of the sample trees from control plots (continuous line) and irrigated-fer-tilised plots (dashed line) of earlywood plotted against ring number (A) and distance from the pith (B). For comparison, resultsfrom two older trees (thin line) of similar diameter as the fertilised trees are included (B).

Table 7. Tests on cell wall thickness, lumen diameter and wallproportion at breast height in the fertilised trees compared tothe control trees.The numbers presented are values of variableτIL in Eq. 1.A positive value means that the fibre characteristicwas larger in the fertilised trees; p-values in parenthesis

Years since the initiation of fertilisation1–3 4–6 7–9 10–12

EarlywoodCell wall thickness

1.3 m –0.084 –0.594 –0.560 –0.414(0.715) (0.000) (0.001) (0.191)

Lumen diameter1.3 m 1.821 2.391 4.547 4.158

(0.097) (0.164) (0.005) (0.009)Wall proportion

1.3 m –3.043 –3.979 –5.177 –5.650(0.081) (0.003) (0.000) (0.000)

LatewoodCell wall thickness

1.3 m –0.859 –2.078 –1.356 –1.208(0.270) (0.000) (0.001) (0.046)

Lumen diameter1.3 m 2.107 3.832 3.316 3.342

(0.086) (0.000) (0.000) (0.004)

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Table 8. Fixed and random regression coefficients, their standard errors in parenthesis and error statistics of the models (Eq. 2) forcell wall thickness as a function of ring number (RN) or distance from the pith (DP)

Model 1 Model 2 Model 3

EarlywoodIntercept 2.524 (0.198) 2.537 (0.176) 2.258 (0.177)log(RN) 0.322 (0.099) 1.610 (0.108)log(DP) 0.256 (0.073) –0.913 (0.093)Variance componentsubpt 0.540 (0.031) 0.426 (0.031) 0.428 (0.031)βbpt × log(RN) 0.635 (0.031)βbpt × log(DP) 0.073 (0.031) 0.034 (0.031)εbpthri 0.284 (0.031) 0.286 (0.031) 0.280 (0.031)Error statisticsE 0.000 (0.031) 0.000 (0.031) 0.000 (0.031)|E| 0.416 (0.031) 0.418 (0.031) 0.413 (0.031)E2 0.284 (0.031) 0.286 (0.031) 0.280 (0.031)

LatewoodIntercept 5.463 (0.336) 5.457 (0.300) 5.097 (0.309)log(RN) 0.319 (0.172) 2.061 (0.230)log(DP) 0.272 (0.130) –1.222 (0.191)Variance componentsubpt 1.539 (0.031) 1.227 (0.031) 1.282 (0.031)βbpt × log(RN) 0.406 (0.031)βbpt × log(DP) 0.233 (0.031) 0.119 (0.031)εbpthri 1.686 (0.031) 1.692 (0.031) 1.675 (0.031)Error statisticsE 0.000 (0.031) 0.000 (0.031) 0.000 (0.031)|E| 1.005 (0.031) 1.005 (0.031) 1.004 (0.031)E2 1.686 (0.031) 1.687 (0.031) 1.675 (0.031)

Table 9. Fixed and random regression coefficients, their standard errors in parenthesis and error statistics of the models (Eq. 2) forlumen diameter as a function of ring number (RN) or distance from the pith (DP)

Model 1 Model 2 Model 3

EarlywoodIntercept 6.533 (1.820) 8.617 (0.889) 7.489 (1.215)log(RN) 8.032 (0.817) 6.713 (0.632)log(DP) 5.559 (0.301) 0.675 (0.689)Variance componentsubpt 46.143 (0.031) 10.893 (0.031) 20.336 (0.031)βbpt × log(RN) 9.307 (0.031)βbpt × log(DP) 1.246 (0.031) 2.728 (0.031)εbpthri 25.263 (0.031) 25.359 (0.031) 25.218 (0.031)Error statisticsE 0.000 (0.031) 0.000 (0.031) 0.000 (0.031)|E| 3.826 (0.031) 3.826 (0.031) 3.820 (0.031)E2 25.231 (0.031) 25.328 (0.031) 25.185 (0.031)

LatewoodIntercept 3.022 (0.779) 3.580 (0.470) 4.523 (0.770)log(RN) 2.315 (0.395) –5.561 (0.319)log(DP) 1.596 (0.194) 5.635 (0.314)Variance componentsubpt 8.433 (0.031) 3.051 (0.031) 8.212 (0.031)βbpt × log(RN) 2.178 (0.031)βbpt × log(DP) 0.521 (0.031) 0.624 (0.031)εbpthri 7.055 (0.031) 6.975 (0.031) 6.886 (0.031)Error statisticsE 0.000 (0.031) 0.000 (0.031) 0.000 (0.031)|E| 2.038 (0.031) 2.022 (0.031) 1.995 (0.031)E2 7.055 (0.031) 6.975 (0.031) 6.886 (0.031)

lumen diameter. In earlywood, wall thickness increasedwith increasing lumen diameter, but in latewood wallthickness and lumen diameter were negatively correlat-ed. In general, correlations between fibre propertieswere surprisingly high even though fibre lengths and di-ameters were not measured from the same fibres as cellwall and lumen properties.

Discussion

Fibre length followed the well-known age trend of coni-fers caused by maturation of the cambium (Dinwoodie1961; Olesen 1982; Frimpong-Mensah 1987; Kucera1994), i.e. fibres were short near the pith and their lengthincreased with decreasing rate from the pith outwards.Asimilar non-linear trend from pith to bark was found bothin the control and fertilised trees, but a faster growth rateled to the formation of shorter fibres.This is in agreementwith earlier studies in different spruce species, where anegative correlation between ring width and fibre lengthhas been reported (Stairs et al. 1966;Yang and Hazenberg1994; Herman et al. 1998).A decrease in fibre length withincreasing growth rate has also been found in Thuja occi-dentalis (Bannan 1960).

As in the case of fibre length, the fibre diameter, cellwall thickness and lumen diameter increased from the

pith outwards both in the control and fertilised trees.Fertilised trees with wider annual rings had wider fibresand lumen diameters, but thinner cell walls than thecontrol trees. This confirms earlier observations byOllinmaa (1959), Denne (1973) and Atmer and Thörn-qvist (1982) who found that mean fibre diameter inPicea abies and P. sitchensis was correlated with ringwidth and rate of shoot elongation. If fibre length is ex-cluded, differences in fibre characteristics between thefertilised and control trees were less apparent when ex-amined with respect to distance from the pith instead ofring number from the pith. Similar results have been re-ported by Olesen (1977, 1982) who found that the fibrediameter of Norway spruce was more strongly correlat-ed with distance from the pith than with ring number.Therefore, fibre dimensions are rather determined bythe number of anticlinal cell divisions taking place in thecambium than by cambium age, i.e. the maturation ofthe cambium is related to its activity (cf. Larson 1969).

Average fibre properties in latewood differed fromthose in earlywood, where earlywood fibres were short-er than those of latewood. Most of the variation in cellwall thickness, lumen diameter and radial diameter of fi-bres was related to variation between earlywood andlatewood, which is in agreement with earlier studies (cf.Stairs et al. 1966; Olesen 1977; Saranpää 1985). The rate

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Table 10. Fixed and random regression coefficients, their standard errors in parenthesis and error statistics of the models (Eq. 2)for wall proportion in earlywood as a function of ring number (RN) or distance from the pith (DP)

Model 1 Model 2 Model 3

Intercept 43.490 (3.528) 41.267 (2.235) 39.976 (1.853)log(RN) –6.443 (1.434) 7.167 (2.232)log(DP) –4.287 (0.700) –9.470 (1.705)

Variance componentsubpt 168.779 (0.031) 65.767 (0.031) 41.553 (0.031)βbpt × log(RN) 27.840 (0.031)βbpt × log(DP) 6.409 (0.031) 3.813 (0.031)εbpthri 6.658 (0.031) 6.247 (0.031) 6.370 (0.031)

Error statisticsE 0.000 (0.031) 0.000 (0.031) 0.000 (0.031)|E| 1.924 (0.031) 1.865 (0.031) 1.878 (0.031)E2 6.016 (0.031) 5.660 (0.031) 5.777 (0.031)

Table 11. Correlation of the mean fibre characteristics in earlywood and latewood; correlations above diagonal and their p valuesbelow diagonal

Fibre Fibre Wall Wall Lumen Lumen Walllengthearly widthearly thicknessearly thicknesslate diameterearly diameterlate proportionearly

Fibre lengthearly – 0.609 0.649 0.156 0.647 0.495 –0.301Fibre widthearly 0.000 – 0.229 0.041 0.872 0.717 –0.736Wall thicknessearly 0.000 0.033 – 0.562 0.368 0.041 0.023Wall thicknesslate 0.137 0.711 0.000 – 0.071 –0.342 0.167Lumen diameterearly 0.000 0.000 0.000 0.502 – 0.739 –0.764Lumen diameterlate 0.000 0.000 0.691 0.001 0.000 – –0.732Wall proportionearly 0.002 0.000 0.812 0.113 0.000 0.000 –

of change from the pith to the bark and the effect of fer-tilisation was, however, similar in early- and latewood.Since the samples were separately prepared from theearlywood and latewood zones of annual rings, determi-nation of fibre properties in the whole wood materialbased on such data is affected by the variation in late-wood content, which in Norway spruce decreases withincreasing growth rate.

Small differences in average fibre dimensions werefound between different stem heights, which confirmsearlier reports in which the fibre dimensions of Norwayspruce changed only slightly at different heights alongthe stem (cf. Dinwoodie 1961; Stern 1963; Saranpää1994).Average fibre length and diameter was less affect-ed by enhanced growth rate in juvenile wood (at a stemheight of 4 m) than in mature wood (at breast height).This result is consistent with studies in which young, in-tensivly cultured poplars were compared with older ma-terial from natural stands (e.g. Snook et al. 1986; DeBellet al. 1998).At least some part of the smaller effect in ju-venile wood may be explained by the different rate ofcell division and maturation in juvenile and maturewood (cf. Panshin and de Zeeuw 1980).The effect of fer-tilisation on fibre extension during their maturationcould, therefore, be masked by the already rapid rate ofcell division in juvenile wood.

Intra-ring variation of fibre characteristics is stronglyinfluenced by the occurrence of juvenile wood (Larson1969). In the present study, the standard deviation of fi-bre length and diameter increased non-linearly with in-creasing ring number and distance from the pith. Thus,the range of intra-ring variation of fibre properties wasnarrower in juvenile wood than in mature wood (cf.Herman et al. 1998). Intra-ring variation increased, how-ever, faster from the pith outwards in juvenile woodthan in outer rings. The rapid growth rate of the fer-tilised trees did not significantly change the standard de-viation of fibre length. On the contrary, intra-ring varia-tion of fibre diameter at breast height and at a height of4 m was increased by fertilisation.

Random tree components were statistically signifi-cant in all the models for fibre properties. In addition,relatively large differences between trees were found inthe rate of change from the pith outwards of fibre diam-eter, cell wall thickness, lumen diameter and wall pro-portion. However, intra-ring variations of fibre lengthand fibre diameter were high compared to random in-ter-tree variation (cf. Dinwoodie 1961; Sirviö andKärenlampi 2000).

It was shown that it is possible to model intra-treevariability of fibre characteristics using ring width andcambial age as independent variables. The models pre-sented are, however, limited by the relatively young ageof the sample trees. Future studies based on a largeramount of material from different geographical loca-tions are necessary in order to develop more generalmodels. Sampling more than two stem heights is alsonecessary to describe the effect of location along thestem more accurately.

Conclusions

Fertilisation affected fibre properties mainly throughthe faster growth rate. Inter-ring variation of the fibredimensions was characterised by the simultaneous in-crease in their average values and standard deviationsfrom pith to bark. Faster growth rate reduced mean fi-bre length and cell wall thickness but increased mean fi-bre and lumen diameter when the fibre properties wereexamined in respect to ring number from the pith. How-ever, fibre and lumen diameter, as well as cell wall thick-ness, were closely related to distance from the pith. Onlythe difference in fibre length between the fertilised andcontrol trees was more apparent with respect to dis-tance over ring number from the pith. In addition, theeffect of enhanced growth rate was less apparent in ju-venile than in mature wood.

The changes in fibre properties could be similar in re-sponse to other silvicultural practices which enhancetree growth. For example, other studies on Norwayspruce have demonstrated that rapid growth rate,caused by low stand density, resulted in similar changesin fibre characteristics as those observed in the presentstudy (e.g. Herman et al. 1998). Therefore, increasinggrowth rate of trees probably causes similar kinds ofchanges in fibre characteristics irrespective of the factorpromoting enhanced growth.

In most forestry practices, stands are normally fer-tilised at an older age and only one or two nutrient ele-ments are added. Therefore, the results presented can-not be used to predict the absolute change in fibrecharacteristics caused by fertilisation in Norway sprucestands. The results rather demonstrate the effects ofgrowth rate on fibre properties and the magnitude ofpotential change and direction that could be caused byoptimising the nutritional status of Norway sprucestands.

Acknowledgements

The study was made possible by financial support from TheAcademy of Finland and The Swedish National Energy Ad-ministration (STEM). We are greatly indebted to Dr. AnteroVarhimo (KCL) for the measurements with the help of Fiber-Lab, and Santtu Hietala, Satu Järvinen, Tapio Järvinen, IrmeliLuovula, Tapio Nevalainen, Carl Räihä, Bengt-Olov Wigrenand Ulla Nylander for skilful technical assistance.

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Received April 23rd 2001

Dr. Harri MäkinenFinnish Forest Research InstituteP.O. Box 1801301 VantaaFinlandE-mail: harri.makinen@metla.fi

Dr. Pekka SaranpääFinnish Forest Research InstituteP.O. Box 1801301 VantaaFinland

Prof. Dr. Sune LinderSwedish University of Agricultural SciencesDepartment for Production EcologyP.O. Box 704275007 UppsalaSweden

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