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Available at www.sciencedirect.com http://www.elsevier.com/locate/biombioe Wood fuel quality of two Salix viminalis stands fertilised with sludge, ash and sludge–ash mixtures Anneli Adler a,b, , Ioannis Dimitriou a , Pa ¨ r Aronsson a , Theo Verwijst a , Martin Weih a a Department of Crop Production Ecology, Swedish University of Agricultural Sciences (SLU), P.O. Box 7043, SE-750 07 Uppsala, Sweden b Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Riia 181, 51014 Tartu, Estonia article info Article history: Received 6 September 2006 Received in revised form 15 December 2007 Accepted 8 January 2008 Available online 7 March 2008 Keywords: Age of shoot population Bark Biofuel Heavy metals Planting density Nutrients Structure of shoot population Willow abstract This study assessed the effects of stand structure and fertilisation with wood ash and/or sludge on wood fuel quality of Salix viminalis. The relative proportions of bark and wood in 1-, 2- and 3-year-old shoot populations were determined. The concentrations of essential elements (N, P, K) and heavy metals (Cu, Zn, Cd, Ni) in bark and wood were used to assess the wood fuel quality in harvestable shoot biomass. Controlled field experiments were conducted on two newly harvested commercial short-rotation willow coppice fields. Five treatments were applied: sewage sludge at the maximum legally permitted amount; ash; two sludge–ash mixtures supplying the maximum and twice the maximum permitted sludge–ash amount; and a control receiving mineral nutrients only. The proportion of bark in the willow stands was decreasing with the age of the shoot population. The shoot population with few large stems, compared to that with many small stems, had a lower proportion of element-rich bark in the harvestable shoot biomass, meaning better quality of the wood fuel. Overall, wood fuel quality in terms of mineral concentrations was influenced by the age of the shoot population at harvest, stand structure, management practices (e.g. planting density, fertilisation) and site conditions (soil type, element availability). Our results imply that harvestable shoot biomass of willows grown as few large stems have better wood fuel quality, compared to harvestable shoot biomass of many small stems. Increased length of cutting cycle improves the wood fuel quality. & 2008 Elsevier Ltd. All rights reserved. 1. Introduction Fast-growing plantations of Salix and Populus are today a viable alternative source of bio-energy, and the profitability of Salix culture can be significantly improved by the use of by- products such as wood ash and sludge [1]. For example, wood ash mixed with sewage sludge can be applied to the energy plantation to meet the nutrient requirements of the Salix stand. The production cost of Salix biomass fuel decreases when the nutrient requirements of the crop can be supplied without artificial fertilisers [2]. During combustion of woody biomass, a large proportion of sulphur (S) and nitrogen (N) are gasified, while the majority of nutrients, trace elements and the acid-neutralising capacity remain in the ash [3,4]. Consequently, the ash can be recycled and used as a fertiliser. However, high concentrations of zinc (Zn) and cadmium (Cd) in biomass fuels can cause problems in ash recycling, since these metals accumulate in the ash during combustion or are emitted to the atmosphere as particulates [5]. Sewage sludge is the solid by-product of wastewater treatment plants. It contains large amounts of phosphorus (P), some N (mainly organically bound), very little ARTICLE IN PRESS 0961-9534/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2008.01.013 Corresponding author at: Department of Crop Production Ecology, Swedish University of Agricultural Sciences (SLU), P.O. Box 7043, SE-750 07 Uppsala, Sweden. Tel.: +46 18 672306; fax: +46 18 672890. E-mail address: [email protected] (A. Adler). BIOMASS AND BIOENERGY 32 (2008) 914– 925

Wood fuel quality of two Salix viminalis stands fertilised with sludge, ash and sludge–ash mixtures

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Available at www.sciencedirect.com

B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5

0961-9534/$ - see frodoi:10.1016/j.biomb

�Corresponding aSE-750 07 Uppsala,

E-mail address: A

http://www.elsevier.com/locate/biombioe

Wood fuel quality of two Salix viminalis stands fertilisedwith sludge, ash and sludge–ash mixtures

Anneli Adlera,b,�, Ioannis Dimitrioua, Par Aronssona, Theo Verwijsta, Martin Weiha

aDepartment of Crop Production Ecology, Swedish University of Agricultural Sciences (SLU), P.O. Box 7043, SE-750 07 Uppsala, SwedenbInstitute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Riia 181, 51014 Tartu, Estonia

a r t i c l e i n f o

Article history:

Received 6 September 2006

Received in revised form

15 December 2007

Accepted 8 January 2008

Available online 7 March 2008

Keywords:

Age of shoot population

Bark

Biofuel

Heavy metals

Planting density

Nutrients

Structure of shoot population

Willow

nt matter & 2008 Elsevieioe.2008.01.013

uthor at: Department ofSweden. Tel.: +46 18 6723

[email protected] (

a b s t r a c t

This study assessed the effects of stand structure and fertilisation with wood ash and/or

sludge on wood fuel quality of Salix viminalis. The relative proportions of bark and wood in

1-, 2- and 3-year-old shoot populations were determined. The concentrations of essential

elements (N, P, K) and heavy metals (Cu, Zn, Cd, Ni) in bark and wood were used to assess

the wood fuel quality in harvestable shoot biomass. Controlled field experiments were

conducted on two newly harvested commercial short-rotation willow coppice fields. Five

treatments were applied: sewage sludge at the maximum legally permitted amount; ash;

two sludge–ash mixtures supplying the maximum and twice the maximum permitted

sludge–ash amount; and a control receiving mineral nutrients only.

The proportion of bark in the willow stands was decreasing with the age of the shoot

population. The shoot population with few large stems, compared to that with many small

stems, had a lower proportion of element-rich bark in the harvestable shoot biomass,

meaning better quality of the wood fuel. Overall, wood fuel quality in terms of mineral

concentrations was influenced by the age of the shoot population at harvest, stand

structure, management practices (e.g. planting density, fertilisation) and site conditions

(soil type, element availability). Our results imply that harvestable shoot biomass of

willows grown as few large stems have better wood fuel quality, compared to harvestable

shoot biomass of many small stems. Increased length of cutting cycle improves the wood

fuel quality.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Fast-growing plantations of Salix and Populus are today a

viable alternative source of bio-energy, and the profitability of

Salix culture can be significantly improved by the use of by-

products such as wood ash and sludge [1]. For example, wood

ash mixed with sewage sludge can be applied to the energy

plantation to meet the nutrient requirements of the Salix

stand. The production cost of Salix biomass fuel decreases

when the nutrient requirements of the crop can be supplied

without artificial fertilisers [2].

r Ltd. All rights reserved.

Crop Production Ecology,06; fax: +46 18 672890.A. Adler).

During combustion of woody biomass, a large proportion of

sulphur (S) and nitrogen (N) are gasified, while the majority of

nutrients, trace elements and the acid-neutralising capacity

remain in the ash [3,4]. Consequently, the ash can be recycled

and used as a fertiliser. However, high concentrations of zinc

(Zn) and cadmium (Cd) in biomass fuels can cause problems

in ash recycling, since these metals accumulate in the ash

during combustion or are emitted to the atmosphere as

particulates [5]. Sewage sludge is the solid by-product of

wastewater treatment plants. It contains large amounts of

phosphorus (P), some N (mainly organically bound), very little

Swedish University of Agricultural Sciences (SLU), P.O. Box 7043,

Page 2: Wood fuel quality of two Salix viminalis stands fertilised with sludge, ash and sludge–ash mixtures

ARTICLE IN PRESS

B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5 915

potassium (K) and heavy metals. For sustainable biomass

utilisation, it is essential to circulate the material flows and to

integrate the biomass ash and sewage sludge within the

natural cycles. Therefore, the minerals in the nutrient cycle

from soil/ash and sludge—to willow roots/stems—to com-

bustion—to ash—to soil should be utilised as completely as

possible. However, since sewage sludge and wood ash may

also contain rather high concentrations of heavy metals [5,6],

the natural cycle of minerals within the process of energy

production from biomass may be disturbed by deposition of

heavy metals in soils under energy forests.

The bark of small-diameter stems can form a large

proportion of the harvestable shoot biomass produced in

short-rotation crops. Bark and wood differ in the relative

amounts of cellulose, hemicellulose, lignin, inorganic materi-

als and extractives, and behave therefore differently during

thermal decomposition [7]. One of the main differences in

chemical composition of wood and bark is the concentration

of inorganic ions; the concentrations of macronutrients and

some heavy metals are significantly higher in bark than in

wood [8–17]. Consequently, the variation in proportion of bark

in a short-rotation willow stand may have pronounced effects

on the characteristics of the wood fuel. High concentrations

of elements decrease the quality of the wood fuel. The

inorganic ions are known to exert a great influence on the

thermal degradation of polysaccharides and lignin [18,19]. It

has been found that a number of ions act as catalysts,

resulting in lowering of decomposition temperature and an

increase in char yield [20,21]. High concentrations of K in

wood fuel decrease the ash melting point [22].

On the other hand, a high proportion of bark may play an

important role in removal of elements from willow vegetation

filters. In vegetation filters, we wish to remove as many

elements as possible through harvesting shoots, but the

biomass fuel should still be of high quality, i.e. the concen-

trations of elements in harvestable shoot biomass should be

low. Hence, proportion of bark plays an important role when

the aim is to remove elements from a vegetation filter and

burn the harvested wood fuel for energy. Taking into account

the above-mentioned differences between bark and wood, a

comparative analysis of different biomass fractions is essen-

tial for evaluation of wood fuel quality.

Earlier studies on short-rotation willow coppice have

assessed the effects of different site conditions and planting

densities on stand structure and harvestable shoot biomass

yields [23–28], while the dynamics of bark proportions in

willow stands and its effect on wood fuel quality has not been

focused yet.

The present study focuses on the particular aspects of wood

fuel quality that can be influenced by management practices

such as the frequency of harvests and the fertilisation with

wastes. Wood fuel quality of willows in terms of proportion of

bark, concentrations of elements in bark and wood (N, P, K

and Zn, Cd, Cu, Ni) and the biomass yield was investigated in

two commercial willow plantations. These plantations had

been established with different planting densities and were

fertilised with sewage sludge, wood fuel ashes and various

mixtures of these. The relationship between the proportion of

bark in the destructively measured shoots and the shoot

diameter at 55 cm height was used to calculate the dry weight

(DW) of bark in non-destructively measured shoots [16]. The

shoots were sampled in wintertime according to commercial

harvesting practices. Our hypothesis was that wood fuel with

a higher proportion of bark would contain more essential

elements and heavy metals compared to wood fuel with

lower proportion of bark, and would thus be of poorer quality.

2. Materials and methods

2.1. Sites

Two newly harvested commercial willow plantations (Salix

viminalis L., clone 78-021), located at Linnes Hammarby

(Hammarby) and Lundby Gard (Lundby) near Uppsala in

central Sweden, were studied. The plantations were estab-

lished in 1997 in a twin row formation with densities of 17,700

and 25,000 cuttings ha�1, at Hammarby and Lundby, respec-

tively. At Hammarby, the distance between the rows alter-

nated at 150 and 75 cm and the distance between the plants in

the rows was approximately 50 cm. At Lundby, the distance

between the rows alternated at 125 and 75 cm and the

distance between the plants in the rows was approximately

40 cm. Both plantations were in the beginning of their second

cutting cycle when we started the experiment.

The soil at Hammarby contained 43% clay (o0.002 mm),

47% silt (0.002–0.06 mm) and 10% sand (0.06–2 mm). The total

N content was 0.19%, the organic C content was 2.08%, and

the pH around 6.6. The soil at Lundby contained 57% clay, 34%

silt and 9% sand. The total N content was 0.27%, the organic C

content was 2.73%, and the pH around 6.2. The mean length

of the growing season in the area is 180–200 days, the mean

rainfall 500–600 mm year�1; the mean air temperature during

the growing season (April–September) is 13 1C (data for

1961–1990) [29].

2.2. Treatments

A randomised block design with five fertilisation treatments

and four replicates was laid out in each of the two fields

(Hammarby and Lundby) in spring 2001. Each plot was

9�9 m2 in size. Fertilisation treatments comprised different

combinations of stabilised dewatered sludge from a nearby

municipal wastewater treatment plant, and wood fuel

ash from the district heating plant in Enkoping in central

Sweden (Table 1). Control plots received 14.5 kg P and

48 kg K ha�1 year�1 as mineral fertiliser during the 3 years of

the experiment. All plots, including control plots, were

additionally fertilised with 100 kg N ha�1 year�1 according to

recommendations [30]. The plant-available fractions of Zn, Ni

and Cd in the soil were less available in the treatments

containing ash compared to the sludge and control treat-

ments (Table 2). In contrast, the plant-available fraction of Cu

in the soil was less available in the sludge and control

treatments compared to the treatments containing ash.

2.3. Proportion of bark in sample pieces and shoots

Shoots were sampled each winter after the first, second and

third growing season. At each site, 15 shoots were sampled to

Page 3: Wood fuel quality of two Salix viminalis stands fertilised with sludge, ash and sludge–ash mixtures

ARTICLE IN PRESS

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B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5916

describe the relationship between the shoots’ diameter at

55 cm height (D55) and the proportion of bark (BPshoot). The

shoots were selected in such a way that the whole range of

D55 found in each stand was represented. The height of the

shoots was measured. The shoots were divided into age

fractions, and from each age fraction three 5-cm long sample

pieces were taken for more detailed analyses. Bark and wood

were separated in all sample pieces. The fresh weight of the

sample pieces was determined. After drying at 85 1C for

4 days, the corresponding oven DW of bark and wood was

determined. The DW of the rest of the age fractions was also

determined. Bark DW of each age fraction was calculated

using the average proportion of bark in three sample pieces of

the respective age. Thereafter, bark DW and wood DW in each

age fraction were added to give the total shoot DW (TDWshoot).

Proportion of bark in the whole shoot (BPshoot) was calculated

as the ratio of bark DW of the shoot (BDWshoot) to TDWshoot.

Non-linear regression between D55 and BPshoot of the destruc-

tively measured shoots was established to estimate the bark

proportion of non-destructively measured shoots:

BPshoot ¼ aþ b�Dc55, (1)

where a, b and c were the estimated parameters (Fig. 1).

2.4. Proportion of bark in willow stands

Another 30 shoots were selected from the stand for establish-

ing the allometric relationship between D55 and TDWshoot.

The selected shoots represented the whole range of D55 found

in the plantation. Non-linear regression of TDWshoot on D55

was used to estimate the DW of non-destructively measured

shoots [31]:

TDWshoot ¼ d� Dg55, (2)

where d and g were the estimated parameters.

To estimate the total stand biomass and shoot size

frequency distribution at Lundby and Hammarby, 12 stools

(i.e. plants that had re-sprouted after harvest) were labelled

within each treatment plot (i.e. n ¼ 12 stools �5 treatments

�4 replications ¼ 240 stools per site). D55 of all living shoots

of the selected stools were measured during the winter after

each of the three growing seasons. Stool mortality was

estimated by counting the number of living stools of a subplot

(4.5�4.5 m2 in the middle of each 9�9 m2 treatment plot) at

the beginning and end of the experiment.

The mean proportion of bark in the whole stand (BPstand)

was estimated using Eq. (1) to calculate BPshoot for non-

destructively measured shoots in each plot and using Eq. (2)

to calculate the TDWshoot of these shoots. Thereafter,

BDWshoot was calculated according to

BDWshoot ¼ BPshoot � TDWshoot (3)

and BPstand was obtained using the following equation:

BPstand ¼

PnBDWshoot � 100P

nTDWshoot, (4)

where n is the number of non-destructively measured shoots.

Thus, the proportion of bark in non-destructively measured

shoots was assumed to represent the proportion of bark in

the whole shoot population.

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ARTICLE IN PRESS

Table 2 – Average plant-available fractions of heavy metals in the 0–15 cm soil layer at Lundby and Hammarby threegrowing seasons after the applications (n ¼ 12) [17]

Site Treatment Zn (mg kg�1 DM) Ni (mg kg�1 DM) Cd (mg kg�1 DM) Cu (mg kg�1 DM)

Lundby Sludge 0.39 0.23 0.009 0.09

Ash 0.22 0.15 0.006 0.10

Sludge+ash 0.22 (0.29) 0.15 (0.18) 0.007 (0.004) 0.13 (0.07)

(Sludge+ash)� 2 0.22 0.15 0.005 0.15

Control 0.36 0.22 0.009 0.09

Hammarby Sludge 0.30 0.10 0.005 0.13

Ash 0.19 0.07 0.003 0.15

Sludge+ash 0.31 (0.22) 0.11 (0.09) 0.004 (0.007) 0.16 (0.11)

(Sludge+ash)� 2 0.21 0.06 0.003 0.17

Control 0.29 0.15 0.006 0.13

Plant-available fractions of heavy metals in the 0–15 cm soil layer at both sites before the experiment are shown in brackets (n ¼ 2, while an

analysed sample consisted of 5 sub-samples).

40

Lundby Hammarby

BP s

hoot

20

30

D55 D55

10 20 3010

00 10 20 30

Fig. 1 – Relationship between D55 (stem diameter at 55 cm

height) and proportion of bark (BPshoot) for 1-year-old

(crosses), 2-year-old (open circles, solid line) and 3-year-old

shoots (filled circles, dashed line) in the second cutting cycle

of Salix viminalis grown at two sites in central Sweden: (a)

2-year-old shoots: BPshoot ¼ �2.0+73.0�D55(�0.35), R2

¼ 0.54;

3-year-old shoots: BPshoot ¼ �2.0+66.60�D55(�0.32), R2

¼ 0.60;

(b) 2-year-old shoots: BPshoot ¼ �2.0+70.14�D55(�0.35), R2

¼ 0.86;

3-year-old shoots: BPshoot ¼ �2.0+52.48�D55(�0.25), R2

¼ 0.81.

B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5 917

2.5. Chemical analyses

The concentrations of N, P and K were analysed for composite

samples of bark or wood for 15 entire shoots per site. Three

shoots from each treatment (one per plot) were collected,

i.e. 15 shoots were sampled per site and age class of the

shoots. Analysis of stem fractions was designed to take into

account concentration differences in different age fractions of

Salix stems. The bark or wood fractions of the three sample

pieces per age fraction were pooled together. The composite

samples of bark for entire shoots were calculated according to

the weight proportion of each age fraction of the shoot. The

composite samples of wood for entire shoots were obtained in

a similar way according to the weight proportion of each age

fraction. Total N, P and K concentrations in bark/wood

samples of the destructively measured 1- and 3-year-old

shoots were analysed by gas chromatography in a Carlo Erba

Elemental Analyser NA 1500 [32]. In total, composite samples

of bark/wood in 60 shoots were analysed.

2.6. Calculations and statistical analyses

The concentration of heavy metals in bark and wood in

different treatments of this experiment, published by Dimi-

triou et al. [16], and the concentrations of N, P and K in bark

and wood, determined in this study, were used to calculate

the wood fuel quality of harvestable shoot biomass. Wood

fuel quality (i.e. element concentration per tonne harvestable

shoot biomass) was calculated as

Wood fuel quality ¼ ½element�bark �BPstand

100

þ ½element�wood � 1�BPstand

100

� �, (5)

where [element]bark and [element]wood are the concentrations

of an element in the bark and wood, respectively, in a shoot

population, BPstand/100 is the mean proportion of bark in the

whole stand and 1�(BPstand/100) is the mean proportion of

wood in the whole stand.

The assumption that BPshoot is independent of treatment

was verified by means of analysis of covariance (ANCOVA),

i.e. by testing the effects of the factors site, age (i.e. age of the

shoot) and treatment on BPshoot, where D55 was used as a

covariate. The response variable BPshoot was power trans-

formed (BPshoot�2 ) prior to the analysis for normality of data.

The effect of site on the height of the destructively measured

shoots was tested by means of ANCOVA using age of the

shoot as a covariate. The effects of site and treatment on the

number of shoots per stool and the number of stools per plot

were tested by means of ANCOVA using age of the shoot as a

covariate.

The effects of site, tissue, treatment and age of the

shoots on the concentrations of N, P and K in the composite

bark/wood samples (see Section 2.5) of the destructively

measured shoots were tested by general linear models

(GLMs). The concentrations of K were log-transformed prior

to the analyses for normality of the data.

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B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5918

GLM was used to test the effects of age of the shoot

population, site and treatment on the BPstand and quality of

the wood fuel. The concentrations of P and Zn were square

root-transformed; the concentration of N was inverse-trans-

formed and the concentrations of K, Cd, Cu and Ni were log-

transformed prior to the analyses for normality of the data.

All statistical analyses were performed with the statistical

packages SA System v. 8.1 [33].

3. Results

3.1. Proportion of bark in shoots

The BPshoot was significantly affected by D55 (Table 3a),

while D55 in turn was determined by the age of the shoot

(po0.001, R2¼ 0.74, n ¼ 100). The variation of BPshoot in

relation to D55 decreased with increasing shoot age (Fig. 1).

The BPshoot of 1-year-old shoots varied greatly and the non-

linear regression described only 32% and 37% of the variation

in BPshoot at Lundby and Hammarby, respectively. The

corresponding non-linear regression model (Fig. 1) after the

second growing season explained 54% of the measured

variation in BPshoot at Lundby and 86% of the variation in

BPshoot at Hammarby. The BPshoot after the third growing

season showed relatively high variability at Lundby, where

BPshoot varied between 21% and 30% (R2¼ 0.60; Fig. 1a). Willow

shoots of the same clone and age from the Hammarby site

had proportions of bark values in the range 19–28% (R2¼ 0.81;

Fig. 1b).

Table 3 – Summary of (a) ANCOVA results for effects ofage of the shoot, site, treatment and stem diameter at55 cm height (D55, covariate) on shoot proportion of bark(n ¼ 3 or 4 per treatment) in the 1-, 2- and 3-year-oldshoots from plantations at Lundby and Hammarby(the model was significant at po0.001, R2

¼ 0.67.The interactions were not significant and are notpresented.) (b) GLM results for the effects of age of theshoot population, site and treatment on proportion ofbark in these willow stands (the model was significant atpo0.001, R2

¼ 0.95)

Factor df F-Value p-Value

(a) Shoot proportion of bark (BPshoot)

D55 (covariate) 1 90.34 0.00001

Age 1 0.30 Ns. (0.58)

Site 1 2.19 Ns. (0.14)

Treatment 4 1.29 Ns. (0.28)

(b) Proportion of bark in the stand (BPstand)

Age 1 1268 0.00001

Site 1 22.99 0.00001

Age�Site 1 4.49 o0.05

Site�Treatment 4 1.11 Ns. (0.36)

Treatment 4 0.36 Ns. (0.84)

Age�Treatment 4 0.22 Ns. (0.93)

Ns. ¼ non-significant.

3.2. Element concentrations in bark and wood

The concentrations of K in the bark and wood were

significantly higher at Lundby compared to Hammarby

(the effect of site: po0.001; the model was significant at

po0.001, R2¼ 0.93, df ¼ 108). The concentrations of K were

significantly higher in the 1-year-old shoots compared to the

3-year-old shoots (the effect of age: po0.001). The 1-year-old

shoots contained 7.7370.26 mg K g�1 DM (dry matter) in the

bark and 1.7870.26 mg K g�1 DM in the wood at Lundby, and

7.2970.25 mg K g�1 DM in the bark and 1.6870.25 mg K g�1 DM

in the wood at Hammarby. The 3-year-old shoots contained

6.1070.26 mg K g�1 DM in the bark and 1.3670.26 mg K g�1 DM

in the wood at Lundby, and 5.5470.25 mg K g�1 DM in the bark

and 0.9770.25 mg K g�1 DM in the wood at Hammarby. The

concentrations of K in the shoots were significantly affected

also by the treatments (the effect of treatment: po0.001). The

mean K concentrations in the bark and wood of 3-year-old

shoots were 6.8170.90 and 1.2770.12 mg g�1 DM, respectively,

in the (sludge+ash)�2 treatment at Hammarby. These values

were significantly higher compared to the mean K concentra-

tions in the bark and wood of the rest of the treatments at the

same site (5.2270.20 and 0.9070.03 mg g�1 DM, respectively).

At Lundby, the mean K concentration was 6.1070.31 mg g�1

DM in the bark and 1.3670.06 mg g�1 DM in the wood in all

treatments.

The concentrations of P in the bark and wood were

significantly higher at Lundby compared to Hammarby and

were affected by age of the shoots (the effect of site: po0.001;

the effect of age: po0.001; the model was significant at

po0.001, R2¼ 0.83, df ¼ 108). The effect of treatment on P

concentrations in the bark and wood was not significant

(p ¼ 0.11). The mean P concentrations in the 1-year-old shoots

were 2.1270.07 mg g�1 DM in the bark and 0.9570.07 mg g�1

DM in the wood at Lundby; and 1.8070.07 mg g�1 DM in the

bark and 0.7870.07 mg g�1 DM in the wood at Hammarby. The

mean P concentrations in the 3-year-old shoots were

1.4870.07 mg g�1 DM in the bark and 0.6070.07 mg g�1 DM

in the wood at Lundby, and 1.1670.07 mg g�1 DM in the bark

and 0.3870.07 mg g�1 DM in the wood at Hammarby.

The concentrations of N were significantly higher in the

bark compared to wood and were significantly higher in

the 1-year-old shoots compared to the 3-year-old shoots

(the effect of tissue: po0.001; the effect of age: po0.001; the

model was significant at po0.001, R2¼ 0.90, df ¼ 108). The

effects of site and treatment on the N concentrations in

the bark and wood were not significant (p ¼ 0.96 and p ¼ 0.90,

respectively). The mean N concentrations of the 1-year-old

shoots were 15.4170.27 mg g�1 DM in the bark and

3.9670.27 mg g�1 DM in the wood. The mean N concentra-

tions of the 3-year-old shoots were 10.1370.27 mg g�1 DM in

the bark and 2.2370.27 mg g�1 DM in the wood.

3.3. Proportion of bark and biomass dynamics atstand level

A higher proportion of shoots with small diameter were

observed at Lundby compared to Hammarby in each year

during the 3 years of the experiment (Fig. 2). After the third

growing season, 41% and 37% of the living shoots recorded in

Page 6: Wood fuel quality of two Salix viminalis stands fertilised with sludge, ash and sludge–ash mixtures

ARTICLE IN PRESS

900

600

300

00 10 20 30 40

0 10 20 30 40

900

600

300

0

0.1

0

0

0.2

0.1

0.2

0.3

Prop

ortio

nPr

opor

tion

Cou

ntC

ount

D55

3rd1st 2nd

Fig. 2 – Shoot size distribution of non-destructively

measured shoots in two willow plantations in Central

Sweden. Diameters of 55 cm height (D55) of all shoots in 240

stools after the 1st, 2nd and 3rd year of the second cutting

cycle at (a) Lundby and (b) Hammarby.

B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5 919

the beginning of the experiment were present in the

plantations at Lundby and Hammarby, respectively.

The number of shoots per stool decreased with the

increasing age of the shoot population (the effect of age as

covariate: po0.001), and was significantly higher at Lundby

compared to Hammarby (the effect of site: po0.001; the

model was significant at po0.001, R2¼ 0.90, df ¼ 119). The

shoots were significantly taller at Hammarby compared to

Lundby after each of the three growing seasons (the model

was significant at po0.001, R2¼ 0.81, n ¼ 69). The height of

the 3-year-old shoot population was 458.1721 cm at Ham-

marby and 377.1722 cm at Lundby. The number of stools per

plot was significantly higher at Lundby compared to Ham-

marby (the effect of site: po0.001; the model was significant

at po0.001, R2¼ 0.37, df ¼ 79; Table 4). After the third growing

season 91% and 93% of the stools recorded in the beginning of

the experiment were present in the plantations at Lundby and

Hammarby, respectively.

BPstand decreased with the increasing stand age and was

significantly higher at Lundby compared to Hammarby after

each of the three growing seasons (ANCOVA, the effect of site

po0.0001, R2¼ 0.94, df ¼ 39; Tables 3b and 4; Fig. 3). The

treatments did not have any significant effect on the BPstand.

The harvestable shoot biomass (yield) increased with the

age of shoot population and was significantly higher at

Hammarby than at Lundby (the effect of site: po0.05; the

model was significant at po0.001, R2¼ 0.87, df ¼ 69; Table 4).

The BPstand decreased with increasing yield (Fig. 4).

3.4. Wood fuel quality

Changes in the proportion of bark in the two willow stands

during the experimental period had a significant effect on the

quality of the wood fuel. BPstand significantly affected the

wood fuel quality in terms of the concentrations of P, K and

Cd in the harvestable shoot biomass (Tables 5 and 6). The

concentrations of all the studied elements per tonne harvest-

able shoot biomass decreased with the increasing age of the

shoot population (Table 5, the effect of age; Table 6). The wood

fuel quality in terms of the concentrations of P, Zn and Cd per

tonne harvestable shoot biomass was significantly higher at

Lundby compared to Hammarby (Table 5, the effect of site;

Table 6). The harvestable shoot biomass yield and the total

energy content of the plantations were higher at Hammarby

compared to Lundby (Table 4).

4. Discussion

The data support the hypothesis that wood fuel with a higher

proportion of bark compared to wood fuel with the lower

proportion of bark contains more essential elements and

heavy metals and is thus of poorer quality. The stand

structure with the higher proportion of small-sized shoots

resulted in the higher proportion of bark in the plantation and

consequently a poorer quality of wood fuel in terms of high

element concentrations.

4.1. Proportion of bark and stand structure

The denser spacing of willow plants at Lundby most likely

promoted the development of more shoots per coppiced plant

(i.e. stool) in this plantation compared to the plantation at

Hammarby (Fig. 2). The proportion of the shoots from the

smaller size classes that had died by the end of the third

growing season (Fig. 2) was higher at Hammarby compared to

Lundby. As a result, the amount of large stems with low

proportion of bark was high at Hammarby. The topography of

the willow field could also have influenced the stand

structure at Lundby. This plantation had a higher proportion

of relatively short and small-sized (low D55) shoots compared

to Hammarby. The plantation at Lundby was covered with

water during early spring each year, indicating that the

experimental site was lying on a waterlogged soil. In an

earlier study, multiple-stem formation (i.e. polycormism) in

mountain birch (Betula pubescens ssp. tortusa (Lebed.) Nyman)

Page 7: Wood fuel quality of two Salix viminalis stands fertilised with sludge, ash and sludge–ash mixtures

ARTICLE IN PRESS

Table 4 – Characteristic parameters of the studied shoot populations in two willow plantations at Lundby and Hammarby,central Sweden (means7SE): shoot diameter at 55 cm height (D55) and the number of non-destructively measured shoots(n), the mean proportion of bark in the whole stand (BPstand), bark dry weight (BDW), wood dry weight (WDW), number ofliving shoots per stool, number of stools per hectare, the yield, net energy content of harvestable shoots on fresh weightbasis (EFW)

Site/age ofshootpopulation

Treatment D55 (n) BPstand

(%)BDW

(t ha�1)WDW(t ha�1)

Livingshoots

(stool�1)

Stools(plot�1)

Yield(t ha�1)

EFW

(GJ ha�1)

Lundby, 1 year Sludge 6.070.1

(752)

27.870.2 4.470.3 11.470.8 15.771.0 2971 15.871.1 3272

Ash 5.870.1

(741)

25.370.5 3.970.2 11.670.7 15.471.2 3071 15.570.9 3173

Sludge+ash 5.970.1

(726)

28.170.2 4.070.4 10.371.1 15.171.0 2672 14.371.5 2773

(Sludge+ash)�2 6.270.1

(797)

28.270.2 4.870.5 12.371.6 16.671.1 2771 17.172.1 3474

Control 5.570.1

(725)

27.870.2 3.570.2 9.271.6 15.171.0 3171 12.772.2 2872

Hammarby, 1

year

Sludge 6.670.1

(472)

23.670.2 4.270.5 13.871.7 9.870.6 2873 18.072.2 2372

Ash 6.970.1

(465)

23.770.2 4.570.5 14.771.8 9.770.5 2572 19.272.3 2474

Sludge+ash 6.770.1

(510)

24.070.2 4.770.4 15.071.6 10.670.7 2373 19.772.3 2475

(Sludge+ash)�2 7.070.1

(511)

23.770.2 4.970.2 15.670.5 10.670.7 2573 20.570.7 2571

Control 8.870.2

(465)

23.770.1 4.270.3 13.371.1 9.770.7 2671 17.571.5 2978

Lundby, 3

years

Sludge 13.870.3

(319)

24.770.2 3.970.3 11.970.8 6.670.5 2771 15.871.1 162711

Ash 13.070.3

(356)

25.370.5 3.970.2 11.670.7 7.470.5 2671 15.570.9 1597 9

Sludge+ash 13.270.3

(335)

25.070.6 3.670.3 10.771.2 7.070.6 2572 14.371.5 146715

(Sludge+ash)�2 13.570.3

(349)

24.670.4 4.270.5 12.971.7 7.370.5 2671 17.172.1 175722

Control 13.770.4

(253)

24.770.6 3.170.5 9.671.7 5.370.4 2772 12.772.2 130722

Hammarby, 3

years

Sludge 19.270.5

(170)

22.070.3 3.970.4 14.171.8 3.570.3 2572 18.072.2 184723

Ash 19.170.5

(191)

22.070.1 4.270.5 15.071. 4.070.3 2371 19.272.3 197724

Sludge+ash 18.170.4

(213)

22.270.2 4.470.4 15.371.6 4.570.3 2471 19.772.0 201721

(Sludge+ash)�2 18.770.4

(214)

22.170.2 4.570.2 15.970.5 4.570.3 2371 20.570.7 2107 7

Control 17.571.5

(183)

22.070.2 3.870.3 13.771.2 3.870.3 2371 17.571.5 179715

EFW was calculated by using a formula of net heating value for fresh material [5] and the yield of the experimental plots.

B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5920

at relatively high soil pH values was proportional to the

severity of nutrient and/or oxygen deficiency in the soil

and was associated with waterlogging of the soil [34]. In

addition to the effect on stand structure, waterlogged soils

may also affect the yield of plantations. Labreque et al. [25]

found that the dry biomass yield at poorly drained willow

plantations was 66% of the yield produced at well-drained

willow plantations in Montreal, Canada. Similar results were

found in this study. The aboveground shoot biomass at

Lundby was 80% of the yield produced in a sloping field at

Hammarby.

The applied treatments did not have any significant

effect either on the number of shoots per stool or on the

number of stools per plot. Consequently, the applied

treatments did not have any effect on the number of shoots

in each plot. This is in accordance with a study conducted

on an abandoned farmland in Southern Quebec, Canada,

where wastewater sludge application in a willow plantation

Page 8: Wood fuel quality of two Salix viminalis stands fertilised with sludge, ash and sludge–ash mixtures

ARTICLE IN PRESS

30

35

20

0

25

HammarbyLundbySite

1 2 3Age (years)

BP sh

oot

Fig. 3 – Proportion of bark in the willow stands (BPstand) after

the 1st, 2nd and 3rd year of the 2nd cutting cycle at Lundby

and Hammarby. Means (7SE) for 5 treatments are shown.

35

00 10 20 30

20

25

30

BP s

tand

(%)

Yield (t ha-1)

Fig. 4 – Relationship between BPstand and harvestable shoot

biomass (yield) after the 1st, 2nd and 3rd year of the second

cutting cycle in two commercial willow plantations at

Lundby and Hammarby (R2¼ 0.44, po0.0001).

BPstand ¼ 35.6+yield(�103). Data are the plot means; n ¼ 3

years�2 sites�20 plots.

Ta

ble

5–

Eff

ect

so

fth

ep

rop

ort

ion

of

ba

rkin

the

sta

nd

(BP

sta

nd),

the

ag

eo

fth

esh

oo

tp

op

ula

tio

n,s

ite

an

dtr

eatm

en

to

nth

eq

ua

lity

of

the

wo

od

fuel,

kg

t�1

(N,P

,K)a

nd

gt�

1

(Zn

,N

i,C

d,

Cu

),g

row

na

tLu

nd

by

an

dH

am

ma

rby

inC

en

tra

lS

wed

en

(n¼

2si

tes�

2a

ges�

5tr

eatm

en

ts�

4re

pli

cati

on

s)

Eff

ect

df

NP

KZ

nN

iC

dC

u

Fp

Fp

FP

Fp

Fp

Fp

Fp

BP

sta

nd

11.2

5N

s.5.2

6o

0.0

56.1

6o

0.0

50.0

2N

s.1.8

7N

s.8.8

5o

0.0

11.9

4N

s.

Age

150.1

4o

0.0

01

17.2

2o

0.0

01

5.0

5o

0.0

517.1

3o

0.0

01

43.2

4o

0.0

01

16.9

0o

0.0

01

166.8

0.0

01

Sit

e1

0.4

8N

s.26.4

7o

0.0

01

2.8

7N

s.379.3

o0.0

01

0.7

3N

s.561.3

o0.0

01

0.5

7N

s.

Tre

atm

en

t4

2.3

8N

s.8.3

7o

0.0

01

12.8

1o

0.0

01

19.8

6o

0.0

01

7.9

5o

0.0

01

3.5

0o

0.0

514.3

8o

0.0

01

Ns.¼

no

tsi

gn

ifica

nt.

B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5 921

did not change the number of stems per plot compared to

control plots [35]. In contrast, wood ash application signifi-

cantly changed the structure of a willow bio-energy planta-

tion in New York State, USA, where wood ash increased

the size of stems and decreased the number of stems,

while the plot biomass production was not changed [36].

In the plantations at Lundby and Hammarby, wood ash

and/or sludge application did not change the structure of

the plantations, while the DW of the shoots in the

(sludge+ash)�2 treatment was increased, resulting in the

increased aboveground shoot biomass in this treatment

(Table 4).

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ARTIC

LEIN

PRES

S

Table 6 – Wood fuel quality of harvestable shoot biomass from two willow plantations in Central Sweden, kg t�1 (N, P, K) and g t�1 (Zn, Ni, Cd, Cu), for the various treatments,if a potential harvest occurred annually or every 3 years

Site, treatment/harvest

interval

N P K Zn Ni Cd Cu

1 year 3 years 1 year 3 years 1 year 3 years 1 year 3 years 1 year 3 years 1 year 3 years 1 year 3 years

Lundby

Sludge 6.7370.04 4.0770.03 1.2670.01 0.7670.01 3.6470.03 2.4970.02 73.170.5 56.870.6 2.9470.02 0.9870.01 1.8970.01 1.1470.01 7.9870.01 2.8470.01

Ash 6.8170.04 3.8370.07 1.1470.01 0.7770.01 3.2670.02 2.3370.05 63.170.3 53.571.3 2.7170.01 0.9670.02 1.7070.01 1.3270.02 7.0370.01 2.7270.02

Sludge+ash 9.0170.03 3.9370.10 1.5070.01 0.8770.01 3.4670.01 2.4970.05 68.270.2 54.371.6 3.3170.01 1.5870.03 1.5870.01 1.1470.02 7.8370.01 3.0270.03

(Sludge+ash)�2 7.7670.03 3.7370.05 1.3770.01 0.8570.01 4.2870.02 2.5570.04 77.870.3 57.071.0 3.2270.01 1.0970.01 1.9470.01 1.1970.01 10.4570.01 3.1970.01

Control 7.8270.04 4.6970.11 1.1370.01 0.8470.01 3.2370.02 2.9970.08 63.270.3 48.871.4 2.5970.01 1.3670.03 1.7870.01 1.2070.02 8.0170.01 2.8670.03

Hammarby

Sludge 7.9070.09 4.0670.04 1.2770.01 0.5870.01 3.5870.04 1.9870.03 39.270.4 33.170.5 2.7970.03 1.0370.01 0.9070.01 0.4870.01 8.4370.02 2.9070.02

Ash 6.6770.02 4.2770.01 0.9070.01 0.5570.01 3.1170.01 1.8870.01 35.970.1 33.170.1 3.0470.01 0.9170.01 0.8370.01 0.4370.01 8.5370.01 2.8770.01

Sludge+ash 7.0070.02 4.0470.03 1.2270.01 0.5370.01 3.5770.01 1.7870.01 47.370.2 32.670.3 3.8470.02 0.8970.01 0.9470.01 0.4870.01 9.3170.01 2.4770.01

(Sludge+ash)�2 6.8370.07 4.3070.03 1.0270.01 0.5470.01 3.4670.04 2.5070.02 40.770.5 32.070.3 2.5570.02 0.7670.01 0.8270.01 0.5570.01 10.0770.01 2.7270.01

Control 6.6670.21 4.0670.03 0.9570.02 0.5570.01 2.6370.10 1.7870.01 39.171.5 25.170.2 2.4470.07 1.0770.01 0.7270.02 0.4970.01 7.5170.06 2.3470.01

The means (7SD) were calculated on the basis of the treatment plots (n ¼ 4).

BIO

MA

SS

AN

DB

IO

EN

ER

GY

32

(20

08

)9

14

–9

25

92

2

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ARTICLE IN PRESS

B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5 923

4.2. Wood fuel quality

Age of the shoot population had a major effect on the quality

of the wood fuel (Table 5). As the proportion of element-rich

bark decreased with the increasing age of the shoot popula-

tion, the concentrations of elements in the harvestable shoot

biomass decreased during the 3 years, improving the quality

of the wood fuel (Table 6). The lower biomass yield at Lundby

resulted in higher proportion of bark in the willow stand

(Fig. 4). The higher BPstand at Lundby (Fig. 3; Table 4) resulted

in higher concentrations of the elements in the harvestable

shoot biomass and, consequently, poorer quality of wood fuel

at this site compared to the wood fuel at Hammarby (Table 6).

The higher proportion of bark means also that the wood fuel

has a higher moisture content compared to the wood fuel

with lower proportion of bark. Moisture content usually

strongly influences the efficiency of the combustion system,

i.e. efficiency decreases with increasing moisture content.

Energy spent on vaporising water can only be recovered if

the combustion system is equipped with an appropriate

heat recovery device, for example a flue gas condensation

unit [5,37].

Fertilisation, soil type and soil pH are important factors

affecting biomass production in willows [24]. Fertilisation

with sludge and/or ash supported the harvestable biomass

production similarly as in the control treatment, where

mineral fertilisers were used (Tables 1 and 4), but reduced

the quality of the wood fuel by increasing concentrations of

the studied elements (except N) in the harvestable shoot

biomass (Table 5). The wood fuel quality was particularly

lower after the first growing season in terms of P and K

concentrations in the treatments containing sludge (Table 6).

However, the ability of fast-growing trees to accumulate

elements that reduce the quality of wood fuel seems to be

limited. The concentrations of Zn in the composite stem

samples of I-214 and Eridano poplar clones (50–80 g t�1)

exposed to industrial waste [38] were in a similar range as

the Zn concentrations in the harvestable shoot biomass of

willows at Lundby (Table 6). Fertilisation of these poplars with

industrial waste increased the total Zn concentration in the

soil by 20%, while the uptake of Zn by the trees was similar to

the uptake of Zn by willows at Lundby. At Lundby, fertilisation

with sludge in legally allowed doses increased the plant-

available fractions of Zn in the soil only by 8% compared to

the control treatments (Table 2).

Due to different absorption capacity for nutrients and trace

elements among species and clones, there are essential

differences in the chemical composition of biofuels. Watson,

Pulford and Riddell-Black [39] found that the Salix clones

S. burjatica ‘Germany’, S. x dasyclados, S. candida and S. spaethii

were more resistant to elevated concentrations of heavy

metals (i.e. produced more biomass on contaminated sites)

and had higher heavy metal concentrations in the wood

compared to the less resistant clones S. viminalis and

S. triandra. These less resistant clones had greater concentra-

tions of Cu and Ni in the bark compared to the wood. The old

Salix clone used in our study (78-021) had also significantly

higher concentrations of all studied elements in the bark

compared to wood. This indicates that the clone used in our

experiments was not an effective accumulator of heavy

metals. An interesting subject to investigate in the future

would be whether clones that accumulate heavy metals in

higher concentrations in wood compared to bark produce

more biomass on contaminated sites. However, higher

concentrations of heavy metals in wood would lower the

wood fuel quality. In this context, plant breeding and genetic

improvements of energy crops offer one possibility to

optimise yield and fuel quality in the future, e.g. to grow

Salix species with high yield and low concentrations of alkali

and heavy metals and other elements that have negative

effect on wood fuel quality (e.g. S and Cl) [5].

The concentrations of N, K (this study) and Cd were ca

4 times higher in the bark compared to wood both in 1- and

3-year-old shoots, and the concentrations of Zn were ca. 8

times higher in the bark compared to wood in the shoots of

both ages [17]. This means that, to ensure good quality of the

wood fuel, willows should be harvested when the proportion

of bark in the stand is relatively low. High N concentrations in

biomass fuels contribute to increased emissions of N oxides

during combustion of N-rich wood fuel, especially bark [37],

but the magnitude is affected by the technology of the

combustion process [5]. Problems with emission of N oxides

can be avoided in larger heat and power plants by the use of

flue gas filters, with which modern district heating plants in

Sweden are usually equipped. In the current study, the

differences in element concentrations between the bark and

wood were relatively larger than those between the treat-

ments. Consequently, the proportion of bark in the stand

should have a greater effect on the wood fuel quality

compared to the fertilisation with sludge and/or wood ash.

Still, the results concerning treatment effects on element

concentrations in this study should be viewed with caution,

since the number of shoots analysed per treatment was

relatively small (n ¼ 3–4). Site significantly affected the

concentrations of the heavy metals in willow bark and wood

in this experiment. The higher concentrations of Zn, Cd and

Ni in the clayey soil at Lundby compared to Hammarby

possibly resulted in higher concentrations of these metals in

the willow bark and wood at this site [17].

High ash content of bark lowers the heat transfer during

combustion [5,22]. The quantity of ash is thus strongly

influenced by the proportion of bark in wood fuels [40,41].

We did not investigate whether the fertilisation with sludge

and/or ash causes the enrichment with mineral elements in

the ashes. These questions should be addressed in the

following studies.

In conclusion, the proportion of bark in the studied willow

stands determined the quality of the wood fuel, since the bark

had significantly higher concentration of majority of the

elements. The proportion of bark in the wood fuel was

dependent on the age of the shoot population and shoot size

frequency distribution of the stand. The application of sludge

and/or ash decreased the wood fuel quality in terms of

increased concentrations of P, K and heavy metals in the bark

and wood. The proportion of bark in a willow stand can be

influenced by the choice of clone and site, planting density

and the frequency of harvests. For example, establishment of

willow fields with relatively low plant densities and extension

of the length of cutting cycles should decrease both the

concentrations of heavy metals in the composite wood fuel

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ARTICLE IN PRESS

B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5924

(kg t�1) and the emissions of nitrous oxides during combus-

tion due to relatively low proportion of bark. Accordingly,

shortening of rotation length should result in an increased

concentration of the above-mentioned elements in the

wood fuel.

Acknowledgements

The authors wish to thank R. Childs, E.-M. Fryk and

C. Segerqvist for technical assistance. Two anonymous

referees and Dr. Almir Karacic commented the manuscript.

Funding for this research was provided by the Swedish

National Energy Administration (project no. P12302-1) and

by the Royal Swedish Academy for Agriculture and Forestry,

which is gratefully acknowledged. Ultuna Egendom and Lars

Helgstrand are gratefully acknowledged for allowing us to use

their willow plantations at Linnes Hammarby and Lundby

Gard, respectively, for our experiments. Johannes Forkman

advised in statistics. Mary McAfee revised the language of an

earlier version of the manuscript.

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