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Scandinavian Journal of Forest Research

ISSN: 0282-7581 (Print) 1651-1891 (Online) Journal homepage: http://www.tandfonline.com/loi/sfor20

Above-ground Woody Biomass Production of

Short-rotation Populus Plantations on Agricultural

Land in Sweden

Almir Karacic , Theo Verwijst & Martin Weih

To cite this article: Almir Karacic , Theo Verwijst & Martin Weih (2003) Above-ground Woody

Biomass Production of Short-rotation Populus Plantations on Agricultural Land in Sweden,

Scandinavian Journal of Forest Research, 18:5, 427-437, DOI: 10.1080/02827580310009113

To link to this article: http://dx.doi.org/10.1080/02827580310009113

Published online: 19 May 2010.

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Above-ground Woody Biomass Production of Short-rotation

Populus

Plantations on Agricultural Land in Sweden

ALMIR KARACIC, THEO VERWIJST and MARTIN WEIH

Department of Short Rotation Forestry, Swedish University of Agricultural Sciences, P.O. Box 7016, SE-750 07 Uppsala, Sweden

Karacic, A., Verwijst, T. and Weih, M. (Department of Short Rotation Forestry, Swedish
University of Agricultural Sciences, P.O. Box 7016, SE-750 07 Uppsala, Sweden). Above-ground
woody biomass production of short-rotation populus plantations on agricultural land in Sweden.
Received Oct. 15, 2002. Accepted Apr. 10, 2003. Scand. J. For. Res. 18: 427

437, 2003.

Although poplars are widely grown in short-rotation forestry in many countries, little is known
about poplar growth performance in Sweden. In this study, above-ground biomass production
was estimated for several hybrid aspen and poplar clones planted at different initial density at
five locations across Sweden. Biomass assessments were based on allometric relationships
between total above-ground woody dry weight and the diameter at breast height. According to a
common harvest practice, tree biomass was partitioned into pulpwood and biomass for energy
purposes. The percentage of pulpwood was strongly determined by clone for DBH

10 cm. The

mean annual increment ranged from 3.3 Mg ha

for balsam poplar in the north to 9.2

Mg ha

for 9-yr-old ‘Boelare’ in southern Sweden. At the same age, hybrid aspen reached

7.9 Mg ha

. The results suggest that poplars and hybrid aspen are superior as biomass

producers compared with tree species commonly grown on agricultural land at these latitudes.
The results are discussed in the light of future wood supply for pulpwood and energy purposes in
Sweden. Key words: Bioenergy, biomass partitioning, biomass production, hybrid aspen, hybrid
poplar, Populus spp., pulpwood, yield.

Correspondence to: A. Karacic, e-mail: Almir.Karacic@lto.slu.se

INTRODUCTION

The future wood market of EU member states will
require a rapid increase in the area under short-
rotation forest to satisfy the need for raw material
from renewable sources (Kuiper et al. 1998). At
present, bioenergy and fibre production within short-
rotation forestry in Europe and North America is
based mainly on poplar species (Populus spp. ) and
their cultivars (Hansen 1991, Heilman et al. 1991,
Makeschin 1999). In Sweden, willow coppicing for
bioenergy has been used for several decades; it has
become an important alternative crop on agricultural
land, with approximately 20 000 ha today. The existing
area of poplar plantations in Sweden does not exceed
300 ha, and little is known about the performance of
poplars grown under short rotation on agricultural
land.

The ability for rapid juvenile growth and an out-

standing growth performance renders poplars suitable
for growth under the fairly fertile conditions provided
by most agricultural soils (Bergez et al. 1989, Heilman
et al. 1994). For example, in the American Pacific
Northwest, fertilised and irrigated plantations reach

yields of above 20 Mg ha

over 4

7-yr-long

rotations (Heilman & Stettler 1985, DeBell et al. 1993,
Scarascia-Mugnozza et al. 1997), and yields of about
30 Mg ha

were recorded in intensively

managed 2 yr coppice systems in France (Pontailler
et al. 1999). However, yields above 10 Mg ha

are rare if irrigation and fertilization are not applied
(Ericsson 1994).

The earliest trials with poplars and aspen in

Sweden, initiated to breed for the industrial needs of
the Swedish Match Company, were started with plant
material originating from Oregon and Washington,
USA (Christersson 1996). The mean annual increment
(MAI) for several clones of hybrid aspen (Populus
tremula L.

P. tremuloides Michx.) in these trials was

estimated to 12 m

, whereas the best producing

poplar

variety

‘Gelrica’

(Populus

canadensis

Mo¨nch.) reached 17 m

by the age of

30 yrs (Persson 1973, Eriksson 1984). Similar

production figures were reported for hybrid aspen on
agricultural land in Denmark (Jakobsen 1976) and
southern Sweden (Elfving 1986, Karlsson 1986, Ilstedt
& Gullberg 1993). Telenius (1999) reported that high
production of dense hybrid aspen and hybrid poplar

Scand. J. For. Res. 18: 427

437, 2003

2003 Taylor & Francis ISSN 0282-7581

DOI: 10.1080/02827580310009113

stands in southern Sweden already occurred after 6 yrs
of growth.

Thus, the expected production figures for poplars in

Sweden are very high, and exceed the production of
Norway spruce [Picea abies (L.) Karst.] and birch
[Betula pendula Roth], the species traditionally chosen
for planting on agricultural land. Spruce, for example,
reaches a maximum MAI of 13 m

(Johansson &

Karlsson 1988) to 14 m

(Eriksson 1976),

whereas the production of birch is 8

10 m

over a period of 50

70 yrs (Elfving 1986,

Sonesson et al. 1994). Johansson (1999a ) found that,
in central Sweden, spruce could produce up to 5.6 Mg
ha

(stem

branches) if grown for bioenergy

purpose in rotations of 30

40 yrs. Elfving (1986)

reported the production of black alder [Alnus gluti-
nosa (L.) Gaertner] on agricultural soils to be about 9
m

, and similar yields have been reported

for young stands of P. tremula L. (Johansson 1999b )
and Alnus incana (L.) Moench. (Granhall & Verwijst
1994).

This study gives an estimate of the growth perfor-

mance of two poplar hybrids, one poplar species and a
clone mixture of hybrid aspen, grown in densely
planted experimental plots and more widely spaced
commercial plantations with locations in northern,
central and southern Sweden. The proportion of
pulpwood in these stands is estimated and possible
advantages and disadvantages of large-scale poplar
culture in Sweden are discussed.

MATERIALS AND METHODS

Sites, planting material and management

The sites included in this study comprise a variety of
agricultural soils in a latitudinal range of between 558
and 648 N (Fig. 1). More detailed information on
climate, soil type, plot design and the number of
sample trees is given in Table 1.

Sites 1 and 2, Bullstofta and Bodarna. These two

experimental sites were established in 1991 to demon-
strate the differences in growth performance between
several tree species that potentially can be used in
short-rotation forestry. Two clones of hybrid poplar
(Populus trichocarpa Hook.

P. deltoides Bartr.) and

one clone mixture of hybrid aspen named ‘Ekebo-mix’
were planted at the plots, 0.1 ha (Bullstofta) and 0.2
ha (Bodarna) large, which were replicated in three
blocks. The hybrid poplar clones ‘Boelare’ and ‘Beau-
pre´’ were obtained as 20-cm-long cuttings from the

Poplar Research Centre in Geraardsbergen, Belgium
(Telenius 1999), whereas ‘Ekebo-mix’ was obtained as
seedlings raised in a greenhouse from root-cuttings
and hardened outdoors for one growing season. In
1995, some plots at both sites were thinned and/or
fertilized (150 kg N ha

), resulting in a split-plot

experimental design (Telenius 1999) with small net-
plots and an edge of at least 6 m (Table 1). The present
study concentrated on non-thinned, fertilized and
non-fertilized, net-plots ending with 54 (Bullstofta)
and 96 (Bodarna) trees available per block for the
assessment of the above-ground biomass. The single
fertilizer application from 1995 did not affect total
biomass production. Both sites were protected by
fences and plots were treated against weeds with
glyphosate during the first 5 yrs.

Sites 3 and 4, Rydsga˚rd and Sa˚ngletorp. Two

commercial poplar plantations, Rydsga˚rd (15.5 ha)
and Sa˚ngletorp (32.5 ha), were established in 1990 and
1991 on sandy agricultural soils near Skurup in
southern Sweden. Both plantations were planted
with Populus maximowiczii Henry

trichocarpa T. &

G. ‘OP42’, obtained from a local forest nursery. At the
age of 10 (Rydsga˚rd) and 9 (Sa˚ngletorp) yrs, each
plantation was divided into three blocks, with four
plots in each block. Consequently, 12 plots, 0.1 ha in
size, with c. 100 trees per plot, were available for
assessing the above-ground biomass in each of these

Fig. 1. Locations of the studied hybrid poplar and hybrid
aspen stands across Sweden.

428

A. Karacic et al.

Scand. J. For. Res. 18 (2003)

two plantations. The plantation in Rydsga˚rd was
protected by a fence, whereas the plantation in
Sa˚ngletorp was unfenced.

Site 5, Innertavle. Clones ‘51’ and ‘910’ of Populus

balsamifera L., originating from Alaska (Latitude
61805? N), were planted in 1986 on the site located
near Umea˚ in northern Sweden. The clones were
systematically mixed within the rows on one plot, 0.1
ha large. After a 4 m border had been defined around
the net-plot, 120 trees were measured in 1999.
Mechanical weeding during the first two growing
seasons after establishment was the only treatment
applied at this site.

Sampling procedure

Biomass estimations were based on a regression
method relating total dry weight (TDW) to the
diameter at breast height (DBH) of single trees
(Verwijst & Telenius 1999). For this purpose, c. 400
trees were randomly selected within diameter strata
and sampled destructively during the period between
1995 and 2002 (Table 1). Trees were harvested
manually after leaf fall, leaving a stump of about 10
cm height. The sampling procedure included the
measurements of DBH, tree height, pulpwood height,
defined as the height where diameter is 5.2 cm, and the
fresh weight of pulpwood and living branches. Hence,
the total biomass of each tree was partitioned into a

pulpwood and a branch section, the latter including
top of the stem (the part above the diameter of 5.2
cm). Stem and branch samples from each tree were
dried to a constant weight at 85

1058C and the dry

weight/fresh weight ratio was used to estimate the
TDW of each sample tree. The DBH of all trees on
net-plots was measured at ages 4

9 yrs in Bullstofta,

11 yrs in Bodarna, 9

12 yrs in Rydsga˚rd and

Sa˚ngletorp, and 14 yrs in Innertavle. The sample trees
were used to develop clone- (species), site- and age-
specific biomass equations.

Regression procedures

For destructively sampled trees, the TDW of whole
trees was related to the DBH for all clones separately
(except for ‘910’ and ‘51’ that were pooled together)
using the function:

TDW bDBH

where b and c are parameters. To calculate TDW for
‘Boelare,’ ‘Beaupre´‘ and ‘Ekebo-mix’, age-, clone- and
species-specific equations were used for the age classes
4, 5, 6

8 and 9

11 yrs, one equation was used for

‘OP42’ at Rydsga˚rd and Sa˚ngletorp (ages 9

12 yrs),

and one for balsam poplars at Innertavle (age 14 yrs).

The amount of pulpwood was expressed as a

percentage of TDW for each destructively sampled
tree. Thereafter, the pulpwood percentage (PP) of the

Table 1. Stand and site characteristics of the study sites and poplar stands in Sweden

Location

Temp.
sum

Precipitation

(mm)

Soil type
(USDA) and
pH

H2O

Clone

Year of
planting

Spacing
(m)

Net-plot
size (m)

No. of
repl.

No. of
sample
trees

Innertavle
(63848? N)

1066

571 (279)

Loam, pH 5.3

910, 51

1986

Bodarna
(60800? N)

1305

607 (362)

Clay, pH 5.7

Ekebo

1991

Bullstofta
(55859? N)

1558

757 (473)

Sandy loam,
pH 6.7

Beaupre´

Boelare

Ekebo

1991

338

Rydsga˚rd
(55831? N)

1500

655 (392)

Sandy loam,
pH 5.8

OP42

1990

Sa˚ngletorp
(55833? N)

1500

655 (392)

Sandy loam,
pH 6.3

OP42

1991

Length of vegetation period calculated at the 58C baseline.

Values in parentheses are the precipitation during the vegetation period (mean value for the period 1960

1990).

Populus balsamifera,

P. tremula

tremuloides (clone mixture),

P. trichocarpa

deltoids,

P. maximowiczii

trichocarpa.

repl.: replications.

Scand. J. For. Res. 18 (2003)

Biomass production of short-rotation poplars

429

TDW of sample trees was related to the DBH using
the function (Richards 1959):

PP ps(1e

(cdDBH)

)

(1=f )

where p, s, c , d and f are parameters, and e is the base
of natural logarithm. Consequently, the dry weight of
pulpwood on net-plots was calculated using four
separate equations obtained for pulpwood percentage
of ‘Boelare’ and ‘Beaupre´’, balsam poplars (‘910’

‘51’), ‘OP42’ and ‘Ekebo-mix’ (Fig. 2).

The volume of all destructively sampled trees was

estimated using the equation (Eriksson 1973):

V 0:03597DBH

1:84297

1:15988

where V represents the volume of whole stem (to the
top) (dm

), and H is the tree height (m). This equation

was obtained for European aspen and was previously
used for estimations of hybrid poplar growth in
Sweden (Persson 1973). To calculate the volume of
trees on net-plots, where tree heights were not
measured, the volume of single trees, calculated by
Eriksson’s equation, was related to the DBH using the
function:

V abDBH

where a , b and c are parameters. Hence, four separate
volume equations were obtained for ‘Boelare’ and

Beaupre´, OP42, Ekebo-mix and balsam poplars
(910

51), with DBH as the only independent vari-

able.

The pulpwood volume was expressed as a percen-

tage of the whole stem volume. For this purpose, 90
trees of 7-yr-old ‘Beaupre´’ and ‘Ekebo-mix’ destruc-
tively sampled in Bullstofta were used, for which whole
stem volumes were recorded by measuring diameters
at the ends of 50-cm-long sections along the stem. The
volume of a whole stem was calculated using the
Smalian’s formula V (Aa)=2L

for each section,

and V (A=3)L

for the top of stem, where A and a

represent the area of both ends of each section, and L

and L

are the lengths of the sections and stem top,

respectively. Thereafter, the percentage of total pulp-
wood fraction of the whole stem volume (PV) for these
trees was related to the DBH using the function:

PV abDBH

where a, b and c were parameters. Another equation
was obtained for the percentage of harvestable pulp-
wood volume (HPV), i.e. the stem fraction that could
be harvested as 3-m-long pulpwood logs (Fig. 3).
Further on, these equations were used to calculate the
percentage of pulpwood volume (of the whole stem
volume) for all the hybrids and poplar species studied.
All regressions and statistics were calculated using the
SYSTAT 9 software package (Anon. 1999).

Fig. 2. Pulpwood biomass expressed as a percentage of the
total biomass of individual sample trees, related to their
diameter at breast height (DBH). The regression lines
marked with different letters approach the asymptote at
significantly different levels, i.e. have significantly (p B

0.05)

different values of the parameter s.

Fig. 3. Total (solid line) and harvestable pulpwood volume
(dashed line) of ‘Boelare’, ‘Beaupre´’ and ‘Ekebo-mix’
expressed as a percentage of the total stem volume, and
related to the diameter at breast height (DBH). Both
regression lines were obtained for 7-yr-old trees.

430

A. Karacic et al.

Scand. J. For. Res. 18 (2003)

RESULTS

Survival and competition

All poplar and hybrid aspen clones were established
successfully, except for ‘Boelare’ and ‘Beaupre´’ in
Bodarna. These two clones suffered serious frost
damage and were completely removed from the site 2
yrs after establishment. By the age of 9 yrs, the
mortality was just below 10% for densely planted
(1

2 m) ‘Boelare’, ‘Beaupre´’ and ‘Ekebo-mix’ in

Bullstofta, and 23% for ‘Ekebo-mix’ in Bodarna. In
Sa˚ngletorp and Rydsga˚rd (3.3

3.3 m spacing), 95%

of planted cuttings survived until December 1999 (at
the age of 9 and 10 yrs), when a storm reduced the
number of living trees by 11% and 24%, respectively
(Table 2). The survival on the plot in Innertavle was
almost 100% at the age of 14 yrs.

The DBH of densely spaced plots reached 9 cm at

the age of 9 yrs, 15 cm at the more widely spaced
plantations in Sa˚ngletorp and Rydsga˚rd at the same
age, and 11 cm at the plot in Innertavle (Table 2). The
mean dry weight of trees in the widely spaced
plantations at Sa˚ngletorp and Rydsga˚rd averaged 60
kg at the age of 9 yrs, which was three to four times
higher than in the densely spaced experimental plots at
Bullstofta and Bodarna (means of 15 kg and 18 kg,

respectively). The coefficient of variance (CV) and
skewness of weight distributions, used as approxima-
tions for competition effects at the plots, tended to be
higher for plots with closer spacing (Table 2).

Total woody biomass production

The total above-ground woody biomass of hybrid
poplar clones ‘Boelare’, ‘Beaupre´’ and ‘OP42’, re-
corded after 9

12 yrs, ranged from 59 to 98 Mg ha

The corresponding yield of 9

11-yr-old hybrid aspen

was 58

77 Mg ha

, whereas the 14-yr-old balsam

poplars at the northernmost site in Innertavle reached
48 Mg ha

The range of MAI was between 3.3 Mg ha

for balsam poplar in Innertavle and 9.2 Mg ha

estimated for densely planted 9-yr-old ‘Boelare’ at
Bullstofta (Table 3). At the age of 9 and 10 yrs, the
MAI in commercial plantations at Sa˚ngletorp and
Rydsga˚rd was 6.6 to 6.8 Mg ha

respectively,

increasing to 8.2 to 8.5 Mg ha

2 yrs later (Fig.

4A).

The current annual increment (CAI) for these two

sites, calculated as a mean for the growing season 2000
and 2001, was 15

17 Mg ha

. At Bullstofta and

Bodarna, the variations of CAI were large at the age
of 5

11 yrs for both hybrid aspen and hybrid poplar,

Table 2. Initial density, survival and development of diameter at breast height (DBH) for hybrid aspen and hybrid
poplar stands grown at various sites across Sweden

Location

Clone

Age
(yrs)

Initial
density

Survival
(%)

Basal area
(m

)

Top
height

(m)

Mean
DBH (cm)

Mean tree
weight (kg)

Skewness

Innertavle

910, 51

1667

99.5

19.06

13.7

11.4

27.9

0.61

0.45

Bodarna

Ekebo

5000

80.4

4.72

3.6

2.2

0.69

0.48

76.7

26.15

13.0

8.9

14.6

0.73

0.99

68.5

32.19

15.5

10.5

22.6

0.70

1.06

Bullstofta

Beaupre´

5000

96.3

10.81

5.0

3.9

0.60

0.46

92.0

31.74

17.3

8.9

15.8

0.69

1.21

Boelare

93.8

9.60

7.2

4.9

3.5

0.56

0.83

90.7

34.55

16.8

9.3

17.9

0.72

1.21

Ekebo

92.9

3.24

5.8

2.8

1.1

0.55

0.13

91.8

32.20

9.1

14.7

0.61

0.26

Rydsga˚rd

OP42

1000

95.7

20.96

16.0

67.1

0.36

0.30

73.0

28.50

18.2

97.5

0.39

0.32

Sa˚ngletorp OP42

1000

95.1

18.76

17.1

15.2

60.5

0.41

0.07

84.3

27.02

21.0

17.8

93.2

0.43

0.02

The coefficient of variance (CV) of tree weight, and skewness of weight distribution were used as a measure of competition
effects in the studied hybrid aspen and poplar stands.

Top height is the height of 30% highest trees in the stand, estimated from the height of destructively sample trees.

At the age of 11 and 12 yrs, respectively, the data are presented for plots and parts of plots not disturbed by wind damage,

except for survival, which is representative for the cumulative area of all plots at Rydsga˚rd and Sa˚ngletorp.

Scand. J. For. Res. 18 (2003)

Biomass production of short-rotation poplars

431

with a mean periodic increment of 10

13 Mg ha

. The maximum CAI (20 Mg ha

) was recorded

in hybrid aspen at Bullstofta at the age of 7 yrs (Fig.
4B).

The pattern of MAI and CAI over time was similar

for all densely planted plots except for hybrid aspen at
Bullstofta (Fig. 4A, B). ‘Boelare’ and ‘Beaupre´’ at
Bullstofta, and the ‘Ekebo-mix’ at Bodarna showed
severe decrease in biomass production during the dry
growing seasons of 1997, 1999 and 2001. In contrast,
the ‘Ekebo-mix’ at Bullstofta increased growth in 1997
and retained a higher CAI than the two hybrid poplar
clones in 1999.

Stem wood (pulpwood) biomass production

In terms of whole stem volume, the total production
ranged from 119 m

in balsam poplar (Inner-

tavle) to 218 m

in 12-yr-old ‘OP42’ (Table 3). In

Rydsga˚rd and Sa˚ngletorp, the MAI reached 18

19 m

after 11 and 12 yrs, whereas the densely planted

plots in Bullstofta produced 22 m

‘Boelare’ after 9 yrs (Fig. 4C). For densely planted
plots of ‘Ekebo-mix’, ‘Boelare’ and ‘Beaupre´’, the
mean periodic increment at the ages of 5

11 yrs

averaged about 30 m

(Fig. 4D).

The production of pulpwood was similar in densely

planted ‘Boelare’ (19 m

) and commercial

plantations at Rydsga˚rd and Sa˚ngletorp (18 m

). When expressed as a percentage of total stem

volume, both the total and harvestable pulpwood
volume (3-m-long pulpwood logs) increased with
DBH (Fig. 3). However, the volume of harvestable
pulpwood was 90

92% of total pulpwood volume

recorded for ‘Beaupre´’ and ‘Boelare’ at the age of 9
yrs, whereas the corresponding percentage was 97

98% for ‘OP42’ at the age of 9

12 yrs.

Pulpwood biomass, expressed as percentage of the

TDW, strongly increased with diameter for trees with a
DBH B

10 cm, and thereafter approached asympto-

tically a percentage of about 75. The slope of the
regression line was similar for all clones, but the clones
were significantly different (p B

0.05) in terms of final

pulpwood percentage, ranging from 72% in the
‘Ekebo-mix’ to 82% in ‘Boelare’ and ‘Beaupre´’ (Fig.
2). The total amount of standing pulpwood biomass
per hectare, species (clone) and age is shown in Fig.
5A. Expressed as a percentage of the total above-
ground biomass per hectare, the pulpwood biomass
increased with age for all clones at Bullstofta and
Bodarna. At the age of 9 yrs, it reached 70% in
‘Ekebo-mix’, and 77% in ‘Boelare’ and ‘Beaupre´’ (Fig.

Table 3. Total and annual above-ground woody biomass production of hybrid poplar and hybrid aspen grown at five
locations in Sweden

Biomass

Stem volume

Pulpwood

Location

Clone

Density
(trees
ha

)

Age
(yrs)

Total
(Mg
ha

)

MAI
(Mg
ha

)

CAI
(Mg
ha

)

Total

)

MAI
(m

)

CAI
(m

)

Total
(Mg
ha

)

Total

)

MAI
(m

)

CAI
(m

)

Innertavle

910

51 1667

3.3

119

8.5

108

7.7

Bodarna

Ekebo

5000

6.4

6.8

149

16.5

16.7

123

13.7

17.0

7.0

8.0

200

18.2

18.5

176

16.0

19.1

Bullstofta

Boelare

5000

9.2

10.8

200

22.2

25.6

169

18.8

24.9

Beaupre´ 5000

8.3

8.9

177

19.7

19.2

147

16.3

18.1

Ekebo

5000

7.9

12.9

183

20.3

33.5

152

16.9

33.1

Rydsga˚rd

OP42

1000

6.8

150

15.0

34.0

143

14.2

33.8

8.2

218

18.1

210

17.5

Sa˚ngletorp OP42

1000

6.6

15.1

132

14.6

37.2

125

13.9

37.1

8.5

16.8

206

18.7

199

18.1

The current annual increment (CAI) was based on the growth during the last growing season (1999 or 2001), except at
Rydsga˚rd and Sa˚ngletorp where the CAI was calculated as a mean of the two last growing seasons (2000 and 2001).

Whole stem volume.

Volume of the stem up to 5.2 cm top diameter.

MAI: mean annual increment.

432

A. Karacic et al.

Scand. J. For. Res. 18 (2003)

5B). At Rydsga˚rd and Sa˚ngletorp, the percentage of
pulpwood biomass per hectare did not change (75%)
during the two last growing seasons (ages 9

11 and

12 yrs, respectively).

DISCUSSION

Three aspects of this study deserve to be emphasized:
(1) the yield of poplar and hybrid aspen compared
with the tree species traditionally grown on agricul-
tural land in Sweden, (2) the partitioning of biomass
into pulpwood and biomass for bioenergy, and (3) the

production potential of large-scale poplar plantations
and its implications for future wood supply in Sweden.

The knowledge of intensive poplar cultivation in

North America and Europe is well documented, but it
cannot be applied to Sweden without modifications.
Moreover, the growth and biomass production in
short-rotation poplar field trials vary greatly owing
to different environmental conditions and cultural
regimens (Ceulemans & Deraedt 1999). Annual bio-
mass production of

20 Mg ha

, reported

from trials in the Pacific northwest, USA (Heilman &
Stettler 1985, Heilman & Xie 1993, DeBell et al. 1996,
Scarascia-Mugnozza et al. 1997) and from coppice
cultures in Europe (Pontailler et al. 1999), should be
seen in the light of the intensive fertilization and
irrigation applied at these sites. In the UK, densely
planted (1

1 m), 4-yr-old ‘Boelare’ and ‘Beaupre´’

had an annual above-ground woody biomass produc-
tion of 6.4

13.6 Mg ha

, whereas the produc-

tion at a spacing of 2

2 m varied between 4.3 and 9.7

Mg ha

(Armstrong et al. 1999). At the same

age, the production of these two clones in the present
trials (Bullstofta) was at the lower margin of the above
range (4

5 Mg ha

). This could be explained

partly by phenological maladjustments pointed out by
Ilstedt (1996) as being the main reason for unsuit-

Fig. 4. Mean (MAI) and current (CAI) annual increment of
poplar and hybrid aspen stands grown in Sweden as related
to age. The yields are expressed as (A, B) total above-ground
woody biomass and (C, D) the total stem volume.

Fig. 5. (A) Total biomass of pulpwood in poplar and hybrid
aspen stands as related to age. The biomass and pulpwood
volume can be related linearly using the conversion coeffi-
cient 2.3 calculated for 9

12-yr-old OP42. (B) Pulpwood

biomass expressed as a percentage of the total biomass
produced per hectare as related to age.

Scand. J. For. Res. 18 (2003)

Biomass production of short-rotation poplars

433

ability of Belgian clones for Swedish climate. The
failure to establish ‘Boelare’ and ‘Beaupre´’ at Bodarna
(central Sweden) confirms these conclusions, but a
vigorous growth of balsam poplars in Innertavle
suggests that, when appropriately selected, poplars
may potentially be grown on agricultural land across
major areas of the country.

Compared with spruce grown on agricultural land

in Sweden, both hybrid aspen and hybrid poplars in
this study exhibited very high biomass production
(Table 4). The MAI of 9.2 Mg ha

in 9-yr-old

clone ‘Boelare’ is among the highest known figures
recorded in forest plantations on agricultural land in
Sweden. Similarly high production figures were re-
corded for naturally regenerated (resprouted) young
(3

18 yrs old) and dense stands (6800

42 000 trees

) of birch, aspen, grey alder and willow-coppice.

In this context, an interesting result was obtained from
the two large commercial plantations at Rydsga˚rd and
Sa˚ngletorp, where equally high annual biomass pro-
duction (8.2 and 8.5 Mg ha

) was achieved with

a relatively low initial density of 1000 trees ha

after

only 11 and 12 yrs of growth. Both the rapid increase
in MAI and the high CAI at Sa˚ngletorp and Rydsga˚rd
(Table 2) suggest that growth is vigorous at the ages of
9

11 (10 to 12) yrs. Therefore, it is still difficult to say

at which age the MAI will reach the maximum in these
plantations. Judged from the ‘Ekebo-mix’ grown at
Bodarna, the maximum MAI at closer spacings (1

m) could be achieved by the age of 11 yrs. In terms of

stem volume, the annual production of the two
commercial plantations at Rydsga˚rd and Sa˚ngletorp
after 11 and 12 yrs (18 m

) was similar to

the maximum MAI of about 17 m

recorded in the old Swedish trials after 25

30 yrs

(Persson 1973, Eriksson 1984). The longer time
needed to achieve these production levels in old trials
was probably due to early thinnings that brought
down the number of trees per hectare to c. 500.

As stated previously, Norway spruce and birch

hitherto have been the most frequently planted tree
species on agricultural land in Sweden. In terms of
MAI, commercial spruce and birch plantations are
inferior to hybrid poplar and hybrid aspen. If estab-
lished by natural regeneration or by resprouting after
the harvest, stands of some pioneer tree species (A.
incana , B. pendula and P. tremula ) become dense,
reaching maximum MAI and a high biomass yields
early in rotation (Table 4). Such stands can be
managed in short rotations and harvested biomass
can be used for bioenergy purposes. Johansson
(1999a ) suggests that spruce grown for bioenergy
purposes could be an alternative crop on agricultural
land. This would require a shortening of the rotation
period to about 35 yrs, abandoning thinnings and
implementing only a final harvest, which would yield
5

6 Mg ha

. An additional positive effect

following the implementation of such a silvicultural
system would be the reduction of risks (e.g. low
density of wood, root-rot, frequent storm damage)

Table 4. Biomass production and some stand characteristics of various tree species commonly planted on farmland in
Sweden

Species

Latitude

Age (yrs)

Density
(trees ha

)

Mean DBH
(cm)

Stem wood
(Mg ha

)

Branch wood
(Mg ha

)

Total
(Mg ha

)

MAI
(Mg ha

)

Reference

Picea abies

57857? N

1507

21.2

157

220

5.5

Johansson (1999a )

Picea abies

60810? N

2000

16.3

105

157

5.1

Picea abies

58825? N

2667

17.2

126

186

5.0

Picea abies

55859? N

880

308

5.6

Nihlga˚rd (1972)

Picea abies

578

608 N

2195

4286

226

3.7

7.4

Johansson &
Karlsson (1988)

Salix spp.

59849? N

20 000

Nordh (2003)

Betula pendula

60809? N

4061

11.3

157

172

6.6

Johansson (1999c )

Betula pendula

65847? N

42 000

3.2

8.0

Populus tremula

60818? N

10 900

8.4

133

156

8.7

Johansson (1999b )

Populus tremula

60815? N

26 143

4.4

8.5

Populus tremula

60822? N

6800

9.2

106

124

8.2

Alnus glutinosa

57859? N

20 000

3.5

7.2

Johansson (2000)

Alnus incana

60827? N

7400

8.2

9.6

Alnus incana

63814? N

16 800

6.6

118

129

8.6

Average for 16 clones (selected and improved) in fertilized trials.

First rotation,

second rotation.

Personal communication (Nils-Erik Nordh, Department of Short Rotation Forestry, Swedish University of Agricultural

Sciences, Uppsala, January 2003).
DBH: diameter at breast height; MAI: mean annual increment.

434

A. Karacic et al.

Scand. J. For. Res. 18 (2003)

present in traditional silviculture with spruce on
agricultural land. According to the present results,
growing poplars in short rotations (10

15 yrs) could

potentially increase the final yields. The shorter
rotation periods would decrease the risks for storm
damage and provide a farmer with an early and
regular income, allowing for better flexibility of the
whole enterprise. However, if only biomass for bioe-
nergy is regarded, the productivity of Salix spp. (7

Mg ha

within a 3

4 yr cutting cycle) is as high

as in poplar stands. The wind damage in Rydsga˚rd and
Sa˚ngletorp that occurred in December 1999 indicates
that poplar plantations also can be sensitive to wind
damage. The proportions and characteristics of such
damage are the subject of a separate study.

The partitioning of above-ground biomass is re-

garded to be under strong genetic control and is an
important commercial characteristic of a clone
(Hinckley et al. 1993). DeBell et al. (1996) found
that biomass allocation in two poplar hybrids was also
affected by initial spacing, where the proportion of
stem biomass was higher in closer spacings. In the
present study, however, the partitioning of tree bio-
mass into pulpwood and biomass for energy purposes
(branches

top of the stem) was also strongly deter-

mined by the clone (Fig. 2). The strong increase in
percentage of pulpwood for the trees with DBH B

cm implies that the percentage of pulpwood per
hectare should be closely related to diameter fre-
quency and hence to the initial spacing. Nevertheless,
despite relatively large differences in mean DBH, the
pulpwood proportion per hectare of closely spaced
‘Boelare’ at the age of 9 yrs was only 5% lower than
the maximum for this clone at DBH of between 10 and
21 cm.

Considering the current harvest practices for thin-

ning of forest stands in Sweden (3-m-long pulpwood
logs), one can expect the volume of harvestable
pulpwood to be very low (c. 50% of total stem
volume) for trees with DBH B

8 cm. At the end of a

9 yr rotation period, the difference between total and
harvestable volume of pulpwood for dense stands of 9-
yr-old Boelare is not very high (8

10%), and it is even

less (2

3%) for more widely spaced plantations

(OP42) after 9

12 yrs of growth. These results

correspond well with those of DeBell et al. (1997),
who argued that single poplar trees in a stand need an
area of at least 6.2 m

to grow to a mean DBH of 15

cm, which is considered a minimum for economic
production of a number of products.

In general, a high planting density is economically

disadvantageous because of higher costs for establish-
ment and harvest. The closer spacings may yield high
economic return, because the high biomass production
is obtained in a short rotation period, but owing to the
high planting density, trees in such plantations are
likely to be exposed to early competition and density-
dependent mortality. In coppiced plantations, density-
dependent mortality is avoided by frequent harvests
(2

4 yrs), but in single-stem plantations it may occur

before the maximum MAI is reached. Although it is
difficult to judge whether mortality is density depen-
dent, the mortality that occurred at the age of 4

yrs in densely planted plots at Bodarna and Bullstofta
was compared against characteristics of weight dis-
tribution, used as a measure of competition. The CV
and skewness of weight distribution were both higher
for densely planted plots than for widely spaced
plantations (Table 2). This is in accordance with the
results of Telenius (1999), who interpreted these higher
values as indications of an intensified competition for
resources among individual trees. However, consider-
ing that the present data are not directly comparable
(i.e. different sites, species and hybrids), the results
shown here can only indicate the general rule and
should be interpreted with care.

All trees at Rydsga˚rd and Sa˚ngletorp that survived

the establishment phase were still alive after 11 and 12
yrs of growth. Survival was equally high at Rydsga˚rd
and Sa˚ngletorp, despite the lack of fence protection at
Sa˚ngletorp and a high wildlife pressure (roe deer,
fallow deer, red deer, hares) from the adjacent wildlife-
protected area at Ha¨ckeberga. This may indicate that
not all poplar clones are appreciated by browsing
animals; this would be an important advantage
compared with hybrid aspen, which has been reported
to be extremely sensitive to browsing by wild animals
(Rytter et al. 2002).

The high biomass yields of the hybrid poplars

studied here suggest that plantations of hybrid poplars
could become a valuable enterprise in Sweden. How-
ever, many Swedish farmers still feel comfortable with
the choice of spruce or birch for planting on aban-
doned agricultural land, implying a well-known silvi-
cultural system, and relying on a well-established
wood market. The interest in deciduous trees has
increased since the early 1990s, however, partly
because of the problems associated with spruce on
agricultural soils, and partly because of environmental
concerns introduced in the Forestry Act of 1994

Scand. J. For. Res. 18 (2003)

Biomass production of short-rotation poplars

435

(Anon. 1994). Indeed, poplar stands of small size are
likely to increase floristic diversity in landscapes
dominated by coniferous forests and/or traditional
agricultural crops (Weih et al. 2003).

If a mean annual production of 18 m

(as found at Rydsga˚rd and Sa˚ngletorp) is assumed, the
establishment of a total of c. 220 000 ha poplar
plantations would be needed to cover the current
annual import to Sweden of about 4 million m

pulpwood from deciduous tree species (Anon. 2000).
Besides pulpwood, poplar stands of this area could
deliver 1.8 TWh energy, which is about 2% of the
current (year 2000) bioenergy production in Sweden.
Established as densely planted energy forest and
assuming an annual yield of 8 Mg ha

, the

same area could support the production of 7.2 TWh
energy (1 Mg :

4.1 kWh), which was 7% of the total

bioenergy production and about 50% of the energy
delivered by Swedish district heating plants in 2000.
The cutting cycle in these short-rotation forests should
be 10

15 yrs, depending on the final product, site

productivity and management intensity.

This study shows that poplars can achieve high

yields in Sweden, and represent an interesting alter-
native crop for planting on agricultural land. Never-
theless, yields will be improved in future by means of
extensive research on poplar breeding and the design
and management of production systems. The most
severe constraint for poplar growth at these latitudes is
probably inadequate in-wintering or budset in many
productive clones and hybrids (Ilstedt 1996, Christers-
son 1996), which requires a breeding programme with
poplars originating from higher latitudes (e.g. British
Columbia and Alaska). Given a relatively large
amount of land area under transition (either from
agricultural land to forest land or from coniferous
forest land to deciduous forest land), poplar cultiva-
tion has a strong potential to expand in Sweden as well
as in other parts of Europe.

ACKNOWLEDGEMENTS

Parts of this work are based on results from field trials,
supported financially by the Swedish Energy Agency.
The biomass assessments were partly carried out
within the project ‘‘Poplars: a multiple-use crop for
European arable farmers (PAMUCEAF)’’, sponsored
financially by the European Union, project FAIR6CT-
98-4193. We wish to thank Richard Childs and Anneli
Tamm, who helped to cut and measure sample trees.

Professor emeritus Lars Christersson contributed with
valuable comments on the manuscript.

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