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Growth and production of a short rotation coppice culture of poplar. III. Second rotation results
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Biomass and Bioenergy 29 (2005) 10–21 www.elsevier.com/locate/biombioe
Growth and production of a short rotation coppice culture of poplar. III. Second rotation results Ilse Laureysens, An Pellis, Jessy Willems, Reinhart Ceulemans Department of Biology, University of Antwerp, Campus Drie Eiken, Universiteitsplein 1, B-2610 Wilrijk, Belgium Received 30 September 2004; received in revised form 3 February 2005; accepted 14 February 2005 Available online 7 April 2005
Abstract This study describes production and growth of the second rotation of 17 poplar (Populus spp.) clones in a short rotation coppice culture (SRC). In addition, the link with leaf characteristics was studied. In April 1996, an experimental field plantation with 10,000 cuttings ha1 was established on a former waste disposal site. In December 1996, January 2001 and February 2004, all stools were coppiced. At the end of the second rotation (2001–2003), highest biomass production was found for P. nigra clone Wolterson with 9.7 Mg ha1 y1. The best performers of the first rotation, i.e. P. trichocarpa P. deltoides clones Hoogvorst and Hazendans, performed poorly in the second rotation, due to heavy rust infections. Two growth strategies were evident: Wolterson had a slow elimination of smaller shoots and had lots of smaller leaves; Hazendans and Hoogvorst had a rapid elimination of smaller shoots and had fewer, larger leaves. We conclude that shoot growth dynamics and leaf size were not the primary production determinants in our poplar SRC. But Melampsora larici-populina remained an important external determinant of biomass production. r 2005 Elsevier Ltd. All rights reserved. Keywords: Populus Spp.; Short rotation coppice; Biomass production; Mortality; Leaf
1. Introduction Short rotation forestry (SRF), i.e. fast-growing tree crops grown in carefully tended plantations for rotations shorter than 15 years [1,2], is limited to a few hectares of experimental plantations in Corresponding author. Tel.: +32 3 820 22 56; fax: +32 3 820 22 71. E-mail address: [email protected] (R. Ceulemans).
Belgium (J.-M. Jossart, 2003, personal communication). Nevertheless, Belgium’s Kyoto target is a 7.5% reduction of fossil fuel emissions by 2008–2012 compared to 1990. Energy crops have an important role to play [3], in particular SRF because of its numerous ecological benefits: it has a positive impact on biodiversity, on nutrient capture and on the carbon circulation in the soilplant atmosphere system, especially on former agricultural land. Furthermore, they protect the soil from water and wind erosion [4–6]. However,
0961-9534/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2005.02.005
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free space is getting very scarce in Belgium. One option is to establish SRF plantations on set-aside land [3]: agricultural land offers optimal soil conditions. However, SRF is not yet profitable without subsidies. Under optimal conditions, smallplot yields achieved by poplar (Populus spp.) are in the order of 20–25 Mg ha1 y1 in trials conducted in the USA (PNWS) [7–9]. Under less optimal conditions, annual yields of 10–15 Mg ha1 are more realistic [10]. Another option is to establish SRF plantations on marginal or slightly polluted land. Trees will stabilize the surface, prevent dustblow, and reduce leaching and run-off of water. SRF on contaminated land provides therefore a means not only for stabilization and decontamination (e.g. heavy metal uptake) of a site, but also for some economic return from the land [5]. However, clones should be selected that grow well under less optimal conditions. In Belgium, poplar and willow are most suitable for SRF. High biomass production is achieved by selecting clones with high growth vigor and disease resistance, and by applying certain agricultural techniques, such as site preparation, weed control and high planting density [1,2]. In addition, cultural management generally involves coppicing, i.e. the cutting of a tree at the base of its trunk to use the ability of the trees to regenerate from the cut stump, resulting in the emergence of new shoots from the stump and/or roots [11]. Coppicing is frequently applied at the end of the establishment year to promote sprouting of many shoots per cutting, which is supposed to increase final biomass production [1,2]. So far, poplar breeding has never focused on the creation of poplar clones specifically suited for a coppice system. Nevertheless, poplar performs rather well in coppice cultures [12]. However, current biomass yields are not high enough to be economically feasible. Therefore, further breeding is necessary to produce clones characterized by superior growth, and resistance to pests and diseases of leaf and stem. In this study, a short rotation coppice culture (SRC) with different poplar clones provided the opportunity to study the relationship between biomass production and some anatomical parameters for a wide genotypic range. In addition, very few studies have quantified biomass produc-
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tivity over more than one rotation cycle in SRC. Therefore, the objectives of this paper were: 1) to quantify the above-ground biomass productivity of 17 clones in a SRC at the end of the second rotation; 2) to study the link between biomass production and shoot growth dynamics; 3) to study the link between biomass production and leaf characteristics; 4) to study clonal differences in biomass distribution to leaves, stem and branches.
2. Materials and methods 2.1. Experimental set-up and plant material In April 1996, a short rotation coppice plantation was established in an industrial area of Boom near Antwerp, Belgium (511050 N, 041220 E). The plantation is situated on an old household waste disposal site, which was covered with a 2-m-thick layer of sand, clay and mixed rubble. The soil was characterized by a high bulk density (heavy clayloam), ranging between 1.22 and 1.62 g cm3, and by a high pH (7.3–8.1). The upper soil horizons contained between 0.8% and 1.8% organic matter. The nutrient and mineral reserves were extremely high in comparison with forest soils, but moderate in comparison with agricultural soils [12]. The site is situated at 5 m above sea level, and has a temperate climate with a mean temperature of 10 1C and a mean annual precipitation of 767 mm. Prior to planting, the area was leveled and cleaned of large stones, plastic, metal and other debris. A rotor tiller was used in the early spring of 1996 for final pre-planting soil preparation. Seventeen poplar (Populus) clones, belonging to five different parentages were studied, i.e. P. trichocarpa T. & G. P. balsamifera L. (T B) clone Balsam Spire; P. trichocarpa P. deltoides Marsh. (T D) clones Beaupre´, Boelare, Hazendans, Hoogvorst, Raspalje and Unal; P. trichocarpa (T) clones Columbia River, Fritzi Pauley and Trichobel; P. deltoides P. nigra L. (D N) clones Gaver, Gibecq and Primo; P. deltoides P. trichocarpa (D T) clones IBW1, IBW2 and
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IBW3; and native P. nigra (N) clone Wolterson. All clones were planted as 25-cm-long dormant, unrooted cuttings, after being soaked in water for 24 h. Cuttings were planted manually to a depth of 20 cm, leaving one or two buds above the soil surface. They were planted in a double-row design with alternating inter-row distances of 0.75 and 1.5 m, and a spacing of 0.9 m between cuttings within the rows, accommodating an overall planting density of 10,000 cuttings ha1. A randomized block design was used with 17 clones 3 replicate plots (except for clone Hoogvorst with six replicates, and for clones IBW1 and Raspalje with two replicates) according to a protocol prescribed by the British Forestry Commission [13]. Individual plot size was 9 m 11.5 m, containing 10 rows of 10 trees each. In the center of each plot, 36 trees (6 6) were used as assessment trees to avoid border effects [14].
were not included in the assessments, unless explicitly mentioned. In January 2001 and February 2004, the plantation was coppiced again to a height of c. 5 cm above soil level (Fig. 1). Mechanical weeding was done frequently during the 1996, 1997 and 2001 growing seasons. In February 1997, a 5-cm-thick layer of mulch was applied to reduce weed growth and to reduce the pH of the soil. In June 1996, June 1997 and April 2001, weeds in between the trees were treated with a mixture of glyphosate (at 3.2 kg ha1) and oxadiazon (at 9.0 kg ha1), as the mechanical weed control was not effective. These herbicides were applied using a spraying device with a hoodcovered nozzle to minimize impact on the trees. During the 1998, 1999 and 2002 growing seasons, the weed vegetation was mechanically cut to ground level with a trimmer. No fertilization or irrigation was applied after the establishment of the experiment.
2.2. Management regime
2.3. Measurements and harvesting
As April and May 1996 were characterized by a prolonged drought, the plantation was irrigated shortly after planting to promote better establishment. In December 1996, all trees were coppiced to a height of c. 5 cm above soil level to create a coppice culture, i.e. a stool composed of a stump with its (coppice) shoots (Fig. 1). The cuttings that did not survive the establishment year were replaced in the spring of 1997 with new 25-cmlong cuttings (40 cm for the clones with a mortality rate higher than 10%), and are referred to as ‘replacement cuttings’ in this study. These replacement cuttings were not coppiced in 1996, and they
Stool mortality was determined annually by counting the number of dead stools among the 36 assessment trees at the end of each growing season. Each year, the overall shoot density was determined by counting the number of surviving shoots in the assessment plots. Shoot diameter (d) of the living shoots was measured every year at the end of the growing season with a digital caliper (Mitutoyo, type CD-15DC, UK) at 22 cm above soil level [15]. For stems thicker than 3 cm, diameter was measured in two perpendicular
Coppice
Cuttings Coppice Replacement cuttings
1996
1997
Establishment
1998
1999
Four-year rotation
2000
2001
Coppice
2002
2003
2004
Three-year rotation
year
Fig. 1. Coppice regime of a short rotation coppice culture of poplar. In April 1996, the stand was established with hardwood cuttings. The stand was coppiced in December 1996, and coppiced again in January 2001. Not-established cuttings were replaced with replacement cuttings in the spring of 1997.
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directions, and the mean value was used in further calculations [15]. In August 2003 (i.e. the third year of the second rotation), 10 shoots per replicate plot were harvested at c. 5 cm above soil level for clones Balsam Spire, Columbia River, Primo, Unal and Wolterson. Shoots were selected using the technique of the quantils of the total, so that the sampled shoots represented the total basal area and its variation within a replicate plot [16,17]. All leaves of each shoot were removed and brought to the laboratory for analysis. Individual leaf area was measured for all leaves with a laser area meter (CID Inc. type CI-203, USA). Dry mass of leaves was determined after drying at 60 1C in a forced air oven to constant mass. Per clone, an allometric power equation was calculated between shoot diameter at 22 cm and total leaf area per shoot. The leaf area index (LAI) was calculated as the total leaf area of all assessment shoots (including replacement cuttings) of a replicate plot per unit ground area of the replicate plot. The number of leaves per shoot was expressed on a shoot basal area basis (NBA). For the 10 harvested trees, five leaves were randomly selected per meter, and specific leaf area (SLA) was calculated as the individual leaf area divided by the individual leaf dry mass. The mean SLA per tree was used in further calculations. For clones Balsam Spire, Columbia River, Primo, Unal and Wolterson, stem and branches were separated and dry mass was determined after drying at 75 1C in a forced air oven to constant mass. The distribution to leaves, stem and branches was calculated as the proportion of total leaf, stem and branch biomass to total above-ground biomass (%). For each replicate plot separately, allometric power relationships (DM ¼ ad b, with a and b as regression coefficients) between (leafless) shoot dry mass (DM) and shoot diameter (d) were used to determine biomass production per plot. For the other clones, five shoots per clone were harvested in December 2003. Stem and branches were dried at 75 1C, and one allometric power relationship between shoot dry mass and shoot diameter per clone was calculated to determine biomass production for each replicate plot. Plot estimates were scaled up to produce dry mass ha1 y1.
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2.4. Statistical analysis Differences in biomass production, stool mortality, number of shoots per stool, biomass distribution and leaf characteristics were tested for significance using analysis of variance. Analyses were performed with the SAS statistical software package (SA System 6.12, SAS Institute Inc., Cary, NC) using the mixed procedure and plot as a replicate [18]. The design was a randomized block design with clone as a fixed factor, and plot as a random factor. The TukeyKramer adjustment was used to control the maximum experiment-wise error rate. Satterthwaite’s procedure was applied to obtain the denominator degrees of freedom. Prior to analysis, data were tested for normality using the ShapiroWilk statistic (proc univariate in SAS). Pearson rank and Spearman rank correlation tests were performed with StatMost 2.50 (DataMost Corporation, Salt Lake City, USA). The level of significance was set to P ¼ 0:05 for all analyses.
3. Results 3.1. Above-ground woody biomass production At the end of the third year in the second rotation (2003), mean annual biomass production ranged between 1.6 Mg ha1 y1 for IBW2 and 9.7 Mg ha1 y1 for Wolterson. Fritzi Pauley and Trichobel also performed relatively well with an average production of 8.2 Mg ha1 y1. The least performing clones were IBW1 and IBW2 with a production of less than 2.0 Mg ha1 y1. The best performers of the first rotation, i.e. Hoogvorst and Hazendans, performed very poorly during the second rotation with a production of, respectively, 3.0 and 3.5 Mg ha1 y1 at the end of 2003 (Fig. 2). 3.2. Stool mortality Above-ground woody biomass production was significantly negatively correlated with stool mortality. At the end of the third year of the second rotation (2003), stool mortality ranged between 6% for Wolterson and 89% for Hoogvorst,
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T×B
Biomass production (Mg ha-1 year-1)
14
T×D T
12
D×N D×T
10
N
8 6 4 2 0 BA
BE BO HA HO RA UN
CO FR TR
GA GI PR
IB1 IB2 IB3
WO
Clone
Fig. 2. Mean annual above-ground woody biomass production of 17 different poplar clones after 3 years of growth in a coppice culture. Mean values of replicate plots and standard errors are presented. Bar patterns refer to parentage. BA: Balsam Spire, BE: Beaupre´, BO: Boelare, HA: Hazendans, HO: Hoogvorst, RA: Raspalje, UN: Unal, CO: Columbia River, FR: Fritzi Pauley, TR: Trichobel, GA: Gaver, GI: Gibecq, PR: Primo, IB1: IBW1, IB2: IBW2, IB3: IBW3, WO: Wolterson, T: P. trichocarpa, B: P. balsamifera, D: P. deltoides, N: P. nigra.
i.e. two and 32 of the 36 original stools, respectively. Boelare, Hoogvorst and Gibecq showed the highest mean stool mortality, i.e. 26, 24 and 24 stools, respectively. Wolterson had the lowest mean stool mortality, i.e. two stools. Hardly any stool mortality was found in the first year after coppicing. However, a relatively high mortality was found during the second year of the rotation. Especially Hoogvorst experienced high stool mortality, averaging 14 stools per plot in 2002 (Table 1). Except for Hazendans and Hoogvorst with an average of three and four dead stools, hardly any stool mortality was observed in 2003. In the spring of 1997, dead cuttings were replaced by new cuttings ( ¼ replacement cuttings) to compensate for the high cutting mortality during the establishment year. These replacement cuttings performed poorer than the coppiced stools throughout both rotations. In well performing plots like those of Wolterson, Hoogvorst, Hazendans and the P. trichocarpa clones, hardly any replacement cuttings survived the first rota-
tion. In all plots, replacement cuttings were smaller and produced less shoots per stool than the coppiced stools. At the end of the second rotation (2003), mean stool biomass of coppiced stools averaged 1.0 kg for IBW2 to 7.1 kg for Gaver, while stool biomass of replacement cuttings averaged 0.2 kg for IBW1 to 2.2 kg for Gaver (Table 1). Mean shoot diameter of coppiced stools averaged 13.83 mm for Balsam Spire to 39.28 mm for Fritzi Pauley; mean shoot diameter of replacement cuttings averaged 13.71 mm for Balsam Spire to 31.97 mm for Fritzi Pauley. Maximum shoot diameter of coppiced stools averaged 30.45 mm for Gibecq to 50.86 mm for Fritzi Pauley; maximum diameter of replacement cuttings averaged 15.27 mm for IBW1 to 35.48 mm for Fritzi Pauley (Table 2). 3.3. Number of shoots All clones sprouted vigorously after coppicing, producing an average of three to 19 shoots per stool after the second coppicing. However, during
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Table 1 Number of dead stools in 2002, total number of dead stools at the end of the second rotation (Cum. stool mortality), and mean stool dry mass of coppiced stools and replacement cuttings in December 2003 of 17 poplar clones Parentage
Clone
TB
Balsam Spire
TD
Beaupre´ Boelare Hazendans Hoogvorst Raspalje Unal
Stool mortality 2002
0 3.3 2.7 1.7 13.8 3.0 5.0
Cum. stool mortality
9.0 (4.0) (1.5) (0.7) (0.3) (3.3) (1.0) (2.1)
18.3 26.0 15.0 23.6 15.5 13.3
(2.2) (2.6) (2.3) (4.1) (8.5) (0.9)
Stool DM (kg) Copp. stools
Replacement cuttings
2.77 (0.5)
1.38 (0.7)
1.60 3.69 1.99 2.76 3.35 1.34
0.16 0.42 0.86 — 0.35 0.48
(0.5) (1.2) (0.3) (0.8) (1.3) (0.7)
(0) (0.1) (0.3) (0.3) (0.3)
T
Columbia River Fritzi Pauley Trichobel
1.7 (1.2) 1.3 (0.9) 3.7 (1.2)
13.3 (0.7) 7.7 (2.0) 9.7 (1.2)
3.64 (0.4) 4.30 (1.2) 3.28 (0.8)
— 1.00 (0.1) 1.08 (0.5)
DN
Gaver Gibecq Primo
0.5 (0.5) 0.3 (0.3) 5.0 (1.5)
23.0 (1.0) 24.3 (0.7) 20.0 (1.5)
7.09 (1.3) 3.01 (1.2) 3.18 (0.2)
2.16 (0.4) 1.22 (0.2) 1.10 (0.3)
DT
IBW1 IBW2 IBW3
2.5 (0.5) 2.7 (0.9) 3.0 (1.2)
19.0 (6.0) 19.7 (2.6) 15.3 (3.3)
1.37 (0.2) 0.99 (0.3) 1.52 (0.2)
0.15 (0.1) 0.20 (0) 0.26 (0.1)
N
Wolterson
1.3 (0.9)
4.0 (1.2)
4.49 (1.7)
0.80 (0.5)
The stand was established in April 1996, and coppiced in December 1996, January 2001 and February 2004. New cuttings ( ¼ replacement cuttings) replaced not-established cuttings. The established cuttings/stools are referred to as coppiced stools. Mean values (standard errors) of replicates are presented.
the 3 years following coppicing (2001–2003), the smaller shoots of a stool were eliminated. Two growth strategies were evident among the 17 clones: a rapid elimination of the smaller shoots leaving one or two stems per stools after 4 years, or a slow elimination leaving numerous shoots per stool. Balsam Spire, Columbia River and Wolterson were characterized by a slow elimination of shoots, leaving an average of 13.4, 8.3 and 14.8 shoots per stool, respectively, at the end of the second rotation. For Fritzi Pauley and Hazendans, on the other hand, an average of 2.6 and 2.4 shoots, respectively, were retained per stool (Table 2). The number of shoots at the end of a rotation was not significantly correlated with final biomass. 3.4. Leaf characteristics Significant clonal differences were found in the number of leaves per unit basal area (NBA), mean
individual leaf area, maximum individual leaf area, SLA and LAI (Table 3). A significantly negative correlation was found between NBA and mean individual leaf area (r ¼ 0:856). So, two growth strategies in terms of number of leaves and leaf size were evident: the production of many small leaves, or of fewer larger leaves. NBA in the third year of the second rotation averaged 60 cm2 for Unal to 123 cm2 for Wolterson. Mean individual leaf area averaged 12.5 cm2 for Wolterson to 27 cm2 for Unal; maximum individual leaf area averaged 69 cm2 for Wolterson to 175 cm2 for Columbia River (Table 3). Above-ground woody biomass production at the end of the second rotation (2003) was significantly positively correlated with NBA (r ¼ 0:586), mean individual leaf area (r ¼ 0:683), LAI (r ¼ 0:781) and SLA (r ¼ 0:552) (Table 4). LAI ranged between two for Unal and six for Wolterson, and was significantly correlated with NBA (r ¼ 0:634), mean
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Table 2 Mean shoot diameter, maximum shoot diameter and mean number of shoots per stool of coppiced stools and replacement cuttings in December 2003 of 17 poplar clones (the stand was established in April 1996, and coppiced in December 1996, January 2001 and February 2004. New cuttings ( ¼ replacement cuttings) replaced not-established cuttings) Parentage
Clone
Mean diameter
Maximum diameter
No shoots per stool
Copp. stools
Replacement cuttings
Copp. stools
Replacement cuttings
Copp. stools
Replacement cuttings
13.4 (2.2)
8.2 (1.8)
TB
Balsam Spire
13.83 (1.86)
13.71 (2.55)
45.95 (2.33)
34.25 (5.68)
TD
Beaupre´ Boelare Hazendans Hoogvorst Raspalje Unal
29.03 33.16 31.04 35.93 29.07 27.99
19.69 20.83 26.29 — 22.24 19.51
37.62 45.10 38.32 44.14 48.11 37.32
20.01(4.67) 22.03 (4.7) 29.42 (5.52) — 27.75 (—) 22.15 (2.92)
3.1 4.1 2.4 2.7 5.3 2.8
Columbia River Fritzi Pauley Trichobel
21.92 (3.9)
—
48.62 (1.4)
—
8.3 (1.7)
4.8 (1.2)
39.28 (1.73) 31.20 (6.27)
31.97 (2.67) 27.70 (9.43)
50.86 (1.17) 48.93 (5.71)
35.48 (0.92) 31.58 (7.42)
2.6 (0.2) 4.0 (0.4)
2.0 (0.8) 2.4 (1.3)
DN
Gaver Gibecq Primo
25.53 (1.56) 21.58 (3.15) 28.25 (1.02)
23.59 (1.36) 19.93 (2.42) 22.79 (2.51)
50.28 (2.53) 30.45 (2.84) 43.23 (0.85)
35.41 (0.54) 27.66 (2.24) 28.76 (3.41)
8.8 (0.7) 6.3 (0.2) 5.4 (0.7)
3.8 (0.5) 4.2 (0.5) 3.2 (0.9)
DT
IBW1 IBW2 IBW3
28.09 (3.69) 24.53 (1.39) 24.64 (1.44)
14.12 (1.83) 19.35 (1.97) 17.50 (2.96)
34.81 (2.23) 33.57 (3.96) 33.94 (1.41)
15.27 (2.98) 20.73 (2.13) 20.04 (3.43)
2.5 (0.4) 3.3 (0.8) 3.4 (0.4)
1.3 (0.3) 1.4 (0.2) 1.4 (0)
N
Wolterson
14.91 (0.88)
—
42.72 (1.34)
—
14.8 (1.1)
8.0 (0.8)
T
(0.86) (2.25) (3.92) (4.53) (6.12) (1.59)
(4.88) (4.84) (7.15) (—) (2.71)
(0.41) (3.8) (3.47) (3.45) (8.39) (2.87)
(0.2) (0.8) (0.3) (0.8) (0.1) (0.2)
1.2 1.6 1.5 1.5 2.9 1.8
(0.2) (0.3) (0.5) (0.4) (0) (0.1)
The established cuttings/stools are referred to as coppiced stools. Mean values (standard errors) of replicates are presented.
Table 3 Number of leaves per unit shoot basal area (NBA), mean individual leaf area (LAa), maximum individual leaf area (LAmax), specific leaf area (SLA) and leaf area index (LAI) in August 2003 of five poplar clones in a short rotation coppice culture Clone
NBA (cm2)
LAa (cm2)
LAmax (cm2)
SLA (cm2 g1)
LAI
Balsam Spire Columbia River Primo Unal Wolterson
85.7 92.4 93.0 60.1 123.1
18.80 22.11 18.67 26.92 12.51
113 (11) 175 (13) 76 (3) 136 (6) 69 (9)
151 150 165 134 156
4.3 5.1 4.8 2.0 6.0
(5.1) (7.0) (5.0) (4.2) (4.8)
(0.56) (0.80) (1.05) (3.85) (0.67)
(3) (2) (6) (3) (4)
(0.5) (0.1) (0.9) (0.2) (0.3)
Mean values (standard error) of replicates are shown.
individual leaf area (r ¼ 0:676), SLA (r ¼ 0:651) (Table 4), maximum individual leaf area (r ¼ 0:556) and number of shoots per stool (r ¼ 0:8). SLA ranged between 134 cm2 g1 for Unal and 165 cm2 g1 for Primo (Table 3).
3.5. Biomass distribution Significant clonal differences were found in the proportion of stem biomass to total above-ground biomass (leaves+stem+branches), averaging 71
ARTICLE IN PRESS I. Laureysens et al. / Biomass and Bioenergy 29 (2005) 10–21 Table 4 Pearson rank correlation coefficients among above ground woody biomass produced after 3 years (2001–2003), leaf area index (LAI), number of leaves per unit shoot basal area (NBA), individual leaf area (LAa) and specific leaf area (SLA).
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Columbia River, Primo, Unal and Wolterson, respectively.
4. Discussion LAI
LAa NBA (cm2) (cm2)
Biomass (Mg ha1) 0.781*** 0.586* LAI 0.634* NBA (cm2) LAa (cm2)
SLA (cm2 g1)
0.683** 0.552* 0.676** 0.651* 0.856*** 0.537 0.518*
***Po0:001; **Po0:01; *Po0:05:
Table 5 Proportion of stem, branch and leaf biomass to total above ground biomass (leaves+stem+branches) of five poplar clones in a short rotation coppice culture at the end of the second rotation (2003) Clone
Stem (%)
Branches (%)
Leaves (%)
Balsam Spire Columbia River Primo Unal Wolterson
74.9 71.4 71.1 71.0 75.2
11.7 12.5 14.0 14.2 12.1
13.4 16.1 14.4 14.9 12.7
(1.8) (0.3) (0.2) (0.2) (2.0)
(1.2) (1.0) (1.0) (0.8) (1.0)
(1.0) (1.1) (1.2) (0.7) (1.0)
to 75%. The proportion of branch to total biomass averaged 12 to 14%; the proportion of leaf to total biomass averaged 13 to 16% (Table 5). No significant clonal differences were found for the proportions of branches and leaves. However, the biomass proportions of all three plant components depended on shoot diameter. For all clones, shoot diameter was significantly negatively correlated with the proportion of stem biomass, and was significantly positively correlated with the proportion of branch biomass. For Balsam Spire, shoot diameter was significantly negatively correlated with the proportion of leaf biomass; for Primo and Wolterson, shoot diameter was significantly positively correlated with the proportion of leaf biomass. When focusing on above-ground woody biomass distribution, significant differences in distribution were found. Stem biomass accounted for 86.8, 85.1%, 83.4%, 84.3% and 86.0% of total woody biomass for Balsam Spire,
Above-ground woody biomass production averaged 1.6–9.7 Mg ha1 y1 at the end of the second rotation (2001–2003). These production levels range within values found in other studies. Annual 4-y rotation yields of 1.2–13.6 Mg ha1 have been reported for various poplar species, depending on clone, soil, climate and management regime [10,19–21]. Biomass production at our site was rather high given the suboptimal soil conditions. The plantation was established on an old waste disposal site with high bulk density (heavy clayloam), high pH (7.3–8.1) and a slight contamination of heavy metals. We have previously shown, however, that heavy metals had no adverse effect on biomass production [22]. Biomass production was primarily determined by cutting and stool mortality. The poor performing P. deltoides P. nigra (D N) clones, P. deltoides P. trichocarpa (D T) clones and P. trichocarpa P. deltoides (T D) clone Boelare all experienced high cutting mortality during the establishment year, as reported earlier [23]. High cutting mortality of the D T and D N clones might be caused by the poor rooting capacity of the mother P. deltoides [1], given the high bulk density of the soil. Possibly, cuttings of 25 cm were too small for a good establishment in this heavy soil. High stool mortality was observed during the second year of both the first (1998) [23] and second rotation (2002). The summers of 1998, 2001 and 2002 were characterized by heavy rust infections (Melampsora larici-populina) in Belgium. Rust, in combination with the bark-killing fungus Discosporium populeum, caused high mortality rates in several poplar plantations (M. Steenackers, 2001, personal communication). In 1998, all clones were infected; in 2002, especially T D clone, Hoogvorst was severely infected and experienced high stool mortality. Two plots of Hoogvorst died nearly entirely, explaining the low biomass production of this clone during the second rotation. Shifts in clonal ranking of biomass production
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were observed between the two rotations. Hoogvorst and Hazendans (T D) were the best performers during the first rotation with, respectively, 10.1 and 10.8 Mg ha1 y1 [12], but had a lower biomass production during the second rotation. Native clone Wolterson was the best performer during the second rotation with 9.7 Mg ha1 y1. P. trichocarpa clones Fritzi Pauley and Trichobel had a relatively high biomass production in both the first and second rotation. P. trichocarpa P. balsamifera clones Balsam Spire, and D N clones Gaver and Primo showed low biomass production in the first rotation [12], but moderate to high biomass production in the second rotation. These shifts in clonal ranking clearly illustrate that long-term studies are needed to identify clones with high biomass production in coppice cultures. In the spring of 1997, replacement cuttings replaced not-established cuttings. All replacement cuttings remained smaller and performed poorer than the coppiced stools throughout both rotations (replacement cuttings were not coppiced in December 1996). Coppicing at the end of the establishment year is a frequently applied technique to promote sprouting of many shoots per stool [1,2]. The early growth rate of coppice shoots is faster than that of seedlings or cuttings, because they benefit from the existing root system, an early onset of growth and a continuous development of new nodes until late into the fall [11]. The high number of shoots per unit area gives a rapid leaf area development, leading to fast crown closure and efficient utilization of space [24]. In this study, shoot growth of coppiced stools was more vigorous than that of the replacement cuttings, causing the replacement cuttings to be dominated by the coppiced stools. Many replacement cuttings did not survive their establishment year, especially in the ‘low-cutting mortality plots’ where little light was available. In Sweden, beating experiments with willow, i.e. planting cuttings in gaps after poor establishment, have also been shown to be unsuccessful, because replacement cuttings become situated in the lower part of the competitive hierarchy [25]. In this study, all clones sprouted vigorously, producing up to seven shoots per stool on average
in the first rotation [23] and up to 19 shoots per stool on average in the second rotation. But the number of shoots per stool decreased progressively during both rotations. During the first year after coppicing, the shoots with the highest growth rate in a stool became the dominant shoots, suppressing all other shoots in the following years [23], as also reported in several other studies [26–28]. The elimination of smaller shoots was common for all clones, but the rate of elimination differed significantly among clones. T D clones Hoogvorst and Hazendans, and T clones Fritzi Pauley and Trichobel had a rapid elimination of smaller shoots, leaving mostly single-stem stools after 4 years in 2000 [23], and up to 4 shoots per stool on average after 3 years in 2003. Balsam Spire and Wolterson, on the other hand, showed a slow elimination of shoots, leaving a mean number of four and three shoots per stool in 2000 [23] respectively, and 11 and 16 shoots per stool in 2003, respectively. This number of shoots per stool in 2000 and 2003, however, was not significantly correlated with biomass production. Individual stool biomass was mostly determined by the diameter of its one or two dominant shoots, as we reported earlier for the first rotation [23]. Suppressed shoots remained always small and had only a small contribution to total stool biomass. The number of shoots produced in the first year after coppicing, on the other hand, was significantly correlated with biomass production [23]. This supports the findings of Tschaplinski and Blake [29], who showed that suppressed shoots support the growth of the dominant shoot by supplying carbon to the lower stem and roots. Early removal of all shoots but the dominant (tallest) shoot resulted in reduced vigor and viability of the dominant shoot [29]. Further research is needed to examine the physiological role of the suppressed shoots. We found significant clonal differences in leaf size and total leaf area, confirming numerous other studies (e.g. [30–32]). Total leaf area is closely related with the accumulated biomass in poplar and this correlation is considered to be genetically controlled [33]. LAI is the most commonly used parameter to quantify leaf area, which expresses total leaf area of a plant canopy per unit land area
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it covers. LAI was significantly correlated with biomass production in our study. In August, LAI ranged between 2 and 6. Pellis et al. [34] and Al Afas et al. [35] reported similar LAI values for the same plantation in the first year of the second rotation (2001). Other studies of closely spaced poplar stands report LAI values of 1–6 in the establishment year [35,36], and between 3 and more than 11 in fully developed canopies [30,36,37–40]. In this study, LAI was significantly positively correlated with the number of leaves (NBA), but significantly negatively with individual leaf size. So a high LAI was obtained with a high number of smaller leaves, e.g. for clones Wolterson and Balsam Spire. Many studies, however, showed that leaf size is a better indicator of productivity in Populus than the number of leaves per tree (e.g. [31,41,42,43]). Our results might be explained by the limited number of clones that were studied. Well performing clones Fritzi Pauley and Trichobel with large leaves were not studied. Moreover, Pellis et al. [34] showed that in the first year of the second rotation a high LAI and biomass production could be achieved by the production of many smaller leaves or of fewer larger leaves. In this study, biomass production was also positively correlated with SLA. This confirms the results of some studies that showed a positive correlation between growth rate and SLA [44,45], but contradicts the findings of several other studies [46–48]. The latter relationship was explained by a higher number of mesophyll cells per unit area or larger mesophyll cells, leading to higher rates of CO2 assimilation; the first relationship by a higher carbon and nitrogen content of leaves with high SLA [46]. Branch production differed significantly among clones when expressed as a percentage of aboveground woody biomass production, although the differences were small. Values ranged well within values reported in other studies, i.e. ranging between 11% and 26%, depending on clone and spacing [7,8,49,50]. Most of the clones in this study were selected for the production of straight, long stems with little branching. Raw-material quality of bark and branches is of lower quality than stem wood [51]. When the end product is energy or pulp, tree size and form are less important.
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In conclusion, native P. nigra clone Wolterson and American P. trichocarpa clones Columbia River, Fritzi Pauley and Trichobel performed best in the second rotation. Large clonal differences in stool mortality, number of shoots per stool, and leaf characteristics were found. Biomass production was primarily determined by cutting and stool mortality. M. larici-populina remains an important external determinant of productivity. High biomass production was also related with high LAI, which was obtained by many small leaves. Further long-term studies and large-plot field studies are necessary to identify clones with high biomass production over several rotation cycles in SRC.
Acknowledgements This study has been supported by a research contract with the Province of Antwerp and by the Fund for Scientific Research Flanders (G 0108.97). All plant materials were kindly provided by the Institute for Forestry and Game Management (Geraardsbergen) and by the Forest Research, Forestry Commission (UK). The project has been carried out in close cooperation with Eta-com B., supplying the grounds and part of the infrastructure, and with the logistic support of the city council of Boom. References [1] Dickmann DI, Stuart KW. The culture of poplars in eastern North America. East Lansing, Michigan: Michigan State University Publications; 1983. [2] Macpherson G. Home-grown energy from short-rotation coppice. Ipswich, UK: Farming Press Books; 1995. [3] European Commission. Energy for the future—renewable sources of energy. White paper for a community strategy and action plan. COM (97) 599 final (26/11/97); 1997. [4] Gordon JC. The productive potential of woody plants. Iowa State Journal of Research 1975;49:267–74. [5] Isebrands JG, Karnosky DF. Environmental benefits of poplar culture. In: Dickmann DI, Isebrands JG, Eckenwalder JE, Richardson J, editors. Poplar Culture in North America. Ottawa, Canada: NRC Research Press, National Research Council of Canada; 2001. p. 207–18. [6] Perttu KL. Ecological, biological balances and conservation. Biomass and Bioenergy 1995;9:107–16.
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[7] Heilman PE, Ekuan G, Fogle D. Above- and belowground biomass and fine roots of 4-year-old hybrids of Populus trichocarpa Populus deltoides and parental species in short-rotation culture. Canadian Journal of Forest Research 1994;24:1186–92. [8] Heilman PE, Stettler RF. Genetic variation and productivity of Populus trichocarpa T. & G. and its hybrids. II. Biomass production in a 4-year plantation. Canadian Journal of Forest Research 1985;15:384–8. [9] Scarascia-Mugnozza GE, Ceulemans R, Heilman PE, Isebrands JG, Stettler RF, Hinckley TM. Production physiology and morphology of Populus species and their hybrids grown under short rotation. II. Biomass components and harvest index of hybrid and parental species clones. Canadian Journal of Forest Research 1997;27: 285–94. [10] Cannell MGR, Smith RI. Yields of minirotation closely spaced hardwoods in temperate regions: review and appraisal. Forest Science 1980;26:415–28. [11] Blake TJ. Coppice systems for short-rotation intensive forestry: the influence of cultural, seasonal and plant factors. Australian Forest Research 1983;13:279–91. [12] Laureysens I, Bogaert J, Blust R, Ceulemans R. Biomass production of 17 poplar clones grown in a short-rotation coppice culture on a waste disposal site and its relation to soil characteristics. Forest Ecology and Management 2003;187:295–309. [13] Armstrong A. The United Kingdom network of experiments on site/yield relationships for short rotation coppice. Edinburgh: Forestry Commission; 1997. [14] Zavitkovski J. Small plots with unplanted plot border can distort data in biomass production studies. Canadian Journal of Forest Research 1981;11:9–12. [15] Pontailler JY, Ceulemans R, Guittet J, Mau M. Linear and non-linear functions of volume index to estimate woody biomass in high density young poplar stands. Annales des Sciences Forestie`res 1997;54:335–45. [16] Cerma´k J, Kucera J. Scaling up transpiration data between trees, stands and watersheds. Silva Carelica 1990;15: 101–20. [17] Cerma´k J, Riguzzi F, Ceulemans R. Scaling up from the individual tree to the stand level in Scots pine. I. Needle distribution, overall crown and root geometry. Annals of Forest Science 1998;55:63–88. [18] SAS Institute Inc. SAS/STAT User’s Guide, Version 6, Fourth Edition. Cary, NC, USA: SAS Institute Inc.; 1990. [19] Armstrong A, Johns C. Effects of spacing on yield from two clones of poplar and two clones of willow grown as energy coppice. Aspects of Applied Biology 1997;49:85–90. [20] Beale CV, Heywood MJ. Productivity of commercial crops of short rotation coppice at six sites in southern England. Aspects of Applied Biology 1997;49:181–8. [21] Strong T, Hansen E. Hybrid poplar spacing/productivity relations in short rotation intensive culture plantations. Biomass and Bioenergy 1993;4:255–61. [22] Laureysens I, Blust R, De Temmerman L, Lemmens C, Ceulemans R. Clonal and seasonal variation in heavy
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ARTICLE IN PRESS I. Laureysens et al. / Biomass and Bioenergy 29 (2005) 10–21 [36] Anderson HW. Biomass production of hybrid poplar grown in minirotation. In: Fayle DCF, Zsuffa L, Anderson HW, editors. Poplar research, management and utilization in Canada. Forest Research Information Note. Ontario: Ontario Ministry of Natural Resources; 1979. [37] Gielen B, Calfapietra C, Sabatti M, Ceulemans R. Leaf area dynamics in a closed poplar plantation under free-air carbon dioxide enrichment. Tree Physiology 2001;21:1245–55. [38] Heilman PE, Hinkley TM, Roberts DA, Ceulemans R. Production physiology. In: Stettler RF, Bradshaw Jr. HD, Heilman PE, Hinckley TM, editors. Biology of Populus and its implications for management and conservation. Ottawa: NRC Research Press; 1996. p. 459–89. [39] Isebrands JG, Promnitz LC, Dawson DH. Leaf area development in short rotation intensive cultured Populus plots. Madison: TAPPI Forest Biology Wood Chemical Conference; 1977. [40] Zavitkovski J, Isebrands JG, Dawson DH. Productivity and utilization potential of short-rotation Populus in the Lake States. In: Thielges BA, Land Jr. SB, editors. Proceedings symposium on cottonwood and related species. Baton Rouge: Louisiana State University; 1976. p. 392–401. [41] Taylor G, Beckett KP, Robinson KM, Stiles K, Rae AM. Identifying QTL for yield in UK biomass poplar. Aspects of Applied Biology 2001;65:173–82. [42] Ceulemans R, Scarascia-Mugnozza G, Wiard BM, Braatne JH, Hinckley TM, Stettler RF. Production physiology and morphology of Populus species and their hybrids grown under short rotation. I. Clonal comparisons of 4-year growth and phenology. Canadian Journal of Forest Research 1992;22:1937–48.
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[43] Isebrands JG, Ceulemans R, Wiard B. Genetic variation in photosynthetic traits among Populus clones in relation to yield. Plant Physiology and Biochemistry 1988;26:427–37. [44] Reich PB, Walters MB, Ellsworth DS. Leaf life-span in relation to leaf, plant, and stand characteristics among diverse ecosystems. Ecology Monograph 1992;62:365–92. [45] Veneklaas EJ, Santos Silva MPRM, den Ouden F. Determinants of growth rate in Ficus benjamina L. compared to related faster-growing woody and herbaceous species. Scientia Horticulturae 2002;93:75–84. [46] Marron N, Villar M, Dreyer E, Delay D, Boudouresque E, Petit JM, et al. Diversity of leaf traits related to productivity in 31 Populus deltoides Populus nigra clones. Tree Physiology 2005;25:425–35. [47] Nelson CJ. Genetic association between photosynthetic characteristics and yield: review of evidence. Plant Physiology and Biochemistry 1988;26:543–54. [48] Thumma BR, Naidu BP, Chandra A, Cameron DF, Bahnisch LM, Liu C. Identification of causal relationships among traits related to drought resistance in Stylosanthes scabra using QTL analysis. Journal of Experimental Botany 2001;52:203–14. [49] DeBell DS, Clendenen GW, Harrington CA, Zasada JC. Tree growth and stand development in short-rotation Populus plantings: 7-year results for two clones at three spacings. Biomass and Bioenergy 1996;11:253–69. [50] Wittwer RF, Immel MJ. A comparison of five tree species for intensive fiber production. Forest Ecology and Management 1978;1:249–54. [51] Phelps JE, Isebrands JG, Jowett D. Raw material quality of short-rotation, intensively cultured Populus clones. I. A comparison of stem and branch properties at three spacings. IAWA Bulletin n.s. 1982;3:193–200.
Biomass and Bioenergy 29 (2005) 10–21 www.elsevier.com/locate/biombioe
Growth and production of a short rotation coppice culture of poplar. III. Second rotation results Ilse Laureysens, An Pellis, Jessy Willems, Reinhart Ceulemans Department of Biology, University of Antwerp, Campus Drie Eiken, Universiteitsplein 1, B-2610 Wilrijk, Belgium Received 30 September 2004; received in revised form 3 February 2005; accepted 14 February 2005 Available online 7 April 2005
Abstract This study describes production and growth of the second rotation of 17 poplar (Populus spp.) clones in a short rotation coppice culture (SRC). In addition, the link with leaf characteristics was studied. In April 1996, an experimental field plantation with 10,000 cuttings ha1 was established on a former waste disposal site. In December 1996, January 2001 and February 2004, all stools were coppiced. At the end of the second rotation (2001–2003), highest biomass production was found for P. nigra clone Wolterson with 9.7 Mg ha1 y1. The best performers of the first rotation, i.e. P. trichocarpa P. deltoides clones Hoogvorst and Hazendans, performed poorly in the second rotation, due to heavy rust infections. Two growth strategies were evident: Wolterson had a slow elimination of smaller shoots and had lots of smaller leaves; Hazendans and Hoogvorst had a rapid elimination of smaller shoots and had fewer, larger leaves. We conclude that shoot growth dynamics and leaf size were not the primary production determinants in our poplar SRC. But Melampsora larici-populina remained an important external determinant of biomass production. r 2005 Elsevier Ltd. All rights reserved. Keywords: Populus Spp.; Short rotation coppice; Biomass production; Mortality; Leaf
1. Introduction Short rotation forestry (SRF), i.e. fast-growing tree crops grown in carefully tended plantations for rotations shorter than 15 years [1,2], is limited to a few hectares of experimental plantations in Corresponding author. Tel.: +32 3 820 22 56; fax: +32 3 820 22 71. E-mail address: [email protected] (R. Ceulemans).
Belgium (J.-M. Jossart, 2003, personal communication). Nevertheless, Belgium’s Kyoto target is a 7.5% reduction of fossil fuel emissions by 2008–2012 compared to 1990. Energy crops have an important role to play [3], in particular SRF because of its numerous ecological benefits: it has a positive impact on biodiversity, on nutrient capture and on the carbon circulation in the soilplant atmosphere system, especially on former agricultural land. Furthermore, they protect the soil from water and wind erosion [4–6]. However,
0961-9534/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2005.02.005
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free space is getting very scarce in Belgium. One option is to establish SRF plantations on set-aside land [3]: agricultural land offers optimal soil conditions. However, SRF is not yet profitable without subsidies. Under optimal conditions, smallplot yields achieved by poplar (Populus spp.) are in the order of 20–25 Mg ha1 y1 in trials conducted in the USA (PNWS) [7–9]. Under less optimal conditions, annual yields of 10–15 Mg ha1 are more realistic [10]. Another option is to establish SRF plantations on marginal or slightly polluted land. Trees will stabilize the surface, prevent dustblow, and reduce leaching and run-off of water. SRF on contaminated land provides therefore a means not only for stabilization and decontamination (e.g. heavy metal uptake) of a site, but also for some economic return from the land [5]. However, clones should be selected that grow well under less optimal conditions. In Belgium, poplar and willow are most suitable for SRF. High biomass production is achieved by selecting clones with high growth vigor and disease resistance, and by applying certain agricultural techniques, such as site preparation, weed control and high planting density [1,2]. In addition, cultural management generally involves coppicing, i.e. the cutting of a tree at the base of its trunk to use the ability of the trees to regenerate from the cut stump, resulting in the emergence of new shoots from the stump and/or roots [11]. Coppicing is frequently applied at the end of the establishment year to promote sprouting of many shoots per cutting, which is supposed to increase final biomass production [1,2]. So far, poplar breeding has never focused on the creation of poplar clones specifically suited for a coppice system. Nevertheless, poplar performs rather well in coppice cultures [12]. However, current biomass yields are not high enough to be economically feasible. Therefore, further breeding is necessary to produce clones characterized by superior growth, and resistance to pests and diseases of leaf and stem. In this study, a short rotation coppice culture (SRC) with different poplar clones provided the opportunity to study the relationship between biomass production and some anatomical parameters for a wide genotypic range. In addition, very few studies have quantified biomass produc-
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tivity over more than one rotation cycle in SRC. Therefore, the objectives of this paper were: 1) to quantify the above-ground biomass productivity of 17 clones in a SRC at the end of the second rotation; 2) to study the link between biomass production and shoot growth dynamics; 3) to study the link between biomass production and leaf characteristics; 4) to study clonal differences in biomass distribution to leaves, stem and branches.
2. Materials and methods 2.1. Experimental set-up and plant material In April 1996, a short rotation coppice plantation was established in an industrial area of Boom near Antwerp, Belgium (511050 N, 041220 E). The plantation is situated on an old household waste disposal site, which was covered with a 2-m-thick layer of sand, clay and mixed rubble. The soil was characterized by a high bulk density (heavy clayloam), ranging between 1.22 and 1.62 g cm3, and by a high pH (7.3–8.1). The upper soil horizons contained between 0.8% and 1.8% organic matter. The nutrient and mineral reserves were extremely high in comparison with forest soils, but moderate in comparison with agricultural soils [12]. The site is situated at 5 m above sea level, and has a temperate climate with a mean temperature of 10 1C and a mean annual precipitation of 767 mm. Prior to planting, the area was leveled and cleaned of large stones, plastic, metal and other debris. A rotor tiller was used in the early spring of 1996 for final pre-planting soil preparation. Seventeen poplar (Populus) clones, belonging to five different parentages were studied, i.e. P. trichocarpa T. & G. P. balsamifera L. (T B) clone Balsam Spire; P. trichocarpa P. deltoides Marsh. (T D) clones Beaupre´, Boelare, Hazendans, Hoogvorst, Raspalje and Unal; P. trichocarpa (T) clones Columbia River, Fritzi Pauley and Trichobel; P. deltoides P. nigra L. (D N) clones Gaver, Gibecq and Primo; P. deltoides P. trichocarpa (D T) clones IBW1, IBW2 and
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IBW3; and native P. nigra (N) clone Wolterson. All clones were planted as 25-cm-long dormant, unrooted cuttings, after being soaked in water for 24 h. Cuttings were planted manually to a depth of 20 cm, leaving one or two buds above the soil surface. They were planted in a double-row design with alternating inter-row distances of 0.75 and 1.5 m, and a spacing of 0.9 m between cuttings within the rows, accommodating an overall planting density of 10,000 cuttings ha1. A randomized block design was used with 17 clones 3 replicate plots (except for clone Hoogvorst with six replicates, and for clones IBW1 and Raspalje with two replicates) according to a protocol prescribed by the British Forestry Commission [13]. Individual plot size was 9 m 11.5 m, containing 10 rows of 10 trees each. In the center of each plot, 36 trees (6 6) were used as assessment trees to avoid border effects [14].
were not included in the assessments, unless explicitly mentioned. In January 2001 and February 2004, the plantation was coppiced again to a height of c. 5 cm above soil level (Fig. 1). Mechanical weeding was done frequently during the 1996, 1997 and 2001 growing seasons. In February 1997, a 5-cm-thick layer of mulch was applied to reduce weed growth and to reduce the pH of the soil. In June 1996, June 1997 and April 2001, weeds in between the trees were treated with a mixture of glyphosate (at 3.2 kg ha1) and oxadiazon (at 9.0 kg ha1), as the mechanical weed control was not effective. These herbicides were applied using a spraying device with a hoodcovered nozzle to minimize impact on the trees. During the 1998, 1999 and 2002 growing seasons, the weed vegetation was mechanically cut to ground level with a trimmer. No fertilization or irrigation was applied after the establishment of the experiment.
2.2. Management regime
2.3. Measurements and harvesting
As April and May 1996 were characterized by a prolonged drought, the plantation was irrigated shortly after planting to promote better establishment. In December 1996, all trees were coppiced to a height of c. 5 cm above soil level to create a coppice culture, i.e. a stool composed of a stump with its (coppice) shoots (Fig. 1). The cuttings that did not survive the establishment year were replaced in the spring of 1997 with new 25-cmlong cuttings (40 cm for the clones with a mortality rate higher than 10%), and are referred to as ‘replacement cuttings’ in this study. These replacement cuttings were not coppiced in 1996, and they
Stool mortality was determined annually by counting the number of dead stools among the 36 assessment trees at the end of each growing season. Each year, the overall shoot density was determined by counting the number of surviving shoots in the assessment plots. Shoot diameter (d) of the living shoots was measured every year at the end of the growing season with a digital caliper (Mitutoyo, type CD-15DC, UK) at 22 cm above soil level [15]. For stems thicker than 3 cm, diameter was measured in two perpendicular
Coppice
Cuttings Coppice Replacement cuttings
1996
1997
Establishment
1998
1999
Four-year rotation
2000
2001
Coppice
2002
2003
2004
Three-year rotation
year
Fig. 1. Coppice regime of a short rotation coppice culture of poplar. In April 1996, the stand was established with hardwood cuttings. The stand was coppiced in December 1996, and coppiced again in January 2001. Not-established cuttings were replaced with replacement cuttings in the spring of 1997.
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directions, and the mean value was used in further calculations [15]. In August 2003 (i.e. the third year of the second rotation), 10 shoots per replicate plot were harvested at c. 5 cm above soil level for clones Balsam Spire, Columbia River, Primo, Unal and Wolterson. Shoots were selected using the technique of the quantils of the total, so that the sampled shoots represented the total basal area and its variation within a replicate plot [16,17]. All leaves of each shoot were removed and brought to the laboratory for analysis. Individual leaf area was measured for all leaves with a laser area meter (CID Inc. type CI-203, USA). Dry mass of leaves was determined after drying at 60 1C in a forced air oven to constant mass. Per clone, an allometric power equation was calculated between shoot diameter at 22 cm and total leaf area per shoot. The leaf area index (LAI) was calculated as the total leaf area of all assessment shoots (including replacement cuttings) of a replicate plot per unit ground area of the replicate plot. The number of leaves per shoot was expressed on a shoot basal area basis (NBA). For the 10 harvested trees, five leaves were randomly selected per meter, and specific leaf area (SLA) was calculated as the individual leaf area divided by the individual leaf dry mass. The mean SLA per tree was used in further calculations. For clones Balsam Spire, Columbia River, Primo, Unal and Wolterson, stem and branches were separated and dry mass was determined after drying at 75 1C in a forced air oven to constant mass. The distribution to leaves, stem and branches was calculated as the proportion of total leaf, stem and branch biomass to total above-ground biomass (%). For each replicate plot separately, allometric power relationships (DM ¼ ad b, with a and b as regression coefficients) between (leafless) shoot dry mass (DM) and shoot diameter (d) were used to determine biomass production per plot. For the other clones, five shoots per clone were harvested in December 2003. Stem and branches were dried at 75 1C, and one allometric power relationship between shoot dry mass and shoot diameter per clone was calculated to determine biomass production for each replicate plot. Plot estimates were scaled up to produce dry mass ha1 y1.
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2.4. Statistical analysis Differences in biomass production, stool mortality, number of shoots per stool, biomass distribution and leaf characteristics were tested for significance using analysis of variance. Analyses were performed with the SAS statistical software package (SA System 6.12, SAS Institute Inc., Cary, NC) using the mixed procedure and plot as a replicate [18]. The design was a randomized block design with clone as a fixed factor, and plot as a random factor. The TukeyKramer adjustment was used to control the maximum experiment-wise error rate. Satterthwaite’s procedure was applied to obtain the denominator degrees of freedom. Prior to analysis, data were tested for normality using the ShapiroWilk statistic (proc univariate in SAS). Pearson rank and Spearman rank correlation tests were performed with StatMost 2.50 (DataMost Corporation, Salt Lake City, USA). The level of significance was set to P ¼ 0:05 for all analyses.
3. Results 3.1. Above-ground woody biomass production At the end of the third year in the second rotation (2003), mean annual biomass production ranged between 1.6 Mg ha1 y1 for IBW2 and 9.7 Mg ha1 y1 for Wolterson. Fritzi Pauley and Trichobel also performed relatively well with an average production of 8.2 Mg ha1 y1. The least performing clones were IBW1 and IBW2 with a production of less than 2.0 Mg ha1 y1. The best performers of the first rotation, i.e. Hoogvorst and Hazendans, performed very poorly during the second rotation with a production of, respectively, 3.0 and 3.5 Mg ha1 y1 at the end of 2003 (Fig. 2). 3.2. Stool mortality Above-ground woody biomass production was significantly negatively correlated with stool mortality. At the end of the third year of the second rotation (2003), stool mortality ranged between 6% for Wolterson and 89% for Hoogvorst,
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14
T×B
Biomass production (Mg ha-1 year-1)
14
T×D T
12
D×N D×T
10
N
8 6 4 2 0 BA
BE BO HA HO RA UN
CO FR TR
GA GI PR
IB1 IB2 IB3
WO
Clone
Fig. 2. Mean annual above-ground woody biomass production of 17 different poplar clones after 3 years of growth in a coppice culture. Mean values of replicate plots and standard errors are presented. Bar patterns refer to parentage. BA: Balsam Spire, BE: Beaupre´, BO: Boelare, HA: Hazendans, HO: Hoogvorst, RA: Raspalje, UN: Unal, CO: Columbia River, FR: Fritzi Pauley, TR: Trichobel, GA: Gaver, GI: Gibecq, PR: Primo, IB1: IBW1, IB2: IBW2, IB3: IBW3, WO: Wolterson, T: P. trichocarpa, B: P. balsamifera, D: P. deltoides, N: P. nigra.
i.e. two and 32 of the 36 original stools, respectively. Boelare, Hoogvorst and Gibecq showed the highest mean stool mortality, i.e. 26, 24 and 24 stools, respectively. Wolterson had the lowest mean stool mortality, i.e. two stools. Hardly any stool mortality was found in the first year after coppicing. However, a relatively high mortality was found during the second year of the rotation. Especially Hoogvorst experienced high stool mortality, averaging 14 stools per plot in 2002 (Table 1). Except for Hazendans and Hoogvorst with an average of three and four dead stools, hardly any stool mortality was observed in 2003. In the spring of 1997, dead cuttings were replaced by new cuttings ( ¼ replacement cuttings) to compensate for the high cutting mortality during the establishment year. These replacement cuttings performed poorer than the coppiced stools throughout both rotations. In well performing plots like those of Wolterson, Hoogvorst, Hazendans and the P. trichocarpa clones, hardly any replacement cuttings survived the first rota-
tion. In all plots, replacement cuttings were smaller and produced less shoots per stool than the coppiced stools. At the end of the second rotation (2003), mean stool biomass of coppiced stools averaged 1.0 kg for IBW2 to 7.1 kg for Gaver, while stool biomass of replacement cuttings averaged 0.2 kg for IBW1 to 2.2 kg for Gaver (Table 1). Mean shoot diameter of coppiced stools averaged 13.83 mm for Balsam Spire to 39.28 mm for Fritzi Pauley; mean shoot diameter of replacement cuttings averaged 13.71 mm for Balsam Spire to 31.97 mm for Fritzi Pauley. Maximum shoot diameter of coppiced stools averaged 30.45 mm for Gibecq to 50.86 mm for Fritzi Pauley; maximum diameter of replacement cuttings averaged 15.27 mm for IBW1 to 35.48 mm for Fritzi Pauley (Table 2). 3.3. Number of shoots All clones sprouted vigorously after coppicing, producing an average of three to 19 shoots per stool after the second coppicing. However, during
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Table 1 Number of dead stools in 2002, total number of dead stools at the end of the second rotation (Cum. stool mortality), and mean stool dry mass of coppiced stools and replacement cuttings in December 2003 of 17 poplar clones Parentage
Clone
TB
Balsam Spire
TD
Beaupre´ Boelare Hazendans Hoogvorst Raspalje Unal
Stool mortality 2002
0 3.3 2.7 1.7 13.8 3.0 5.0
Cum. stool mortality
9.0 (4.0) (1.5) (0.7) (0.3) (3.3) (1.0) (2.1)
18.3 26.0 15.0 23.6 15.5 13.3
(2.2) (2.6) (2.3) (4.1) (8.5) (0.9)
Stool DM (kg) Copp. stools
Replacement cuttings
2.77 (0.5)
1.38 (0.7)
1.60 3.69 1.99 2.76 3.35 1.34
0.16 0.42 0.86 — 0.35 0.48
(0.5) (1.2) (0.3) (0.8) (1.3) (0.7)
(0) (0.1) (0.3) (0.3) (0.3)
T
Columbia River Fritzi Pauley Trichobel
1.7 (1.2) 1.3 (0.9) 3.7 (1.2)
13.3 (0.7) 7.7 (2.0) 9.7 (1.2)
3.64 (0.4) 4.30 (1.2) 3.28 (0.8)
— 1.00 (0.1) 1.08 (0.5)
DN
Gaver Gibecq Primo
0.5 (0.5) 0.3 (0.3) 5.0 (1.5)
23.0 (1.0) 24.3 (0.7) 20.0 (1.5)
7.09 (1.3) 3.01 (1.2) 3.18 (0.2)
2.16 (0.4) 1.22 (0.2) 1.10 (0.3)
DT
IBW1 IBW2 IBW3
2.5 (0.5) 2.7 (0.9) 3.0 (1.2)
19.0 (6.0) 19.7 (2.6) 15.3 (3.3)
1.37 (0.2) 0.99 (0.3) 1.52 (0.2)
0.15 (0.1) 0.20 (0) 0.26 (0.1)
N
Wolterson
1.3 (0.9)
4.0 (1.2)
4.49 (1.7)
0.80 (0.5)
The stand was established in April 1996, and coppiced in December 1996, January 2001 and February 2004. New cuttings ( ¼ replacement cuttings) replaced not-established cuttings. The established cuttings/stools are referred to as coppiced stools. Mean values (standard errors) of replicates are presented.
the 3 years following coppicing (2001–2003), the smaller shoots of a stool were eliminated. Two growth strategies were evident among the 17 clones: a rapid elimination of the smaller shoots leaving one or two stems per stools after 4 years, or a slow elimination leaving numerous shoots per stool. Balsam Spire, Columbia River and Wolterson were characterized by a slow elimination of shoots, leaving an average of 13.4, 8.3 and 14.8 shoots per stool, respectively, at the end of the second rotation. For Fritzi Pauley and Hazendans, on the other hand, an average of 2.6 and 2.4 shoots, respectively, were retained per stool (Table 2). The number of shoots at the end of a rotation was not significantly correlated with final biomass. 3.4. Leaf characteristics Significant clonal differences were found in the number of leaves per unit basal area (NBA), mean
individual leaf area, maximum individual leaf area, SLA and LAI (Table 3). A significantly negative correlation was found between NBA and mean individual leaf area (r ¼ 0:856). So, two growth strategies in terms of number of leaves and leaf size were evident: the production of many small leaves, or of fewer larger leaves. NBA in the third year of the second rotation averaged 60 cm2 for Unal to 123 cm2 for Wolterson. Mean individual leaf area averaged 12.5 cm2 for Wolterson to 27 cm2 for Unal; maximum individual leaf area averaged 69 cm2 for Wolterson to 175 cm2 for Columbia River (Table 3). Above-ground woody biomass production at the end of the second rotation (2003) was significantly positively correlated with NBA (r ¼ 0:586), mean individual leaf area (r ¼ 0:683), LAI (r ¼ 0:781) and SLA (r ¼ 0:552) (Table 4). LAI ranged between two for Unal and six for Wolterson, and was significantly correlated with NBA (r ¼ 0:634), mean
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Table 2 Mean shoot diameter, maximum shoot diameter and mean number of shoots per stool of coppiced stools and replacement cuttings in December 2003 of 17 poplar clones (the stand was established in April 1996, and coppiced in December 1996, January 2001 and February 2004. New cuttings ( ¼ replacement cuttings) replaced not-established cuttings) Parentage
Clone
Mean diameter
Maximum diameter
No shoots per stool
Copp. stools
Replacement cuttings
Copp. stools
Replacement cuttings
Copp. stools
Replacement cuttings
13.4 (2.2)
8.2 (1.8)
TB
Balsam Spire
13.83 (1.86)
13.71 (2.55)
45.95 (2.33)
34.25 (5.68)
TD
Beaupre´ Boelare Hazendans Hoogvorst Raspalje Unal
29.03 33.16 31.04 35.93 29.07 27.99
19.69 20.83 26.29 — 22.24 19.51
37.62 45.10 38.32 44.14 48.11 37.32
20.01(4.67) 22.03 (4.7) 29.42 (5.52) — 27.75 (—) 22.15 (2.92)
3.1 4.1 2.4 2.7 5.3 2.8
Columbia River Fritzi Pauley Trichobel
21.92 (3.9)
—
48.62 (1.4)
—
8.3 (1.7)
4.8 (1.2)
39.28 (1.73) 31.20 (6.27)
31.97 (2.67) 27.70 (9.43)
50.86 (1.17) 48.93 (5.71)
35.48 (0.92) 31.58 (7.42)
2.6 (0.2) 4.0 (0.4)
2.0 (0.8) 2.4 (1.3)
DN
Gaver Gibecq Primo
25.53 (1.56) 21.58 (3.15) 28.25 (1.02)
23.59 (1.36) 19.93 (2.42) 22.79 (2.51)
50.28 (2.53) 30.45 (2.84) 43.23 (0.85)
35.41 (0.54) 27.66 (2.24) 28.76 (3.41)
8.8 (0.7) 6.3 (0.2) 5.4 (0.7)
3.8 (0.5) 4.2 (0.5) 3.2 (0.9)
DT
IBW1 IBW2 IBW3
28.09 (3.69) 24.53 (1.39) 24.64 (1.44)
14.12 (1.83) 19.35 (1.97) 17.50 (2.96)
34.81 (2.23) 33.57 (3.96) 33.94 (1.41)
15.27 (2.98) 20.73 (2.13) 20.04 (3.43)
2.5 (0.4) 3.3 (0.8) 3.4 (0.4)
1.3 (0.3) 1.4 (0.2) 1.4 (0)
N
Wolterson
14.91 (0.88)
—
42.72 (1.34)
—
14.8 (1.1)
8.0 (0.8)
T
(0.86) (2.25) (3.92) (4.53) (6.12) (1.59)
(4.88) (4.84) (7.15) (—) (2.71)
(0.41) (3.8) (3.47) (3.45) (8.39) (2.87)
(0.2) (0.8) (0.3) (0.8) (0.1) (0.2)
1.2 1.6 1.5 1.5 2.9 1.8
(0.2) (0.3) (0.5) (0.4) (0) (0.1)
The established cuttings/stools are referred to as coppiced stools. Mean values (standard errors) of replicates are presented.
Table 3 Number of leaves per unit shoot basal area (NBA), mean individual leaf area (LAa), maximum individual leaf area (LAmax), specific leaf area (SLA) and leaf area index (LAI) in August 2003 of five poplar clones in a short rotation coppice culture Clone
NBA (cm2)
LAa (cm2)
LAmax (cm2)
SLA (cm2 g1)
LAI
Balsam Spire Columbia River Primo Unal Wolterson
85.7 92.4 93.0 60.1 123.1
18.80 22.11 18.67 26.92 12.51
113 (11) 175 (13) 76 (3) 136 (6) 69 (9)
151 150 165 134 156
4.3 5.1 4.8 2.0 6.0
(5.1) (7.0) (5.0) (4.2) (4.8)
(0.56) (0.80) (1.05) (3.85) (0.67)
(3) (2) (6) (3) (4)
(0.5) (0.1) (0.9) (0.2) (0.3)
Mean values (standard error) of replicates are shown.
individual leaf area (r ¼ 0:676), SLA (r ¼ 0:651) (Table 4), maximum individual leaf area (r ¼ 0:556) and number of shoots per stool (r ¼ 0:8). SLA ranged between 134 cm2 g1 for Unal and 165 cm2 g1 for Primo (Table 3).
3.5. Biomass distribution Significant clonal differences were found in the proportion of stem biomass to total above-ground biomass (leaves+stem+branches), averaging 71
ARTICLE IN PRESS I. Laureysens et al. / Biomass and Bioenergy 29 (2005) 10–21 Table 4 Pearson rank correlation coefficients among above ground woody biomass produced after 3 years (2001–2003), leaf area index (LAI), number of leaves per unit shoot basal area (NBA), individual leaf area (LAa) and specific leaf area (SLA).
17
Columbia River, Primo, Unal and Wolterson, respectively.
4. Discussion LAI
LAa NBA (cm2) (cm2)
Biomass (Mg ha1) 0.781*** 0.586* LAI 0.634* NBA (cm2) LAa (cm2)
SLA (cm2 g1)
0.683** 0.552* 0.676** 0.651* 0.856*** 0.537 0.518*
***Po0:001; **Po0:01; *Po0:05:
Table 5 Proportion of stem, branch and leaf biomass to total above ground biomass (leaves+stem+branches) of five poplar clones in a short rotation coppice culture at the end of the second rotation (2003) Clone
Stem (%)
Branches (%)
Leaves (%)
Balsam Spire Columbia River Primo Unal Wolterson
74.9 71.4 71.1 71.0 75.2
11.7 12.5 14.0 14.2 12.1
13.4 16.1 14.4 14.9 12.7
(1.8) (0.3) (0.2) (0.2) (2.0)
(1.2) (1.0) (1.0) (0.8) (1.0)
(1.0) (1.1) (1.2) (0.7) (1.0)
to 75%. The proportion of branch to total biomass averaged 12 to 14%; the proportion of leaf to total biomass averaged 13 to 16% (Table 5). No significant clonal differences were found for the proportions of branches and leaves. However, the biomass proportions of all three plant components depended on shoot diameter. For all clones, shoot diameter was significantly negatively correlated with the proportion of stem biomass, and was significantly positively correlated with the proportion of branch biomass. For Balsam Spire, shoot diameter was significantly negatively correlated with the proportion of leaf biomass; for Primo and Wolterson, shoot diameter was significantly positively correlated with the proportion of leaf biomass. When focusing on above-ground woody biomass distribution, significant differences in distribution were found. Stem biomass accounted for 86.8, 85.1%, 83.4%, 84.3% and 86.0% of total woody biomass for Balsam Spire,
Above-ground woody biomass production averaged 1.6–9.7 Mg ha1 y1 at the end of the second rotation (2001–2003). These production levels range within values found in other studies. Annual 4-y rotation yields of 1.2–13.6 Mg ha1 have been reported for various poplar species, depending on clone, soil, climate and management regime [10,19–21]. Biomass production at our site was rather high given the suboptimal soil conditions. The plantation was established on an old waste disposal site with high bulk density (heavy clayloam), high pH (7.3–8.1) and a slight contamination of heavy metals. We have previously shown, however, that heavy metals had no adverse effect on biomass production [22]. Biomass production was primarily determined by cutting and stool mortality. The poor performing P. deltoides P. nigra (D N) clones, P. deltoides P. trichocarpa (D T) clones and P. trichocarpa P. deltoides (T D) clone Boelare all experienced high cutting mortality during the establishment year, as reported earlier [23]. High cutting mortality of the D T and D N clones might be caused by the poor rooting capacity of the mother P. deltoides [1], given the high bulk density of the soil. Possibly, cuttings of 25 cm were too small for a good establishment in this heavy soil. High stool mortality was observed during the second year of both the first (1998) [23] and second rotation (2002). The summers of 1998, 2001 and 2002 were characterized by heavy rust infections (Melampsora larici-populina) in Belgium. Rust, in combination with the bark-killing fungus Discosporium populeum, caused high mortality rates in several poplar plantations (M. Steenackers, 2001, personal communication). In 1998, all clones were infected; in 2002, especially T D clone, Hoogvorst was severely infected and experienced high stool mortality. Two plots of Hoogvorst died nearly entirely, explaining the low biomass production of this clone during the second rotation. Shifts in clonal ranking of biomass production
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were observed between the two rotations. Hoogvorst and Hazendans (T D) were the best performers during the first rotation with, respectively, 10.1 and 10.8 Mg ha1 y1 [12], but had a lower biomass production during the second rotation. Native clone Wolterson was the best performer during the second rotation with 9.7 Mg ha1 y1. P. trichocarpa clones Fritzi Pauley and Trichobel had a relatively high biomass production in both the first and second rotation. P. trichocarpa P. balsamifera clones Balsam Spire, and D N clones Gaver and Primo showed low biomass production in the first rotation [12], but moderate to high biomass production in the second rotation. These shifts in clonal ranking clearly illustrate that long-term studies are needed to identify clones with high biomass production in coppice cultures. In the spring of 1997, replacement cuttings replaced not-established cuttings. All replacement cuttings remained smaller and performed poorer than the coppiced stools throughout both rotations (replacement cuttings were not coppiced in December 1996). Coppicing at the end of the establishment year is a frequently applied technique to promote sprouting of many shoots per stool [1,2]. The early growth rate of coppice shoots is faster than that of seedlings or cuttings, because they benefit from the existing root system, an early onset of growth and a continuous development of new nodes until late into the fall [11]. The high number of shoots per unit area gives a rapid leaf area development, leading to fast crown closure and efficient utilization of space [24]. In this study, shoot growth of coppiced stools was more vigorous than that of the replacement cuttings, causing the replacement cuttings to be dominated by the coppiced stools. Many replacement cuttings did not survive their establishment year, especially in the ‘low-cutting mortality plots’ where little light was available. In Sweden, beating experiments with willow, i.e. planting cuttings in gaps after poor establishment, have also been shown to be unsuccessful, because replacement cuttings become situated in the lower part of the competitive hierarchy [25]. In this study, all clones sprouted vigorously, producing up to seven shoots per stool on average
in the first rotation [23] and up to 19 shoots per stool on average in the second rotation. But the number of shoots per stool decreased progressively during both rotations. During the first year after coppicing, the shoots with the highest growth rate in a stool became the dominant shoots, suppressing all other shoots in the following years [23], as also reported in several other studies [26–28]. The elimination of smaller shoots was common for all clones, but the rate of elimination differed significantly among clones. T D clones Hoogvorst and Hazendans, and T clones Fritzi Pauley and Trichobel had a rapid elimination of smaller shoots, leaving mostly single-stem stools after 4 years in 2000 [23], and up to 4 shoots per stool on average after 3 years in 2003. Balsam Spire and Wolterson, on the other hand, showed a slow elimination of shoots, leaving a mean number of four and three shoots per stool in 2000 [23] respectively, and 11 and 16 shoots per stool in 2003, respectively. This number of shoots per stool in 2000 and 2003, however, was not significantly correlated with biomass production. Individual stool biomass was mostly determined by the diameter of its one or two dominant shoots, as we reported earlier for the first rotation [23]. Suppressed shoots remained always small and had only a small contribution to total stool biomass. The number of shoots produced in the first year after coppicing, on the other hand, was significantly correlated with biomass production [23]. This supports the findings of Tschaplinski and Blake [29], who showed that suppressed shoots support the growth of the dominant shoot by supplying carbon to the lower stem and roots. Early removal of all shoots but the dominant (tallest) shoot resulted in reduced vigor and viability of the dominant shoot [29]. Further research is needed to examine the physiological role of the suppressed shoots. We found significant clonal differences in leaf size and total leaf area, confirming numerous other studies (e.g. [30–32]). Total leaf area is closely related with the accumulated biomass in poplar and this correlation is considered to be genetically controlled [33]. LAI is the most commonly used parameter to quantify leaf area, which expresses total leaf area of a plant canopy per unit land area
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it covers. LAI was significantly correlated with biomass production in our study. In August, LAI ranged between 2 and 6. Pellis et al. [34] and Al Afas et al. [35] reported similar LAI values for the same plantation in the first year of the second rotation (2001). Other studies of closely spaced poplar stands report LAI values of 1–6 in the establishment year [35,36], and between 3 and more than 11 in fully developed canopies [30,36,37–40]. In this study, LAI was significantly positively correlated with the number of leaves (NBA), but significantly negatively with individual leaf size. So a high LAI was obtained with a high number of smaller leaves, e.g. for clones Wolterson and Balsam Spire. Many studies, however, showed that leaf size is a better indicator of productivity in Populus than the number of leaves per tree (e.g. [31,41,42,43]). Our results might be explained by the limited number of clones that were studied. Well performing clones Fritzi Pauley and Trichobel with large leaves were not studied. Moreover, Pellis et al. [34] showed that in the first year of the second rotation a high LAI and biomass production could be achieved by the production of many smaller leaves or of fewer larger leaves. In this study, biomass production was also positively correlated with SLA. This confirms the results of some studies that showed a positive correlation between growth rate and SLA [44,45], but contradicts the findings of several other studies [46–48]. The latter relationship was explained by a higher number of mesophyll cells per unit area or larger mesophyll cells, leading to higher rates of CO2 assimilation; the first relationship by a higher carbon and nitrogen content of leaves with high SLA [46]. Branch production differed significantly among clones when expressed as a percentage of aboveground woody biomass production, although the differences were small. Values ranged well within values reported in other studies, i.e. ranging between 11% and 26%, depending on clone and spacing [7,8,49,50]. Most of the clones in this study were selected for the production of straight, long stems with little branching. Raw-material quality of bark and branches is of lower quality than stem wood [51]. When the end product is energy or pulp, tree size and form are less important.
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In conclusion, native P. nigra clone Wolterson and American P. trichocarpa clones Columbia River, Fritzi Pauley and Trichobel performed best in the second rotation. Large clonal differences in stool mortality, number of shoots per stool, and leaf characteristics were found. Biomass production was primarily determined by cutting and stool mortality. M. larici-populina remains an important external determinant of productivity. High biomass production was also related with high LAI, which was obtained by many small leaves. Further long-term studies and large-plot field studies are necessary to identify clones with high biomass production over several rotation cycles in SRC.
Acknowledgements This study has been supported by a research contract with the Province of Antwerp and by the Fund for Scientific Research Flanders (G 0108.97). All plant materials were kindly provided by the Institute for Forestry and Game Management (Geraardsbergen) and by the Forest Research, Forestry Commission (UK). The project has been carried out in close cooperation with Eta-com B., supplying the grounds and part of the infrastructure, and with the logistic support of the city council of Boom. References [1] Dickmann DI, Stuart KW. The culture of poplars in eastern North America. East Lansing, Michigan: Michigan State University Publications; 1983. [2] Macpherson G. Home-grown energy from short-rotation coppice. Ipswich, UK: Farming Press Books; 1995. [3] European Commission. Energy for the future—renewable sources of energy. White paper for a community strategy and action plan. COM (97) 599 final (26/11/97); 1997. [4] Gordon JC. The productive potential of woody plants. Iowa State Journal of Research 1975;49:267–74. [5] Isebrands JG, Karnosky DF. Environmental benefits of poplar culture. In: Dickmann DI, Isebrands JG, Eckenwalder JE, Richardson J, editors. Poplar Culture in North America. Ottawa, Canada: NRC Research Press, National Research Council of Canada; 2001. p. 207–18. [6] Perttu KL. Ecological, biological balances and conservation. Biomass and Bioenergy 1995;9:107–16.
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