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Available at www.sciencedirect.com

B I O M A S S A N D B I O E N E R G Y ] ( ] ] ] ] ) ] ] ] – ] ] ]

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

�Corresponding auE-mail address:

Please cite this aculture of popj.biombioe.2007.1

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

Growth and production of a short-rotation coppice cultureof poplar—IV: Fine root characteristics of five poplar clones

N. Al Afasa, N. Marronb, C. Zavallonia, R. Ceulemansa,�

aDepartment of Biology, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk, BelgiumbUMR 1137, INRA-UHP Ecologie et Ecophysiologie Forestieres, F-54280 Champenoux, France

a r t i c l e i n f o

Article history:

Received 10 April 2007

Received in revised form

14 November 2007

Accepted 15 November 2007

Keywords:

Fine root biomass

Root length

Genotype

Root area index

Nitrogen concentration

Leaf area index

Populus spp.

R

nt matter & 2007 Elsevieioe.2007.11.007

thor. Tel.: +32 3820 2256; fReinhart.Ceulemans@ua.

rticle as: Al Afas N, Marlar—IV: Fine root c1.007

RECTED PROOFa b s t r a c t

Below-ground characteristics of five Populus clones, belonging to different species and

parentages, were studied during the second growing season of the third rotation of a high-

density coppice culture. Size (length, area and volume), biomass, nitrogen and carbon

concentrations of three classes of fine roots (diameter classes of 0–1, 1–2 and 2–5 mm) were

determined for four different soil layers. Fine root biomass varied significantly among

clones and among soil layers. Clone Primo (Populus deltoides� Populus nigra) had the highest

root biomass and the longest fine roots, while clone Hazendans (Populus trichocarpa� P.

deltoides) had the lowest root biomass and shortest fine roots. The topsoil layer (0–5 cm) was

very rich in fine roots; the fine root biomass and distribution of all clones decreased with

increasing soil depth. Fine root area index (diameter classes of 0–1 and 1–2 mm) varied

among clones, with higher values for clones Wolterson and Primo (3.6 and 3.7,

respectively), while clones Hazendans and Columbia River had lower values (1.7 and 2.2,

respectively). The absence of a significant correlation between fine root traits and above-

ground biomass leads to the conclusion that above-ground biomass was not a reliable

indicator of below-ground biomass in poplar, probably because of the age of the plantation

in our study (stump age of 10 years). Fine root area index was positively correlated with leaf

area index for all clones and at all soil depths, i.e., clones with a high fine root area index

also had a high leaf area index. We conclude that leaf area index can be an indicator of root

area.

& 2007 Elsevier Ltd. All rights reserved.

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UNCO1. Introduction

Roots are a large and metabolically active fraction of total tree

biomass [1,2]. Among all roots, fine roots represent only a

small fraction of total tree biomass, but fine root production

and turnover are significant components of the carbon

turnover (up to 40%) [3]. In most ecosystems, a major portion

of carbon fixed through photosynthesis is allocated to the

production and maintenance of fine roots, and therefore they

represent a major sink of carbon [4]. Additionally, fine roots

are an extremely important component of tree root systems,

r Ltd. All rights reserved.

ax: +32 3820 2271.ac.be (R. Ceulemans).

ron N, Zavalloni C, Ceulharacteristics of five

as they are the main links between the plant and the soil,

allowing water and nutrient uptake. At a larger scale, fine

roots also play a crucial role for nutrient, carbon and energy

cycling in forest ecosystems. The fast turnover rate and the

short life span of fine roots enable a great flexibility to

changing soil environmental conditions [5]. However, sam-

pling and quantification of fine roots are more time consum-

ing than those of other compartments of the tree, and the

methods are usually less precise [6]. Moreover, the root

system is known to be very dependent on environmental

conditions and on genetic background. The tight dependence

73

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emans R. Growth and production of a short-rotation coppicepoplar.... Biomass and Bioenergy (2007), doi:10.1016/

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Table 1 – Soil characteristics of the experimental short-rotation coppice plantation of poplar

Soil characteristics

pH 7.7 (0.001)

OM (%) 1.25 (0.001)

N (mg/100 g) 122.5 (2.5)

P (mg/100 g) 16.4 (2.1)

Bulk density (g cm�3) 1.46 (0.02)

Mean (standard error) soil pH, organic matter (OM), total nitrogen

(N), plant available phosphorus (P) and bulk density are presented.

B I O M A S S A N D B I O E N E R G Y ] ( ] ] ] ] ) ] ] ] – ] ] ]2

RRECT

of fine roots on a variety of abiotic and biotic factors, as well

as on the edaphic composition and soil depth, makes the

analysis of this below-ground compartment even more

difficult [7]. Differences in standing biomass of roots among

different tree species are frequently observed. Although we

have some general knowledge of the vertical distribution and

seasonal dynamics of fine roots of trees, information on fine

root growth and development is still needed for many tree

species. Thus, fine roots remain one of the least accessible,

but most important, study areas in terrestrial ecosystems [8].

Poplars are among the fastest growing trees at temperate

latitudes and are of considerable commercial importance [9].

Above-ground variability in productivity and in functional

and structural determinants has already been widely docu-

mented for the Populus genus [10]. Leaf characteristics and

their links with biomass production are also well documented

in poplar. High variations in morphological and physiological

leaf traits among clones, parentages and species, as well as

close links between foliage characteristics and productivity,

have been extensively reported for the genus [11–14]. How-

ever, the poplar root system remains the most poorly studied

and understood portion of the plant [15]. Indeed, only a very

limited number of studies have investigated the (fine) rooting

system of poplar trees. These few studies have shown that

the morphology and physiology of poplar roots vary among

species and among clones [16]. For example, Nguyen et al. [17]

and Pregitzer et al. [18] reported that two poplar clones

differed in fine root development, root carbohydrate content

and nitrogen dynamics. Most roots were found in the top

50 cm layer of soil when trees were excavated in an experi-

mental plantation of 2-year-old hybrid clones [19]. Poplar

clones seem to differ primarily in number and size of roots,

and in the depth and orientation of root growth. Significant

differences among families and among both male and female

parentages have been observed in the amount of vertical

rooting [20].

The objectives of the current study were to document the

variability among several Populus parentages in terms of roots

by comparing the fine root systems of five poplar clones

growing in a short-rotation plantation for which the above-

ground characteristics have already been extensively de-

scribed during the last decade [11,12,21–26]. More precisely,

we aimed:

105

(1)

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Plcuj.b

COto explore the variability in fine root biomass among five

poplar clones belonging to different parentages as a

function of soil depth and

(2)

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UNto relate below-ground root characteristics to above-

ground biomass production and leaf area index (LAI).

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2. Materials and methods

2.1. Site and stand description

In April 1996, 17 poplar (Populus) clones were planted on an

experimental field site of 0.56 ha in an industrial zone at

Boom, province of Antwerp (Belgium, 511050N, 41220E, 5 m

ease cite this article as: Al Afas N, Marron N, Zavalloni C, Ceullture of poplar—IV: Fine root characteristics of fiveiombioe.2007.11.007

ED PROOF

elevation above sea level). The plantation was situated on a

capped landfill site, covered with a 2 m thick layer of sand,

clay and mixed rubble. As this was a rather old landfill site

(over 40 years), no gas leakage was detected any more. The

soil was characterised by a bulk density between 1.22 and

1.62 g cm�3 (heavy clay-loam soil), and a pH between 7.3 and

8.1. The upper soil horizon contained between 0.8% and 1.8%

organic matter. The nutrient and mineral reserves were high

in comparison with forest soils, but moderate in comparison

with agricultural soils [24]. As the time of plantation was only

few years after the capping of the landfill site, a clear soil

profile was not yet fully developed. Soil characteristics are

listed in Table 1.

Hardwood cuttings (25 cm long) 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 within rows accommodating an

overall density of 10,000 trees ha�1. A randomised block

design with 17 clones�3 plot replications was adopted

according to the protocol prescribed by the UK Forestry

Commission [27]. Each monoclonal plot (n ¼ 100 trees) had a

double border row, leaving 36 assessment trees in the centre

of each plot [28]. The plantation was irrigated shortly after

planting and mechanical weed control was applied to

promote optimal establishment.

2.2. Plant material and management regime

The cuttings that did not establish in 1996 were replaced in

the spring of 1997 with new hardwood cuttings. At the end of

the establishment year in December 1996, as well as after the

first rotation cycle of 4 years, all shoots were cut back to a

height of 5 cm to create a multi-shoot coppice system. No

fertilisation or irrigation was applied after the establishment

of the experiment. Chemical weed control was applied during

the course of the plantation only when mechanical weeding

became insufficient. On three occasions (in June 1996, June

1997 and May 2001), herbicides—a mixture of glyphosate at

3.2 kg ha�1 and oxadiazon at 9.0 kg ha�1—were applied using a

spraying device with a hood-covered nozzle to reduce the

impact on trees. In January 2001 and February 2004, the

plantation was coppiced again to a height of 5 cm above soil

level, achieving three rotations of the coppice culture. Further

details on the plantation including site management, history

and plant materials can be found in [21,25].

emans R. Growth and production of a short-rotation coppicepoplar.... Biomass and Bioenergy (2007), doi:10.1016/

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B I O M A S S A N D B I O E N E R G Y ] ( ] ] ] ] ) ] ] ] – ] ] ] 3

UNCORRECT

All 17 poplar clones in the plantation had been selected for

superior biomass production and disease resistance. The five

clones selected for the current study belong to various

parentages and hybrid groups: clones Balsam Spire (BS,

Populus trichocarpa�Populus balsamifera, T�B), Columbia

River (CR, P. trichocarpa, T), Hazendans (HD, P. trichocar-

pa�Populus deltoides, T�D), Primo (PR, P. deltoides�Populus

nigra, D�N) and Wolterson (WO, P. nigra, N).

2.3. Fine root sampling

Fine roots (0–5 mm in diameter) were sampled by collecting

soil cores using a metal auger with a length of 25 cm and an

inner diameter of 4.8 cm (split-tube sampler, Eijkelkamp, The

Netherlands) in August 2005 (i.e., the second growing season

of the third rotation of the plantation). Twelve random

samples per clone (four samples per plot) were collected

mid-way between two rows of trees. Samples from four

different soil layers (at depths of 0–5, 5–10, 10–15 and

15–30 cm) were put in plastic bags and stored separately at

�20 1C until processed. Fine roots were manually separated

from the soil, and living roots were separated from dead roots

based on their lighter colour and greater resilience. Living

roots were also stiffer and showed a better cohesion between

the cortex and the periderm [29]. Roots were washed by tap

water in pots and put in Petri dishes with water.

2.4. Fine root morphology, biomass, carbon and nitrogen

Roots in Petri dishes were scanned on a flatbed scanner (AGFA

SnapScan, 1212). Pictures were filtered and pre-processed

with the GIMP 2.0 program (GNU Image Manipulation

Program, http://www.gimp.org). Root length, area and volume

were measured by processing the pictures on a DT-scan

program. Three classes of root diameter were identified:

diameter class 1 (0–1 mm), diameter class 2 (1–2 mm) and

diameter class 3 (2–5 mm). Relative root length (%) for each

diameter class was measured as (total root length of one

specific class�100)/P

total root length of all classes in one

soil layer. After scanning, fine roots were dried for 2 days at

75 1C; dry root mass was determined and expressed as mg of

root per cm3 of soil. The dry samples of the different soil

layers were ground; nitrogen (N) and carbon (C) concentra-

tions were determined with an NC element analyser

(NC-2100, Carlo Erba Instruments, Italy) and the C to N ratio

(C:N) was subsequently calculated.

Fine root (0–2 mm diameter) area index (RAI) was calculated

as:P

root area of all samples of diameter classes 1 and 2 per

replicate plot/core area, and expressed as m2 m�2 (dimen-

sionless). Fine root area density (RAD) was calculated asP

root area of all samples (of diameter classes 1 and 2 only)

per replicate/core volume, and expressed in cm2 cm�3.

2.5. Leaf area index

In August 2005, 10 shoots per replicate plot (of the 36 central

assessment trees) were harvested at 5 cm above soil level for

each of the five clones. All leaves of each shoot were removed

and brought to the laboratory for analysis. Individual leaf area

was measured for all leaves using a leaf area meter (CID Inc.

Please cite this article as: Al Afas N, Marron N, Zavalloni C, Ceulculture of poplar—IV: Fine root characteristics of fivej.biombioe.2007.11.007

ROOF

type CI-203, USA). The LAI was calculated from the total leaf

area of the 10 shoots and scaled-up to all assessment shoots

of each replicate plot per unit ground area (36 m2) of the plot.

2.6. Above-ground standing biomass

At the end of the second growing season of the third rotation

(2005), the diameter of all shoots was measured at 22 cm

above soil level with a digital calipers (Mitutoyo, type CD-

15DC, UK). Standing above-ground biomass was estimated

per clone using allometric power relationships between shoot

diameter and total shoot dry mass: M ¼ aDb, with a and b as

regression coefficients, D as shoot diameter and M as shoot

dry mass. These allometric relationships were established

and validated in 2002 during the second growing season of

the second rotation [25].

2.7. Statistical analyses

Data analyses were performed with the Statistical Package

SPSS (SPSS, Chicago, IL). Means were calculated with their

standard error (7SE) and compared using two-way ANOVA.

Clone and soil layer were considered as the main factors.

Interclonal comparisons were followed by a Scheffe test. All

differences were considered significant at Pp0.05. Linear

correlations between parameters were tested for significance

using Pearson’s correlation coefficient.

D P3. Results

3.1. Fine root biomass

Biomass of fine roots of diameter classes 1 and 2 from the

0–30 cm soil layers differed significantly among clones

belonging to the different parentages. The mean fine root

biomass ranged between 2.5 mg cm�3 for clone Hazendans

and 4.4 mg cm�3 for clone Primo. The highest biomass of fine

roots over the examined profile occurred in the upper 5 cm

layer of the soil, whereas the lowest was measured at a soil

depth of 15–30 cm for all clones (Fig. 1). Fine root biomass in

the deepest soil layer (15–30 cm) varied only slightly among

clones.

3.2. Fine root morphology and root area index

Total root length of diameter class 1 differed significantly

among clones and among soil layers (Fig. 2). At a soil depth of

0–5 cm, clones Primo and Wolterson had the highest root

length, while clones Hazendans and Columbia River had the

lowest values in the same soil layer. Similar variation in root

length among clones was observed at the other soil depths

examined. Clone Balsam Spire always showed intermediate

values (Fig. 2, top panel). In contrast, relative fine root lengths

of diameter classes 2 and 3 (1–2 and 2–5 mm in diameter,

respectively) were high for clones Hazendans and Columbia

River, and low for clones Primo and Wolterson for most soil

layers (Fig. 2, bottom panels). Relative root length of diameter

class 1 was high in all soil layers for clones Primo and

Wolterson. Total root length (sum of all root diameter classes)

emans R. Growth and production of a short-rotation coppicepoplar.... Biomass and Bioenergy (2007), doi:10.1016/

CORRECTED PROOF

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-30

-25

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-15

-10

-5

00

Balsam Spire

Columbia River

Hazendans

Primo

Wolterson

So

il d

ep

th (

cm

)

Fine root biomass (mg cm-3)

1 2 3

Fig. 1 – Fine root biomass density of five poplar clones as a function of soil depth for clone Balsam Spire (P. trichocarpa�P.

balsamifera), clone Columbia River (P. trichocarpa), clone Primo (P. deltoides�P. nigra), clone Hazendans (P. trichocarpa�P.

deltoides) and clone Wolterson (P. nigra). Mean values (7SE) of three replicates per clone.

0-5cm

5-10cm

10-15cm

15-30cm

Wolterson

80 85 90 95 100

Primo

0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8

HazendansColumbia RiverBalsam Spire

Relative root length (%)

Absolute root length (m)

So

il d

ep

th (

cm

)

0 80 85 90 95 1000 80 85 90 95 1000 80 85 90 95 1000 80 85 90 95 1000

0-5cm

5-10cm

10-15cm

15-30cm

Fig. 2 – Absolute (top panels; m) and relative root length (bottom panels; %) of three diameter classes of fine and small roots of

five poplar clones as a function of soil depth. Clone Balsam Spire (P. trichocarpa�P. balsamifera), clone Columbia River (P.

trichocarpa), clone Primo (P. deltoides�P. nigra), clone Hazendans (P. trichocarpa�P. deltoides) and clone Wolterson (P. nigra) are

shown. Root length class 1 (white bars) ¼ root diameter 0–1 mm; root length class 2 (black bars) ¼ root diameter 1–2 mm; and

root length class 3 (hatched bars) ¼ root diameter 2–5 mm. Mean values (7SE) of three replicates per clone.

B I O M A S S A N D B I O E N E R G Y ] ( ] ] ] ] ) ] ] ] – ] ] ]4

UNshowed a high clonal variation, especially in the 0–5 cm soil

layer (Fig. 3).

Fine RAI of diameter classes 1 and 2 varied among clones.

RAI ranged between 1.77 for clone Hazendans and 3.56 for

clone Wolterson (Table 2), and decreased with increasing soil

depth (data not shown). Fine RAD was high for clones

Wolterson and Primo, while it was low for clone Hazendans

(Table 2). RAD varied significantly among soil layers (data not

shown). The topsoil layer (0–5 cm) had higher RAD than the

other soil layers for all clones (data not shown).

Please cite this article as: Al Afas N, Marron N, Zavalloni C, Ceulculture of poplar—IV: Fine root characteristics of fivej.biombioe.2007.11.007

3.3. Fine root nitrogen and carbon

Fine root N concentration did not differ significantly among

clones, but differences in N concentration were detected

among the different soil layers. For instance, fine root N

concentration of clone Wolterson was significantly higher in

the 0–5 cm topsoil layer (1.18%70.07) than in the deepest

15–30 cm soil layer of the profile (0.81%70.03). Fine root C

concentration differed significantly among clones and ranged

between 36.2% for clone Primo and 44.3% for clone Balsam

emans R. Growth and production of a short-rotation coppicepoplar.... Biomass and Bioenergy (2007), doi:10.1016/

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Soil depth (cm)

0

2

4

6

8

10

Balsam Spire

Columbia River

Hazendans

Primo

WoltersonTo

tal ro

ot

len

gth

(m

)

0-5 cm 10-15 cm5-10 cm 15-30 cm

Fig. 3 – Total root length (sum of diameter classes 1, 2 and 3) of fine and small roots of five poplar clones in the second growing

season of the third coppice rotation. Mean values (7SE) of three replicates per clone.

Table 2 – General mean7standard error of fine root areaindex and fine root area density for the five poplar clones(three replicates per clone; root diameter classes 1 and 2)

Clone Parentage Rootarea

index

Root areadensity

(cm2 cm�3)

Balsam

Spire

T�B 3.2970.38 0.1670.02

Columbia

River

T 2.2270.14 0.1170.01

Hazendans T�D 1.7770.15 0.0970.01

Primo D�N 3.5370.29 0.1870.01

Wolterson N 3.5670.29 0.1970.01

B: P. balsamifera, D: P. deltoides, N: P. nigra; and T: P. trichocarpa.

B I O M A S S A N D B I O E N E R G Y ] ( ] ] ] ] ) ] ] ] – ] ] ] 5

RSpire. C concentration was not significantly different for fine

roots sampled from different soil layers.

O 105

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UNC3.4. Leaf area index and above-ground standing biomass

LAI varied significantly among clones, and ranged between

2.1 for clone Hazendans and 6.0 for clone Wolterson. Clone

Primo (5.4) also had a high LAI, while clones Columbia River

and Balsam Spire had intermediate values (4.1 and 4.7,

respectively). Mean cumulated above-ground biomass (after

2 years) differed significantly among clones and ranged from

14.6 tonne ha�1 for clone Hazendans to 25.8 tonne ha�1 for

clone Wolterson. Clone Primo was also very productive,

showing values very similar to the ones of clone Wolterson

(above-ground biomass of 22.0 tonne ha�1 after 2 years). Clone

Columbia River showed a relatively high standing biomass

Please cite this article as: Al Afas N, Marron N, Zavalloni C, Ceulculture of poplar—IV: Fine root characteristics of fivej.biombioe.2007.11.007

D PRO(18.1 tonne ha�1), while clone Balsam Spire had an intermedi-

ate value of 15.9 tonne ha�1.

3.5. Correlations among traits

To avoid redundancy of the analogous results on root length,

area and volume, root length was primarily used in the

following analysis. Fine root biomass was significantly and

positively correlated with root length for all three diameter

classes (Table 3 and Fig. 4). Fine root biomass was also

positively correlated with N concentration and negatively

with the C:N ratio (Table 3). Furthermore, significant correla-

tions were found among fine root lengths of diameter classes

1, 2 and 3, as well as between fine root length of classes 1 and

2 and root N concentration. In general, root parameters were

not significantly correlated with above-ground standing

biomass (Table 3). However, a significant positive correlation

between LAI and RAI was found (Fig. 5).

4. Discussion

Three aspects of fine root growth deserve particular attention.

First, rapid rates of root length extension are known to be

usually well correlated with above-ground growth, at least in

young trees [4,30]. However, in mature and older trees, above-

and below-ground growth can become independent, which

could be the case in our study. Older trees rely not only on

current photosynthates but also on stored reserves in

permanent organs such as coarse roots and trunks. A second

important aspect of root growth is the fact that C allocation to

root growth appears to be under strong genetic control. There

are several reports of genotypic and species-level differences

in root growth [18,20]. The third aspect of root growth needing

particular attention is the understanding of root plasticity in

response to different environmental conditions. For instance,

emans R. Growth and production of a short-rotation coppicepoplar.... Biomass and Bioenergy (2007), doi:10.1016/

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Table 3 – Linear correlations (Pearson’s coefficients) between fine root parameters (biomass, root length of diameter class 1(0–1 mm), class 2 (1–2 mm), class 3 (2–5 mm), nitrogen (N), carbon (C) concentration and C:N ratio) with above-groundbiomass and root area index

Root traits

Biomass Length Nitrogen Carbon C:N ratio Root area index

Diameter class

1 2 3

Root traits

Length

Diameter class

1 0.80***

2 0.85*** 0.92***

3 0.70*** 0.77*** 0.86***

Nitrogen 0.30* 0.33* 0.31* ns

Carbon ns ns ns ns ns

C:N ratio �0.33* ns �0.36 ns �0.85*** 0.42**

Root area index 0.81*** 0.98*** 0.95*** 0.82*** 0.32* ns �0.26*

Above-ground biomass ns ns ns ns ns ns ns ns

Levels of significance are indicated by asterisks; ns ¼ non significant; *Pp0.05; **Pp0.01; ***Pp0.001.

r =0.70

0

1

2

3

4

0 0.1 0.2 0.3

Class 3 (diameter 2-5 mm)

r = 0.85

0 0.4 0.8 1.2 1.6 2

r = 0.80

0

1

2

3

4

0 4 6 8 10

Class 1 (diameter 0-1 mm)

Root length

Fin

e r

oo

t b

iom

ass (

mg

cm

-3)

Fin

e r

oo

t b

iom

ass (

mg

cm

-3)

Class 2 (diameter 1-2 mm)

2

A B C

Fig. 4 – Relationships between fine root biomass and root length of class 1 (A), class 2 (B) and class 3 (C). Mean values of three

replicates per clone are presented. Linear correlations (Pearson’s coefficients) and their level of significance are indicated in

Table 3.

r = 0.91

0

2

4

6

8

0

Le

af

are

a in

de

x

Root area index

Balsam Spire

Columbia River

Hazendans

Primo

Wolterson

X

1 2 3 4 5

Fig. 5 – Relationship between root area index and leaf area

index. Mean values (7SE) of three replicates per clone are

shown. Both indices are dimensionless (m2 m�2).

B I O M A S S A N D B I O E N E R G Y ] ( ] ] ] ] ) ] ] ] – ] ] ]6

Please cite this article as: Al Afas N, Marron N, Zavalloni C, Ceulculture of poplar—IV: Fine root characteristics of fivej.biombioe.2007.11.007

elevated atmospheric CO2 and soil N availability have been

shown to increase root production and mortality of the hybrid

poplar clone Eugenei (P. deltoides�P. nigra) [4].

4.1. Clonal variability

In this study, significant variation in fine root biomass was

found among different poplar clones. This confirms what has

been reported in few studies on the clonal variability in fine

root growth of poplar [31,32]. The variability in fine root

biomass is related to the large genotypic variations in above-

ground leaf and stem characteristics, and to the genetic

variation between parents and their hybrids [11–14,25,33]. For

instance, Nguyen et al. [17] and Pregitzer et al. [18] observed

for two hybrid poplar clones in a short-rotation culture (SRC)

that the clones differed significantly in fine root production.

Clone Tristis (Populus tristis�P. balsamifera) produced more

and longer fine roots in the 0–30 cm soil layer than did clone

Eugenei (P. deltoides�P. nigra), even though Tristis trees were

emans R. Growth and production of a short-rotation coppicepoplar.... Biomass and Bioenergy (2007), doi:10.1016/

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much smaller. Similarly, the vigorous P. trichocarpa root

growth differed from the poor root growth of P. deltoides [34],

while the hybrids between P. trichocarpa and P. deltoides

showed intermediate root growth characteristics. The hy-

brids’ fine root production ranged between 1.0 and

5.0 tonne ha�1 year�1 [35]. Thus, interclonal differences in fine

root production occur in the genus Populus. Compared with

other tree species, fine roots of poplar are long, relatively

unbranched and very thin. In addition, species within the

genus Populus have very similar fine root morphology and

anatomy [36]. Genotypic variation could be related to the

plasticity of the response to environmental conditions such

as drought, frost or pathogens. Dickmann et al. [31] reported

that clone Eugenei was quite plastic in its above-ground

response to environmental changes, while no below-ground

response was observed. Clone Tristis was just the opposite,

showing only minor above-ground responses, but with major

morphological below-ground responses. This phenomenon

may be related to the competition among clones growing in a

SRC, because this kind of management reduces the fine root

production and induces an immediate increase in mortality

of fine roots, especially after coppicing [31].

4.2. Root profiles

In our study, all clones showed higher values of fine root

biomass in the topsoil layer (0–5 cm). This has also been

shown in other studies on poplar [31,32] and on other genera

[37–40], whereby fine root biomass production was largest in

the upper 15 cm of the soil layer. The differences in fine root

biomass in soil layers could be related to the C inputs via

coarse roots, which could vary with soil depth, nutrient

availability (especially N), microbial activity and soil char-

acteristics such as temperature and moisture [35]. These

latter factors are more dynamic in the topsoil layer and

consequently stimulate the fine root turnover more inten-

sively in the upper soil layers than in the deeper ones [40].

Fine root production is very dynamic in poplar, reaching its

highest rate in summer, which coincided with our sampling

period [15].

Fine RAI and RAD were high in the topsoil layer (0–5 cm). A

number of forest ecosystem studies have reported that fine

roots were most abundant in the uppermost soil layer while

their density steadily decreased with increasing depth [41–43].

On the contrary, the study of Lopez et al. [44] showed that root

distribution of Holm oak growing in a Mediterranean forest

was higher at the 10–20 cm soil depth than at the topsoil

horizon of 0–10 cm.

4.3. Relationships among traits

Fine root length of diameter class 1 was highest in all clones

and at all soil layers, followed by the length of root diameter

class 2. Clones with high root biomass had higher root length

in the different diameter classes, because of the positive

correlation between fine root biomass and fine root length.

Many studies conducted on fine roots have arbitrarily defined

roots with o2 mm as fine roots assuming that all roots within

this diameter category act in a similar manner. In reality,

poplar roots o2 mm in diameter can include new roots,

Please cite this article as: Al Afas N, Marron N, Zavalloni C, Ceulculture of poplar—IV: Fine root characteristics of fivej.biombioe.2007.11.007

D PROOF

suberised roots and woody roots [15]. Fine root production

and mortality are highly dynamic and often simultaneous

[35,45,46]. The positive correlation between fine root length

and N concentration can be explained by the general

importance of N for growth.

RAI is comparable to LAI in terrestrial ecosystems, and is in

most systems substantially larger [47]. In this study, fine RAI

correlated positively with LAI. Thus, clones with high LAI had

high RAI. Moreover, fine RAI in the present study was in the

range reported by Larcher [48], but it was higher than the RAI

values of 0.01–0.4 (for roots o5 mm in diameter) reported by

Addington et al. [49] for pine trees growing in xeric and mesic

habitats. Along the same line, RAI values estimated for

Belgian beech and spruce forests were 6.8 and 5.4 m2 m�2

for fine roots o5 mm in diameter, with fine roots o1 mm in

diameter comprising 85–90% of total RAI [50].

In SRC the root system is of major importance for regrowth

and production of the culture after each rotation cycle. It is

evident that the coarse roots play a dominant role in this

regrowth after coppice. However, to maximise growth and

production of the culture, e.g. for bio-energy purposes, an

optimal nutrient and water supply to the leaves and the

above-ground plant organs are crucial. This nutrient and

water supply depends on the root exploration by fine and

small roots. An improved knowledge of the characteristics of

fine roots as provided by this study is therefore necessary.

Furthermore, the C consumption by the short-living fine roots

is a very important aspect of the C balance of an SRC

plantation.

In conclusion, our study confirmed that clonal as well as

profile variations of fine root traits exist in an SRC plantation.

All root traits exhibited highest values in the topsoil layer

(0–5 cm). Positive correlations were found among fine root

biomass, N concentration and root length. Moreover, the

above-ground biomass could not be considered as a relevant

indicator of fine root biomass, possibly because of the smaller

dependence of above-ground from below-ground character-

istics due to the age of the plantation (stump age of 10 years).

Two strategies were recognised among the five poplar clones:

clones with high root density and high LAI (clones Primo and

Wolterson), and clones with low fine root density and low LAI

(clone Hazendans). LAI was a reliable indicator of fine RAI.

Further root studies are necessary to extrapolate these

conclusions to more poplar clones and over different rota-

tions of SRC.

Acknowledgements

This study was supported by a research contract of the

Province of Antwerp. The project was carried out in close

cooperation with Eta-com B., who supplied the grounds and

part of the infrastructure, and with the logistic support of the

city council of Boom. All plant materials were kindly provided

by the Research Institute for Nature and Forestry (INBO,

Geraardsbergen) and by Forest Research of the Forestry

Commission (UK). We are grateful to Zakareya Hleibie and

Ine Pauwels for help with field data collection, to Fred

Kockelbergh for image analysis, to Nadine Calluy for nitrogen

and carbon analysis, and to Ivan Janssens and Jan Cermak for

emans R. Growth and production of a short-rotation coppicepoplar.... Biomass and Bioenergy (2007), doi:10.1016/

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critical comments on an earlier version of the manuscript.

The first author was supported by a fellowship from the

Syrian University (Al Baath).

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