Site-related set-back by weeds on the establishment of 12 biomass willow clones

Site-related set-back by weeds on the establishmentof 12 biomass willow clones

J ALBERTSSON*, D HANSSON†, N-O BERTHOLDSSON* & I �AHMAN**Department of Plant Breeding, Swedish University of Agricultural Sciences, Alnarp, Sweden, and †Department of Biosystems and

Technology, Swedish University of Agricultural Sciences, Alnarp, Sweden

Received 24 June 2013

Revised version accepted 25 February 2014

Subject Editor: Corn�e Kempenaar, WUR, the Netherlands

Summary

Ten commercial clones of willow and two breeding

clones were studied for their ability to compete with

weeds during the establishment year at three different

sites in southern Sweden. Cuttings were planted

according to commercial practice in April, and the two

treatments, ‘Weeded’ and ‘Unweeded’, were laid out in

a strip-plot design. Weeds in the ‘Weeded’ treatment

were removed mechanically and by hand hoeing. Wil-

low plant shoot weight and plant mortality were mea-

sured after the first growing season to evaluate the

initial effect of weeds. In addition, weed flora, weed

aboveground biomass, soil properties, shoot damage

and soil moisture were assessed during the growing

season. Plant mortality was <1% in the ‘Weeded’

treatment at the three study sites, while in ‘Unweeded’

it was significantly higher, 2.7%, 24.6% and 37.4%.

Weeds reduced willow plant shoot weight by 93.4%,

94.0% and 96.1% at the three sites. Only one site

showed clonal differences in shoot growth reduction,

as well as in plant mortality. These results show the

importance of weed control in willow plantations, as

growth of all clones tested were dramatically hampered

by weeds during the first growing season, regardless of

trial site conditions. Moreover, conditions at certain

sites, such as soil properties in combination with weed

cover, may cause high plant mortality during the

establishment year in this perennial biomass crop.

Keywords: bioenergy, biomass, growth reduction, plant

mortality, Salix, short-rotation coppice, weed competi-

tion.

ALBERTSSON J, HANSSON D, BERTHOLDSSON N-O & �AHMAN I (2014). Site-related set-back by weeds on the establish-

ment of 12 biomass willow clones. Weed Research 54, 398–407.

Introduction

Willow shrubs (Salix spp.) can be managed as

short-rotation coppice (SRC) to produce renewable

feedstock for bioenergy production. These perennial

willow plantations are harvested normally at intervals

of 2–4 years and have an expected lifespan of at least

20 years (Karp et al., 2011; Verwijst et al., 2013).

Sweden, together with the United Kingdom and

United States of America, has played an important

role in developing willow SRC as an agricultural crop

(Larsson, 1998; Smart & Cameron, 2008), which is

now grown on approximately 12 000 hectares in

Sweden (Jordbruksverket, 2012). The Swedish govern-

ment has stated that by the end of 2020, more than

50% of the energy used in the country should come

from renewable sources (Regeringskansliet, 2009), and

willow SRC is part of this transition. The estimated

yield range in well-managed Swedish plantations is

5.4–7.1 oven dry tonnes shoot biomass per hectare

Correspondence: J Albertsson, Department of Plant Breeding, Swedish University of Agricultural Sciences, P.O. Box 101, SE-23053 Alnarp,

Sweden. Tel: (+46) 708 658318; Fax: (+46) 40 462272; E-mail: [email protected]

© 2014 European Weed Research Society 54, 398–407

DOI: 10.1111/wre.12086

per year (Mola-Yudego & Aronsson, 2008). Willow

SRC produces 11–24 times the amount of energy used

as input to the plantation (Rowe et al., 2009; B€orjes-

son & Tufvesson, 2011). Thus, its energy ratio is high

compared with that of other bioenergy crops, such as

the annual crops wheat (Triticum aestivum L.) and

maize (Zea mays L.) (B€orjesson & Tufvesson, 2011).

This high energy ratio and the fact that neither

fungicides nor insecticides are used in the production

(�Ahman, 2001) contribute to willow having the best

environmental profile among the arable bioenergy

crops grown in Sweden. The only pesticides used in

commercial plantations are herbicides, as willow,

especially during its establishment phase, is known to

be very sensitive to competition from other plants

(Labrecque et al., 1994; Clay & Dixon, 1995; Sage,

1999). A yield reduction in willow SRC of more than

95% due to competition from weeds in the first year

after planting has been found in one plantation in

England (Clay & Dixon, 1995). Both the low planting

density of 1–2 cuttings m�2 and the use of unrooted

cuttings in plantations contribute to this unfavourable

weed effect. Poor establishment of the crop negatively

affects future yields, probably during the whole

lifespan of the plantation (Clay & Dixon, 1997). In a

Swedish survey (Helby et al., 2006), inferior growth

due to weeds was the most common reason for

complete or partial removal of willow plantations.

Through plant breeding, many new willow clones

with higher yields and improved resistance to pests

and diseases, such as leaf rust and certain insects,

have been developed (Larsson, 1998; �Ahman & Lars-

son, 1999; Karp et al., 2011). However, it is still

unknown whether willow clones differ in weed com-

petitive ability, as observed in other crops (Murphy

et al., 2008), and therefore, no deliberate selection for

competitive ability against weeds has yet been made.

If differences exist, it might be possible to further

improve the environmental profile of willow SRC by

breeding.

In field studies, it is hard to separate by which

means plants interact. Therefore, the word competition

is here used in a broad sense (cf. H�akansson, 2003).

Ten commercial clones of willow and two breeding

clones were studied during their first growing season.

The aim was to investigate whether there were

differences between clones in their abilities to compete

with weeds during the establishment year, measured as

willow growth and plant mortality with and without

weeds. As this ability may differ depending on

environmental conditions, we performed the studies at

three sites that differed in soil properties, but only

marginally in weather conditions.

Materials and methods

Site information

The study was conducted at three sites, designated

J€arnv€agen (J), Popplarna (P) and Staketet (S), located

within 1.5 km from each other and close to Campus

Alnarp of the Swedish University of Agricultural Sci-

ences (55°390N, 13°50E) in southern Sweden. The sites

differed in terms of soil properties and preceding crops

(Table 1). All three sites were surrounded by a 90-cm-

high fence with a mesh size of 25 mm. Precipitation

during the establishment year 2010 was 730 mm, which

was above normal compared with the yearly mean pre-

cipitation of c. 600 mm for this area. However, the

rainfall from mid-June until the end of July 2010 was

low, 18 mm, compared with the normal mean precipi-

tation of c. 50 mm in June and c. 60 mm in July

(SMHI, 2012).

Soil characteristics

At each site, 24 soil samples were taken, between dou-

ble rows, to a depth of 25 cm along a diagonal line

crossing the field, on 21 July 2011. The soil samples

from each site were mixed thoroughly in a bucket and

sent to Agrilab AB (Uppsala, Sweden) for analysis.

Method KLK 1965:1 was used for the analysis of

organic matter, ISO 10390 for pH analysis and SS

27123 and SS 27124 for soil texture analysis.

Site preparation

In the autumn prior to planting, sites S and P were

ploughed and site J was mechanically cultivated by a

stubble cultivator. The stubble cultivator was also used

Table 1 Soil and other characteristics at sites J, P and S

Site pH

Organic

matter (%)

Clay

(%)

Silt

(%)

Sand

(%)

Previous crop

in 2009

Site size

(ha)

J 6.9 2.8 14.0 20.0 63.2 Sugar beet 0.76

P 7.6 19.6 22.0 43.5 14.8 Rye 0.76

S 6.7 2.8 15.0 31.0 51.2 Barley 0.91


Weed effects on willow short-rotation coppice establishment 399

in spring 2010 at site J to ensure that the soil was culti-

vated to a depth of at least 20 cm. All sites were har-

rowed the day before planting.

Plant material

Ten commercial clones, all from Lantm€annen SW Seed

AB’s breeding programme, were planted at the three

sites (Table 2). At site S, two additional breeding

clones were included. Cuttings were 18 cm long, with

an upper mean diameter of 10.4 and a standard devia-

tion of �2.2 (10% of the cuttings sampled). Sites J, P

and S were planted by hand on 20, 15 and 13 April

2010 respectively. All cuttings were supplied by profes-

sional nurseries and had been stored a maximum of

3 months at approximately �4°C until the day of

planting.

Experimental design

The trials were laid out in a strip-plot design with two

treatments (‘Weeded’ and ‘Unweeded’) and 10 com-

mercial willow clones (plus two additional breeding

clones at site S) in four blocks. Within each block,

clones and treatments were randomised to rows and

columns respectively. There were a total of 80 plots at

sites J and P, and 96 plots at site S. Weeds were

removed mechanically by a row cultivator and by hand

hoeing in the ‘Weeded’ treatment during the whole

growing season of 2010. The aim of the weed control

in the ‘Weeded’ treatment was to remove weeds before

they began to compete with the willow plants. There-

fore, the number of weeding occasions differed, due to

variations in weed growth between the sites. Each plot

was 9 m 9 7 m in size and contained 80 plants in four

double rows, with 10 plants in each row. Paired rows

within the double row were 0.75 m apart, with 1.5 m

between double rows. Plants were 0.70 m apart within

rows, resulting in a final plant density of about 13 000

plants ha�1. This double row system is the common

planting method used in Swedish commercial planta-

tions (Jordbruksverket, 2012). A border row of the wil-

low clone Tora was planted around the trials.

Shoot biomass measurements

The measurements of shoot biomass were taken during

January and February 2011 when plants were dor-

mant. In order to avoid border effects, measurements

were taken in central 6.75 m 9 5.60 m net plots, each

containing of 48 plants. Plants were considered to be

living, if living tissue was found 5 cm above the

ground. At site S, living shoots from individual plants

were cut (5 cm above ground) and weighed separately

(accuracy �0.5 g), allowing these plants to be moni-

tored individually over the years. However, even

though only shoots from alternate plants were sam-

pled, this was too time-consuming, and the method

was abandoned at the other two sites. At sites J and P,

all living shoots from each net plot were pooled and

weighed. Dry matter content was determined based on

10 randomly chosen shoots from every plot. Stem

pieces from these shoots were cut at 20 cm distance

from both sides of the gravity centre point and subse-

quently dried at 65°C for at least 1 week until constant

weight was reached. The weight of fresh and dry stem

pieces was determined with an accuracy of �0.01 g.

Total shoot dry weight per plot was calculated by mul-

tiplying the total fresh weight of all sampled shoots for

each net plot by the dry matter content. Plant shoot

dry weight was calculated as total shoot dry weight

per net plot divided by number of living plants in that

net plot.

Plant mortality

As mentioned above, plant mortality was assessed dur-

ing the shoot biomass measurements after the first

growth season. In addition, plant mortality was non-

destructively assessed during the growth season in

August 2010 in the ‘Unweeded’ treatment by sampling

alternate plants in every net plot. Dead plants were

split in two categories: plants without shoots and

Table 2 Willow clones included in the study and their genetic

background

Clone Genetic background*

Gudrun S. dasyclados†Karin S. dasyclados†, S. schwerinii,

S. viminalis

Klara S. dasyclados†, S. schwerinii,

S. viminalis

Linnea S. schwerinii, S. viminalis,

S. eriocephala, S. triandra

Lisa S. schwerinii, S. viminalis

Stina S. aegyptiaca, S. schwerinii,

S. viminalis, S. lanceolata

Sven S. schwerinii, S. viminalis

SW Inger S. triandra, S. viminalis

Tora S. schwerinii, S. viminalis

Tordis S. schwerinii, S. viminalis

98‡

58‡

*Gabriele Engqvist, Lantm€annen SW Seed AB, pers. comm.;

I. �Ahman, unpublished.

†Sometimes referred to as S. burjatica.

‡Breeding clones. Only planted at site S. Genetic background not

available.


400 J Albertsson et al.

plants with dead shoots. In addition, living plants were

examined for browsing by voles, hares and rabbits. A

plant was rated as browsed if the dominant shoot of

the plant had been cut off below 20 cm.

Weed measurements

The ground flora was assessed during 10–13 June

2010. A 25 cm 9 50 cm frame was placed on the

ground between the double rows at three places in a

diagonal pattern within each plot in the ‘Unweeded’

treatment. Inside each frame, the five most common

weeds in terms of ground cover were recorded. Weed

aboveground biomass was estimated by measuring

percentage reflectance [1009 (down sensor reading/up

sensor reading)] at 813 and 661 nm with an MSR16

instrument (Cropscan, Rochester, MN, USA). Relative

vegetation index (RVI) was calculated as the 813/661

ratio (cf. Bertholdsson, 1999). Measurements were

taken in a circular area with a diameter of 75 cm,

between double rows diagonally at three places in each

plot at sites J, P and S on 22, 17 and 21 June 2010

respectively.

Soil moisture measurements

The volumetric water content (VWC) was measured

with a Fieldscout TDR 300 m (Spectrum Technologies

Inc.) equipped with two 12-cm rods on 29 July 2010

(after nearly 1.5 months of dry weather). Soil moisture

was measured in clone Tora ‘Weeded’ and ‘Unweeded’

plots. Means of nine samples, taken between the dou-

ble rows, were calculated for each plot.

Statistical analysis

Statistical analysis of clonal differences (only commer-

cial clones) in willow shoot dry weight per plant was

performed using the PROC MIXED procedure in SAS

9.3 (SAS, 2012). Interactions were found between sites

and clones and between sites and treatments when data

from all three sites (strip-plot design) were analysed,

and between clones and treatments when sites were

analysed one by one; thus, treatments were analysed

separately at each site. RVI was used as a covariate in

the ‘Unweeded’ treatment to account for differences in

weed aboveground biomass between plots. Growth

reduction was calculated using mean plant shoot dry

weight and the formula 1009 [(‘Weeded’ –

‘Unweeded’)/‘Weeded’] for every block and clone. Dif-

ferences in growth reduction between commercial

clones and between sites were analysed using PROC

MIXED. PROC MIXED was also used to analyse

weed aboveground biomass (RVI) differences between

sites (only in plots with commercial clones). The RVI

data were square-root-transformed before analysis to

fulfil the assumptions of normally distributed residuals.

Differences in plant mortality between treatments were

analysed at each site with the nonparametric Wilcoxon

signed-rank test (PROC UNIVARIATE) due to non-

normal distribution of the data. Plant mortality data

comparing sites and comparing commercial clones per

site in the ‘Unweeded’ treatment were arcsine-trans-

formed before analysis (PROC MIXED). Due to non-

normally distributed residuals, the nonparametric

Friedman test (PROC FREQ) was performed to ana-

lyse differences between clones per site in the ‘Weeded’

treatment. Data from the plant assessment in August

were analysed using Friedman test (due to non-nor-

mally distributed residuals), except for differences

between clones at site P. The latter analyses were per-

formed using PROC MIXED (arcsine-transformed

data). PROC MIXED was also used to analyse differ-

ences in soil moisture between sites. In all PROC

MIXED analyses, multiple comparisons were made

with Tukey’s post hoc test (a = 0.05). Treatment, site

and clone were considered fixed factors and block a

random factor in the analyses. Correlation analyses

(PROC CORR) were performed for each site, with

RVI and shoot dry weight per plot as variables. Rela-

tionships between willow shoot dry weight per plot

and RVI were tested separately for each commercial

clone, using the Pearson partial correlation, where the

effect of sites was adjusted for by the PARTIAL state-

ment.

Results

Shoot growth

There was a significant difference in shoot growth

reduction between sites (F2,9 = 6.8, P = 0.016). The

post hoc test showed no differences between site P

(94.0%) and the other sites, but site J (96.1%) had a

significantly larger growth reduction than site S

(93.4%). Clonal differences in growth reduction were

only found at site P according to ANOVA (F9,26 = 2.8,

P = 0.019). However, the differences were not large

enough for Tukey’s test to separate between clones.

There were significant differences between clones in

plant shoot dry weight in both the ‘Weeded’ and the

‘Unweeded’ treatments at all three sites when treat-

ments and sites were analysed separately (Table 3).

The clone Linnea ranked highest in mean plant shoot

dry weight at all three sites in ‘Weeded’ and at two of

the sites in ‘Unweeded’.



Plant mortality

After the growth season, the ‘Unweeded’ treatment had

significantly higher plant mortality than ‘Weeded’ at all

three sites (Fig. 1). The plant mortality differed between

sites in ‘Unweeded’ (F2,9 = 23.1, P < 0.001), with site S

having significantly lower plant mortality (2.7%) than

the other two sites (site P: 24.6%, site J: 37.4%). There

were no significant differences in plant mortality

between clones within site J (F9,27 = 1.0, P = 0.477) or

site S (F11,33 = 1.0, P = 0.449) in the ‘Unweeded’ treat-

ment. However, at site P, clonal differences were found

(F9,27 = 2.6, P = 0.025), with Lisa having significantly

higher plant mortality than Gudrun and Karin. At this

site, the lower leaves of willow plants became yellow and

necrotic, regardless of treatment and clone. No clonal

differences were found in plant mortality in the

‘Weeded’ treatment at either site.

In the growth season assessment, the percentage of

dead plants without shoots were 0.4, 8.6 and 0.2

(v2 = 17.6, d.f. = 2, P < 0.001), and the percentage of

dead plants with dead shoots were 37.0, 5.0 and 1.6

(v2 = 18.2, d.f. = 2, P < 0.001) in the ‘Unweeded’

treatment at sites J, P and S respectively. There were

no significant differences between clones in the percent-

age of dead plants without shoots at site P. Due to

few dead plants without shoots at sites J and S, no sta-

tistical analysis was performed for this variable at

these sites. There was a significant difference in per-

centage of dead plants with dead shoots between the

clones at site P (F9,27 = 3.0, P = 0.014). The post hoc

test showed that Lisa had a significantly higher mean

than Gudrun, Stina, Sven and Tordis. No significant

differences between clones in percentage of dead plants

with dead shoots were found at the other sites.

Browsed living plants were only found at site P where

6.9 % of the plants were damaged by herbivorous

mammals. No clonal differences in browsed plants

were found.

Weeds

Altogether, more than 20 different weed species were

identified during the weed assessments (Table 4). The

sites differed in weed flora, but Chenopodium album L.

Table 3 Plant shoot dry weight in 12 willow clones grown with and without weed control and growth reduction after one growth sea-

son, at three sites. Within column, least square means followed by different letters are significantly different according to Tukey’s test

Clone

Site J Site P Site S

Plant shoot dry weight

(g)Growth

reduction

(%)


(g)Growth

reduction

(%)


(g)Growth

reduction

(%)Weeded Unweeded Weeded Unweeded Weeded Unweeded

Gudrun 63.1 b 2.5 b 95.8 85.7 de 3.3 b 95.1 54.4 d 3.4 b 92.8

Karin 90.4 b 3.1 b 96.6 61.1 de 3.9 b 93.1 104.9 c 4.4 b 95.5

Klara 115.1 ab 4.3 ab 95.9 96.2 cd 8.0 a 90.8 139.1 bc 8.6 ab 93.4

Linnea 161.0 a 5.8 a 96.3 199.3 a 9.2 a 95.5 191.1 a 12.0 a 93.5

Lisa 81.5 b 2.8 b 96.1 142.0 b 8.1 ab 95.6 143.3 abc 12.2 a 91.8

Stina 73.3 b 4.2 ab 94.3 85.9 de 6.1 ab 92.7 149.6 abc 11.9 a 92.3

Sven 98.6 b 2.5 b 97.2 138.4 b 5.7 ab 95.4 147.8 abc 8.0 ab 94.0

SW Inger 93.9 b 2.8 b 96.9 146.9 b 5.6 ab 96.2 165.2 ab 8.9 ab 94.6

Tora 83.7 b 3.9 ab 95.4 52.9 e 5.2 ab 90.6 123.6 bc 7.7 ab 94.2

Tordis 109.5 ab 3.9 ab 96.3 132.6 bc 7.4 ab 94.7 163.6 ab 12.9 a 91.8

98* 144.7 abc 8.6 ab 94.3

58* 159.6 ab 11.6 a 92.5

P-value <0.001 <0.001 0.196 <0.001 0.001 0.019† <0.001 <0.001 0.082

F-value 4.7 5.7 1.5 36.1 4.7 2.8 12.7 6.7 1.9

d.f. 9, 27 9, 26 9, 27 9, 27 9, 25 9, 26 11, 33 11, 32 11, 33

*Breeding clones. Only planted at site S.

†No significant differences between clones according to Tukey’s test.

10

15

20

25

30

35

40

45

Plan

t mor

talit

y (%

)

0

5

Site J

***

Site P

Unweeded

***

Site S

Weeded

**

Fig. 1 Mean plant mortality per treatment and site. Data are

from 40 plots (48 plants plot�1) at sites J and P, and from 48

plots (24 plants plot�1) at site S. **P < 0.01 and ***P < 0.001

for differences between treatments according to the Wilcoxon

signed-rank test.



was present in more than 50% of the samples, at all

three sites. Other common species were Fallopia convol-

vulus (L.) A. L€ove and Tripleurospermum inodorum

(L.) Sch. Bip. at site J, Galium aparine L. and Stellaria

media (L.) Vill. at site P, and F. convolvulus and G.

aparine at site S.

Willow shoot dry weight per plot was significantly

negatively correlated with weed aboveground biomass,

as measured by the Cropscan instrument, at all three

sites when all clones were analysed together (Fig. 2).

All clones responded with lower shoot dry weight per

plot as the weed aboveground biomass increased

(Table 5). However, the correlation was only signifi-

cant for seven of the 10 clones tested. Mean RVI was

significantly higher at site P (Table 6) than at the other

sites (F2,9 = 12.0, P = 0.003).

Soil moisture

Soil moisture was significantly higher (F1,9 = 624.1,

P < 0.001) in the ‘Weeded’ than in the ‘Unweeded’

treatment when all sites were analysed together. There

were also significant differences in soil moisture

between all three sites in the ‘Unweeded’ treatment,

and site J was significantly dryer than the other two

sites in the ‘Weeded’ treatment (Table 6).

Table 4 Incidence of the five most common weed species in

0.125 m2 samples at the three sites. Samples were from three ran-

domly placed rectangles between willow double rows in each

clone and replicate in the ‘Unweeded’ plots. The weed assess-

ments were made during the period 9–13 July 2010. Species

names according to Artdatabanken (2012)

Species

Percentage of frames

Site J Site P Site S

Capsella bursa-pastoris

(L.) Medik.

0.0 25.8 0.0

Chenopodium album L. 97.5 75.8 58.3

Cirsium arvense (L.) Scop. 2.5 1.7 2.8

Euphorbia helioscopia L. 0.0 0.0 1.4

Fallopia convolvulus

(L.) A. L€ove

72.5 3.3 95.8

Galium aparine L. 0.8 60.0 85.4

Jacobaea vulgaris

Gaertn.

2.5 0.0 0.0

Lamium purpureum L. 17.5 50.0 3.5

Matricaria chamomilla L. 0.0 0.0 24.3

Myosotis arvensis

(L.) Hill

0.0 39.2 0.0

Persicaria lapathifolia

(L.) Delarbre

0.0 45.0 4.9

Poaceae* 0.8 0.0 0.0

Polygonum aviculare L. 44.2 0.0 25.0

Rumex crispus L. 0.0 0.8 0.0

Senecio vulgaris L. 4.2 0.0 0.0

Silene noctiflora L. 16.7 22.5 0.0

Solanum nigrum L. 2.5 30.8 6.9

Sonchus spp.† 1.7 25.8 0.7

Stellaria media (L.) Vill. 1.7 84.2 0.7

Thlaspi arvense L. 3.3 0.0 0.0

Trifolium repens L. 0.0 0.0 0.7

Tripleurospermum

inodorum (L.) Sch. Bip.

63.3 34.2 0.7

Urtica urens L. 0.0 10.8 0.0

Veronica spp.‡ 30.0 0.0 2.1

Viola arvensis Murray 4.2 1.7 28.5

*All grasses recorded collectively.

†Sonchus arvensis L. and Sonchus asper (L.) Hill.

‡Veronica arvensis L., Veronica agrestis L. and Veronica persica

Poir.

0

100

200

300

Shoo

t dry

wei

ght p

er p

lot (

g)

Relative vegetation index (RVI)

r = –0.47 **

0

100

200

300

400

500

600

700

0 2 4 6 8

0 2 4 6 8 10 12 14 16 18

Shoo

t dry

wei

ght p

er p

lot (

g)Relative vegetation index (RVI)

r = –0.56 ***

0

100

200

300

400

500

0 1 2 3 4 5

Shoo

t dry

wei

ght p

er p

lot (

g)

Relative vegetation index (RVI)

r = –0.31 *

Site P

Site J

Site S

Fig. 2 Total willow shoot dry weight per plot in relation to RVI

as an estimation of weed aboveground biomass. Measurements of

percentage of reflectance at 813 and 661 nm were taken at sites J,

P and S on 22, 17 and 21 June 2010 respectively. RVI was calcu-

lated as the 813/661 ratio. *P < 0.05, **P < 0.01,

***P < 0.001.



Discussion

There is currently no efficient way to replace dead

willow plants in an SRC plantation (Verwijst, 1996).

Thus, first-year plant mortality due to weeds will

probably affect the biomass production of a willow

plantation in the long term more than merely the

growth reduction in the establishment phase, even

though plant shoot weight reduction can be as high as

c. 95% (Clay & Dixon, 1995; this study). Surviving

willow plants may use the gaps of dead neighbour

plants, but weeds will also thrive there and compete

further with the willow plants. No previous study

investigating the effect of weeds on willow growth

(Labrecque et al., 1994; Clay & Dixon, 1995; Volk,

2002) has recorded increased plant mortality due to

weed competition during the establishment year. How-

ever, Volk (2002) found that treatments with weed

control had significantly lower plant mortality at the

end of the first 4-year rotation period than treatments

with no weed control. Similar to our study, Otto et al.

(2010) showed weed-related plant mortality in SRC-

managed poplar during the first growing season. The

studies on willow cited above included different clones

to those used in the present study and therefore, clonal

differences in weed-related mortality rates cannot be

ruled out. However, we found clonal differences in

plant mortality in just one of three sites. Likewise, in

this same site, only small differences in weed-related

growth reduction between clones were found, despite

the fact that the plant material represented a consider-

able genetic variation.

In other crops, there is genetic variation in ability

to compete with weeds. For example, Murphy et al.

(2008) evaluated 63 wheat cultivars and found that the

best suppressed weed weight by more than 500% com-

pared with less competitive cultivars. Studies showing

differences in weed competition abilities predominantly

involved cereal crops with a plant density of 150–450per m2 (Korres & Froud-Williams, 2002; Murphy

et al., 2008; Bertholdsson, 2010), but cultivar differ-

ences have also been found in soyabean grown at ~70plants m�2 (Vollmann et al., 2010) and maize grown at

5 plants m�2 (Roggenkamp et al., 2000). As the ability

of a crop to compete well with weeds increases with

increasing plant density (O’Donovan et al., 1999; Wei-

ner et al., 2001), willow managed as SRC suffers from

the low initial biomass produced by the 1–2 cuttings

m�2. The plant density of emerging weeds in a new

willow plantation is commonly 100-fold higher than

the density of the willow cuttings (J. Albertsson,

unpublished). Thus, we expect the weeds to affect the

willow more than the willow affects the weeds. In our

study, the amount of weeds significantly explained the

variation in first-year plant growth, similarly as

reported by Sage (1999) but for grow-back after

first-year coppice. As in our study, Sage (1999) studied

different clones, but did not perform any clonewise

analyses. All the clones in our study showed the same

tendency of decreased shoot biomass growth as the

weed aboveground biomass increased. However, the

correlation coefficient varied between clones, and this

may indicate that the clones responded somewhat

differentially to weed pressure.

Whereas willow plant growth reduction due to weeds

varied only marginally between our study sites

(93.4–96.1%), plant mortality was nearly 14-fold higher

at the site with highest numerical mortality (site J) than

at the site with lowest (site S). This large variation in

plant mortality cannot be explained by differences in

weather conditions, as the sites were <1.5 km apart.

Also, all sites were planted within 8 days. The sites

differed in amount and species composition of weeds,

soil properties and herbivore damage to shoots. Site S,

Table 5 Pearson partial correlation coefficient per clone, with

adjustment for the site effect, between willow shoot dry weight

per plot and weed aboveground biomass (RVI), measured by

Cropscan, at all sites (n = 12)

Clone r P-value

Gudrun �0.69 *

Karin �0.60 NS

Klara �0.66 *

Linnea �0.80 **

Lisa �0.27 NS

Stina �0.77 **

Sven �0.80 **

SW Inger �0.63 *

Tora �0.31 NS

Tordis �0.87 ***

NS, Not significant.

*P < 0.05, **P < 0.01, ***P < 0.001.

Table 6 Relative vegetation index (RVI) measured by Cropscan

in the ‘Unweeded’ treatment and volumetric water content

(VWC) of the soil in the ‘Weeded’ and ‘Unweeded’ treatments at

the three sites. VWC measurements were taken on 29 July 2010

and RVI measurements at sites J, P and S on 22, 17 and 21 June

2010 respectively. Data presented as mean � SE. Means in each

column followed by different letters are significantly different

according to Tukey’s test

Site RVI � SE

VWC (%) � SE

Unweeded Weeded

J 3.9 � 0.3 b 3.8 � 0.4 c 16.7 � 0.8 b

P 9.6 � 0.7 a 9.3 � 0.5 a 23.2 � 1.0 a

S 2.1 � 0.2 b 6.4 � 0.5 b 21.9 � 0.8 a



the site with the lowest plant mortality (2.7%), also had

the lowest numerical weed aboveground biomass. How-

ever, weed aboveground biomass did not explain the dif-

ferences between sites J and P in willow mortality, as site

J had lower weed aboveground biomass than site P, but

site J still had higher numerical plant mortality (37.4%

vs. 24.6% at site P). Site J had a more sandy soil than

the other sites, which may have lowered its water-hold-

ing capacity, and hence the availability of water for the

cuttings. Moisture measurements taken at the end of the

dry period support this, as site J had the lowest VWC

and also the greatest reduction in VWC due to weeds.

Also, this site was not ploughed prior to planting, possi-

bly limiting the plant root growth and limiting water

uptake. At site J, almost all dead plants had developed

shoots before the dry period, and surviving plants grew

well again after precipitation in late July. Hence, the

high plant mortality in the ‘Unweeded’ treatment at this

site is most likely a matter of water shortage due to low

precipitation, low water-holding capacity of the soil and

competition for water with weeds. The reason for rela-

tively high plant mortality at site P in the ‘Unweeded’

treatment is more complex. Similar to site J, lack of

water was probably responsible for the majority of the

dead plants with developed shoots at the assessment in

August. Furthermore, as 6.9% of the living plants were

damaged by browsing at this time, it is possible that her-

bivorous mammals were responsible for removal of

shoots for the dead plants without any shoots. It is also

likely that several of the browsed plants died later in the

season, as the already weak plants were then much

shaded by the tall weeds. Hence, herbivorous mammals

favoured by the predator-protecting weed cover and site

P conditions, such as soil texture suitable for burrowing,

might explain much of the plant mortality in the

‘Unweeded’ treatment at site P. One more factor

contributing to plant mortality at this site might be the

high soil pH, possibly related to the specific symptoms of

yellow and necrotic lower leaves found just here. Nota-

bly, site P was the only site where we found clonal differ-

ences in plant mortality and growth reduction in the

‘Unweeded’ treatment. Several of the weed species at our

sites have been found to be allelopathic (Carlson et al.,

2008). However, we found no obvious relationship

between occurrence of these species and plant mortality.

The growth reduction and part of the plant mortal-

ity we observed in willow might have been lower if the

precipitation would have been higher during June and

July 2010. However, both weeds and willow plants

grow better if sufficient water is available. Hence, in

case of drought, drip irrigation, with one dripper per

willow plant, might be a possible measure to favour

the willow plants more than the weeds. Nevertheless,

even though drip irrigation would be applied, some

weed control is probably still needed to ensure a

proper establishment, for example to reduce competi-

tion for nutrients and light and to counteract browsing

mammals. There is also the question of whether the

willow plant density should be increased or whether

plants could be distributed differently in order for the

willow to better compete with weeds during crop estab-

lishment (cf. Weiner et al., 2001).

As stated above, only small clonal differences were

found in growth reduction due to weeds. However, sig-

nificant differences between clones in plant shoot

weight were found at all sites, in both ‘Unweeded’ and

‘Weeded’ treatments. For example, the clone Linnea

had significantly larger shoot biomass than Karin and

Gudrun regardless of site and treatment. Gudrun is

considered to be a slow starter (Weih & Nordh, 2002),

and Karin has recently been withdrawn from the mar-

ket due to poor growth (Anon, 2012). Linnea is a new

commercial clone and extensive growth data are there-

fore lacking. However, this study might suggest that

Linnea establishes well even when part of the growth

season is hot and dry.

Conclusions

The high plant mortality at two of the sites and the

generally large growth reduction found in this study

clearly shows that weeds have to be controlled in SRC

willow during the first growing season, regardless of

the clone used. As stated above, drought was probably

an important factor for plant mortality. However, after

the dry period, weed competition for light and nutri-

ents probably contributed relatively more to the

growth reduction. The small clonal differences in

growth reduction due to weed competition observed

here may indicate that breeding for weed competitive

ability is not a feasible way to improve the environ-

mental profile of willow SRC. However, further studies

with other willow species/clones should be conducted

to clarify this. These studies should include clones

known to differ in water and nutrient uptake efficiency

(cf. Weih & Nordh, 2002), plant growth rhythm, can-

opy characteristics, allelopathic abilities and resistance

to herbivores. Also, more long-term effects of weed

competition on various willow clones are needed

before any final conclusions can be drawn on their

weed competitive abilities.

Acknowledgements

The authors thank the Swedish Research Council for

Environment, Agricultural Sciences and Spatial Plan-

ning for funding this research, Fatih Mohammad for

technical assistance and Jan-Eric Englund for valuable



advice on statistical analyses. We also thank Lantm€an-

nen SW Seed AB for providing the two breeding clones.

References

�AHMAN I (2001) Hantering av skadeg€orare i energiskog av

Salix. Sveriges Uts€adesf€orenings Tidskrift 111, 98–103.�AHMAN I & LARSSON S (1999) Resistensf€or€adling i Salix f€or

energiproduktion. V€axtskyddsnotiser 2, 17–19.ANON (2012) Willow Varietal Identification Guide. Available

at: http://www.teagasc.ie/publications/2012/1494/Willow_

Identification_Guide_2012.pdf (last accessed 16 September

2013).

ARTDATABANKEN (2012) Swedish Taxonomic Database.

Available at: https://www.dyntaxa.se/ (last accessed 5

November 2012).

BERTHOLDSSON NO (1999) Characterization of malting barley

cultivars with more or less stable grain protein content

under varying environmental conditions. European Journal

of Agronomy 10, 1–8.BERTHOLDSSON NO (2010) Breeding spring wheat for

improved allelopathic potential. Weed Research 50, 49–57.B€ORJESSON P & TUFVESSON LM (2011) Agricultural crop-

based biofuels – resource efficiency and environmental

performance including direct land use changes. Journal of

Cleaner Production 19, 108–120.CARLSON ML, LAPINA IV, SHEPHARD M et al. (2008)

Invasiveness Ranking System for Non-Native Plants of

Alaska. US Forest Service, US Department of Agriculture,

Alaska, USA.

CLAY D & DIXON F (1995) Vegetation management in the

establishment of poplar and willow short-rotation coppice.

In: Proceedings 1995 Brighton Crop Protection Conference

(ed. FM MCKIM), 979–984. British Crop Protection Council,

Farnham, UK.

CLAY D & DIXON F (1997) Effect of ground-cover vegetation

on the growth of poplar and willow short-rotation coppice.

Aspects of Applied Biology 49, 53–60.H�AKANSSON S (2003) Weeds and Weed Management on Arable

Land: An Ecological Approach. CABI Publishing,

Cambridge, MA, USA.

HELBY P, ROSENQVIST H & ROOS A (2006) Retreat from Salix

—Swedish experience with energy crops in the 1990s.

Biomass and Bioenergy 30, 422–427.JORDBRUKSVERKET (2012) Handbok f€or Salixodlare. Available

at: http://www2.jordbruksverket.se/webdav/files/SJV/

trycksaker/Pdf_ovrigt/ovr250.pdf (last accessed 4

November 2012).

KARP A, HANLEY SJ, TRYBUSH SO et al. (2011) Genetic

improvement of willow for bioenergy and biofuels. Journal

of Integrative Plant Biology 53, 151–165.KORRES NE & FROUD-WILLIAMS RJ (2002) Effects of winter

wheat cultivars and seed rate on the biological

characteristics of naturally occurring weed flora. Weed

Research 42, 417–428.LABRECQUE M, TEODORESCU T, BABEUX P, COGLIASTRO A &

DAIGLE S (1994) Impact of herbaceous competition and

drainage conditions on the early productivity of willows

under short-rotation intensive culture. Canadian Journal of

Forest Research 24, 493–501.

LARSSON S (1998) Genetic improvement of willow for short-

rotation coppice. Biomass and Bioenergy 15, 23–26.MOLA-YUDEGO B & ARONSSON P (2008) Yield models for

commercial willow biomass plantations in Sweden.

Biomass and Bioenergy 32, 829–837.MURPHY K, DAWSON J & JONES S (2008) Relationship among

phenotypic growth traits, yield and weed suppression in

spring wheat landraces and modern cultivars. Field Crops

Research 105, 107–115.O’DONOVAN J, NEWMAN J, HARKER K, BLACKSHAW R &

MCANDREW D (1999) Effect of barley plant density on

wild oat interference, shoot biomass and seed yield under

zero tillage. Canadian Journal of Plant Science 79,

655–662.OTTO S, LODDO D & ZANIN G (2010) Weed-poplar

competition dynamics and yield loss in Italian short-

rotation forestry. Weed Research 50, 153–162.REGERINGSKANSLIET (2009) Sveriges nationella handlingsplan

f€or fr€amjande av f€ornybar energi enligt Direktiv 2009/28/

EG och Kommissionens beslut av den 30.6.2009.

Government Offices of Sweden. Available at: http://www.

regeringen.se/content/1/c6/14/90/23/968a6b5e.pdf (last

accessed 10 July 2012)

ROGGENKAMP GJ, MASON SC & MARTIN AR (2000) Velvetleaf

(Abutilon theophrasti) and green foxtail (Setaria viridis)

response to corn (Zea mays) hybrid. Weed Technology 14,

304–311.ROWE R, STREET N & TAYLOR G (2009) Identifying potential

environmental impacts of large-scale deployment of

dedicated bioenergy crops in the UK. Renewable and

Sustainable Energy Reviews 13, 271–290.SAGE R (1999) Weed competition in willow coppice crops:

the cause and extent of yield losses. Weed Research 39,

399–411.SAS (2012) SAS 9.3 Product Documentation. Available at:

http://support.sas.com/documentation/93/index.html (last

accessed 4 November 2012).

SMART LB & CAMERON KD (2008) Genetic improvement of

willow (Salix spp.) as a dedicated bioenergy crop. In:

Genetic Improvement of Bioenergy Crops (ed. W

VERMERRIS), 377–396. Springer, New York, USA.

SMHI (2012) Nederb€ord. Available at: http://www.smhi.se/

klimatdata/meteorologi/nederbord (last accessed 9 August

2013).

VERWIJST T (1996) Stool mortality and development of a

competitive hierarchy in a Salix viminalis coppice system.

Biomass and Bioenergy 10, 245–250.VERWIJST T, LUNDKVIST A, EDELFELDT S & ALBERTSSON J

(2013) Development of sustainable willow short rotation

forestry in northern europe. In: Biomass Now - Sustainable

Growth and Use (ed. MD MATOVIC), 479–502. InTech,Rijeka, Croatia. doi: 10.5772/55072.

VOLK T (2002) Alternative Methods of Site Preparations and

Coppice Management During the Establishment of Short-

Rotation Woody Crops. PhD thesis, State University of

New York, College of Environmental Science and

Forestry, Syracuse, USA.

VOLLMANN J, WAGENTRISTL H & HARTL W (2010) The effects

of simulated weed pressure on early maturity soybeans.

European Journal of Agronomy 32, 243–248.



WEIH M & NORDH NE (2002) Characterising willows for

biomass and phytoremediation: growth, nitrogen and

water use of 14 willow clones under different irrigation and

fertilisation regimes. Biomass and Bioenergy 23, 397–413.

WEINER J, GRIEPENTROG H-W & KRISTENSEN L (2001)

Suppression of weeds by spring wheat Triticum aestivum

increases with crop density and spatial uniformity. Journal

of Applied Ecology 38, 784–790.



Site-related set-back by weeds on the establishment of 12 biomass willow clones

Documents

Transcript of Site-related set-back by weeds on the establishment of 12 biomass willow clones