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Biomass production of four willow clones grown as shortrotation coppice on two soil types in Denmark

Lisbeth Sevel a,b,*, Thomas Nord-Larsen b, Karsten Raulund-Rasmussen b

aHedeDanmark A/S, 12 Klostermarken, DK-8800 Viborg, Denmarkb Forest and Landscape, University of Copenhagen, 23 Rolighedsvej, DK-1958 Frederiksberg C, Denmark

a r t i c l e i n f o

Article history:

Received 29 August 2011

Received in revised form

4 May 2012

Accepted 21 June 2012

Available online 11 August 2012

Keywords:

SRC

Salix

Yield

Soil type

Harvest

Non-destructive sampling

* Corresponding author. HedeDanmark A/S,E-mail address: lise@hededanmark.dk (L.

0961-9534/$ e see front matter ª 2012 Elsevhttp://dx.doi.org/10.1016/j.biombioe.2012.06.0

a b s t r a c t

Ambitious targets for reducing emissions of carbon dioxide have created a demand for

renewable sources of energy. Short rotation coppice (SRC) willow has the potential for

meeting part of this demand. In this study, an experiment including four commercial

clones of willow grown on two different soil types in northern Denmark is reported. Annual

biomass production was estimated after the first and second growing season in the first

rotation using a non-destructive method and total biomass production was measured by

harvesting of the willow after the second growing season. The non-destructive method

showed a large increase in annual biomass production from the first to the second growing

season. Based on the harvested willow, average annual biomass production of the four

clones ranged from 5.2 to 8.8 odt ha�1 yr�1 with a significant effect of both soil type and

clone. The interaction between clones and soil types was also significant, indicating that

different clones may be better suited for different soil types. On average, estimates of

annual biomass production obtained by non-destructive estimation exceeded those ob-

tained by destructive methods by 1.2 odt ha�1 yr�1. This bias indicates a need to revise

commonly used methods for assessment of biomass production in SRC willow.

ª 2012 Elsevier Ltd. All rights reserved.

1. Introduction In Denmark, short rotation coppice (SRC) willow has

Biomass such as straw, wood and waste is by far the largest

source of renewable energy in Europe, and is responsible for

about 58 percent (2008) of the total renewable energy

consumption within EU 27 [1]. In Denmark, 18% of the gross

energy consumption originates from renewable sources of

which 82% originates from biomass and waste [1]. As

a consequence of ambitious targets to reduce the carbon

dioxide emissions from combustion of fossil fuels in the EU by

20% [2] and in Denmark by 30% in 2020 [3], the demand for

renewable sources of energy has increased substantially and

this increase is likely to continue in the future. Much of the

additional demand is expected to be met by an increase in the

use of energy from biomass [3].

12 Klostermarken, DK-88Sevel).ier Ltd. All rights reserve30

recently gained considerable focus as an important source to

increase the national biomass production. A national subsidy

scheme for establishment of 30,000 ha of energy crops,

including SRC willow, was created during 2010e2012 [4].

SRC willow is so far of limited use in Denmark and expe-

riences and well proven recommendations are sparse. Based

on previous studies in other countries, several factors e.g. soil

properties such as nutrient and water availability [5e7],

climatic conditions [8,9], plant density [10e14], rotation length

[11,14,15], weed and pest control [16], and the choice of clone

[17e19] are all influencing biomass production.

Development of new high producing willow clones was

initiated in Sweden in 1987 by Svalof-Weibull AB [20]. The

main purpose of the breeding program was to develop high

00 Viborg, Denmark. Tel.: þ45 21353758; fax: þ45 87281259.

d.

b i om a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 6 6 4e6 7 2 665

yielding clones resistant towards pests, frost, and diseases,

and with morphology suitable for mechanical harvesting.

From 1996 to 2002 several new clones were developed in

cooperation between Svalof-Weibull and Long-Aston research

in UK. Overall these breeding programs have increased the

biomass yield substantially since the 1980es [21e23].

Several studies have investigated biomass production of

SRC willow clones developed in the above mentioned

breeding programmes. The biomass production has been

shown to vary greatly with reported yields between 5 and 12

oven dried tonnes per ha per year (odt ha�1 yr�1) with

extremes between 2 and 18 odt ha�1 yr�1 [18,19,22,24e26].

Much of this variation is suggested to be related to effects of

clones and site properties [20,25]. Biomass production in

these studies is either measured using a destructive or a non-

destructive method. The destructive method includes

harvest of the entire plot or of subplots. This is contrary to

the non-destructive method in which a functional relation-

ship between the diameter of a shoot and its biomass content

is used to estimate the total biomass production based on

diameter measurements, either of all shoots in the plot or

a sample thereof.

Despite the apparent differences in biomass productivity

for different clones and sites, only little effort has been made

to assess the productivity of different willow clones in

Denmark and their interaction with different soil types. To

increase knowledge of SRCwillow production in Denmark and

on comparable sites elsewhere, we selected four of the

currently most used and promising commercial Swedish

willow clones and estimated their biomass production in the

first rotation on two contrasting soil types, a well drained

sandy soil and an artificially drained organic soil, respectively.

We hypothesized that biomass production would differ

among the four different clones and that the clones would

respond differently on the two soil types. Annual biomass

production was estimated by a non-destructive method in

year one and two and total biomass production wasmeasured

using a destructive method in year two.

Fig. 1 e Schematic overview of the experimental design. Blocks

(A, B and C), plots representing individual clones are separated

lines, only the four central double rows, where data were collec

shown as boxes. The order of the clones is shown for the sand

2. Materials and methods

2.1. Sites and experimental design

The experiment was established in May 2008 on the

commercial SRC willow farm, Ny Vraa Bioenergy in the

Northern part of Denmark (EUREF89, UTM zone 32 N, N:

556500, E: 6337744). Annual mean temperature is 7.5 �C and

annual precipitation is 689 mm of which 306 mm falls during

the growing season (MayeSeptember) [27].

Four clones were planted as 20 cm cuttings by a planting

machine (Egedal Energy Planter) in May 2008 on two sites

about 1 km apart but with different soil conditions. On each

site, two soil pits were dug and the soil profiles were described

[28]. The first site was awell drained, nutrient poor, sandy soil.

The A-horizon was 24 cm deep and the majority of the fine

roots were found here. The soil is classified as an Udepts [28],

due to an umbric epipedon and a cambic horizon. Prior to

establishment of the willow, the site laid fallow for one year

and before that the site had been grownwith rape. The second

site was a poorly drained organic soil with parent material of

very fine sand. A lithologic shift in 60 cm soil depth from sand

to clay resulted in poorly drained conditions. The ground-

water table was situated in 110 cm soil depth and artificial

drain tubes were found in 120 cm soil depth. The soil had

a histic epipedon and was classified as a Humaquepts [28].

Before establishment of thewillow, this site laid fallow for five

years. In the autumn before establishment both sites were

treated with an herbicide (Round-Up (360 g active ingredient

L�1), dose: 2 L ha�1). Before planting the site was ploughed,

harrowed and levelled out. The four clones planted were the

Svalof-Weibull clones Inger (Salix triandra � S. viminalis,

EU11635), Sven (S. viminalis � (S. schwerinii � S. viminalis),

EU5285), Tora (S. schwerinii � S. viminalis, EU0627) and Tordis

((S. schwerinii � S. viminalis) � S. viminalis, EU9288). Each site

was divided into three blocks (A, B, C), each consisting of 32

double rows of willow (Fig. 1). The distance between double

are separated by thick full lines and represented letters

by thick dotted lines and rows are indicated by thin dotted

ted are shown. Subplots for non-destructive sampling are

y soil; the order was different on the organic soil.

b i om a s s an d b i o e n e r g y 4 6 ( 2 0 1 2 ) 6 6 4e6 7 2666

rows was 1.5 m and the distance between single rows in

a double row was 0.75 m. Cuttings were planted 0.70 m apart

resulting in approximately 12,000 cuttings per hectare. Each

block was divided into four plots, each consisting of eight

double rows planted with one clone. To avoid edge effects,

data was collected only within the 4 central double rows of

each plot. The rows were 80 m long on the sandy soil and

106 m long on the organic soil resulting in a plot area (four

double rows) of 720 and 954 m2 on the sandy and organic soil,

respectively. Within each plot four subplots were established

for the non-destructive sampling.

In June after planting, both sites were treated with

a herbicide (Stomp, 4 L ha�1). In February 2009, the willowwas

cut back and in May 2009 both sites were fertilized with 120 kg

nitrogen ha�1 (NPK, 21-3-10). During summer 2009 weed

control was done by a line cultivator. Fertilizer was not added

in 2010. All plots were harvested in March 2011.

2.2. Biomass production estimates

2.2.1. Non-destructive methodNon-destructive sampling was conducted in March 2010 of

one year old shoots (representing the 2009 growing season,

root age two years), and in 2011 immediately before harvest-

ing of two year old shoots (representing the 2010 growing

season, root age three years).

The four subplots for non-destructive sampling were

located on an aerial photograph to ensure representation of

the entire plot. In the field subplots were located based on the

sketch made on the aerial photo without consideration of

plant growth or mortality and the coordinates were recorded

by a GPS. Each subplot consisted of ten willow plants within

a single row. The row length (m) covered by the ten plants and

the distances between the rows (both the distance between

single rows within a double row and the distance between

Table 1 e Data from the non-destructive sampling carried out ais the number of diametermeasurements conducted on the 12 sdiameter measured at 90 cm above ground. Standard deviatio

Clone Site N Average D90 (mm) Minim

2010 2011 2010 2011 2010

Non-destructive sampling (total from 12 subplots)

Inger Sandy 563 466 11.5 (5.0) 17.3 (7.6) 3

Organic 404 276 13.9 (6.5) 27.2 (9.5) 3

Sven Sandy 697 556 11.3 (4.7) 17.2 (7.0) 3

Organic 340 288 12.4 (4.3) 21.1 (6.9) 3

Tora Sandy 528 406 11.7 (5.6) 19.4 (8.4) 3

Organic 377 318 12.5 (5.3) 23.6 (9.8) 3

Tordis Sandy 571 476 12.1 (5.1) 18.1 (7.3) 3

Organic 304 301 13.3 (4.9) 21.7 (8.5) 3

Sample shoots for parameter estimation

Inger Sandy 15 15 12.8 (6.5) 20.9 (9.2) 4

Organic 15 15 13.5 (7.1) 24.5 (13.0) 5

Sven Sandy 15 15 14.3 (5.2) 16.8 (9.1) 3

Organic 15 15 15.2 (5.9) 20.9 (12.1) 6

Tora Sandy 15 15 13.4 (7.0) 18.9 (11.3) 3

Organic 15 15 13.6 (6.6) 22.6 (11.2) 4

Tordis Sandy 15 15 12.3 (6.4) 17.4 (9.1) 3

Organic 15 15 14.0 (6.5) 23.0 (11.6) 5

double rows) were measured to estimate the area of the

subplot. As a consequence of varying plant and row distances,

the subplot areas varied slightly (CV ¼ 12.6%). As suggested by

Nordh & Verwijst [29], shoot diameters 90 cm above ground

(D90) were measured on every shoot of the ten plants by

a digital calliper (Masser Racal 500, Masser Calliper, Savcor

Forest).

For each site, clone, and sampling occasion (year 2010 and

2011), 15 shoots were harvested outside the subplots for

establishment of the functional relationship between shoot

diameter and biomass (Table 1). The shoots were cut 10 cm

above ground, which resembles the approximate harvesting

height of the one-row JF willow harvester used for harvesting

of the experiment. The shoots were weighed in the field on

a Salter spring balance (�50 g) when >1000 g and on a Super

Samson Salter spring balance (�5 g) when <1000 g. If D90 was

�15 mm, the whole shoot was cut into pieces and dried until

constant weight at 105 �C. If D90 was >15 mm a 40 cm piece

was sampled around themass equilibrium point of each shoot

in accordance with Telenius [30]. The sample was weighed

(Kern FOB Bench scale (�1 g)) and dried at 105 �C until constant

weight. Using the estimated moisture content and the

measured fresh weight of each shoot, dry shoot biomass was

calculated. On a subsample of shoots, bothmethods (drying of

whole shoots and drying of a 40 cm subsample) for obtaining

dry biomass were applied in order to test the 40 cm sub-

sampling method.

A preliminary analysis showed that the diameter distri-

bution of shoots measured in subplots differed significantly

between the two sites, the four clones, and the two sampling

occasions. However, within each of these groups only minor

differences in the diameter distribution of the shoots

measured in subplots and the shoots harvested for estab-

lishing the relationship between shoot diameter and biomass

were observed (KolmogoroveSmirnov GoF test) (SAS 9.2). This

fter the growing season 2009 (in 2010) and 2010 (in 2011). Nubplots established for each site and clone.D90 is the shootn is shown in brackets.

um D90 (mm) Maximum D90 (mm) Shoots per plant

2011 2010 2011 2010 2011

6 23 35 4.7 3.9

8 30 47 4.0 2.8

6 23 33 5.8 4.6

6 23 37 3.4 2.9

6 26 39 4.4 3.4

6 23 43 3.8 3.2

6 22 33 4.8 4.0

6 27 41 3.0 3.0

8 23 34

6 26 45

6 20 30

7 22 38

6 24 35

7 26 39

6 22 32

6 25 39

b i om a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 6 6 4e6 7 2 667

analysis indicated that individualmodels should be developed

for each site, clone, and sampling occasion and that the har-

vested shoots used for parameter estimation in equation (1)

were representative for the shoots in the subplots. The rela-

tionship between individual shoot biomass and D90 was found

to be well described by a power function [31,32]:

bmiclm ¼ bclmDackm90;iclm; (1)

where bmiclm is the biomass (kg dry weight) of the ith shoot, of

the cth clone on the lth site, at themth sampling occasion, and

a and b are parameters to be estimated. Estimates of a and

bwere obtained using nonlinear regression (PROCNLIN of SAS

9.2) [33] and are shown in Table 2. Using the parameter esti-

mates for each clone, site and sampling occasion, dry matter

was estimated for each shoot of the ten plants within each

subplot. Biomass per hectare was estimated by summation of

individual shoot biomass and dividing by the area covered by

the ten plants. Biomass per hectare for the entire plot was

estimated as the average biomass per hectare of the four

subplots.

2.2.2. Destructive methodIn April 2011 the experiment was harvested with a one-row JF

willow harvester (JF Z192). The stump height was approxi-

mately 10 cm above ground.Willow chips from each plot were

blown into separate containers with known weight and the

total weight of container and chips was measured on

a weighbridge with a precision of �10 kg. The amount of

willow chips was between 1400 and 4100 kg fresh weight per

plot. A representative sample of willow chips of approxi-

mately 1 kg was sampled from each container. The sample

was weighed in the field (Kern FOB Bench scale; �1 g) and

subsequently dried at 105 �C until constant weight in the

laboratory. Based on wet and dry weights, the moisture

content was estimated and used for estimation of the total dry

weight of harvested biomass.

2.3. Data analysis

2.3.1. Biomass productionAnnual biomass production of the 2009 growing season and

the 2010 growing season (non-destructive method) and the

average annual production of the two growing seasons

Table 2e Parameter estimates and coefficient of determinationbiomass (equation (1)).

Clone Site Growing season 2009

a b

Inger Sand 1.7722E-04 2.3525

Organic 3.0714E-04 2.1501

Sven Sand 2.9151E-04 2.2124

Organic 1.3337E-04 2.4675

Tora Sand 2.6088E-04 2.2480

Organic 1.9624E-04 2.2930

Tordis Sand 1.0118E-04 2.5918

Organic 5.4376E-05 2.7664

(destructive method) was analysed using the generalized

linear model:

BMjkl ¼ aþ b½site� þ g½block� þ k½clone� þ 4½site� � ½clone� þ εjkl;

(2)

where BMjkl is the annual biomass production (odt ha�1 yr�1),

estimated using either the destructive or the non-destructive

method of the jth plot, kth block and lth site and εjlk w N(0,

s2) is the random error. All analyses were carried out using the

GLM procedure by use of SAS 9.2 [33].

2.3.2. Destructive versus non-destructive methodThe destructive and the non-destructive methods were

compared by regressing of predicted values from the two

methods. The regression was tested for zero intercept and

unit slope using ordinary t-tests. First this test was conducted

for the individual shoots harvested in relation to the non-

destructive method and thereafter for the aggregated plot-

level biomass production estimated by the destructive and

non-destructive methods.

3. Results

3.1. Non-destructive method

In the 2009 growing season, the average biomass production

of the one-year old shoots of all four clones was 4.6 and

3.5 odt ha�1 yr�1 on the sandy and organic soil, respectively

(Fig. 2). The highest production was observed for the clone

Sven on the sandy soil (5.0 odt ha�1 yr�1) and the lowest was

observed for Tordis on the organic soil (3.1 odt ha�1 yr�1). The

difference in average production between the two sites was

highly significant (P< 0.001). Furthermore, average production

of the different clones varied between sites i.e. the interaction

between clones and site was significant (P ¼ 0.003). No

significant effect of the blocks was observed (P ¼ 0.512).

Contrary to the 2009 growing season, average biomass

production in the 2010 growing season was larger on the

organic soil (13.9 odt ha�1 yr�1) than on the sandy soil

(10.4 odt ha�1 yr�1). The highest production of the two-year

old shoots was observed for Tora and Inger on the organic

soil (16.9 and 15.5 odt ha�1 yr�1, respectively) and the lowest

for Tordis and Sven on the sandy soil (9.4 and 10.3 odt ha�1 yr�1,

for the relation between individual shoot diameter (D90) and

Growing season 2010

R2 a b R2

0.988 9.5230E-04 1.9917 0.900

0.951 1.0096E-04 2.6115 0.991

0.982 1.9883E-04 2.4717 0.996

0.998 1.8882E-04 2.4799 0.997

0.994 2.2588E-04 2.4190 0.992

0.984 2.3463E-04 2.4017 0.992

0.998 2.4361E-04 2.3908 0.997

0.977 5.6902E-05 2.8156 0.997

Table 3 e Average biomass production for four clones(Inger, Sven, Tora, Tordis) and two sites (sandy, organic) forthe destructive harvest sampling in 2011. Standarddeviations of dry matter production are in parentheses.Mean values of a columnwith the same superscript letterare not significantly different.

Clone Sandy Organic Average

odt ha�1 yr�1

Inger 5.19a (0.61) 8.66b (0.33) 6.93ab (1.95)

Sven 5.77a (0.99) 6.14a (1.04) 5.95b (0.93)

Tora 6.49a (1.41) 8.77b (0.73) 7.63a (1.60)

Tordis 6.26a (0.62) 6.93a (0.19) 6.60ab (0.55)

Average 5.93a (0.97) 7.62b (1.30) 6.78 (1.42)Fig. 2 e Box plots (25 and 75th percentiles) of the annual

dry matter production of the non-destructive sampling for

2009 (year 1 left) and 2010 (year 2 right) for the sandy (S)

and organic (O) site.

b i om a s s an d b i o e n e r g y 4 6 ( 2 0 1 2 ) 6 6 4e6 7 2668

respectively). Significant differences in average production

were observed between the two sites (P ¼ 0.001), the different

clones (P < 0.001), and between different clones on different

sites (P ¼ 0.048). The interaction between sites and clones was

mainly attributed to Inger and to some extend to Tora that

grew faster on the organic soil. As in the 2009 growing season,

no effect of the different blocks on biomass production was

found (P ¼ 0.481).

The annual average biomass production of all clones

increased significantly (P< 0.001) from 4.1 to 12.0 odt ha�1 yr�1

from the 2009 to the 2010 growing season (Fig. 2). The average

annual production of both growing seasons was between 7.1

and 10.1 odt ha�1 yr�1 for the different clones and sites. The

average annual production varied between sites (P ¼ 0.009)

and between different clones (P ¼ 0.011), and the interaction

between site and clone was significant as well (P ¼ 0.047). We

recorded no plant mortality in any of the subplots from the

2009 growing season to the 2010 growing season.

3.2. Destructive method

Based on harvest of the experiment in 2011 of the two year old

shoots, average annual dry matter production (average of

three plots) ranged from5.2 (Inger on the sandy soil) to 8.8 (Tora

on the organic soil) odt ha�1 yr�1 and effects of sites, clones

and their interaction were all significant (P < 0.001, P ¼ 0.022

and P ¼ 0.017, respectively). Based on a subsequent analysis

conducted for the two sites separately, the interaction effect

seems to be related to the higher productivity of Inger and Tora

on the organic soil while no difference between clones were

found on the sandy soil (Table 3). As for the non-destructive

method no block effects were observed.

3.3. Destructive versus non-destructive method

Wegenerally found a very good correlation (R2¼ 0.99) between

the estimates of individual shoot biomass (equation (1)) and

actual measured shoot biomass (Fig. 3). When applying the

model for individual shoots to the subplots and subsequently

calculating total biomass production for the subplots and

entire plots, the biomass production estimated by the non-

destructive method was on average 1.2 odt ha�1 yr�1 larger

than the amount harvested and the difference was highly

significant (P < 0.0001). The differences between the two

methods were unrelated to predicted values, site, blocks or

clones (P > 0.05) and thus data from the non-destructive

analyses could be used in the analyses of biomass production.

4. Discussion

Based on the destructivemethod, average biomass production

over the two-year first rotation ranged from 5.2 to

8.8 odt ha�1 yr�1. We found a clear difference in biomass

production of the four clones and a generally higher produc-

tion on the organic soil than on the sandy soil. Further, the

interaction between clone and site was significant, indicating

that some clones may be better suited for specific sites. These

results support our overall hypothesis that the biomass

production of different SRC willow clones differs and is

influenced by soil properties.

Biomass production in our investigation is within the range

reported in comparable studies made in the 1990es. In

a Danish study of older willow clones intensively grown in

Denmark, annual production was found to range between 4.0

and 10.4 odt ha�1 yr�1 in the years 1989e1991 with an average

of 7.5 odt ha�1 yr�1 [34]. In another Danish study two clones

(L78183 (S. viminalis) and Bjørn (S. schwerinii� S. viminalis)) were

continuously measured from 1993 to 2006 [35,36]. Production

in the first rotation was 5.0 odt ha�1 yr�1 for L78183 and

8.3 odt ha�1 yr�1 for Bjørn [37].

In a study established in 1990 in Sweden, 12 Swedish clones

were tested on a clay soil. The average biomass production

ranged between 6.8 and 9.5 odt ha�1 yr�1 during the first three

rotations. Significant differences in production were observed

between clones but not between the three rotations. The

rotation length was four years and the production was esti-

mated by a harvest of experimental plots [12]. In UK large

variation among different clones was found in a study at four

different sites established in 1991 with clones both from the

Swedish and the UK programs. Estimated by harvesting of 10

subplots each consisting of 10 plants, the average biomass

production in the first rotation ranged between 6.5 and

Fig. 3 e Biomass from the destructive versus the non-destructive sampling of individual shoots (left) and for the entire plots

(right). Triangles are for the organic site and black circles are for the sandy site. Lines represent unit slope and zero intercept.

b i om a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 6 6 4e6 7 2 669

14.6 odt ha�1 yr�1 [19]. Based on a non-destructive method in

a study of a 49-site network of yield trials also in UK,

production of 16 clones grown on different soil types ranged

between 4.1 and 10.5 odt ha�1 yr�1 in the first rotation [25].

Overall biomass production in the first rotation has been re-

ported to range between 4.0 and 14.6 odt ha�1 yr�1.

Three of the clones (Inger, Tordis and Sven) used in our study

were first released commercially in 2000 and 2003 and

consequently not many studies have been carried out

including these clones. Contrary, Tora released in 1995 [18] is

included in several studies. In a trial of 22 sites (the different

sites were not described) in Sweden, the average biomass

production of Tora in the first rotation of two year old shoots

were 7.3 odt ha�1 yr�1 [18]. This is of the same level as we

found for Tora estimated by destructive harvest (6.5 and

8.8 odt ha�1 yr�1 on the sandy and organic soil respectively

and an average of 7.6 odt ha�1 yr�1). Biomass production for

Tora grown on two different fields at Long Ashton Research

Center, UK (two year old shoots, first rotation) was 11.7 and

15.2 odt ha�1 yr�1, respectively [19,24]. In both assessments

biomass production estimation was done by harvesting of

small subplots (26 and 6 plants).

Table 4 e Yield and recommendations of the four clonesfrom Lantmannen SW seed (former LantmannenAgroenergi) tested in this investigation (Personalcommunication Engqvist G, Lantmannen SW seed AB,2011 [39]).

Clone Rel.yielda

Yield Frostdamages

Soil type

Inger 141 High 5 Best on sandy soils

Sven 136 Average 5 More clayey soils

Tora 155 High 3 More clayey soils

Tordis 131 High 5 Best on sandy soils

a Relative yield compared to the reference clone L78183.

4.1. Difference between soil types

In our investigation, biomass production varied between the

sandy and the organic soil, the different clones, and the two

growing seasons. Biomass production across both years of the

rotation was highest on the organic soil, but in the first

growing season production was highest on the sandy soil. The

relatively slow growth on the organic soil during the first

growing season was probably a consequence of frost damage

caused by late night frost after the willow had started to grow

in the spring 2009 (visual observations).

The higher overall biomass production on the organic soil

indicates that this soil is better suited for willow growth

compared to the well drained sandy soil. The high ground-

water table and the clay layer below 50 cm are probably

responsible for a high amount of availablewater and a low risk

of drought during the summer. On the contrary, the willow on

the well drained sandy site has a higher risk of suffering from

summer drought. A higher nutrient content on the poorly

drained site due the clay layer may also influence the willow

growth positively.

Soil properties, especially plant available water, are often

reported to influence the biomass production of SRC willow

[5e7]. The mean yield from four harvests of the clone Tora

(shoot age¼ 2, root age¼ 3) at two different sites (Long Ashton

research centre and Loughgall) in 1996e1999 was 11.7 and

9.1 odt ha�1 yr�1 respectively [19]. The higher production at

long Ashtonwas explained bymore favourable soil conditions

(fertile brown argillic subsoil) compared to Loughgall (heavy

clay loam). The same trend is reported by Macalpine et al.

[24] with a higher production of Tora at Long Ashton

(15.2 odt ha�1 yr�1) than at Rothamsted (13.3 odt ha�1 yr�1)

having a soil with lower content of plant available water.

The growth of different clones varied between different

sites, indicating that different clones may be adapted to

different sites. According to Lantmannen SW seed (former

Lantmannen Agroenergi) Inger and Tordis are recommended

for sandy soils whereas Sven and Tora are best on more

clayey soils (Table 4). However, in this experiment, biomass

production of the four clones grown on the sandy soil was not

found to differ and hence no indication of a higher tolerance

to sandy soil conditions of Inger and Tordis is supported by our

b i om a s s an d b i o e n e r g y 4 6 ( 2 0 1 2 ) 6 6 4e6 7 2670

results. Neither did our results fully support the recommen-

dation from Lantmannen SW seed that Tora and Sven should

perform better on a clayey soil. We found that Tora performed

well on the organic soil with a clayey subsoil but that Sven only

performed at the same level as on the sandy soil. Surprisingly,

Inger performed very well on the organic soil and at the same

level as the best performing Tora.

This investigation only examined the first rotation. Several

studies have shown that biomass production is higher in the

second and the third rotation [19,25,37].

4.2. Destructive versus non-destructive method

In assessments of biomass production of SRC willow various

destructive and non-destructive methods are commonly

used. The destructive methods include weighing of harvested

willow shoots or chips and determination of the moisture

content of a subsample. Subsequently, biomass production

per hectare is estimated from the fresh weight, the moisture

content and the area of the sampled willow. Although

destructive method may have errors and uncertainties, the

method is complete and usually used as the reference.

However, this method is obviously not suitable when the

experiment is to be continued. The non-destructive methods

are usually based on the allometric relationships between the

diameter at some height above ground and the biomass of

a single shoot of a subsample of shoots. Several models

describing this relationship are used [25,32]. After calibration

of the shoot diameter-biomass models, the biomass produc-

tion on subplots of entire plots is estimated by measuring

shoot diameters and up scaling to plot size or per hectare by

use of the functional relationship. The choice of biomass

estimation method is often a compromise between several

factors such as aim of the study, time available, the size of

area available and desired precision [32].

The very high correlation between biomass and D90 of

single shoots found in the non-destructive method is in

agreement with findings of similar studies [31,32,38,39]. As

also suggested by Arevalo et al. [39], our results indicate that

differences in the allocation of biomass within shoots among

different clones and sites require that models for estimating

biomass from shoot diameter should be calibrated for each

site, clone and sampling occasion.

Estimates of biomass production based on non-destructive

method varied substantially between the four subplots within

the individual plots. The average within-plot coefficient of

variation was 22.5% and was larger on the sandy site. This

variation is probably related to the local variability in growth

and establishment success combined with the applied

sampling regime used in this study where subplots were laid

to describe this variability. The variability in estimating

biomass production by non-destructive samplingmethods are

rarely described in studies on biomass production in SRC

willow, but may cause a high degree of uncertainty which is

commonly not taken into account when evaluating biomass

production based on these methods.

When comparing biomass production assessed by the

destructive and non-destructive methods at plot level, we

observed a highly significant difference in the biomass

production (Fig. 3) (i.e. the intercept differed from zero

whereas the slope of the regression line of biomass estimated

by destructive versus non-destructive methods did not differ

from unity). This difference may mainly be related to the

sampling procedures. In this study, subplots were placed to

ensure representation of the plot variation based on a sketch

made on an aerial photo. However, this procedure might have

caused an unintended error as each subplot was not selected

randomly. To eliminate this error in the future a method for

random lay out of subplots should be developed e.g. by

random selection of rows and distances within each row.

Another factor which may explain a part of the observed

difference in production between the twomethods is possible

differences in stumpheight [31]. The stipulated harvest height

was 10 cm in both methods. The harvest in the non-

destructive method was done using a hand saw and a tape

measure and this method is considered very accurate. The

harvest height from the destructive harvest by the JF harvester

was inspected during the harvest process. The surface of the

field was plane resulting in only small differences in the

harvest height which we believe are not enough to cause the

observed difference in total biomass production. Also unin-

tentional loss in the harvest process from chips not entering

the container and shoots not cut off by the harvest machine

may add to some of the differences However, in this study,

care was taken in the harvest process to minimize the loss of

wood chips andwe conclude that this error cannot explain the

observed differences.

A comparison of destructive and non-destructive methods

of individual plants was conducted by Nordh & Verwijst [29].

They found a systematic overestimation by the non-

destructive method compared to the destructive method on

plant level, and suggested that this was caused by their

sampling procedure and their definition of living shoots.

However, they also concluded that the non-destructive

method is suitable for biomass estimation in 4-year old

commercial grown SRC willow. Only few studies have

compared commercial harvest of willow with biomass

production estimated using a non-destructive method.

Lowthe-Thomas et al. [40] compared harvested biomass

production with biomass production from a non-destructive

method comparable to our method in nine willow clones

planted by the layflat method in the first rotation. Contrary to

our findings, they found that the production based on the non-

destructive method was 2.5 times lower than the biomass

production observed for the destructive method. The reason

for this underestimation was not discussed.

Another study in UK done by the UK Forest Research [41]

has been working with the development of predictive empir-

ical non-destructive yield models for SRC willow and poplar.

Besides the allometric relationship between diameter and the

dry matter content they also included site specific variables

such as soil type and climatic factors. Only a limited amount

of model validation was possible but preliminary investiga-

tions indicated an overestimation by the non-destructive

models over the observed commercial harvest which corre-

sponds to the trend observed in this study. They concluded

that much further work on validation of the models under-

lying the non-destructive method is needed.

Based on our findings and the work from UK, care should

be taken when comparing biomass production estimated

b i om a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 6 6 4e6 7 2 671

from destructive and non-destructive methods as an over-

estimation using the non-destructive method might occur.

This should be kept in mind when evaluating biomass

production from different investigations where different

biomass productionmethods are used. It is obvious that there

is a need for further investigations of errors and uncertainties

when estimating biomass production in SRC willow.

5. Conclusions

The results of this study support the overall hypotheses that 1)

the biomass production of different SRC willow clones differs,

2) production depends on the soil properties, and 3) that

production also depends also on cloneesoil interactions.

Average biomass production over the two-year rotation for

the individual clones ranged between 5.2 and 8.8 odt ha�1 yr�1

based on the harvested willow biomass. We found a higher

production on the organic soil than on the sandy soil. The two

clones Inger and Tora showed amarkedly higher production on

the organic soil than on the sandy soil. Comparison of the

harvest and non-destructive method showed a clear differ-

ence in estimated biomass at aggregated plot-level. This

difference calls for a further investigations of the twomethods

at subplot, plot and field scales.

Acknowledgements

This study was funded by Ny Vraa Bioenergy, HedeDanmark

A/S, Dalgas Innovation and the Danish Agency for Science

Technology and Innovation. We greatly acknowledge Ny Vraa

Bioenergy for provision of fields for the two experimental

sites, management of the site and harvest. We thank

laboratory technician Lise Bak for a great work with weighing

willow shoots for moisture content determination and Ph.d.

Morten Ingerslev and senior scientist Uffe Jørgensen for help

and discussions regarding the experimental design and the

evaluation of the results.

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