Will energy crop yields meet expectations?

Stephanie Y. Searle

, Christopher J. Malins

The International Council on Clean Transportation, 1225 I St NW, Ste 900, Washington DC 20008, USA

The International Council on Clean Transportation, 11 Belgrave Road, IEEP Offices 3rd Floor, London SW1V 1RB, UK

a r t i c l e i n f o

Article history:

Received 30 June 2013
Received in revised form
23 December 2013
Accepted 4 January 2014
Available online 1 February 2014

Keywords:

Energy crop
Yield
Biomass
Biofuel
Cellulosic ethanol
Renewable energy

a b s t r a c t

Expectations are high for energy crops. Government policies in the United States and
Europe are increasingly supporting biofuel and heat and power from cellulose, and
biomass is touted as a partial solution to energy security and greenhouse gas mitigation.
Here, we review the literature for yields of 5 major potential energy crops: Miscanthus spp.,
Panicum virgatum (switchgrass), Populus spp. (poplar), Salix spp. (willow), and Eucalyptus spp.
Very high yields have been achieved for each of these types of energy crops, up to
40 t ha

in small, intensively managed trials. But yields are significantly lower in semi-

commercial scale trials, due to biomass losses with drying, harvesting inefficiency under
real world conditions, and edge effects in small plots. To avoid competition with food,
energy crops should be grown on non-agricultural land, which also lowers yields. While
there is potential for yield improvement for each of these crops through further research
and breeding programs, for several reasons the rate of yield increase is likely to be slower
than historically has been achieved for cereals; these include relatively low investment,
long breeding periods, low yield response of perennial grasses to fertilizer, and inappli-
cability of manipulating the harvest index. Miscanthus

giganteus faces particular chal-

lenges as it is a sterile hybrid. Moderate and realistic expectations for the current and
future performance of energy crops are vital to understanding the likely cost and the po-
tential of large-scale production.

Introduction

1.1.

Policy relevance

Biomass is currently seen as a potentially major part of carbon
mitigation strategies in the U.S. and EU. The U.S. Renewable
Fuels Standard 2

[1]

mandates a high volume of cellulosic

biofuel to be produced in 2022; although this mandate is un-
likely to be met

[2]

, it has led to the commercialization of

several cellulosic biofuel production processes in the U.S. In
the EU, the Renewable Energy Directive

[3]

calls for 20% of total

energy to be sourced from renewables by 2020, and biomass is
a major component of this plan, both in the heat and power
sector and as transportation fuels. In addition, the Fuel
Quality Directive

[4]

mandates a 6% greenhouse gas (GHG)

reduction in transport fuels by 2020, further incentivizing
biofuels. The European Commission has proposed introducing
double and quadruple counting to the RED for biofuels from
non-food sources, including agricultural and forestry residues
and dedicated bioenergy crops

[5]

Looking forward, the EU is currently considering providing

regulatory support for biofuels beyond 2020, and cellulosic
biofuel may receive continued support in the U.S. as well.

Abbreviations: SRC, short-rotation coppice; GHG, greenhouse gases; DoE, U.S. Department of Energy.

* Corresponding author. Tel.:

þ1 202 534 1612.

E-mail addresses:

stephanie@theicct.org

(S.Y. Searle),

chris@theicct.org

(C.J. Malins).

Tel.:

þ44 7905 051 671.

Available online at

www.sciencedirect.com

ScienceDirect

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

b i o m a s s a n d b i o e n e r g y 6 5 ( 2 0 1 4 ) 3 e1 2

http://dx.doi.org/10.1016/j.biombioe.2014.01.001

Other regions are likely to consider biomass as part of their
GHG mitigation strategies that does not compete with food.
Dedicated energy crops are a likely candidate to meet this
increasing demand for sustainable biomass.

How much sustainable biomass can we count on to meet

future targets? Energy crop yield is a critical part of the
answer, and will also partially determine the GHG reduction
from such biomass. It is more efficient, both in terms of re-
sources and money, to manage and harvest more biomass
from the same plot of land.

1.2.

Expectations of energy crop yields

Expectations are high for energy crop yields. The U.S. Envi-
ronmental Protection Agency has quoted the literature for
expected Miscanthus yields of 10e40 t ha

in the U.S.

[6]

(for reference, today’s maize yields are

w10 t ha

[7]

while the Department of Energy (DoE) has supported switch-
grass research for over a decade based on expectations of
commercially viable yields up to 33 t ha

[8]

. Well-cited

studies projecting future biomass potential have assumed
energy crop yields of 18 t ha

(global average)

[9]

and

10.5e22.9 t ha

[10]

. Additionally, expectations are high

for future yield growth of energy crops; the U.S. DoE assumed
yield growth of up to 5% per year (for comparison, maize yields
have increased on average

w2% per year over the past 60 years

[11]

Whether or not commercial energy crop production is

likely to meet these expectations is the topic of this review.
We focus on currently attainable yields of 5 major candidates
for large scale energy crop production: Miscanthus spp.,
Panicum virgatum (switchgrass), Salix spp. (willow), Populus
spp. (poplar), and Eucalyptus spp. In the following sections, we
compare reported yields from small and large scale experi-
ments with geographical context, discuss environmental im-
plications of energy crop production, and the future potential
for yield improvement.

Overview of studies

2.1.

Miscanthus

Miscanthus is a genus of perennial grasses native to Asia and
Africa that use the C4 photosynthetic pathway. The type most
commonly discussed as a potential energy crop is Miscanthus
giganteus Greef et Deu, thought to be a hybrid of Miscanthus

sinensis and Miscanthus sacchariflorus

[12]

. As M. sinensis

natively grows in cooler, northern temperate climates and M.
sacchariflorus is better suited to a warmer climate, M.

giganteus thrives in the temperate zone but is intolerant to
both cold and hot extremes

[12e15]

giganteus is a sterile triploid

[16]

and at the present is

propagated vegetatively by transplanting rhizomes, although
producing seed from the parent species has been attempted

[17]

Miscanthus yields are low in the first 1e2 years after

establishment, and stabilize or slowly increase after the 3rd
year

[15]

. Yields have been known to decline in stands after 10

years

[17,18]

, and re-establishment of stands may be neces-

sary after this time period.

Reported yields of M.

giganteus are shown in

Table 1

, and

full details are shown in the

supporting online material

. Most

research performed on Miscanthus spp. for biomass produc-
tion has been in Europe, with some at the University of Illinois
in the U.S.

[19]

. Yields range from 5 to 13 t ha

on poor

soils or marginal land

[15,18]

and from 7 to 44 t ha

arable land with either sufficient precipitation or irrigation

[15,20e24]

(all yield measurements quoted here are presented

in dry mass). Yields are highest (13e44 t ha

) in warm

temperate regions such as Greece

[15,25e27]

Previous reviews of Miscanthus yields have reported

typical yields of 12 t ha

[28]

and ranges of

8.8e16 t ha

[29]

and 5e44 t ha

[15]

with the higher

yields generally associated with very well-irrigated and
fertilized Miscanthus on arable land in warm temperate re-
gions. Virtually all the studies reviewed here on Miscanthus
were conducted on small plots, but in a broad review of Eu-
ropean Miscanthus trials, Scurlock

[30]

gave 7e9 t ha

yields that can be expected at field scale, which is somewhat
lower than is typically measured in small plots. In some
studies, Miscanthus yield has been found not to respond to
nitrogen fertilization

[20,31]

. Yields have been found to be

somewhat

higher

with

increased

fertilization

[27]

(15.8 t ha

vs. 12.7 t ha

, both measured in Spring),

but it is likely that the yield response of Miscanthus to fertil-
izer is muted at best. Still, it has been advised to add a modest
amount of fertilizer to Miscanthus stands, even on good soil,
to avoid nutrient depletion

[20]

giganteus does not thrive in colder climates; Jorgensen

[16]

experienced 15% mortality of this hybrid in its first winter

in Denmark, and in later experiments, no M.

giganteus sur-

vived the first winter

[22]

. M. sinensis is more hardy in northern

regions where it has been found to have higher yields than the
hybrid

[16]

Table 1 e Range of reported yields in the literature for energy crops by climatic zone, land quality, and plot size; all values in
t ha

Total

range

Climate

Land quality

Plot size

Cold

temperate

Temperate

Warm

temperate

Subtropical/

tropical

Agricultural Marginal Small (

<1 ha) Large (>1 ha)

Miscanthus

giganteus

5 to 44

7 to 11

5 to 38

13 to 44

7 to 44

5 to 13

5 to 44

7 to 9

Switchgrass

1 to 35

1 to 15

7 to 35

8 to 13

3 to 9

4 to 35

2 to 9

Willow SRC

0 to 21

4 to 15

2 to 15

4 to 21

0 to 13

Poplar SRC

0 to 35

3 to 9

0 to 28

5 to 18

2 to 11

0 to 35

2 to 3

Eucalyptus

0 to 51

3 to 51

0 to 33

14 to 51

0 to 17

3 to 51

4 to 5

b i o m a s s a n d b i o e n e r g y 6 5 ( 2 0 1 4 ) 3 e1 2

2.2.

Switchgrass (

P. virgatum)

Switchgrass (P. virgatum L.) is a perennial grass native to North
America, where it is dominant in tall grass prairies. Like
Miscanthus, this species uses the C4 photosynthetic pathway.
It is an obligate out-crosser and cannot be self bred

[32,33]

Upland and lowland varieties of switchgrass are available.

Unsurprisingly, most research on switchgrass is conducted

in the U.S. Yields are sometimes as low as 1e2 t ha

especially in poorly drained soils

[34,35]

and with inadequate

precipitation (

Table 1

;

supporting online material

), while other

studies have found yields as high as 35 t ha

in warm

climates

[36,37]

. Most plot-scale studies in temperate areas

and on arable or moderate quality soils achieve yields of
5e10 t ha

[21,33,38e40]

. In a nationwide trial program,

the U.S. Department of Energy found high variation of yields
in switchgrass, from 5.5 to 21.6 t ha

[41]

. Previous liter-

ature reviews have reported typical yields of

w8 t ha

[29]

10 t ha

[28]

and 10.9 for monocultures

[42]

. There is not a

clear difference in yield between lowland and upland
switchgrass varieties, although lowland varieties have been
found to be best suited to warmer latitudes, while upland
varieties thrive at colder latitudes in the U.S

[43]

It has been observed that yields in switchgrass are lower at

field scale than in small experimental plots (

Table 1

;

Supporting online material

). One study

[44]

reported relatively

lower yields (4.2e8.8 t ha

) in large fields managed by

commercial farmers. In a comparison between small plots
and

<1 ha fields, another

[38]

found somewhat lower yields at

scale (2.1e6.8 t ha

) than in small plots (4e6.8 t ha

both measured in Spring), although the small plots received
more fertilizer (112 kg ha

vs. 0e56 kg ha

of nitrogen). That

being said, like Miscanthus, switchgrass has been found to
have only a slight yield response to nitrogen fertilizer. One
study

[39]

reported no yield benefit above a modest addition of

56 kg ha

nitrogen fertilizer, and another

[34]

found no N

response in switchgrass grown on poor soil in mixture with
other native grasses. In any case, modest fertilizer additions
are likely necessary to prevent longterm soil depletion, as with
Miscanthus.

Switchgrass can be grown in mixtures with other native

grasses, which may enhance the crop’s ability to support
biodiversity and reduce the need for fertilizer when legumes
are included in the mix. However, this results in lower yields
of both the switchgrass and the mixture as a whole. A review
of such studies found yields of switchgrass grown in mono-
culture (10.9 t ha

) to be greater than those in mixtures

(4.4 t ha

) and yields of all species in the mixture com-

bined (6.9 t ha

), although when legumes were part of the

mixture, total yields were similar (9.9 t ha

) to those of

monocultures

[42]

2.3.

Poplar (

Populus)

Poplar (Populus spp.) is a genus of deciduous flowering trees
native to most of the northern Hemisphere; some species are
also commonly known as “aspen” or “cottonwood.” The genus
has large genetic diversity and hybridizes easily

[18]

. Poplar

grows well in temperate regions, and requires either irrigation
or generous precipitation to thrive

[14]

. Most of the research

on poplar for bioenergy production has focused on short-
rotation coppice (SRC), a method whereby the stem(s) of the
plant is repeatedly cut and new growth sprouts from the
stump. This is thought to produce higher growth rates,
although poplar in SRC may not have this desired effect for
poplar

[14]

Poplar yields (in SRC or single stem) can be quite variable e

one study found very low yield for one genotype and as high as
24 t ha

for another

[45]

. Most studies report yields in the

5e10 t ha

range for both SRC

[46e48]

and plantations

[49,50]

. Previous reviews give short-rotation and SRC poplar

yields of 7 t ha

[28,29]

and ranges of 3e9 t ha

and 6e12 t ha

. See

and

online material for detail

One reason poplar is of interest as a biomass crop is that it

can tolerate poor soil conditions. For example, one study
measured yields of Populus spp. in SRC at 2.2 to 11.4 t ha

in a former landfill site in Belgium

[46]

. On the other hand,

Bungard

Hu¨ttl

[49]

reported

much

lower

yields

(3e6 t ha

) for Populus spp. grown at a former mining site

than were found in the same clones grown on better quality
soils (12 t ha

Poplar has been found to produce lower yields at large-

scale than in small experimental plots. In a review of poplar
yields in the U.S., Hansen

[53]

writes that yields from small

plots range from 5 to 27.8 t ha

, while field scale yields are

2.9 t ha

. Similarly, a review found that the clone “Tristis”

yielded 25e30 t ha

in small plots but only 10 t ha

field scale

[52]

Poplar can be grown in mixture with grass, although Scholz

& Ellerbrock

[47]

found this to decrease yields e Populus max-

imowiczii

nigra yields were 41% lower when grown with grass

than without (averaged across fertilizer treatments).

2.4.

Willow (

Salix)

Willow (Salix spp.) is a genus of deciduous flowering trees in
the same family as poplar (Salicaceae). Like poplar, the willow
genus has high genetic diversity and readily hybridizes, but
requires adequate precipitation or irrigation (

[51]

and not

tolerant of cold climates, and is susceptible to weeds

[54]

and

herbivory

[55]

Yields are typically in the 5e10 t ha

range

[47,54,56,57]

(

Table 1

;

Supporting online material

), although

higher yields were found on previously agricultural land in
Canada

[55]

, and in the hybrid S.

‘Aquatica gigantea’ grown in

Ireland (8.8e17.0 t ha

)

[58]

. Previous reviews have given

estimates of 7

[28]

and 7.5 t ha

[29]

2.5.

Eucalyptus

Eucalyptus (Eucalyptus spp.) is a genus of largely evergreen
flowering trees, native to Australia, where they are dominant
in tree flora, although some species are native to New Guinea,
Indonesia, and the Philippines. Eucalyptus can have high
yields and are thus of increasing interest for bioenergy pro-
duction. It is the only type of energy crop reviewed here that
thrives in tropical and subtropical regions; because govern-
ments in the U.S. and Europe have expressed the most interest
in supporting energy crop development, most research in this

b i o m a s s a n d b i o e n e r g y 6 5 ( 2 0 1 4 ) 3 e1 2

field has been on temperate species, and little information on
using tropical species other than Eucalyptus for bioenergy
production is available.

Reported Eucalyptus yields are quite variable (

Table 1

;

supporting online material

). Yields have been found to be

10e17 t ha

for Eucalyptus grown in SRC or as single

stems

[59e61]

. One study found very low yields of

0.4 t ha

(1.5 t ha

with irrigation and fertilizer) on

highly degraded land

[62]

while another found yields as high

as 51 t ha

when extrapolated from a small plot of 5 trees

[63]

. Reviews give a range of yields from 4 to 24 t ha

[64,65]

. One study estimated that Eucalyptus yields in Brazil

are generally 5 t ha

on poor soil and up to 20 t ha

productive coastal land, while interviews with government
officials provided estimates of

w1 t ha

on poor land and

w3 t ha

on pasture land

[66]

Unlike switchgrass and poplar, Eucalyptus yields have

been found to be higher when grown in a mixture with le-
gumes than in monoculture. One study

[67]

reported 55%

higher yields in Eucalyptus saligna in a

w50:50 mixture with the

legume Albizia falcataria (10.7 t ha

) than in monoculture

(6.9 t ha

), while another

[68]

measured similar yields (14

vs. 15.5 t ha

) in E. saligna grown in a 50:50 mixture with

the legume Facaltaria moluccana. Furthermore, yields in
monoculture declined after 7 years, while those in mixtures
did not over a 10-year period

[68]

Growing Eucalyptus at scale has multiple environmental

concerns. Eucalyptus is drought-tolerant but consumes large
amounts of water when available and will outcompete other
plants in areas where it is not native

[69]

. For this reason,

Eucalyptus has been used to drain swamps. Various species of
Eucalyptus have been invasive on several continents

[70]

Lastly, Eucalyptus produces high amounts of isoprene, a vol-
atile compound that interacts with NO

to produce ozone,

which in turn inhibits growth in food crops, poplar, and
Eucalyptus itself

[71e74]

Yields at scale

3.1.

Commercial scale vs. small plot

In the overview of studies, we gave several examples where
average yields were lower at field scale than in small experi-
mental plots, in Miscanthus

[30]

, switchgrass

[38,44]

, and

poplar

[52,53]

. Indeed, the highest biomass yield of any study

reviewed here (51 t ha

[63]

) was extrapolated from only 5

plants. In some of these studies the reported differences were
small

[38]

but other studies observed a striking contrast.

Hansen

[53]

writes, “current record small-plot yields still

exceed field trials by 4e7 times”, while another study

[35]

observes “actual yields in commercial production switch-
grass fields are considerably lower and more variable than
commonly reported in the literature.” Indeed, “typical”
switchgrass yields given in review papers (8e10 t ha

) are

above the entire range given in several individual studies

[35,38,44]

, even for small plots

[34,39]

, suggesting a bias in

review papers towards predicting high yields.

There are two main reasons why measured yields are

lower at scale. The first is that energy crops in small plots can

be carefully hand-harvested, preventing significant biomass
loss, but in harvesting a large field mechanically, some losses
are unavoidable. Owens

[75]

gave an example for a switch-

grass experiment where the harvester had to raise its cutter
blade to avoid damage to its tires, resulting in reduced
biomass collection. The other reason has to do with edge ef-
fects

[52]

. Especially in high-density biomass plantations,

plants growing on the edge of the plot receive more light and
can thus grow faster than plants in the middle of the plot. The
proportion of plants benefitting from this edge effect is
reduced with increasing plot size. Some researchers may have
set up with border plots to control edge effects for perennial
grasses, reducing the influence of this factor. Lastly, even if
one clone has high yields in one location, it may have poor
yields in another

[54]

and some yield penalty should be ex-

pected with poor clone-site matching at large-scale

[51]

Although energy crop researchers are aware of these dif-

ferences between large and small plots, these differences are
not obvious to policymakers, investors, and other non-
specialist readers who may expect commercial-scale yields
to be as high as those measured on small plots. Primary
research articles on energy crop yields should consider mak-
ing this distinction clear to avoid misinterpretation.

3.2.

Biomass drying

The storage of any crop requires low moisture content to
avoid rotting. For energy crops this is no different. Switchgrass
and Miscanthus biomass yields are often highest in early
autumn, but the plants have high water content at this time.
Drying the crop post-harvest is energy-intensive and expen-
sive; at commercial-scale production, farmers leave some but
not all types of perennial grasses uncut in the field over
winter, during which it senesces and loses water content.
Some biomass loss is associated with this drying period, some
of which is likely due to translocation to the roots and some to
lodging. Here we are not referring to the difference between
fresh and dry measured weight, but actual dry mass loss in the
over winter period.

Miscanthus loses about 35% of its biomass during the

drying period (estimates of biomass loss range from 25% to
50%

[14e16,22,30]

. Switchgrass yield loss is typically around

20%

[21,38,41]

Woody biomass (poplar, willow, Eucalyptus) must also be

dried from a water mass fraction of around 50e60% at time of
felling to

w20% for storage. One study

[52]

has suggested

drying poplar biomass on the field during the summer to
reduce the water content. Presumably this would require
springtime harvest, when biomass yields are off-peak due to
autumn translocation. Thus, it is possible that peak yields of
poplar, willow and Eucalyptus are also not representative of
those that may be achieved at scale.

3.3.

Geography

For this review, we selected a variety of energy crops that can
be complementary in geographic range. M.

giganteus,

switchgrass, poplar, and willow thrive in temperate regions,
with the grasses being at least somewhat tolerant to drought.
M.

sinensis and, to some extent, some species of Populus are

b i o m a s s a n d b i o e n e r g y 6 5 ( 2 0 1 4 ) 3 e1 2

cold tolerant and thrive in Canada and northern Europe.
Eucalyptus is grown in tropical and subtropical climes.

There are other potential energy crops well-suited to hot

climates, such as Energycane (Saccharum spontaneum) and
Giant Reed (Arundo donax). The majority of research on energy
crops, however, has been conducted on temperate species in
the U.S. and Europe, as these are the two regions with the
highest level of policy support for biomass at the present. It is
likely that interest in tropical biomass will increase as other
nations increase their targets for bioenergy (e.g. Brazil). It is
also possible that some nations in temperate regions with a
shortage of arable land (e.g. China) will invest in biomass
projects in tropical countries in the future.

3.4.

Marginal land

In the overview of studies, we gave some examples of lower
yields on poor-quality “marginal land” (land that is not well-
suited to food production) than on arable land that has been
used for food production

[15,36,49]

. This effect may seem

obvious, but its importance may be underappreciated. In gen-
eral, most highly productive land is already under agriculture e
cropped area comprises 12% of global land area, and when
pasture is included that rises to 42% (calculated from Ref.

[76]

While developed countries in the Northern Hemisphere have
been slowly setting aside cropland as yields have improved

[77]

abandoned agricultural land tends to be of the lowest quality

[78]

. Cereal and grain yields on set-aside land are typically found

to be 50e90% those achieved on currently cropped areas

[79e81]

, and it is reasonable to assume yields will be even lower

on poor-quality land that has never been brought into produc-
tion of commodity crops. Although the energy crops discussed
here, and in particular the perennial grasses, may fare relatively
better on marginal land than cereals, lower yields than those
measured on quality land should be expected.

The price of biomass may be high enough to incentivize

crop switching on arable land from food crops to energy crops.
Azar & Larsen

[66]

show that, with Eucalyptus in Brazil, “the

value of the higher yields that can be expected on ‘good’ lands
generally outweighs the additional cost associated with
acquiring that land.” Indeed, there is a trade off between food
production, nature conservation, and bioenergy production
that can already be observed with sugarcane plantations in
Brazil. Large-scale displacement of food crops from arable
land would tend to raise food prices and hence food insecurity
across the globe, much as biofuels contributed to the food
price spike of 2008

[82]

. It is critical that government policies

include safeguards to preserve existing cropland for food
production, and thus energy crop production must be viable
on currently unused marginal land.

After reviewing yields in the literature for these five energy

crops in the context of scale, land quality, and geography, we
provide ranges of yields that can be expected for large-scale,
commercial production in

Table 2

3.5.

Environmental concerns

There are potential environmental benefits and risks with
each of the energy crops discussed here. Some have invasive
potential in areas where the crop is not native (Eucalyptus, M.
sinensis), and in fact Eucalyptus has already been invasive in
several countries

[69]

. Arundo donax (Giant Reed) and Pennise-

tum purpureum (Napier grass) also have invasive potential, at
least in the U.S.

[83,84]

. On the other hand, plantations of

native species, such as switchgrass in the U.S., may support
increased biodiversity, especially if it is planted in mixtures
with other species and is planted on degraded land.

There is also potential for some energy crops to increase

soil carbon when grown on degraded or previously agricul-
tural land, although the effect of cultivating energy crops on
soil carbon has not been determined definitively. M.

gigan-

teus

[18,23]

and P. virgatum

[85]

have been reported to increase

soil carbon. In general, conversion of agricultural or degraded
land, and sometimes even forest, to grassland is associated
with soil carbon increase

[86e88]

Although somewhat drought-tolerant, some energy crop

types are high consumers of water when it is available. Euca-
lyptus can drain water from soils and lead to widespread
senescence of other plants in the area

[69]

. One review

[28]

points

out that perennial biomass crops may intercept rainfall during a
larger part of the year than cereals and grains, and that “this
could have a significant impact on water storage in drier regions
if the crops were grown over large areas.” Water resources will
be a major concern for biomass crops grown in dry regions.

Biomass crops, like any cultivated plant, have the potential

to cause pollution to water and air from fertilizer and poten-
tially pesticide and herbicide application, but the evidence
suggests this is not likely to be as large a problem as it is for
cereals and grains. Miscanthus and switchgrass in particular
have often been found not to respond to nitrogen fertilizer at
levels above

w56 kg ha

nitrogen

[20,31,34,39]

, compared

to a 20-year average of 149 kg ha

nitrogen for U.S. maize

production (calculated from Ref.

[89]

). Interestingly, one study

[90]

found nitrate leaching from willow to be 90% lower than

from barley in Denmark, even at high levels of fertilizer.

The major opportunities for achieving environmental

benefits with energy crop production are likely in rehabili-
tating degraded land with perennial grasses. The greatest
risks for environmental damage are invasive species intro-
duction and excessive water consumption.

Table 2 e Yields of energy crops that can be expected at commercial scale on land that is marginal for agriculture, by
climatic zone; all values in t ha

Cold temperate

Temperate

Warm temperate

Tropical/subtropical

Miscanthus

3e5

7e15

Switchgrass

2e7

5e10

Willow SRC

0e10

4e13

Poplar SRC

3e8

4e10

Eucalyptus

5e15

b i o m a s s a n d b i o e n e r g y 6 5 ( 2 0 1 4 ) 3 e1 2

Potential for yield improvement

Yield improvements can be expected for all the energy crops
reviewed here, but in the words of Karp & Shield

[14]

, “to

propose even greater rates of yield improvement than have
been achieved to date in annual arable crops is a bold claim
that requires detailed substantiation.”

4.1.

Why energy crops face greater challenges than

cereals and grains

The market for biomass, and thus financial incentives, is un-
likely to ever be as large as for food crops. One study calcu-
lated that the potential land area available to energy crops in
2050 is likely to be only one-tenth that used for food crops
today (excluding pasture)

[91]

Cereal and grain yields have benefitted from an increase in

the harvest index, or the ratio of grain to the rest of the above-
ground plant

[92]

. But changing the harvest index would not

affect yields of biomass crops or could even reduce net pri-
mary productivity if photosynthate is reallocated inefficiently.

Much of the increase in food crop yields has been from

increased use of inorganic fertilizers and irrigation, but it is
widely understood that water resources are becoming scarce
across much of the world

[93e95]

. As discussed above,

perennial grass yields do not usually respond to fertilizer
application above a modest threshold.

There is some potential to improve yields of biomass crops

through conventional breeding and genetic modification. But
while breeding cycles of annual cereal crops are very short, on
the order of a few months, perennial biomass crops take much
longer to reach maturity. Willow may flower in 2e4 years

[96]

and Eucalyptus in 2e10 years

[97]

, at which point breeding

may be possible. Researchers may need to wait even longer for
the completion of a rotation cycle to identify the highest
yielding

varieties

for

optimal

breeding

combinations

[14,41,46]

. Similarly, any modification in biomass yields

through genetic modification will take years to fully detect;
while such yield increases are possible, they cannot be ex-
pected in the short-medium term. So while there is some
potential to improve yields of these crops, progress will almost
certainly be slower than it has been for cereals.

Some who expect high yield growth for energy crops cite

increased photosynthetic efficiency as one mechanism

[98]

But most attempts to improve the process of photosynthesis
have resulted in disruption to another process in the plant

[99]

. For example, in one study genetically modified Arabi-

dopsis plants with increased stomatal conductance achieved
30% higher photosynthetic rates

[100]

, but these plants almost

certainly require more water and would very likely face higher
mortality from water limitation in the field. It will likely be
decades before advances in photosynthetic efficiency through
genetic modification will be available for any crop.

One major factor that may thwart potential yield gains is

climate change. With global warming, temperatures and CO

concentrations will rise, and rainfall patterns will change.
Crop yields may benefit from an increase in atmospheric CO

concentration and from higher temperatures in cold
temperate regions, but these benefits are likely to be offset by

yield declines in hotter regions and from an increase in
droughts

[101]

. Direct yield benefits from CO

fertilization are

likely to be limited for Miscanthus and switchgrass as these
are C4 plants

[102]

. Global warming is also expected to in-

crease weather variability and extreme weather events

[103e105]

, which will depress crop yields

[106]

. For example,

the 2010 Moscow heat wave that destroyed 20% of Russia’s
cropland

[107]

has been statistically linked to climate change

[103]

. Even if energy crops fare better with these pressures

than cereals, a climate-driven depression in yields of food
crops could drive agricultural extensification and reclamation
of set-aside land, pushing energy crops onto lower quality
land.

4.2.

Breeding Miscanthus

Miscanthus faces particular challenges at yield improvement.
Virtually all Miscanthus is propagated vegetatively using rhi-
zomes, or through micropropagation (using plant tissue cul-
ture to create new plants in a laboratory) as the hybrid is
triploid and does not produce seed. Both methods are
expensive and labor-intensive; cost will to some extent inhibit
commercial viability of Miscanthus. There has been recent
and limited success of producing hybrid seeds from M. sinensis

[17]

, but generally there has not been much progress in pro-

ducing seeds of M.

giganteus from it parent species (M.

sinensis and M. sacchariflorus)

[96]

. Additionally, Miscanthus

has a narrow genetic base

[15,30]

, further narrowing the scope

for yield improvement.

Discussion

Expectations for energy crop yields are high, based on studies
that have achieved impressive yields on small, intensively
managed plots. In a review of the literature on five major
energy crops, we have found that yields are reduced when
produced at semi-commercial scale, when grown on sub-
optimal land, and after taking into account biomass loss
with drying.

Realistic expectations for energy crop yields are critical for

success of the bioenergy industry. Energy crops are more
expensive to produce at lower yields, as more time, energy,
and resources are sunk into each harvested tonne. In addition,
greenhouse gas savings will generally be lower than expected
as more fuel, fertilizer, and land (with associated conversion
emissions) are consumed per tonne biomass. Commercial
ventures and government policies alike are likely to fail to
meet their goals if they are premised on overly optimistic yield
projections. Biomass production costs are heavily affected by
yields (lower yielding fields are more expensive to harvest per
tonne

[108]

), and so projects with lower than expected yields

will have higher costs and thinner profit margins than antic-
ipated; projected yields must thus weigh heavily on project
selection. From the perspective of bioenergy policy, lower
than expected yields will make targets more difficult and
more costly to meet. It is imperative for governments and
other investors to have realistic yield expectations and to
optimize growth in the emerging bioenergy industry.

b i o m a s s a n d b i o e n e r g y 6 5 ( 2 0 1 4 ) 3 e1 2

Looking to the coming decades, more research is needed to

develop cultivars that can thrive on poor soils with little
water, especially for woody biomass like poplar and willow.
Some research is being conducted to screen clones of energy
crops on saline soils or soils with mining contaminants

[109,110]

. But the opportunity is largest on dry land e deserts

make up over one-third of non-forested land on the planet
(calculated from Refs.

[111]

, although only a fraction of this

land could ever be brought into production). Lastly, more
research will be needed to understand which energy crops are
the best candidates for tropical and subtropical plantations.

Conclusions

A broad review of the literature has found that yields are
reduced when produced at semi-commercial scale, when
grown on sub-optimal land, and after taking into account
biomass loss with drying. Although energy crop researchers
may take these factors into account in their own work, poli-
cymakers and other non-specialists often quote projections of
energy crop yields that are representative only of carefully
controlled small plot experiments, rather than those that can
be reasonably expected at commercial scale. If policymakers
continue to base expectations on energy crop yields quoted
out of context, these expectations will not be met.

Acknowledgments

We thank the ClimateWorks Foundation for providing funding
for this review.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.biombioe.2014.01.001

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