REVIEWS REVIEWS REVIEWS
Growing fuel: a sustainability assessment of
willow biomass crops
Timothy A Volk1, Theo Verwijst2, Pradeep J Tharakan1, Lawrence P Abrahamson1, and Edwin H White1
Global energy use projections predict that biomass will be an important source of renewable energy in the
coming decades. Short-rotation woody crops will be the prime source of this biomass. However, the sustainability of woody crops has been questioned. Using internationally accepted forest sustainability criteria, an assessment of willow biomass crops indicates that they are sustainable compared to agricultural
land and the fossil fuel-based energy systems they will replace. Assessing each criterion also reveals aspects
of willow crop systems that should be addressed to further improve sustainability. Biological characteristics and management of willow create a structurally diverse habitat for an array of species and protect soil
and water resources. Biomass from willow crops can be used to produce energy with no net addition of CO2
to the atmosphere. The implementation of good management practices will maintain productivity over
multiple rotations. Rural development and environmental benefits associated with deployment and use
will accrue to the local community because of the willow system’s short supply chain. The economic valuation of these benefits are necessary for the deployment of woody crops, which in turn can help society
become more sustainable.
Front Ecol Environ 2004; 2(8): 411–418
T
he development and use of biomass resources for bioproducts and bioenergy has become a critical priority
in North America, Europe, Australia, and elsewhere.
This is driven by growing concerns about environmental
impacts associated with the use of fossil fuels, national
energy security, sustainability of natural resources, and
the need to revitalize rural economies (National
Research Council 2000). Biomass can come from a number of different sources, including forests, agricultural
crops, various residue streams, and dedicated woody or
herbaceous crops. There has been increasing interest in
developing dedicated woody crops grown on short rotations, primarily because of the multiple environmental,
rural development, and social benefits associated with
In a nutshell:
• Biomass from short-rotation woody crops can be grown sustainably as a feedstock for energy and products currently produced
from fossil fuels
• Woody crops enhance biodiversity, protect soil and water
resources, are CO2 neutral, and provide rural development benefits
• Addressing weaknesses in the production and use of woody
crops would improve the sustainability of the system
• Valuation of environmental and rural development benefits
associated with woody crops is essential for the creation of a
commercial enterprise
1
Faculty of Forest and Natural Resources Management, State
University of New York College of Environmental Science and
Forestry, Syracuse, NY 13210, USA (tavolk@esf.edu); 2 Department
of Short Rotation Forestry, Swedish University of Agricultural
Sciences, PO Box 7016, SE-750-07 Uppsala, Sweden
© The Ecological Society of America
their production and use (Abrahamson et al. 1998;
Verwijst 2001).
Biomass currently provides about 10.5% of the global
primary energy supply (IEA 2002), although some is not
being produced and used sustainably. Most projections of
global energy use predict that biomass will be a more
important component of primary energy sources in the
future, and that woody crops will be the primary source of
biomass (Berndes et al. 2003). In addition to combustion
and gasification conversion pathways for power and heat
production, woody crops represent a uniform, locally
available feedstock for the production of liquid fuels,
chemicals, and other advanced materials currently made
from petroleum products.
Woody crop systems involve genetically improved
plant material grown on open or fallow agricultural land.
They require intensive site preparation, nutrient inputs,
and short rotations (3–10 years). In northern temperate
areas, woody crop development has focused on willow
shrubs (Salix spp) and hybrid poplar (Populus spp),
while eucalyptus (Eucalyptus spp) has been a model
species in warmer climates.
Willow has several characteristics that make it ideal for
woody crop systems, including high yields obtained in a
few years, ease of vegetative propagation, a broad genetic
base, a short breeding cycle, and the ability to resprout
after multiple harvests. There are about 450 species of
willow worldwide (Argus 1997), ranging from prostrate,
dwarf species to trees that are over 40 m high. The willows used in woody crop systems are primarily drawn from
the subgenus Caprisalix (Vetrix), which includes over 125
species worldwide. While they share many characteriswww.frontiersinecology.org
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Sustainability of willow biomass crops
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1.50 m
0.76 m
0.61 m
TA Volk et al.
mined by site conditions (Mitchell 1999).
The perennial nature, rapid growth, diffuse fibrous root
systems, and tolerance of high planting density makes willow ideal for a wide range of applications across the landscape. It has been used in Europe and North America for
phytoremediation, nutrient recycling and management,
living snowfences, riparian buffers, and alternative landfill
covers. By establishing woody crops in strategic positions
across the landscape, biomass can be produced while
simultaneously addressing other environmental concerns
(Verwijst 2001).
Sustainability of willow biomass crops
Figure 1. Willow biomass crops planted in the typical doublerow configuration. Plants have just sprouted in the spring, after
being cut back to ground level (coppiced) during the winter. Each
willow plant will develop multiple (6–15) stems. Arrows indicate
the recommended spacing for willow biomass crops, resulting in a
plant density of just over 14 000 per hectare.
tics, their growth habits, life history, and resistance to
pests and diseases vary. This diversity is important in the
successful development of woody crops.
Willow production and applications
Willow has been used for soil conservation, windbreaks,
basket and furniture production, and ornamental planting
for centuries. Its use declined at the beginning of the 20th
century, as other materials became available. However,
interest in willow has grown dramatically, particularly in
Sweden, since the energy crisis of the 1970s. Willow cultivation there has expanded from a few hectares in 1970 to
about 17 000 ha at present. The cultivation of about
200 000 ha is envisaged in Sweden alone during the next
15 years (Verwijst 2001). Other countries are also developing plans for large areas of woody crops.
Willow crop cultivation occurs on agricultural or open,
fallow land and requires a knowledge of both forestry and
agronomy. A double-row configuration is used, with a
density of 10 000–20 000 plants per hectare (Figure 1),
depending on soil conditions, rotation length, and
desired dimensions of the end product. Unrooted cuttings
are planted following agricultural-type site preparation
and complete weed control that is begun in the fall prior
to planting. Cover crops can be established on sites prone
to erosion. Planting machinery cuts 12–24-cm long sections from 100–200 cm dormant whips, then plants them
flush with the soil surface.
Willow is typically harvested on a 3–4-year cycle, using
modified agricultural equipment that cuts and chips the
biomass in a single operation. Willow plants resprout vigorously after each harvest. Seven to eight harvests are possible from a single planting (Figure 2). Nutrients, particularly nitrogen (N) in organic or inorganic form, are
applied after each harvest, the rates and types being deterwww.frontiersinecology.org
The concept of sustainability is based on human values
associated with environmental, economic, and social
components that need to be integrated and balanced.
Since sustainability is based on values that will change
among stakeholders and across both temporal and spatial
scales, it is not an absolute (Bridge et al. 2003; Floyd
2002). Differences in values associated with sustainability
often result in disagreements among stakeholders on
effective, objective mechanisms to assess the sustainability of biomass systems and, in particular, woody crops.
National and international groups have set protocols that
use criteria and indicators to assess sustainable forest management in different regions of the world. While these protocols were not designed specifically for woody crops, they
provide an objective framework to assess widely agreed
upon values relevant to woody crop systems. Criteria reflect
the forest values that stakeholders identify and want to sustain or enhance; they are the goals or outputs from the system. For each criterion, there are sets of indicators, which
are measurable parameters used to assess whether the goals
are being achieved. While indicators provide qualitative
and quantitative information about the system, they are
measurement and monitoring systems and do not have specific standards or targets (Bridge et al. 2003; Floyd 2002).
Criteria and indicators provide a framework to assess the
current state of forests and determine if improvements are
being made over time, or if changes in management practices or policies make the system more sustainable.
Despite the variation in forest types and stakeholders, a
suite of common criteria has emerged from the six different
international protocols used to assess sustainable forestry.
These include (1) conservation of biological diversity; (2)
conservation of soil and water resources; and maintenance
of (3) the forest ecosystem’s contribution to global carbon
cycles, (4) forest ecosystem productivity and health, (5)
socioeconomic benefits, and (6) a policy and legal framework (Floyd 2002). Depending on the international protocol, there are 25 to 67 indicators for these criteria, many of
which are conceptually similar between protocols. While
many of the indicators are relevant to woody crops, others
are not; trying to assess all the indicators for woody crops is
therefore not possible here. However, the agreed upon criteria provide a useful framework to discuss the sustainabil© The Ecological Society of America
TA Volk et al.
Sustainability of willow biomass crops
Three years after coppice, willow are
5-8 meters tall and ready to be harvested.
(7-8 rotations).
Proper area preparation is essential
in establishing a successful crop.
Trees are about two meters tall one
year after coppice.
Four weeks after planting, the willow
plants are 10–20 cm tall.
With an established root system, the plants
resprout vigorously after being coppiced.
Figure 2. Willow biomass crop growth cycle. Once the crop is established, seven to eight harvests at 3–4-year intervals are possible
(indicated by the cycle of green arrows) before the crop needs to be replanted. After each harvest, the willow resprouts the following
spring and develops rapidly.
ity of willow biomass crops in northern temperate zones.
Assessments of the sustainability of woody crops using
these criteria will be made by comparing their performance to fallow or agricultural land (the land use they
will replace), or to fossil-fuel systems currently used to
produce electricity.
Conservation of biological diversity
A common misconception about woody crops is that
monocultures are planted across the landscape, creating
“biological deserts”. In fact, mixtures of different species
and hybrids are established by planting blocks of different
varieties across a field (Figure 3) or random mixtures of
varieties within each row. These mixtures enhance structural and functional diversity and reduce the impact of
pests and diseases (McCracken and Dawson 2001). At a
landscape level, willow crops provide diverse habitats
because they are managed on 3-year coppice rotations.
Each spring, about one third of the planted area in a region
is in a more open habitat as the willows regrow following
harvesting. The other two thirds of the area is shrub habitat. The combination of different varieties and stages of
growth creates a three-dimensional, structurally diverse,
and stable habitat for a range of wildlife (Rich et al. 2001).
The impact of willow crops on biodiversity has been
assessed by monitoring organisms both above and below© The Ecological Society of America
ground. Between 24 and 41 species of birds regularly use
woody crops (Sage 1998; Dhondt and Wrege 2003; Figure
4) and show preferences for different willow varieties
(Dhondt et al. in press). Diversity in woody crops is
greater than on agricultural land and comparable to natural habitats, including shrub land, eastern deciduous
forests, and traditional coppice (Sage 1998; Dhondt and
Wrege 2003). Soil microarthropods, organisms essential
for the decomposition of organic matter and nutrient
cycling, have often been used as indicators of belowground biodiversity. The diversity and density of soil
microarthropods under willow immediately after planting
is similar to that in agricultural fields that are tilled annually, but lower than in undisturbed fallow fields. Four
years after planting, however, the density and diversity of
soil microarthropods under willow are similar to levels in
nearby undisturbed, fallow fields (Minor et al. 2004).
Because of the perennial nature of the crop, it is anticipated that these levels will be maintained or increased.
The North American and European landscape is
becoming increasingly fragmented into smaller, disconnected patches. Fragmentation reduces species diversity
and can create ecological traps for nesting birds. This
occurs when breeding birds are attracted to a particular
patch of habitat but are not able to maintain a viable population because of low reproductive success. In upstate
New York, nesting success in willow fields was similar to
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Sustainability of willow biomass crops
TA Volk et al.
woody crops. Currently, weeds are aggressively
controlled during establishment because they are
a primary factor limiting production. Initial studies indicate that the economic threshold for weed
populations is high enough to increase plant
diversity without compromising production
(Sage 1999). Actively establishing shade-tolerant
herbaceous species with slow growth and low
transpiration rates under willow crops can
enhance plant and animal diversity and potentially improve biological control of pests (Tucker
et al. 1997). Further work on IPM approaches is
necessary so that the concepts can be applied on a
commercial scale.
414
Conservation of soil and water quality
Figure 3. Different varieties of willow biomass crops planted as blocks across resources
a field. These plants are three-quarters of the way through the first growing
season, after being coppiced.
Willow crop management has been designed to
complement biological processes in maintaining or improving site conditions. It also effectively
addresses concerns about nutrient depletion and soil compaction associated with repeated harvesting. A critical
design feature is harvesting after leaf fall, when the
translocation of leaf, branch, and stem nutrients to roots
for winter storage has occurred. These nutrients are essential for vigorous sprouting the following spring. Nutrients
that are not translocated from the foliage are returned to
the system in litter, which, when mineralized, can supply a
third to two thirds of the annual nutrient demand
(Ericsson et al. 1992). Research and years of commercialscale applications indicate that willow crops do not
decrease soil nutrient levels or have other detrimental
impacts on soil properties (Jug et al. 1999; Volk 2002).
However, monitoring and assessment of soil conditions is
required over the 20–30-year operational life of willow, to
ensure this remains the case.
Nitrogen is added in the spring, at the beginning of each rotation, when the crop is
actively growing (Danfors et al. 1998; Volk et
al. 1999). This ensures that the maximum
amount of N is assimilated by the crop, rather
than lost from the site or used by weeds. The
amount applied should replace the quantity
removed during harvest, in order to maintain
soil productivity rather than trying to boost
site productivity. Nitrogen is applied as synthetic fertilizer or organic residuals, such as
biosolids or manure, that would otherwise be
managed as waste streams. There is negligible
N leaching from willow crops when it is
applied to actively growing plants, even with
higher than recommended annual applications
(Mortensen et al. 1998; Aronsson et al. 2000).
Figure 4. Wood thrush (Hylocichla mustelina) nesting in willow biomass The need for other nutrients, and rates of
crops. Declining shrub habitat has resulted in the wood thrush being listed as a application, should be determined at each site,
species of concern in New York State.
based on soils information.
Courtesy of A Dhondt, Cornell University
other natural habitats (Dhondt and Wrege 2003). Other
negative impacts of fragmentation can be reduced by
strategically planting woody crops to act as “ecological
corridors”, through which species can migrate between
patches, and as buffers around natural areas (Sage 1998).
While the structural and plant species diversity of woody
crops is much greater than annual agricultural systems
(Weih et al. 2003), it can be improved. Plant diversity can be
enhanced without compromising productivity by actively
planting and managing vegetation in unplanted areas in and
around the crop, which typically amount to 4–7% of a field
(Figure 5). The proportion and active management of open
areas could be increased if the benefits of improved diversity
were valued through programs such as the conservation
reserve program in the US (Tharakan et al. in press.)
Adopting an integrated pest management (IPM)
approach to weeds is another way of increasing diversity in
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© The Ecological Society of America
TA Volk et al.
The three colors represent 1-, 2-, and 3-year-old willow
biomass crops. This age distribution is maintained by harvesting the portion of the field with 3-year-old willow each
dormant season. An annual harvest of a portion of the crop
provides a regular stream of income for the landowner.
Sustainability of willow biomass crops
Spaces are left between the crops to facilitate access to the crop
for harvesting and maintenance. Headlands, up to 10 m wide, at
the ends of the rows provide space to turn harvesting equipment
around. Planting these spaces with a selected mix of species
can increase plant diversity
Figure 5. Representation of a typical field planting design for willow biomass crops. The area contains three age classes, with a number
of different willow varieties in each. Mixing varieties across the field increases landscape diversity and may help reduce impacts from
potential pests and diseases. (Modified from Anon 1996, created by J Ballard.)
In conventional agriculture, soil erosion is a serious
threat to long-term productivity because it reduces rooting
depth and removes nutrient-rich topsoil. Once willow
crops are established, erosion potential is very low because
of their perennial nature, extensive root system, and the
litter layer that develops. However, soil erosion during
establishment is a concern. Recommended site preparation results in limited vegetation cover on the field for
about a year and a half, from the time of fall plowing until
the crop canopy begins to close in the second growing season. Soil losses during that time can be comparable to
losses from annual row crops, depending on the site, soil
type, and weather patterns. Systems that provide soil
cover during the establishment phase, without compromising production, are being developed, based on experience with cover crops in orchard and vine crops (Malik et
al. 2001; Volk 2002). Additional research is needed, so
that these methods become more widely adopted.
Although the amount of traffic on fields with woody
crops is a fraction of that which occurs with agricultural
crops, the potential for soil compaction exists. Harvesting
when the soil is wet and not frozen has the greatest
potential to cause substantial soil compaction, so harvesting under these conditions should be avoided. Developed
willow crop root systems are extensive and diffuse enough
to support harvesting equipment when soils are relatively
dry (Mitchell et al. 1999).
Forest ecosystems and global carbon cycles
Woody crops influence global carbon cycles by taking up
carbon through photosynthesis and sequestering it in
above- and belowground biomass, and by displacing fossil
fuels as a source of energy and products. Woody crops will
© The Ecological Society of America
have the greatest impact on global carbon cycles by displacing fossil fuels, because woody-crop biomass used to
generate electricity is carbon neutral (Mann and Spath
1997; Matthews 2001; Heller et al. 2003). This means
that the amount of carbon released during the production, harvest, transportation, and conversion of the biomass is equal to the amount taken up by the growing crop
(Figure 6). The production and conversion of willow biomass can therefore produce electrical energy with no net
addition of CO2 to the atmosphere. This is an important
benefit, in view of the growing concerns about the
impacts of increasing CO2 emissions from fossil fuel combustion. If the 40 million ha of land available for woody
crops in the US were planted and harvested to offset coal
use for energy production, up to 76% (0.30 Pg of C per
year) of the carbon offset targets for the US under the
Kyoto Protocol could be met (Tuskan and Walsh 2001).
Portions of the willow production system produce other
greenhouse gases, so the production of electricity from
woody crops has a small global warming potential (39–52
kg CO2 equivalent per MWh of electricity), depending on
the conversion technology. This is the same order of magnitude as wind and building integrated photovoltaic systems (Heller et al. 2004). While progress can be made to
reduce these emissions, especially through better use of N
fertilizers, they are far lower than emissions from coal (over
1000 kg CO2 equivalent per MWh) or natural gas (600 kg
CO2 equivalent per MWh) (Mann and Spath 1999).
Willow crops can be thought of as solar collectors that
convert the sun’s energy and store it as woody biomass.
The net energy ratio from the production and harvest of
willow biomass ranges from 1:29 to 1:55 (Matthews 2001;
Heller et al. 2003). This means that for every unit of nonrenewable fossil fuel energy used to grow and harvest wilwww.frontiersinecology.org
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Sustainability of willow biomass crops
TA Volk et al.
416
100% Carbon closure
Assumes 0.25t / ha/yr
increase in soil carbon
Feedstock
production
Transportation
Power plant
construction
Net CO2 emissions:
• 46g CO2 equivalent/kwh
assuming no change in
soil carbon
• 0g CO2 equivalent/kwh
for 0.25t/ha/yr increase in
soil carbon.
used to keep pests below economic injury level by minimizing
the frequency, duration, extent,
and intensity of outbreaks (Tucker and Sage 1999). Better information on acceptable pest levels
needs to be developed so that IPM
can be more widely implemented.
One aspect of IPM that is actively
employed is planting each field
with different varieties that are
resistant to various pests and weed
competition (Mc-Cracken and
Dawson 2001).
Maintenance of socioeconomic
benefits
This criterion is the most difficult
to assess accurately, but is often
cited as an important value by
many stakeholders. While some
impacts, such as job creation and
monetary gains, can be measured
in economic terms, other important aspects, such as local community participation and the involvement of local and
regional government and non-government organizations,
are harder to quantify.
Willow’s low energy density, which limits transportation distances, and its organic nature, which limits storage time, result in a short spatial and temporal supply
chain. This characteristic means that crop and energy
expenditures are recirculated through the local economy,
which should translate into economic and social benefits
(IEA 2003). In contrast, about 80% of the energy used in
New York State is currently imported, resulting in a net
outflow of $1800 per person per year (NYSERDA 2001).
If a small portion of those funds were recirculated through
the local economy via the deployment of woody crops,
the positive impacts would be substantial.
Biomass energy has the potential to create more jobs per
unit of energy than other renewables or fossil fuels. In
Europe, almost 80 000 jobs related to woody and herbaceous energy crops will be created by 2020 (ECOTEC
1999). About 75 direct and indirect jobs will be created
for every 4000 ha of willow crops that are established in
New York state, generating over $500 000 per year in revenue for local and state governments (Proakis et al. 1999).
Due to the short supply chain of woody crops, active
participation by local stakeholders in project planning,
development, and operation is required to ensure success
and the realization of socioeconomic benefits (Anon
1996). Failure to engage and respond to local concerns
not only undermines the sustainability of the system, but
has resulted in the failure of projects, almost half the time
due to local opposition (Upreti in press).
Figure 6. Carbon cycle and energy return on investment results from life-cycle analysis of
willow biomass crops. The amount of CO2 produced during production, transportation, and
use of willow biomass is offset by the CO2 taken up during photosynthesis by the plants
growing in the field. Numbers in red indicate the flow of one unit of non-renewable fossil-fuel
energy into the system (left side) and the amount of biomass (center) or electrical energy
(right) produced. (Modified from Mann and Spath 1997, created by J Ballard.)
low, between 29 and 55 units of stored energy in biomass
are produced. If this biomass is transported to a power
plant and used to generate electricity, the net energy ratio
is 1:11 when the biomass is co-fired with coal (Heller et
al. 2003) and 1:16 for a gasification system (Mann and
Spath 1997). In contrast, the net energy ratio for ethanol
produced from corn is 1:1.3 (Shapouri et al. 2002) and
1:0.4 for electricity generated from coal and natural gas
(Mann and Spath 1999).
Maintenance of forest ecosystem productivity
and health
Consistently high production for seven or more rotations
is important for both biological and economic success.
Yields have increased up to 130% from first to second
rotation (Larson 2001; Volk et al. 2001) and were maintained or increased through the fourth rotation (Larson
2001; Figure 7). Part of the yield increase is due to
changes in above- and belowground allocation patterns.
More biomass is allocated to the aboveground portion in
later rotations, since the root system developed during
the first rotation remains intact. Data and experience
indicate that the productive capacity of woody crops can
be maintained with proper management. Poor management decisions, such as harvesting during the growing
season or failing to manage competing vegetation during
establishment, will undermine long-term productivity.
A variety of pests, including insects, diseases, weeds, and
browsing mammals, have the potential to limit willow crop
production. Integrated pest management (IPM) should be
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© The Ecological Society of America
TA Volk et al.
Concerns are expressed that deployment of woody crops
will compete for land used for food production. However,
in both North America and Europe, government programs
pay landowners to take millions of hectares of agricultural
land out of production. Woody and herbaceous energy
crops are projected to supply 1.5 quadrillion Btus annually
in the US by 2025 (EIA 2003). At current yields of 10 odt
per hectare per year, this would require only about 25% of
the 40 million ha of surplus and fallow agricultural land
that is suitable for these crops (Graham 1994). With
recent advances in breeding, the proportion required
could be closer to 15%, thereby avoiding conflict with
land used for agricultural production.
Biomass from woody crops is currently not cost-competitive with fossil fuels such as coal (Tharakan et al. in
press). Much of the price differential, however, can be
attributed to “market failure”. In conventional energy
markets, the social and environmental costs associated
with the production and use of fossil fuel use are mostly
ignored, resulting in an overprovision of fossil fuel-based
energy relative to socially efficient levels (Hohmeyer,
1992). Meanwhile, the positive benefits associated with
woody crops are not valued, leading to an underprovision
of this resource. In order to deploy woody crops, as well as
other forms of renewable energy, and reap the benefits
associated with them, society needs to be willing to pay
for some of their positive benefits or for the negative
social costs associated with fossil fuels (Tharakan et al.
in press).
Summary
Using the set of criteria discussed above, woody crops
are sustainable, in comparison to the agricultural land
practices and the current fossil fuel energy systems that
they will replace. They support a wide array of species
both above- and belowground and improve biodiversity
across the landscape. Their perennial nature, extensive
fine root systems, and ability to coppice provide protection and enhancement of soil and water attributes.
Biomass from woody crops produces electricity with no
net addition of CO2 to the atmosphere, in contrast to current fossil fuel systems. Commercial-scale experience and
research results indicate that woody crop productivity
can be maintained over multiple rotations. The short
supply chain ensures that the rural development and
environmental benefits associated with deployment and
use will accrue to the local community.
As with any form of land use, sustainability requires the
implementation of good management practices based on
a sound understanding of the system’s processes, properties, and interactions with the local community. Failure
to follow these practices could undermine the sustainability of the system.
Using these criteria as a framework for woody crops
shows that the system is sustainable. It also reveals that
there are weaknesses that can and should be addressed.
© The Ecological Society of America
Sustainability of willow biomass crops
417
Figure 7. Changes in yield over multiple rotations for five
different willow varieties grown as woody crops in Sweden
(Larsson 2001). L78183 is a reference variety, while the others
are hybrids, produced using traditional breeding techniques.
As the system is improved and implemented across the
landscape, its benefits should be reassessed using these
and other criteria to ensure that woody crops continue to
contribute to society’s quest for sustainability.
Acknowledgements
This work was supported by the US Department of Energy
through the Biomass Power for Rural Development
Program, the US Department of Agriculture Cooperative
State Research Education and Extension Service, and the
New York State Energy and Research and Development
Authority. The authors thank Jennifer Ballard for refining
and creating the figures.
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