INVITED EDITORIAL

Large-scale bioenergy from additional harvest of forest

biomass is neither sustainable nor greenhouse gas neutral

ERNST-DETLEF SCHULZE*, CHRISTIAN KO ̈ RNER † , BEVERLY E. LAW ‡ ,

HELMUT HABERL § and SEBA ST IAAN LUY S SAERT¶

*Max-Planck Institute for Biogeochemistry, Box 100164, 07701, Jena, Germany, †Institute of Botany, University of Basel, 4056,

Basel, Switzerland, ‡Department of Forest Ecosystems and Society, Oregon State University, 321 Richardson Hall, Corvallis,

OR, USA, §Institute of Social Ecology Vienna, Alpen-Adria University Klagenfurt, Wien, Graz Schottenfeldgasse 29, 1070,

Vienna, Austria, ¶Laboratoire des Sciences du Climat et de l’Environnement, IPSL, CEA CNRS UVSQ, Centre d’Etudes Ormes

des Merisiers, 91191, Gif-sur-Yvette, France

Abstract

Owing to the peculiarities of forest net primary production humans would appropriate ca. 60% of the global

increment of woody biomass if forest biomass were to produce 20% of current global primary energy supply.

We argue that such an increase in biomass harvest would result in younger forests, lower biomass pools,

depleted soil nutrient stocks and a loss of other ecosystem functions. The proposed strategy is likely to miss its

main objective, i.e. to reduce greenhouse gas (GHG) emissions, because it would result in a reduction of biomass

pools that may take decades to centuries to be paid back by fossil fuel substitution, if paid back at all. Eventually, depleted soil fertility will make the production unsustainable and require fertilization, which in turn

increases GHG emissions due to N2O emissions. Hence, large-scale production of bioenergy from forest biomass

is neither sustainable nor GHG neutral.

Keywords: bioenergy, biomass, ecosystem function, forestry, greenhouse gas emission, human appropriation of net primary

production

Received 28 December 2011 and accepted 31 January 2012

Climate change impacts resulting from fossil fuel combustion challenge humanity to find energy alternatives

that would reduce greenhouse gas (GHG) emissions.

One important option in this context is bioenergy. There

is a wealth of literature on actual yields of different

energy crops and production systems (WBGU, 2009;

NRC, 2011). Beringer et al. (2011) estimate that 15–25%

of global primary energy could come from bioenergy in

the year 2050. A prominent recent assessment suggested

that bioenergy provision could even be up to

500 EJ yr1

, more than current global fossil energy use

(Chum et al., 2012) and that GHG mitigation could be

sustained under future climate conditions (Liberloo

et al., 2010).

Western and developing countries are on a course to

increase bioenergy production substantially. For example, the United States enacted the Renewable Fuels Standard as part of the 2005 Energy Policy Act and

amended it in 2007, mandating the use of renewable

fuels for transportation from 2008 to 2022 and beyond.

In addition, 20% of all EU energy consumption is to

come from renewable sources by 2020 with bioenergy

as a focal point in this effort (COM, 2006a). In 2005, the

European Commission adopted the Biomass Action

Plan (COM, 2005) and in 2006 the Strategy for Biofuels

(COM, 2006b), both of which aim to increase the supply

and demand for biomass. Strategies that could substantially diminish our dependence on fossil fuels without

competing with food production include substitution

with bioenergy from forests (Tilman et al., 2009), either

by direct combustion near the source or by conversion

to cellulosic ethanol. There are important questions

about GHG reduction, economic viability, sustainability

and environmental consequences of these actions.

Greenhouse gas reduction

The general assumption that bioenergy combustion is

carbon-neutral is not valid because it ignores emissions

due to decreasing standing biomass and contribution to

the land-based carbon sink. The notion of carbon-neutrality is based on the assumption that CO2 emissions

from bioenergy use are balanced by plant growth, but

Correspondence: Ernst-Detlef Schulze, tel. + 49 3641 576 100,

fax + 49 3641 577 102, e-mail: dschulze@bgc-jena.mpg.de

© 2012 Blackwell Publishing Ltd 1

GCB Bioenergy (2012), doi: 10.1111/j.1757-1707.2012.01169.x
this reasoning makes a ‘baseline error’ by neglecting the

plant growth and consequent C-sequestration that

would occur in the absence of bioenergy production

(Searchinger, 2010; Hudiburg et al., 2011), and it ignores

the fact that fossil fuels are needed for land management, harvest and bioenergy processing.

Recent life cycle assessments cast doubt on the existence of emission savings of bioenergy substitution from

forests. In the Pacific Northwest United States, policies

are being developed for broad-scale thinning of forests

for bioenergy production, with the assumed added benefit of minimizing risk of crown fires. This includes forests of all ages and thus timeframes of biomass

accumulation. However, a recent study suggests that

more carbon would be harvested and emitted in fire

risk reduction than would be emitted from fires (Hudiburg et al., 2011). Furthermore, policies allow thinning

of mesic forests with long fire return intervals, and

removal of larger merchantable trees to make it economically feasible for industry to remove the smaller

trees for bioenergy. These actions would lead to even

larger GHG emissions beyond those of contemporary

forest practices (Hudiburg et al., 2011).

Increased GHG emissions from bioenergy use are

mainly due to consumption of the current carbon pool

and from a permanent reduction of the forest carbon

stock resulting from increased biomass harvest (Holtsmark, 2011). When consumption exceeds growth,

today’s harvest is carbon that took decades to centuries

to accumulate and results in a reduction of biomass

compared to the current biomass pool (Holtsmark, 2011;

Hudiburg et al., 2011). Hence, it is another example of

‘slow in and fast out’ (Ko ̈rner, 2003). Consequently,

reduction in forest carbon stocks has been shown to at

least cancel any GHG reductions from less use of fossil

fuel over decadal time spans (Haberl et al., 2003;

Mc-kechnie et al., 2011). Boreal forests with relatively

low carbon sequestration potential may take centuries

before permanent reduction of the carbon stocks resulting

from increased bioenergy harvest is repaid by reduced

emissions from fossil fuels (Holtsmark, 2011). For more

productive temperate regions, an infinite payback time

was found implying that lower GHG emissions are

achieved through C-sequestration in forests rather than

through bioenergy production (Hudiburg et al., 2011).

Recent studies of the differences in timing of CO2

emissions from bioenergy production and forest carbon

uptake (Cherubini et al., 2011a,b) suggest that the

‘upfront’ CO2 emitted during biomass harvest and combustion stays in the atmosphere for decades before the

CO2 is removed by the growing forest. It results in a

‘pulse’ of warming in the first decades of bioenergy

implementation. This contrasts calls for a rapid reduction of the growth rate of climate forcing (Friedlingstein

et al., 2011) required to achieve the policy of limiting

warming to 2 °C.

The initially reported emission savings from forest

bioenergy are based on erroneous assumptions in the

accounting schemes. Studies that corrected these errors

suggest that forest management that reduces the current

biomass pool is unlikely to result in the envisioned

emissions savings at all, and certainly not over the next

decades.

Economic viability

Emerging technologies such as biofuel refineries and

combined heat and power plants have to compete

against established technologies applied in coal, gas and

nuclear power plants. In the United States, a recent

National Research Council report concluded that only

in an economic environment characterized by high oil

prices (e.g. >$191 per barrel), technological breakthroughs (cellulosic ethanol) and at a high implicit or

actual carbon price would biofuels be cost-competitive

with petroleum-based fuel (NRC, 2011). Hence, incentives favouring bioenergy (i.e. production quota, subsidies, tax cuts) will be needed to complement or even

replace fossil fuel-based technologies (Schneider & Kaltschmitt, 2000; Ryan et al., 2006; Ahtikoski et al., 2008;

NRC, 2011).

Schemes favouring the economics of one practice or

technology over another often lead to unanticipated

side-effects. For example, side-effects have been documented for the Common Agricultural Policy of the

European Union (Macdonald et al., 2000; Stoate et al.,

2001), and forest-based bioenergy production would

seem to be similar. In Germany, where bioenergy is subsidized, the market price for woody biomass increased

from 8 to 10 € m3 in 2005 to 46 € m3 for hardwood

and 30–60 € m3 for coniferous wood in 2010. Prices for

woody biomass for bioenergy now reach 60–70% of saw

log prices (Waldbesitzerverband, 2010; wood sales by

one of the authors). Such prices discourage the production of quality timber and make root extraction and total

tree use attractive options despite the documented unfavourable effects on soil carbon, soil water and nutrient

management (Johnson & Todd, 1998; Johnson & Curtis,

2001; Burschel & Huss, 2009; Peckham & Gower, 2011).

For the German example, the price increase is driven

by the installation of distributed bioenergy plants and

the competitive market of other uses for biomass, such

as wood for production of cellulose. Although the

details will differ among regions and countries, increasing imports by developed nations is the most likely

response to an increasing wood demand (Seintsch,

2010), because total wood harvest has not substantially

changed in the developed world (i.e. ~1.4 9 109 m3

© 2012 Blackwell Publishing Ltd, GCB Bioenergy, doi: 10.1111/j.1757-1707.2012.01169.x

2 E.-D. SCHULZE et al.
between 1990 and 2010 in Europe and North America,

FAO, 2010). Increased imports are likely to be met

through land-use (intensity) change in other regions

(lateral transfer of emissions). In the case of increased

imports, these are most likely met by harvesting previously unmanaged forests or forest plantations. Thus,

similar to crop-based production systems, forest-based

bioenergy requires additional land, contrary to previous

expectations (Tilman et al., 2009). Increased wood

imports, thus, represent a global footprint of local

energy policies and should be accounted for in life cycle

assessment of wood-based bioenergy.

Reduced manufacturing residue losses and other

technological advances such as glued wood-based elements initiated a trend towards shorter rotations and

thus younger forests. However, the economics of bioenergy production supported by existing subsidy schemes

is expected to reduce rotation length to its lowest limit

and promote questionable management practices and

increased dependency on wood imports. Further, high

prices for biomass will discourage forest owners from

investments in long rotations, resulting in a shortage of

quality timber. Given the time required to produce

high-quality timber, such shortage cannot be remedied

by short-term (economic) incentives.

Environmental consequences

Homogeneous young stands with a low biomass resulting from bioenergy harvest are less likely to serve as

habitat for species that depend on structural complexity.

It is possible that succession following disturbance can

lead to young stands that have functional complexity

analogous to that of old forests; however, this successional pathway would likely occur only under natural

succession (Donato et al., 2011). A lower structural complexity, and removal of understory species, is expected

to result in a loss of forest biodiversity and function. It

would reverse the trend towards higher biomass of

dead wood (i.e. the Northwest Forest Plan in the United

States) to maintain the diversity of xylobiontic species.

Cumulative impacts of bioenergy-related management activities that modify vegetation, soil and hydrologic conditions are likely to influence erosion rates and

flooding and lead to increased annual runoff and fish

habitat degradation of streams (Elliot et al., 2010).

Young uniform stands with low compared to high

standing biomass have less aesthetic value for recreation

(Tahvanainen et al., 2001) and are less efficient in avalanche control and slope stabilization in mountains

owing to larger and more frequent cutting (Brang,

2001). A potential advantage is that younger forests

with shorter rotations offer opportunities for assisted

migration, although there is great uncertainty in

winners and losers (species, provenances, genotypes) in

a future climate (Larsen, 1995; Millar et al., 2007; Pedlar

et al., 2011). Plantations, however, largely contribute to

pathogen spread, such as rust disease (Royle & Hubbes,

1992).

Forests offer several important ecosystem services in

addition to biomass and some would be jeopardized by

the bioenergy-associated transition from high to low

standing biomass. Agriculture provides a visible example for abandoning most ecosystem services except biomass production (Foley et al., 2005); communities in

intensive agricultural regions often rely on (nearby) forested water sheds for drinking water, recreation and

offsetting GHG emissions from intensive agriculture

(Schulze et al., 2009).

Sustainability

From a historical perspective, a transition from forest

biomass burning to fossil fuels literally fuelled the

industrial revolution, and consequently, caused rapid

climate change. However, the collapse of biomass use

enabled the recovery of largely degraded forest ecosystems (Gingrich et al., 2007). Partly due to recovery from

previous (mis)use, C-sequestration is especially strong

over Europe (Ciais et al., 2008; Luyssaert et al., 2010)

and the United States (Williams et al., 2011). As such,

C-sequestration can be considered a side-effect of the

transition of energy sources from wood to fossil fuels

(Erb et al., 2008). Industrial-scale use of forest biomass

for energy production would likely reverse this trend or

at least reduce the carbon sink strength of forests (Haberl et al., 2003; Holtsmark, 2011; Hudiburg et al., 2011).

The historical forest resource use in Europe and the

United States is the present day situation in Africa. For

example, southern African miombo forests have been

degraded into shrubland as a result of charcoal production, where charcoal is the main energy source for rural

communities even at a very low level of total energy

consumption (Kutsch et al., 2011).

A widespread misconception is that the most productive forests are necessarily the strongest carbon sinks.

Actually, net primary productivity of forests is typically

negatively correlated with the cumulative amount of

carbon stored in biomass (Fig. 1). In reality, old forests

show lower NPP but store the largest amount of carbon

(Luyssaert et al., 2008; Hudiburg et al., 2009; Bugmann

& Bigler, 2011) because slow growing forest live longer

than fast growing forest (Schulman, 1954; Bigler & Veblen, 2009). Hence, on areas currently forested, any fast

rotation management and use for fossil fuel substitution

is reducing forest carbon sequestration. At regional

scales, a permanent increase in annual wood harvest

results in a permanent reduction in the amount of

© 2012 Blackwell Publishing Ltd, GCB Bioenergy, doi: 10.1111/j.1757-1707.2012.01169.x

BIOENERGY 3
carbon stored in forests at the regional scale due to a

lower average stand age (Ko ̈rner, 2009; Holtsmark, 2011).

Globally, ~7% of global forest net primary production

(NPP) outside wilderness areas is used by humans

annually (Haberl et al., 2007a). In Europe, human appropriation of forest NPP reaches ~15% (Luyssaert et al.,

2010). Thus, even in the absence of industrial production of wood-based bioenergy, humans already seize a

remarkable share of forest production. To produce 20%

of current primary energy consumption from woodbased bioenergy, as suggested by policy targets, it

would require more than doubling the global human

appropriation of NPP (HANPP) to 18–21% (Table 1;

ratio of row 1 and 6). Such an increase in human appropriation would have serious consequences for global

forests. Due to its nature, much of forest NPP cannot be

harvested, e.g. fine root NPP, NPP for mycorrhizal associations and NPP in volatile organic emissions. Further,

forests are harvested after decades of growth; hence,

much of the NPP is already consumed by herbivores,

added to the litter pool or decomposed in the detritus

food chains long before harvest, e.g. leaves, fruits, fine

Fig. 1 Land management trade-off: maximizing productivity vs. carbon stocks. Given fixed resource availability, land managers can

maintain highly productive ecosystems with a low standing biomass such as grasslands. The dominant tissues are leaves and roots

with a low C/N ratio (~50). The same resources could be used to grow forest. With time forest accumulate considerable amounts of

carbon in their biomass but forest that grow old have a lower net primary production than young forest and grasslands. Woody biomass has high C/N ratios (~400) and with an increasing share of woody biomass in the total biomass, the C/N ratio of the ecosystem

decreases. Consequently, the time integral of productivity will be lower for an old forest compared with grassland, but at the same

time, the time integral of nitrogen export will be lower for an old forest (closed nitrogen cycle) compared with a grassland (open

nitrogen cycle). Hence, increasing the biomass pool size is the sustainable way of capitalizing from forests in the C-sequestration vs.

C substitution debate. Ranges in the figure are for temperate ecosystems based on (Van Tuyl et al., 2005; Luyssaert et al., 2007, 2008;

Schulze et al., 2009; Keith et al., 2009).

Table 1 Global HANPP in forests in the year 2000 and future HANPP that would result from providing 20% of world primary

energy from forest harvest. NPP denotes net primary production and HANPP the human appropriation of net primary production.

Using a gross caloric value of 19 kJ g1 forest biomass or 38 kJ g1 biomass carbon and a net caloric value of 41.9 GJ for 1 ton of oil

equivalent. Conversion from net to gross calorific value was based on the following multipliers (gross/net): coal 1.1, oil 1.06, natural

gas 1.11 and biomass 1.1 (Haberl et al., 2006)

Global C-flux

(PgC yr1

)

Energy

equivalent (EJ yr1) Source

(1) Current NPP of forest ecosystems 27–29 1030–1100 Haberl et al. (2007a) and

Pan et al. (2011)

(1a) Belowground NPP (40%) 10–11 – Luyssaert et al. (2007)

(1b) Leaf + twigs NPP (30%) 8.4–8.7 – Luyssaert et al. (2007)

(1c) Aboveground woody NPP (30%) 8.4–8.7 330 Luyssaert et al. (2007)

(2) Primary energy use in 2006–2008 – 550 IEA (2008) and BP (2009)

(3) Global fossil energy use in 2006–2008 6–7 450 IEA (2008) and BP (2009)

(4) Additional fuel wood to produce

20% of primary energy

2.3 87 From 3 and 5

(5) NPP lost in harvest (10–30%) 0.5–1.4 19–53 From 2 and 6

(6) New HANPP level in forests 4.4–5.3 170–200 From 2, 6 and 7

© 2012 Blackwell Publishing Ltd, GCB Bioenergy, doi: 10.1111/j.1757-1707.2012.01169.x

4 E.-D. SCHULZE et al.
roots, mycorrhiza and plants in early succession stages.

Last, part of the NPP could be harvested but typically

has no economic value, e.g. perennials, mosses and

lichens. Consequently, the maximum HANPP is about

30% of the total NPP; hence, the proposed HANPP of

18–21% already represents ca. 60% of the global increment of woody biomass (Table 1; ratio of rows 1c and

6). Note that our maximum level of harvestable increment of woody biomass is most likely overestimated

because the estimate did not account for economic (e.g.

distance to population centre), logistic (e.g. steep mountain slopes) and legal (e.g. conservation areas) constraints on harvest. In addition to the increased GHG

emissions that would result from such a programme

due to reduced biomass stocks (see above), this increase

in human appropriation of forest production would

likely contribute to forest biodiversity loss, according to

recent evidence on the correlation between HANPP and

species richness (Haberl et al., 2005, 2007b).

Typically, the most fertile lands are in urban and agricultural use (Scott et al., 2001), leaving the poorer soils

for forest use. The industrial-scale of envisioned forest

bioenergy production would export substantial amounts

of nutrients, further depleting the soil nutrient stock,

particularly if wood removal includes relatively nutrient-rich biomass residues (slash) and root stocks (Peckham & Gower, 2011) as for total tree use. Nutrient and

cation losses would have to be compensated for by fertilization, which in turn increases GHG emissions and

increases N and P levels in nearby rivers leading to

eutrophication of aquatic ecosystems (for a crop related

example see Secchi et al., 2011).

A persistent 60–70% appropriation of woody biomass

increment for bioenergy production from forest harvest

over decades will erode current biomass pools, lower

average stand age, deplete soil fertility and could thus

only be sustained by amendments to nitrogen and

phosphorous-depleted soils, activities that also produce

GHG (N2O) emissions.

Conclusion

Although bioenergy from forest harvest could supply

~20% of current energy consumption, this would

increase human appropriation of NPP in forests to ~20%

which is equivalent to 60–70% of the global increment

in woody biomass. We argue that the scale of such a

strategy will result in shorter rotations, younger forests,

lower biomass pools and depleted soil nutrient capital.

This strategy is likely to miss its main objective to

reduce GHG emissions because depleted soil fertility

requires fertilization that would increase GHG emissions, and because deterioration of current biomass

pools requires decades to centuries to be paid back by

fossil fuel substitution, if paid back at all. Further,

shorter rotations would simplify canopy structure and

composition, impacting ecosystem diversity, function

and habitat. In our opinion, reasonable alternatives are

afforestation of lands that once carried forests and

allowing existing forests to provide a range of ecosystem services. Yet, on arable or pasture land, such a strategy would compete with food and fodder production.

Society should fully quantify direct and indirect GHG

emissions associated with energy alternatives and associated consequences prior to making policy commitments that have long-term effects on global forests.

Reasonable alternatives for reducing GHG emissions on

the order of the proposed bioenergy substitution

include increased energy efficiency and reduced waste

of energy via technological improvements and behaviour modification. There is a substantial risk of sacrifying forest integrity and sustainability for maintaining or

even increasing energy production with no guarantee to

mitigate climate change.

Acknowledgements

E. D. S. thanks the Max-Planck Society for support. B. E. L.’s

research was supported by the Office of Science (BER), US

Department of Energy. H. H. acknowledges funding by the

Austrian Science Funds FWF (project P20812-G11), by the

Austrian Academy of Sciences (Global Change Programme)

and by the Austrian Ministry of Research and Science

(BMWF, proVision programme). S. L. is funded by ERC

Starting Grant 242564. C. K. was supported by ERC advanced

grant 233399.

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