Residential Wood Heating:
the Forest, the Atmosphere
and the Public Consciousness
This paper was written for the 1993 Air and Waste
Management Association annual conference by Dr. Ole Q. Hendrickson,
Forest Ecologist, Forestry Canada and John Gulland, Gulland Associates
Inc.
"We maintain that current practices of obtaining
and using wood energy are the foundation on which an expanded
residential wood heating sector should rest, and that such an expansion
is compatible with maintaining forest biodiversity and reducing
greenhouse gas emissions."
It is generally agreed by both energy and forestry
scientists that, provided harvesting is conducted in a sustainable
manner, the combustion of wood for energy production is essentially
carbon dioxide neutral when the normal forest regeneration period is
considered. When wood combustion replaces the consumption of fossil
fuels, however, the net reduction in carbon dioxide release is almost
immediate.
In addition to the requirement of
sustainable forestry practices, the maintenance of site biodiversity
must also be considered. A preliminary review of the literature reveals
that periodic selective harvesting can actually have a positive impact
on the biodiversity of the forest. Despite the fact that the harvesting,
processing and transportation of wood fuel invariably consumes fossil
fuels, it has been shown in case studies that the energy return on
investment can easily exceed a ratio of 25:1.
Approximately 20 percent of the single
family dwellings in Canada are heated to some extent with wood and the
potential exists for an increasing contribution of wood fuel to
residential energy requirements. However, there is evidence of confusion
among the public regarding the environmental impact of woodburning,
particularly as it relates to CO2 emissions and carbon storage in
forests. This confusion could impede the increased use of wood for
residential heating because it calls into question the appropriateness
of using wood for energy purposes.
The forms of residential wood energy use
that have evolved in rural North America provide important but neglected
models of sustainable development. This could serve as the central theme
of a public information program to clarify the role of wood energy in
the reduction of greenhouse gas emissions.
The formation of an effective cartel of oil-producing nations in the
early 1970s and the accompanying rise in the price of oil prompted
developed nations to increase research in alternative fuels, including
biomass. One collective response was the founding of the International
Energy Agency (IEA) in 1974, and the signing of an IEA Forest Energy
Agreement in 1978 (renamed the Bioenergy Implementing Agreement in
1986). Individual countries also formed biomass energy programs. These
programs, however, largely overlooked the growing public use of wood
burning appliances and the need for a sustained supply of wood fuel for
residential use. The need to encourage a transition from fossil fuels to
renewable energy sources such as biomass is again high on the
international agenda. It received a boost from the 1992 United Nations
Conference on Environment and Development (UNCED) in Rio. One of the
UNCED documents states:
The need to control atmospheric emissions of greenhouse and other
gases and substances will increasingly need to be based on efficiency in
energy production, transmission, distribution and consumption, and on
growing reliance on environmentally sound energy systems, particularly
new and renewable sources of energy.
This document defines "new and
renewable energy sources" as solar thermal, solar photovoltaic,
wind, hydro, biomass, geothermal, ocean, animal and human power.
Another stimulus for a renewed interest in
biomass fuels is the merging of the disciplines of ecology and
economics. To encourage a gradual displacement of non-renewable energy
sources with renewables, some economists favour "severance
taxes" when non-renewable resources are extracted, or when
"conditionally renewable resources" such as biomass are
extracted in unsustainable fashion. Other economists argue that the
failure of the market to account for environmental, social and economic
costs caused by fossil fuel use (collectively referred to as
externalities) should be alleviated by carbon taxes or other energy
pricing reforms.
In this paper, we will develop the thesis
that the forms of wood energy use that have evolved in rural North
America provide important but neglected models of sustainable
development. We maintain that current practices of obtaining and using
wood energy are the foundation on which an expanded residential wood
heating sector should rest, and that such an expansion is compatible
with maintaining forest biodiversity and reducing greenhouse gas
emissions. A public information campaign that builds upon the positive
views of current residential wood energy users could lead to a
widespread social consensus that fuelwood must be considered as a key
factor in sustainable forest management planning.
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As is true for any form of timber harvest, cutting trees for
residential fuelwood may have a variety of impacts on forest ecosystems.
Some areas that have received considerable scientific attention include
changes in forest biodiversity, soil loss and compaction, altered
nutrient cycling processes, changes in microclimate, and effects on
streams and riparian zones. A thorough treatment of these topics is
beyond the scope of this paper. However, a brief analysis of impacts of
residential wood heating on forest biodiversity is provided.
Two types of residential wood heating can
be distinguished in North America. The principal form is single-unit,
owner-operated wood stoves and furnaces. The second, and far less common
form is multi-unit systems that generally require a full-time skilled
operator (i.e., multi-unit housing and district heating systems).
Fuelwood markets are largely informal and unregulated. Some fuelwood
cutting is done by permit on public lands, but most probably occurs on
private lands. Home owners largely burn "chunk wood" as
opposed to wood chips or pellets.
Although many studies have examined
impacts of wood energy use on the forest environment, nearly all have
focused on industrial scale systems. Industrial systems generally
involve clearcutting with intensified biomass removal and/or shortened
rotation periods, and are largely irrelevant to current patterns of
residential wood energy supply and use. Studies of partial harvesting
systems are likely to be a better guide to current impacts of
residential fuelwood cutting on the forest.
In forest regions with shade-tolerant
species able to regenerate under a full or partial canopy, fuelwood
markets may help subsidize "improvement cuts" designed to
remove inferior trees and leave trees with potential for high-value saw
logs. Apart from its economic benefits, periodic harvesting of northern
hardwoods also can have positive effects on biodiversity. Compared to
uncut controls, selectively cut deciduous forests in southern Ontario
had higher value for conservation of herbaceous plants. Studies in the
Lake States have shown that uneven-aged selection cuts provide higher
diversity of regenerating tree species, as well as greater long-term
economic returns. Although species diversity can also be high following
clearcutting of northern hardwoods, this practice greatly degrades
future commercial stand potential. The traditional practice of cutting
all stems greater than a certain diameter (e.g., 20 cm) tends to promote
formation of sugar maple monocultures of low diversity.
Some landowners feel that an
environmentally superior practice is to only harvest dead trees for
firewood. However, this foregoes opportunities to increase wood lot
biodiversity and economic returns by selective cutting of live trees.
Furthermore, dead snags provide critical nesting habitat for birds and
certain mammals. A well managed wood lot provides a steady economic
return and is therefore less vulnerable to pressures for conversion to
non-forest use.
Single-unit residential users,
particularly those in central North America, prefer higher quality
hardwoods, such as red oak or sugar maple. This preference is largely
based on tradition, but could be modified by public information
programs. It could be pointed out to these users that home heating with
wood in western and northern parts of the continent is done with
softwood species.
Some of the potential for future expansion
in residential markets is likely to be associated with district heating
systems, which may be fuelled with any available species. A concern is
that increased demand for wood chips may perpetuate practices such as
clearcutting that have allowed hardwood forests to reach their current
degraded condition over much of eastern and central North America.
Public forestry agencies have a critical role to play in ensuring that
demand for wood fuel is regulated within constraints of long- term
restoration and sustainability of forest ecosystems.
It is still unclear what proportion of
future residential energy supplies may be provided by short rotation
intensively cultured plantations of hybrid poplar or willow. These are
comparable to agricultural crops in their needs for site preparation,
weed and insect control, fertilization, and intensive breeding programs
to select fast growing stock. Low yields and high input costs currently
make wood energy plantations non-viable. If this changes in the future,
they will probably be established on marginal crop and pasture lands,
where the available pool of skilled farm labour and soil quality are
sufficiently high to support the required intensity of management.
Conversion of natural forests to energy plantations should be resisted
owing to negative impacts on biodiversity, watershed quality, and so
forth.
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Carbon (C) storage in managed forests has been examined in a number
of recent studies whose findings can be placed in the context of
residential wood energy. A complete carbon accounting requires that
attention be given not only to C stored in trees and soil, but also in
forest products. On the negative side of the ledger are expenditures of
fossil fuel C for forest management. For the purposes of this paper, the
key consideration in forest carbon accounting is the use of biomass as a
renewable fuel to offset non- renewable fossil fuels. Fuelwood use
benefits the atmosphere because it causes no net increase in CO2
if forests are managed sustainably, while displacing the use of fossil
fuels that do cause such a net increase.
Some foresters have maintained that
old-growth forests, whose net carbon uptake is around zero, should be
converted to young plantations with active carbon uptake. However,
carbon is released to the atmosphere at a much faster rate following
cutting of a high-biomass old-growth forest (owing to decay of logging
residues, mill wastes, forest products, etc.) than carbon is absorbed by
the plantation that replaces it. As a result, old-growth conversion
leads to massive losses of stored carbon, and can not be recommended as
a measure to combat increasing atmospheric CO2
levels. Old-growth forests may deserve protection for other features
such as their high diversity of taxonomic groups such as lichens and
bryophytes.
Another topic of great current interest is
the potential for carbon storage by afforesting currently unproductive
lands. Dewar and Cannell have modelled long-term carbon storage in soil,
trees, and forest products under different plantation management
scenarios. They found that short rotations do not achieve a high carbon
storage, owing to limited inputs of carbon to soils, and lower average
long-term carbon storage in biomass. Other things being equal, extended
rotations are preferable for carbon storage. Lifetimes for forest
products are poorly known and inject some uncertainty into this
conclusion, but evidence suggests that wood products may represent less
than a quarter of total carbon when soils are included.
One area of concern is the impact of wood
smoke on local air quality. This is being addressed through government
regulations that control particulate emissions from residential
appliances:
. . . the increased use of wood as an alternative to fossil fuels
could not be promoted if the particulate emissions from wood stoves were
creating significant airshed contamination. By forcing manufacturers to
design cleaner burning stoves, the regulations have helped the industry
to develop the image of woodburning as an environmentally friendly
alternative to fossil fuels.
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In the case of native northern hardwood forests, it should be
possible to simultaneously maintain high long-term average biomass C and
acceptable yields of fuelwood (and other products).
Wood lot owners who forego the short-term
economic rewards of clearcutting in favour of selective logging of
shade-tolerant species under natural regeneration regimes are helping
reduce atmospheric CO2 levels, and should be
recognized for their contribution. This is the normal practice on
well-managed private forests in southern Canada. Figure 1 shows that
selective cutting of tolerant hardwoods can easily lead to long-term
average carbon storage levels 50 percent higher than those achievable
with clearcutting.
Figure 1. Impact on forest carbon storage of
50 year rotation clearcutting versus 25 year
rotation selective harvesting in a northern
hardwood forest.
Certain forest types can not be managed
under selection regimes. For these forests, extended rotations can still
provide significant gains in long-term average C storage. Clear-cut
harvesting at so-called "financial maturity" can result in
long-term C storage only 20 percent of maximum, partly owing to lags in
C accumulation during early stand development. Early harvesting is also
likely to increase net fossil fuel C costs per unit biomass harvested,
as outlined in the following section.
Where clearcutting is the normal
management prescription, the question arises as to whether intermediate
thinnings and final harvest residues should be used for bioenergy. These
residues normally decay quickly and would cause little net increase in
soil C, so it might seem best to use them to displace fossil fuels.
However, many authors have pointed out that use of "unmerchantable"
branches and foliage represents a small net gain in energy in exchange
for a large net nutrient drain on the site. In Sweden, use of harvest
residues for bioenergy is restricted on nutrient-poor sites (e.g.,
shallow or sandy soils). Scientific evidence for declining site
productivity with intensive harvesting is limited, but this is
inherently a long-term concern and regulatory agencies should consider
the need for a precautionary approach. Industrial full-tree harvesting
systems that leave piles of branches and tops by the roadside are
clearly the worst of all possible worlds, as the slash provides no
benefits either in terms of energy or nutrition.
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Obtaining maximum net carbon productivity, or net carbon yield per
unit time, is the main goal for an energy plantation. An increased
long-term average C storage in above ground biomass and soil becomes a
secondary consideration. Although there may be an increase in long-term
average carbon storage on the site, this is a one-time benefit that (in
theory) will be outweighed in time by repeated displacements of fossil
fuel C in the form of harvested biomass C.
Calculating net carbon yield requires that
fossil fuel C costs (for stock production, site preparation, planting,
initial fertilization and weed control, etc.) be deducted from biomass C
gains. Some of these costs are independent of rotation length, and will
therefore be reduced on a rotation-averaged basis if rotations are
extended. Other costs, such as harvesting and transportation, increase
somewhat for older plantations, but do not keep pace with biomass
increases owing to economies of scale in handling larger diameter
materials.
Table 1. Impacts of fixed fossil fuel costs on optimal rotation age
in a bioenergy plantation.
------------------Age----------------
0-4 5-8 9-12 13-16 17-20
-----------------------------------------------------------
Annual C gain 5.0 4.0 3.0 2.0 1.0
Cumulative biomass-C 20.0 36.0 48.0 56.0 60.0
Average C gain 5.0 4.5 4.0 3.5 3.0
Average fuel C cost 5.0 2.5 1.75 1.25 1.0
Net annual C gain 0.0 2.0 2.25 2.25 2.0
Note: Fossil fuel costs (for site preparation, planting,
early weed control, etc.) are assumed to be 20 carbon
units, regardless of final age of harvesting.
Table 1 gives a hypothetical example of a hybrid poplar plantation in
which mean annual C increment of biomass is maximum during years 1-4,
and declines steadily thereafter. If no fossil fuel is required to
produce this biomass, the optimum rotation age is clearly four years.
However, if we assume that fixed fossil fuel C costs amount to 20 units
per rotation, the first four years of growth are needed just to recoup
our investment in fossil fuel C. In order to maximize the difference
between mean annual increment and these fixed fossil fuel costs,
harvesting should be delayed until age 12-16, even though growth has
decreased dramatically by that time. Allowing for reduced establishment
costs in subsequent rotations via coppicing and including data for costs
that vary with biomass would reduce this optimum rotation age somewhat,
but the point remains that energy considerations do not favour shortened
rotations in forestry. Assuming a constant fossil fuel C cost per unit
biomass C produced in an energy plantation makes no more sense than
assuming that the relation between GNP and energy consumption is
constant. One important consideration is that as trees mature, they
become progressively more efficient in the use of nitrogen, phosphorus,
and other essential elements. Energy costs to replace these elements
over successive rotations are reduced accordingly.
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A number of recent studies have examined how fossil fuel energy costs
for harvesting, processing and transporting woody biomass compare with
the energy content of materials delivered to the user. Gingerich and
Hendrickson recently conducted a case study of a whole-tree chipping
operation that supplies the district heating system for the University
of Prince Edward Island. They measured an "energy return on
investment" (EROI) at 27.6:1 (i.e., one unit of non-renewable
fossil fuel energy was required to provide 27.6 units of renewable
energy in the form of wood chips). A 240 km round trip to transport the
chips accounted for the largest fraction of total fossil fuel use, and
was targeted for further improvements in EROI.
Mechanization of harvesting tends to
involve a trade-off between labour inputs and fossil fuel inputs. Higher
manual labour inputs (e.g., chain saws instead of mechanical harvesters)
and lower transport distances in most private wood lot harvesting
operations would tend to increase EROI.
It is likely that EROI will be quite high
for farm wood lot management, somewhat lower for district heating
systems fuelled with wood chips, still lower for use of wood pellets
made from mill wastes, and lowest in short-rotation energy plantations.
Some authors feel that full accounting of fossil fuel costs of stock
production, site preparation, planting, weed control, fertilizers,
harvesting, processing, etc. for bioenergy plantations will reveal that
they provide essentially no net energy gains. There is clearly a need
for EROI analysis to supplement economic studies of energy plantations.
One might also ask why so little attention is given to wood lot
management, when this appears to provide clear benefits in terms of low
fossil fuel inputs and high long-term biomass C storage.
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According to Statistics Canada figures almost 1.4 million households,
or about 21 percent of the single family dwellings in Canada, report the
use of wood as either a principal or supplementary heating fuel (14
percent of total households). The use of wood as the principal heating
fuel is reported by 6.5 percent of those who live in single family
dwellings and as a supplementary heating fuel by 14.2 percent. These
percentages have not changed substantially during the past decade.
Among the households that use wood for
heating, there has been a gradual shift away from central heating with
wood using furnaces towards space heating using stoves and fireplaces.
In 1982, those reporting wood as their principal heating fuel burned the
wood with equal frequency in central furnaces and heating stoves. By
1991, the relationship had changed significantly to the extent that
central heating was about half as frequent as space heating. A very
large majority (94%) of those who use wood as a supplementary fuel do so
using a space heating stove.
This shift in equipment usage patterns
reflects the concurrent evolution of woodburning appliance technology.
In recent years the technology of space heating appliances such as wood
stoves, fireplace inserts and heating fireplaces has developed rapidly,
due in part to environmental legislation in the United States that
mandates low emission combustion systems in these classes of appliance.
Another development that has influenced the selection patterns is the
"glass air wash" systems that keep door glass clear for
unobstructed viewing of the fire. At the same time, Canadian houses have
been made more energy efficient, making whole-house heating with a space
heater more effective. Together, these developments have improved the
consumer appeal of space heating equipment and have supported the move
of the wood heating system from the furnace room to the living or family
room.
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Although few studies have explored the attitudes of householders to
their wood heating activities, those reviewed for this paper reveal
strong positive feelings about the practice of heating the house with
wood. A 1983 study conducted for the Canadian federal government found
that 97 percent of primary users and 91 percent of supplementary users
were either very satisfied or somewhat satisfied with their wood heating
activities, despite the time and effort required. The same study showed
that these householders express considerable enthusiasm for woodburning
in general; 91 percent of primary users and 70 percent of supplementary
users were either strongly or mildly enthusiastic about woodburning.
Householders appear to develop a commitment to the use of wood fuel for
heating and this tends to become part of their self-image.
This assessment was confirmed more clearly
by a 1987 study conducted in Ontario using qualitative social research
methods. Sponsored by a multi-stake-holder wood heat safety steering
committee, the aim of the study was to investigate, through focus group
research methodology, the attitudes of householders to their wood
heating activities and specifically, their responses to various safety
messages. The following excerpts from the report reveal some key
insights into the motivation and attitudes of people who heat with wood:
It is clear that most respondents view
wood heating to be a positive and worthwhile activity.
. . . . for the majority of Peterborough
respondents using a wood stove or insert, wood heating had become a
family-centred activity. In many cases all family members are involved
in fuelling of the appliance and the wood appliance plays an important
role in family life. Unprompted commentary focused on this aspect of
wood heating more than any other. Respondents related how their family
behaviour had been altered by the introduction of the appliance. A
typical comment was: "We installed the wood stove in the living
room and pretty soon everyone started spending the evening there. Before
we put in the stove we never sat in the living room, we were always in
the rec. (sic) room watching TV."
Although difficult to substantiate, the
commentary during Peterborough groups suggested a strong linkage between
wood heating and some underlying values and beliefs. It is clear that
respondents believe wood heating to be an inherently good thing to do,
partly because it provides a means of exercising control over an aspect
of their lives.
The fact that wood heating clearly
functions as a focal point in family life is also highly reinforcing and
appears to be consistent with underlying values.
It is evident that many people take
pride in their ability to heat their home with a woodburning appliance,
in demonstrating that they possess the physical strength and knowledge
to do the job, and that they have mastered the various necessary skills.
Heating their homes with wood provides many people with feelings of
satisfaction and self- reliance.
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While some members of the general public might have a basic
understanding of the function of forests in relation to global warming
as a storage medium for carbon, it is apparent that there is little
understanding of how wood energy can help reduce net emissions of
greenhouse gases. If an average person were told that the destruction of
tropical rain forests is one of the causes of global warming, it would
be reasonable for that person to question the burning of wood for home
heating. It is also likely that urban-based energy and environment
policy-makers who lack experience with wood energy may also
misunderstand its potential in reducing net greenhouse gas emissions.
For those who do not use it, the dismissal of wood heating as
environmentally unfriendly can be done without hesitation or doubt.
Items in news media reports and other public information sources
contribute to this confusion. Ms. Laura Porcher, co-ordinator of a
Victoria, British Columbia task group on atmospheric change, had her
remarks reported in the local newspaper as follows:
Porcher said that even without the
chemicals, wood-burning is harmful to the atmosphere. "The
by-product of combustion is carbon dioxide, blamed for global warming.
Trees release carbon dioxide as they decay, but we’re speeding up the
process when we should be trying to slow it down."
In another case, the nationally
distributed list, "50 Ways You Can Help to Save Our Earth",
contained the following item:
Don’t buy a wood-burning stove. Not
only will it contribute to our growing air pollution problem, it will
use the trees we so desperately need to clean the air.
In neither case was an alternative view or
correction of the facts provided to the public. Although we know of no
study to support the view, we suspect that even people who currently
heat with wood may be confused and perhaps doubtful about the
appropriateness of their use of wood for heating. Confusion is likely to
occur because the public has not been exposed to the idea of the
complete carbon/carbon dioxide cycle that trees are part of (Figure 2),
nor to the concept of wood energy being inherently more appropriate than
fossil fuels because it is derived from a renewable source.
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One of the principal forms of wood energy production and consumption
is the supply of firewood from privately managed wood lots to
residential users of the fuel. This represents an important but
neglected model of sustainable development. The model has several
relevant features, including a reasonable economic return and a high
energy return on investment.
An increase in the use of wood as a fuel
for residential heating can occur within the framework prescribed by
current principles of environmental sustainability. This framework could
be generally described by the following points:
- The integrity of the forest, including the trees, the soil and the
site, is maintained.
- Species diversity within the managed forest is maintained or
enhanced.
- The requirement for the use of non-renewable fossil fuels is
reduced, resulting in reduced concentration of greenhouse gases in the
atmosphere.
- Air shed pollutants are minimized and those that are released do not
produce health impacts on the population.
The latter item may require regulatory
limitations on some forms of residential woodburning in densely
populated urban areas and in areas with poor airshed ventilation.
A significant percentage of the Canadian
population has made a commitment to the use of wood for home heating.
This commitment to wood burning appears to be linked to underlying
personal values related to family life and a desire for independence.
There may be confusion among the public
regarding the mechanism by which the use of wood as an energy source can
help to reduce net carbon dioxide emissions. In fact, there is some
evidence to suggest that this lack of understanding may extend to those
responsible for developing energy and environmental policies.
There are two likely consequences of this
confusion. First, local airshed impacts from residential wood heating
(with conventional equipment) may lead policy-makers to support harsh
regulatory controls, such as outright bans, without consideration of the
potential benefits. Second, confusion among the general public may limit
the acceptance of wood fuel as an appropriate means of heating houses.
In order for wood energy to reach its
potential as an environmentally acceptable renewable energy source,
those responsible for forestry, energy and environment policy, as well
as the general public would require information regarding the conditions
under which wood fuel can be used to reduce net CO2
emissions. The production of fuel wood from well-managed private wood
lots could be promoted as an appropriate model of sustainable energy
development.
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