Residential Wood Heating:
The Forest, Atmosphere and 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." Hendrickson, Gulland.
Table of Contents
- Biodiversity in Forests Managed for Fuelwood
- Using Forests to Offset Rising Atomospheric CO2
- Maintaining High Biomass Carbon in Natural Forests
- Limitations of Bioenergy Plantations
- Energy Return on Investment
- Extent of Residential Wood Heating in Canada
- User Attitudes Towards Woodburning
- Does the Public Understand Greenhouse Gases?
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.
Biodiversity in Forests Managed for Fuelwood
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.
Using Forests to Offset Rising Atmospheric CO2
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."
Maintaining High Biomass C in Natural Forests
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.
Limitations of Bioenergy Plantations
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.
Annual C gain
|Cumulative Biomasss 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.
Energy Return on Investment
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.
Extent of Residential Wood Heating in Canada
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.
User Attitudes Towards Wood Burning
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.
Does the Public Understand Greenhouse Gases?
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.
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.