CARBON & NUTRIENT CYCLING: Is Forest Bioenergy Sustainable?
by Hamish Kimmins

Bioenergy was one of the first values people harvested from forests. Firewood for heating, cooking, and, later on, energy for industrial and transportation needs has been a major component of human, social, and technological evolution. Firewood continues to be a major source of energy in developing countries that still have forests, although it’s been displaced in most developed countries over the past half century by coal, oil, natural gas, nuclear, and hydro energy.

As the human population grew and the need for a variety of wood products increased, and as forest fuel was replaced by fossil fuel, timber became the major economic resource provided by most forests. Firewood became relatively unimportant and vast quantities of forest biomass remained in the forests following exploitative timber harvesting, or harvesting of old forests in which a large proportion of the tree biomass was unusable. However, with growing concerns about climate warming and its association with the almost exponential increase in fossil fuel combustion, forests are once again being valued as a source of biomass for energy, and as a way of sequestering and storing atmospheric carbon.

Without question, bioenergy and carbon storage are legitimate forest products just like water, wood, wildlife, and biodiversity. However, as we have learned so many times, excessive focus on any one forest value tends to result in negative impacts on other values, leading to problems with their sustainability. A preoccupation with timber economics can lead to problems with soils, water, wildlife, recreation and aesthetics. An uninformed preoccupation with certain aspects of biodiversity may lead to failures to sustain other measures of biodiversity, and several of the important social values provided by forests. Similarly, an excessive preoccupation with biofuel without adequate consideration of other forest values including the sustainability of tree growth would be unwise. All forest values are important, and while the relative ranking in importance varies from place to place and from time to time, it is a basic requirement of sustainable, multi-value forest management and stewardship today that the implications of management for any one resource on the sustainability of other resources be considered in developing forest management policy, plans, and practices.

There are two important questions that must be addressed in seeking assurance that the resurgence of interest in forests as bioenergy producers is consistent with sustaining the many other values desired from forests by society: (1) Is the harvest of forest biomass for bioenergy sustainable?; and (2) What is the best strategy with respect to optimizing the role of forests in carbon storage and sequestering? This article focuses on the first of these two questions. 

Like so many questions in forestry and ecology, there is no simple answer to this question. Determining factors include: 
• the type of forest (species, age, timber volume)
• the nutritional demands of the tree species involved
• the frequency of harvesting (the rotation length in even-aged stands or the frequency of entry in partially harvested stands)
• what proportion of the tree is removed (only the stems, or stems
plus one or more of branches, leaves, stumps and roots) 
• whether slash and forest floor material is harvested 
• the history of natural disturbance (as it has affected accumulation of organic matter and nutrients in the soil and vegetation)
• the risks of future natural disturbance that would remove organic
matter and nutrients
• the depth, texture, geology, and fertility of the soil
• other values that are desired from forests
• the value tradeoffs implicit in managing forests for bioenergy

The broader question as to whether bioenergy harvesting is a component of sustainable forestry includes these factors, plus the effects on hydrology, soil erosion and slope stability, soil animals and microbes (and thus on soil fertility, plant growth, and productivity), and above ground diversity of plants and animals. The waste biomass in the forest is not ecological waste. Branches, leaves, stumps, roots, large decomposing logs, and standing dead trees (snags) are important components of the forest ecosystem, providing energy and habitat for soil animals and microbes, habitat and food sources for small and medium-sized vertebrate and countless invertebrate animals. They help to sustain soil humus levels, and supply nutrients slowly over time for uptake by trees to support their future growth and productivity. 

The expression “no free lunch” comes to mind. Everything we take out of the forest prevents the use of the energy and nutrients contained therein by other forest organisms, removes the habitat contributions of those materials, and affects the important hydrological role of organic matter in forests. We know that a substantial amount of forest biomass can be harvested periodically without long-term negative consequences, but for every ecosystem and every value there will be some frequency of biomass harvest with some intensity of removal, beyond which forest ecosystem function and biological diversity will be impaired. Annual harvesting of branches and forest floor litter by landless German peasants in the early 1800s led to a yield decline in pine forest that was a major stimulus for the development of modern forest science. This resulted in a government-initiated study by an eminent chemist of the day who concluded in 1876 that excessive biomass removal will threaten future productivity on nutrient-poor soils, and that predictions of future stand growth should incorporate nutritional assessments.

Harvesting of bioenergy conducted at infrequent intervals on sites with fertile soils and high rates of re-accumulation of nutrients should not threaten long-term tree growth. If such harvesting of bioenergy, in addition to logs for timber, is conducted within a landscape pattern of forests of different ages, and “islands” of forest are left with high loadings of decomposing logs, snags and organic matter, it should sustain most values. Such “islands” (e.g. the retention patches in variable retention silviculture) provide reservoirs of organisms that can re-colonize intensively harvested areas once organic matter and nutrients have re-accumulated. In contrast, one or more of short rotations/frequent entries, low soil fertility, a lack of organic matter and low nutrient legacies from the past, and/or lack of retention patches may fail to sustain a variety of forest values and may not even sustain bioenergy production over the long term.

Considering the complexity of this issue, how can we evaluate the sustainability of bioenergy? Experience has often served forestry better than the results of disciplinary, reductionist science. While experience continues to guide farmers and fishermen in systems where the results of management changes can be detected empirically within a few years, the long time scale in forestry reduces the value of experience alone in the face of changing climates and new management systems for which we lack long experience. The best available solution at present is to combine what relevant experience we have with our rapidly increasing understanding of how ecosystems work - the key ecosystem processes that are responsible for sustainability. Only then can we make informed estimates and predictions of the relative sustainability for multiple values or the various ways we can manage forests, including harvesting them for bioenergy. 

It has been argued that the environmental risks posed by climate warming and the urgent need to replace fossil fuel-based energy with renewable bioenergy sources outweigh concerns over the next few decades about the effects of forest bioenergy harvesting on soil fertility and biodiversity. With continued climate warming, these values are at risk anyway. Solar, geothermal, hydro, wind, and wave/tide sources of energy offer alternatives to fossil fuel, but many of these have significant environmental problems, require considerable capital investment, and will take years or decades to be brought on line. In contrast, unused forest biomass is seen as readily available, and requires little capital or technical development. The removal, reduction, or concentration of post-logging slash and/or the disturbance of deep, slowly decomposing forest floors by burning or mechanical means is sometimes needed before a harvested area can be regenerated, especially in old forests, and rather than disposing of this biomass, why not use it for bioenergy?

Agricultural crops, such as corn and soybean, which are grown for biofuel require fertilizers and fossil fuels to manage and harvest the annual crop. In the tropics, soybean production for biofuel has accelerated the clearing of tropical rainforests, and the fertilizer use for corn production in the southern US is contributing to the nutrient enrichment of the Mississippi River that it is thought to be causing a large and expanding “dead zone” in the gulf of Mexico. Growing food crops on agricultural land for biofuel is causing food shortages and increases in the price of basic foods such as corn and grains. In contrast, forests grown with low intensity management generally do not require fertilizers and have relatively little fossil fuel demand per unit of bioenergy produced. Unused forest biomass does not compete with food, and thus appears to be a much more environmentally and socially-friendly source of bioenergy than agricultural crops if managed with sensitivity to ecosystem function.

Acceptance of most alternative sources of energy (nuclear, hydro, wind, tidal) requires detailed environmental impact assessments. However, it seems that there is a readiness to rush into using forest bioenergy as a short-term way of contributing to a reduction in the release of greenhouse gases from fossil fuels without adequate assessment of the ecosystemic consequences. The harvest of unused forest biomass as biofuel is acceptable as long as appropriate environmental assessments are undertaken to define levels of organic matter and nutrient removal that are consistent with ecosystemic sustainability and acceptable value tradeoffs.

The Canadian Forest Service asked that such a capability assessment be developed in the mid-1970s (the FORCYTE model). Ecosystem management simulation models have been designed over the past thirty years specifically to assess the sustainability of multiple forest values under a wide range of alternative forest management systems interacting with natural disturbance agencies (e.g. fire, insects, wind, and climate change). This decision support system is based on our current understanding of the ecology of forest productivity. The key to assessment of the sustainability of bioenergy harvesting from an ecosystemic perspective is to base this on a model that represents the key ecosystemic processes that underlie the sustainability of ecosystemic primary production. Using forests as an energy system must incorporate this approach, as was asserted in Germany nearly 130 years ago.

The main model in this family of ecosystem assessment tools, FORECAST (FORestry and Environmental Change ASessmenT) can be used to examine stand-level sustainability questions - alternative scenario and value tradeoff assessments. The output from the model – forecasts of possible forest futures - can be used to replace the output concerning timber volume from conventional stand-level growth and yield models that is generally the driver of large landscape and management unit timber supply models. This converts a timber supply model from a simple timber inventory control tool into a landscape-level timber supply, wildlife habitat supply, aesthetics (through interactive, three-dimensional visualization), carbon budget (carbon storage in live vegetation, decomposing organic matter and soils; carbon sequestration and carbon release), fossil fuel energy requirement as well as employment and economic assessment decision support tool. Because such ecosystem-driven landscape models are multi-value, they can be used in multi-value scenario and value tradeoff analyses. 

As ecosystem management models, the FORECAST family of decision support tools have proven valuable in forest certification, since they are able to assess the utility of various indicators of sustainability criteria, undertake credible analyses of value tradeoffs, and facilitate the transition from static, snapshot assessments of sustainability to assessments of the dynamic ever-changing character of stands and landscape that are the true objective of sustainability analysis.

The urgency of the climate change issue does suggest use of some proportion of unused forest biomass for bioenegy. However, using all the unused forest biomass would have little long-term impact compared with the carbon and climate consequences of burning all the world’s coal, oil and gas. Rather than threatening the many other values of forests by harvesting waste biomass, it is better to work on reducing our consumption of fossil fuels. Like carbon credit trading, use of the remaining underutilized forest organic matter does not get to the heart of the problem, which is burning fossil fuels. In the short run there may be merit in using forest bioenergy to meet our national commitments, but this political expediency cannot be supported in the absence of a careful, ecosystem-specific assessment of the consequences for nutrient cycling and other forest values.

Hamish Kimmins is Professor Emeritus of Forest Ecology in the department of Forest Sciences at UBC, Vancouver.