There is widespread confusion about how “green” it is to burn wood instead of other fuels. The basic information lacking in much of the discussion is also missing from much discussion about adding or removing carbon dioxide from the atmosphere.
Green plants, including trees, take up CO2 in photosynthesis and store it in their molecular structure. This is the source of all “bioenergy”. If those molecules are oxidized soon after they are stored, the CO2 is then put back into the atmosphere. So, corn may take up CO2 in large amounts. But that CO2 is put back into the atmosphere in only a few months when the corn is used as animal food or feedstock for industrial processes such as producing ethanol that is soon oxidized. Such processing of CO2 does little to change the amount of CO2 in the atmosphere. It may add CO2 from the industrial processing.
If we are to avoid extreme effects of climate change, the rate of increase of CO2 in the atmosphere must be reduced within the next two or three decades and the effect must last several decades. Removal of CO2 from the atmosphere by trees can affect the amount of atmospheric CO2. If the trees that store the CO2 are processed or die and are decomposed when young and small, there are essentially no effects on atmospheric CO2, just like corn. The same is true for tree leaves that fall in autumn and are soon decomposed except some of the carbon in those leaves becomes incorporated into organic matter in the soil where it may continue to “sequester” that carbon for many years. That same long-term storage happens if the CO2 is stored in the wood of long-lived trees. Cellulose and lignin may store carbon for several decades. If the wood is harvested and used to build human infrastructure, and those buildings are cared for, the carbon in that lumber also may stay out of the atmosphere for many decades. If the trees continue living in the forest, that carbon is taken out the atmosphere for the lifetime of the trees.
But if the harvested wood is burned for heating, the CO2 that the tree stored is released back into the atmosphere at the top of the chimney. And that smoke may also contain some gases other than CO2. Data show that a calorie of heat can be obtained from coal with less CO2 released than if wood is burned. A full analysis also needs to account for the CO2 released by the mining and the transport of the coal compared to the harvest and transport of the wood. So wood is the preferred fuel for heating a cabin in the boreal forest but coal is preferred for a cabin near a coal mine.
Politically the question becomes: how long does it take to return the CO2 from the atmosphere to sequestered storage in the ecosystems on earth thus avoiding increasing atmospheric heating of earth. That point of view fails to account for the many other variables that are affected by a system that just manages the flow of carbon. The economics of managing flows takes over from ecological processes. Variables without simple economic values and incompletely understood variables are not considered.
As the density of humans and their demands for fuel increases, the discussion becomes more complex. For example, harvesting of more wood from landscapes that are primarily agricultural can reduce the amount of habitat for many species, increasing the probability of local loss of species. Replacing harvested forests in heavily settled areas is often done with tree plantations. Commonly, these plantations lack ecological integrity and are closer to fibre farms than to habitats.
Fibre farms can’t take the place of semi-natural ecosystems. When woodlots make up too little of the total landscape, local extinctions increase and biodiversity decreases. And it’s not just birds and squirrels. Direct effects on humans from high diversity forests are not well-understood but it is clear that forests do have psychological effects on us. There are valid questions about whether some of those effects shape mental development of children.
Both direct payment and compensatory carbon credits are being offered for areas of regrowth to compensate for forest removal and to neutralize CO2 additions from power generation facilities by “carbon sequestration”. To regrow mature forest trees with the capacity to store much carbon in the permanent woody parts of the trees takes several decades. For oaks or maples in Southern Ontario, perhaps 50 to 70 years. Promises of faster growth probably refer to woody species that produce wood with very much shorter longevity, such as pines. The slow growing trees also have better rates of build up and longevity of soil organic matter. There is some resistance to programs of compensation for “sequestration” of areas that would regrow with no management inputs and no production losses.
There no single clear answer to “how green is wood as a fuel?”. Your particular answer will require both knowledge of basics from scientific research and judgment from common sense.
More information: Frontiers in Ecology and Environment No. 6, Vol. 13, August 2015:299