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Providing balance to the debate on carbon, forestry and old growth

North Cowichan Mayor Al Siebring recently referenced an opinion piece written by some of Canada’s senior timber industry officials who support the notion that it is better to log trees for profit before they grow old and turn into carbon and methane emitters. The Mayor said on Facebook that the industry comments were “apropos to the discussion about our Municipal Forest Reserve,” which could be interpreted as providing justification for continued harvesting and not allowing our forests to reach maturity.
This is a nuanced subject area deserving a balanced and informed touch. As such, I have included, below, key passages from the study, Forestry and Carbon in BC, released earlier this year by the highly respected Dr. Jim Pojar, a long-time researcher for the BC Ministry of Forests, lecturer, and best-selling author, including co-author of the Plants of Coastal BC, the Bible of such field guides. Of course, there are many good reasons for maintaining old-growth forests, including biodiversity, hydrology, resilience to wildfires, recreation and culture. Regrettably, North Cowichan is bankrupt of old growth. Let’s hope that the pending consultation process for the Six Mountains is the beginning of a brighter, richer future for our forests.

MYTH #1 — Forestry is carbon neutral.
It could be but usually isn’t. At the scale of a forest stand, the conversion by logging of mature and old forests to young forests results in an increased release of carbon immediately, and for several years thereafter. This is because a) clearcutting generally leaves minimal carbon sinks (living trees and other plants) on the cutblock; b) a large pulse of carbon is lost immediately after logging due to the removal of trees and to the associated fossil fuel emissions; and c) disturbance to the soil and the original vegetation, and sometimes warming of the site, results in an increased rate of decomposition of coarse woody debris, litter, and soil organic matter, whereby losses of CO2 due to respiration exceed the amount fixed through photosynthesis by the regenerating forest—for at least a decade. Moreover, in managed forests, the overall carbon store is reduced if the secondary forests are managed on typical commercial rotations. The oldest stands typically have the largest stores of carbon. At the scale of a large landscape (say 300,000-500,000 ha) or of the entire province and if forest management is performed sustainably, it is possible that forestry-related emissions could be offset by uptake of carbon dioxide by the unharvested forests. It should be emphasized that the underlying carbon budget calculations are complex and depend on assumptions about a future with much uncertainty around carbon dynamics in a rapidly changing environment. Logging primary, mature and old forests and converting them to secondary, managed forests can reduce total carbon storage by 40-50% or more, even when off-site storage of carbon in wood products in buildings is factored in. The carbon dynamics are sensitive to rotation length, proportion of felled wood that becomes wood products in long-term storage (reportedly 25-40% for BC wood used domestically), and longevity of storage. Construction materials such as lumber, plywood, and laminated beams can last for many decades but wood products include paper and pulp materials (office paper, toilet tissue, paper towels, cardboard packaging, disposable diapers) as well as pallets and pellets, all of which have much shorter lifespans. Conventional short rotations and relatively short ‘life cycle’ even of long-lasting wood products (often reckoned to be 50-70 years in both cases, although some storage persists beyond 100 years) probably result in a significant one-time net loss of about 100-300 tonnes C/ha. A managed secondary forest could — in principle — recapture the lost forest carbon if allowed to regrow long enough to fully recover its carbon stock, which could be achieved more quickly and easily in most interior forests than in coastal or interior wetbelt forests. 2 “Oversimplification, and the second part is mostly false.”
MYTH #2 — Young forests take up more carbon than they emit and are ‘carbon sinks’; mature and old forests take up less carbon than they emit, are ‘carbon sources’, and contribute to climate warming.
That is an oversimplification and the second part of it is mostly false. Forests both absorb and release carbon throughout their life, from regeneration after disturbance through youth and maturity to old age. This results in a dynamic balance that changes over time, depending on stand age and on type and intensity of disturbance. The relative balance between uptake and emission determines whether a particular forest ecosystem is a net carbon sink or a source. After a stand-initiating disturbance, young forests are net carbon sources for several years until the amount of carbon they take up exceeds the carbon they emit through respiration and decomposition. Some old forests (sources) emit more carbon than they fix but most (sinks) fix more than they emit, depending on levels of within-stand mortality, decay, and growth. Net carbon uptake in old forests does level off or decrease, but total storage increases. Old forests usually store much more carbon on site than do young post-logging forests. Depending on how they naturally function, how they are disturbed, and how they are managed, forests can therefore either mitigate or contribute to greenhouse gas emissions and climate change. 3 “Death is inevitable, but beside the point.”
MYTH #3 — Mature and old forests are not permanent carbon banks because inevitably the trees die; the forests will succumb to wildfire, insects, disease, drought, and logging.
Death is inevitable but in this matter beside the point, which is about the time value of carbon currently stored in forests. Indeed some existing forests will succumb or are already on the way out but BC forests will not disappear overnight. And some of these forests grow very old—ancient even— and carry on functionally intact for a long time, for several centuries or even millenia. If stand-replacing disturbances are rare or infrequent, as they are in wet coastal forests and many wet subalpine forests and interior wetbelt forests, the majority of the landscape will be occupied by old forests and most of them will just keep ticking along, taking up and storing carbon. Trees can get very old but they don’t live forever. If a forest does not experience a standreplacing disturbance (like wildfire, beetle attack, blowdown, clearcutting), as it ages individual or small groups of trees continually die and are replaced in what is called gap dynamics. The forest carries on with new recruits. Moreover, although all BC forests will eventually be replaced—suddenly, episodically, or gradually—currently they are carbon banks and their stored carbon has much greater time value now and in the crucial next three decades than anticipated, post-logging carbon storage recouped over the ensuing seven or more decades. Regardless of whether BC forests are a net source or a sink at any given moment, they continue to store megatonnes of carbon as long as they still have trees on site—even if the trees are dead.
View the full Pojar report:

— Larry Pynn


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