Ancient forests, wood products and carbon emissions


Science is not black or white, writes Alice Palmer.

This spring, British Columbia (BC) launched its Mass timber action plan, based on the principle that wood construction has a lower carbon footprint than building alternatives such as concrete and steel. Meanwhile, environmental groups continue to threaten to protest ancient logging across British Columbia, in part based on allegations about the carbon footprint of logging.

Intrigued by this apparent contradiction in climate claims, I took a closer look at the science behind the carbon impacts of forestry and wood use, starting with the emissions “big picture” global carbon.

Why build with wood ?

According estimates According to the US Energy Information Administration (EIA), 11% of global greenhouse gas emissions come from the construction of buildings, with at least 8% of global carbon emissions coming from cement industry only. Current cement-making processes require limestone to be fired in high-temperature kilns to produce lime, which releases a lot of carbon into the atmosphere.

In contrast, wood products used in construction have a much lower carbon footprint. Trees sequester (absorb) carbon dioxide from the air as they grow, using it to form wood. When used in buildings, that carbon stays sequestered for as long as that building lasts.

In effect, studies showed that log buildings, which are thick wood panels made from small pieces of wood glued together, emit 30-40% less carbon from their construction than their concrete and steel counterparts.

How are solid wood and ancient forests connected?

Today, 27 percent of BC’s timber harvest (by area) comes from old-growth forests (roughly defined in BC as over 250 years old on the coast and over 140 years old in the interior). While much of the wood used for solid wood comes from younger trees, some will likely be old growth.

Ancient forests contain stores of carbon absorbed from the atmosphere. Given this fact, can BC forest products, including solid wood, really be considered carbon friendly?

The answer is complicated.

Why the concern over ancient logging?

Trees absorb carbon as they grow. But once they die, the carbon they have accumulated is slowly released into the atmosphere as they rot.

Over time, the amount of living and dead carbon in a forest can accumulate – especially in forests that do not burn frequently, such as the moist forests of coastal British Columbia where old-growth forests are found in plenty. Disturbances such as insect outbreaks, fires, and clearcutting can release this stored carbon more quickly into the atmosphere.

While young forests that emerge from such disturbances eventually reabsorb the carbon that was released, this process can take time. The argument against old-growth logging is that the disturbance caused by logging mature forests could release more carbon than would be absorbed by the younger trees that will grow in their place.

However, we also know that young forests tend to sequester more carbon than older forests because they have more densely spaced trees.

Carbon modeling gives us answers

Since it is impossible to measure emissions from an entire forest, forest researchers use mathematical models to determine how much carbon a forest captures and stores. These models incorporate a wide range of variables, both on forests (such as tree growth rates and forest composition) and on the human activities that take place within them (such as logging methods, use of forest products and reforestation).

Researchers from Oregon State University and American society CORRIM consortium (among others) have modeled the carbon impacts of logging on different forest types, including old-growth forests in British Columbia and the Pacific Northwest of the United States.

In general, models have revealed that clearcut forests initially become a source of carbon, as decaying debris emits more carbon dioxide than newly established seedlings. However, after about 12 years, the young trees catch up with their carbon absorption capacities, bringing this forest to a neutral carbon balance. At about 30 years old, the trees begin to add a lot of bulk and the forest quickly sequesters carbon, turning the forest into a net carbon sink.

Eventually, the new forest reabsorbs the carbon that was released after logging and continues to sequester it rapidly in the future. Models estimate this reabsorption time at 90 to 200 years, depending on the age of the logged forest (old or very old), how the area was logged and how the wood was used.

The verdict? It is best to have a mix of old and young forests.

Old forests store carbon. Young forests sequester carbon. A combination of the two is the best carbon scenario. The challenge is to regulate the rate of exploitation so that the area of ​​very young forests emitting more carbon than they sequester is balanced by a greater area of ​​carbon-intensive adolescent to middle-aged forests.

In a nutshell, the models suggest that harvesting old-growth forests can be carbon-friendly, especially if used in products like solid wood. But there are caveats. Obviously, the first caveat is that you should not connect too quickly. We should also avoid logging the oldest and most carbon-laden forests. And it’s important that we engage in practices that minimize carbon emissions from logging.

Silvicultural practices are also an important part of the conversation. Replanting new trees quickly after a disturbance and letting the new forest grow long enough to maximize carbon sequestration are other ways to ensure carbon-friendly logging. Finally, we need to use the resulting forest products for as long and as efficiently as possible, ideally replacing more carbon-intensive products such as concrete and steel with wood products like solid wood.

Ultimately, carbon modeling shows us that building with wood – even old wood – box have a positive impact on the climate, if done with the carbon footprint in mind.

Dr. Alice Palmer is a freelance writer, researcher and consultant with over 20 years of experience in the Canadian forest industry. This article was originally published in Sustainable forests, resilient industry – a new sub-stack by Alice Palmer.

Posted on August 04, 2022


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