It’s naive to assume that all forests are equal when considering carbon sequestration. Carbon sequestration is the natural process by which organic matter, such as trees, remove carbon dioxide from the atmosphere and hold it within their biomass.
One of the largest reasons why carbon sequestration and subsequent carbon emissions from logging varies so much involves the differences between secondary growth forests and old growth forests. Understanding the differences, and the wide-ranging effects the differences cause, can assist with intelligent timber selection making the best choices in terms of sustainability, longevity and competency.
Secondary growth forests refer to forests that contain trees younger than approximately 30 – 40 years. Secondary growth forests are often referred to as industrial tree farms, or forest plantations, but this isn’t always the case. For example, there are large tracts of secondary growth in the Amazon where the old growth was removed during the initial rise of industrial-scale logging during the 70’s. Although most areas of deforested old growth rainforest cannot regrow as the nutrients have been washed out of the soil, topographical variances mean that in some places, trees can re-establish themselves naturally.
However, given that we are primarily concerned with the carbon impact of the logged timber rather than the growth of the tree in this article, when we discuss secondary growth we are referring to industrial-scale forest plantations. These are areas of land, often farmland, that has been purposely planted with tree saplings usually of the same or a limited range of species to increase stocks of readily available timber and for profit.
Because a quicker turnaround from planting to harvesting is more desirable than a longer turnaround, secondary growth comprises of fast-growing softwoods such as Pine, Douglas Fir and Cedar.
Secondary Growth Forests: Kielder Forest – UK, Stanley Park – Canada, Agua Salud Forest – Panama
Old growth forests are forests that have grown naturally and show no signs of human interference or ecological disruption. It is estimated that 34% of the Earth’s forests are old growth forest. Forests can be called old growth areas after the trees are approximately 150 years old. They are known as climax communities because the forest has reached a steady state through ecological succession in the development of vegetation.
Old growth forests typically contain the most valuable timbers which boast excellent durability, stability and aesthetic characteristics but opinion is split whether logging old growth should be allowed. Logging companies maintain that sections of old growth can be sustainably logged, whereas environmentalists argue that it is impossible to ‘sustainably’ harvest old growth.
Old Growth Forests: Tongass National Park – Alaska, Congo Basin Tropical Hardwood Forests – DR Congo, Amazon Rainforest – Brasil
Carbon Sequestration During Growth. It is a common misconception that once a tree’s growth has slowed after 100 years or so, the amount of carbon it ‘inhales’ decreases. This isn’t true; mature old growth trees actually take in more carbon per tree compared to secondary growth trees, which leads one to imagine that old growth forests are better from a carbon sequestration point of view.
However, in secondary growth forests the trees can be planted much closer together. This means that although on a tree v tree basis old growth will absorb more carbon, when comparing old growth v secondary growth on a forest-wide basis secondary growth forests will actually sequester more carbon overall. Effectively, the greater number of trees more than compensates for the greater size of trees in old growth forests.
Effect of Logging on Carbon Sequestration. Many trees in old growth forests have stood for centuries accumulating tremendous amounts of carbon in their biomass. If these trees were logged and the timber used, the general assumption is that the timber will be replaced after several decades, especially with outdoor applications. This means that the sequestered carbon is being released much faster into the atmosphere than it would naturally – through dead wood falling to the forest floor and gradually decomposing.
The situation is the same for secondary growth timber with regards to expected lifespan and subsequent carbon release. However, the advantage with secondary growth timber is that it grows much faster, meaning that the long term, overall effect is more carbon neutral rather than carbon negative.
The Local Factor. There isn’t much old growth forest left in the UK. Any remaining old growth in the UK, such as Savernake Forest, are heavily protected for good reason and must be left alone. Trees cannot be removed even it were for maintenance purposes rather than commercial purposes. This means that for timber used in the UK, the choice is heavily biased in favour of secondary growth because any old growth timber has to be imported from abroad, with high carbon emissions from transport.
The Biodiversity Factor. Remember that carbon sequestration isn’t the only variable at play here. Old growth forests harbour a far greater range of animal and plant species, so are much more valuable than secondary growth from a biodiversity viewpoint. This is because secondary growth forests are planted so closely together that little light can penetrate through to top layer onto the forest floor beneath to encourage undergrowth.
Using timber from secondary growth forests is an environmental anomaly; both sustainability and profit happily coexist. From all points of view, secondary growth is superior to old growth for carbon sequestration, and while logging companies can make more money harvesting old growth forests, secondary growth is still financially attractive.
Given a few decades, this argument between old growth and secondary growth may well be irrelevant because all the large-scale old growth forests will have gone – although old growth deforestation has slowed during the last decade, it is still rapidly disappearing.
This lends even greater weight to the importance of using timber from secondary growth forests. The only argument for using timber from old growth forests is their desirable properties such as durability, stability and a lack of knots, but these come with a heavy environmental cost. Surely at this stage of the climate crisis we should overlook the relatively minor shortcomings of secondary growth timber for the sake of our planet’s health.