Lisa Kellman of the Environmental Sciences Research Center at St. Francis Xavier University, Canada, and her team have been investigating the impact that forest harvesting has on the underlying soil. It seems that the damage goes deeper than previously thought and lasts for much longer than traditionally assumed.
When we buy a new wardrobe made from sustainably sourced wood, we tend to assume that the trees that were harvested to make the product were replaced by new trees, which continue to mop up carbon dioxide from the atmosphere and stabilize carbon in the soil. The Forest Stewardship Council for example, one of the largest and most well-known certification schemes in the UK, says that its label “guarantees that the trees that are harvested are replaced or allowed to regenerate naturally”.
However, Kellman’s research shows that simply planting a new tree isn’t always enough to negate the damage of cutting down an old one. She has been studying carbon and nitrogen storage in the soil at a red spruce forest in Nova Scotia, Canada. The red spruce has been harvested in a similar manner since the 1940s but the forest also contains an area of virgin old-growth trees, providing an ideal reference point.
Kellman and colleagues have studied soil profiles down to a depth of 50 cm at five different sites across the forest. Each site was similar in all aspects apart from the age of the trees, which ranged from freshly planted one-year-olds to ancient trees older than 125 years.
The researchers found that storage of carbon in the soil reached a minimum approximately 30 years after harvesting, by which point carbon was around 50% of its original level. The storage didn’t bounce back to its pre-cut level until approximately 100 years after harvesting.
What’s more, the study revealed that the greatest losses occurred below a depth of 20 cm in the soil – a region deeper than most previous studies of soil-carbon storage have examined. Worryingly, the study also showed a similar picture for soil nitrogen but with even longer recovery times.
So what does all of this mean for sustainably sourced wood? According to Kellman, many current certification schemes may not be allowing enough time for the woodland to regenerate. “My research suggests that long rotation times – on the order of 80 to 100 years – are critical for sustainably managing forests, and ensuring that carbon lost to the atmosphere is ultimately returned to the soils,” she told environmentalresearchweb. “Shortening harvest rotation times have the potential to make the resource unsustainable, and to contribute to the rise in atmospheric greenhouse gases.”
Ultimately we may have to curb our appetite for new wooden products and become a bit more patient about waiting for trees to grow.
