by Caroline Schultz

It has been more than a decade since old-growth forest conservation took centre stage as a critical conservation issue in Ontario. But it is essential that we look again at why old-growth conservation is an imperative. The challenges of protecting endangered species, such as threatened woodland caribou, and conserving biodiversity are as pressing as ever. And now that we’ve woken up to the need to address climate change aggressively, we have to look at the role of old-growth forests in our battle to control greenhouse-gas emissions.

One of the biggest assumptions about old-growth forests – and probably a misleading one – is that they are carbon neutral, which means that they emit as much carbon to the atmosphere through decomposition as they take in and store as they grow. Therefore, in the battle against climate change, the focus on forests as carbon sinks has been on young forests, which scientists have long believed to consume more atmospheric carbon dioxide than they produce.

Because of these assumptions, climate change mitigation efforts have targeted the development of new carbon sinks by replacing lost forests with young fast-growing ones – an intrinsic element of the Kyoto Protocol. Currently, the Kyoto Protocol contains no incentives for leaving the world’s existing primary and old-growth forests intact.

But a team of international researchers led by Sebastiaan Luyssaert of the University of Antwerp has challenged the idea that old-growth forests are not a global carbon sink. The team’s study, published in the scientific journal Nature, found that most old-growth forests continue adding to their carbon stores and act as net carbon sinks over centuries. The team compiled data on ecosystem productivity (the annual difference between carbon dioxide uptake and release) from 519 different studies undertaken in temperate and boreal forests, and found that forests of all ages are more likely to store carbon than release it. While carbon storage slows somewhat in forests beyond 80 years of age, it continues to occur in forests that are 300 to 800 years old.

Luyssaert’s team showed that primary boreal and temperate forests in the northern hemisphere alone sequester at least 1.3 billion tonnes of carbon per year. These forests comprise about 15 percent of the world’s total forest cover and account for about 10 percent of net global ecosystem productivity – in other words, they are a major carbon sink. The assumption that old-growth forests are carbon neutral has been built into numerous ecological models of carbon flux and into carbon-accounting schemes for greenhouse-gas mitigation, according to the authors. They highlight the importance of revising carbon-accounting rules and giving credit for leaving old-growth forests intact. They also expect that if old-growth forests are disturbed, much of the carbon in these forests, including soil carbon, would move back into the atmosphere.

In Ontario and Canada, putting old-growth forest protection first would give a double win – for biodiversity and for climate.