Assessments Require Strict Carbon Accounting

The Issue

The eyes of scientists and politicians have recently turned toward Canadian boreal forests as a possible cure for the excessive carbon dioxide in the atmosphere. Satellite data and field experiments have indicated that global warming could stimulate the growth of boreal forests, thereby removing some of our carbon from the air. Can boreal forests be a sink of carbon? Mike Apps (Canadian Forest Service) will argue that only a strict accounting of the carbon budget of entire forest areas could answer this question.

The Science

• As a forest matures under natural conditions, its rate of carbon uptake from the atmosphere gradually decreases to zero. A mature stand is more susceptible to increased mortality from insects, diseases and forest fires, eventually returning the carbon stored in vegetation and soils to the atmosphere. In turn, seedling regrowth begins a new cycle of carbon uptake. Human disturbances such as harvesting and fire control add to the natural disturbances driving this cycle. Changes in disturbance regimes, both natural and human-induced, maintain forests in a state of constant disequilibrium.

• Scientists from the Canadian Forest Services are studying the variations over time of the carbon storage capabilities of Canada's forests, which represent roughly 10% of the world's forested areas. Their methods rely on inventories of age-class distributions of forests, which conventiently reflect past disturbances. They found that the forests of Canada have moved from a sink of atmospheric carbon in the first * of this century to a source in the last decades, due to a change in disturbance regimes. The change in disturbance regime is possibly related to climate change, while human impacts are still negligible in this region.

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