12 February 2014
Increasing both forest stocks and timber harvest will buy time while we learn more about how trees absorb carbon, say Valentin Bellassen and Sebastiaan Luyssaert.
The best way to manage forests to store carbon and to mitigate climate change is hotly debated. Trees absorb carbon dioxide from the atmosphere, and wood can be a substitute for fossil fuels and carbon-intensive materials such as concrete and steel. In the past few decades, the world's forests have absorbed as much as 30% (2 petagrams of carbon per year; Pg C year−1) of annual global anthropogenic CO2 emissions1 — about the same amount as the oceans. Two-thirds of forests are managed.
Much has been learned about the carbon cycle in forests, but there are still too many gaps in our knowledge. New observations have called long-accepted theories into question: the finding that unharvested forests, for example, are absorbing more carbon than they release2, accounting for half the sink, is contrary to the tenet of ecology, known as Odum's framework, that carbon flows in natural forests should be in equilibrium.
This carbon-sink behaviour of mature forests is attributed to large-scale environmental changes that violate the assumption of the steady conditions underlying Odum's framework: higher atmospheric CO2 concentrations are accelerating tree growth worldwide and nitrogen emitted by industry, agriculture and fossil-fuel burning is increasingly fertilizing managed forest soils in Europe, China and the eastern United States.