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The Carbon-Sequestering Status of Old-Growth Forests
Volume 11, Number 45: 5 November 2008

It has long been thought that ageing forests do not accumulate carbon. In fact, the authors of a paper recently published in Nature (Luyssaert et al., 2008) write that "a decline in net primary production is commonly assumed in ecosystem models," and they say that this assumption "has led to the view that old-growth forests are redundant in the global carbon cycle." But is this long-held view correct?

The eight researchers, who hail from six different countries (the UK, USA, Belgium, France, Germany and Switzerland), decided to investigate the perceived wisdom of the day as it pertains to this subject. In doing so, they compiled and analyzed real-world data from 519 forest-plot studies (approximately 30% boreal and 70% temperate) where the studied trees had not been subjected to experimental treatments such as fertilization and irrigation. This effort revealed, in their words, that in forests of 15 to 800 years of age, net ecosystem production (NEP, the net carbon balance of the forest including soils) "is usually positive; that is, the forests are CO2 sinks."

What is more, they indicate that in contrast to the fact that real-world data "consistently indicate that carbon accumulation continues in forests that are centuries old," young-growth forests "are very often conspicuous sources of CO2 [our italics] because the creation of new forests (whether naturally or by humans) frequently follows disturbance to soil and the previous vegetation, resulting in a decomposition rate of coarse woody debris, litter and soil organic matter that exceeds the net primary productivity of the regrowth."

The international team of scientists also notes that "old-growth forest stands with tree losses do not necessarily become carbon sources," because "the CO2 release from the decomposition of dead wood adds to the atmospheric carbon pool over decades, whereas natural regeneration or in-growth occurs on a much shorter timescale," and this latter phenomenon more than compensates for the slower and smaller carbon losses from the decaying trees.

As for the significance of their findings, Luyssaert et al. indicate that under the Kyoto Protocol, leaving forests intact was not perceived as an anthropogenic activity to be rewarded. However, they write that "because old-growth forests steadily accumulate carbon for centuries, they contain vast amounts of it," and they say that these forests "will lose much of this carbon to the atmosphere if they are disturbed." Hence, they make a strong case for the proposition that "carbon-accounting rules for forests should give credit for leaving old-growth forest intact."

Sherwood, Keith and Craig Idso

Luyssaert, S., Schulze, E.-D., Borner, A., Knohl, A., Hessenmoller, D., Law, B.E., Ciais, P. and Grace, J. 2008. Old-growth forests as global carbon sinks. Nature 455: 213-215.