Old-growth forests

There has been a long-standing view that old-growth forests are carbon neutral because it was thought that photosynthesis (carbon dioxide uptake) was equivalent to respiration (carbon dioxide release) (Luyssaert, 2008). However, this understanding is based on ten years of data collection from one site (Luyssaert, 2008). Unfortunately this has led to old-growth forests not being protected by international treaties.

Luyssaert et al (2008) investigated the view that old-growth forests are carbon neutral by compiling data from 519 sites. They found that in forests aged between 15 and 800 years the net carbon balance is usually positive. Therefore old-growth forests can continue to remove carbon dioxide from the atmosphere and store the sequestered carbon dioxide in woody tissue and soil.

Contrastingly, Luyssaert et al (2008) found that young forests can be sources of carbon dioxide because the disturbance resulting from creating a new forest causes a greater decomposition rate than the net primary production of the regrowth.

Another important issue which arises from the article is that it is more unlikely for an old-growth forest to become a carbon source than an even-aged plantation. If an old-growth forest was to undergo some disturbance then there is usually another canopy of trees which can continue the productivity of the ecosystem. Also tree mortality and regeneration is quick whereas decomposition can be on a decadal scale. Therefore contribution of carbon dioxide to the atmosphere through decomposition is a slow process and so old-growth forests do not necessarily become carbon sources. Even-aged plantations do not have the same insurance of a secondary canopy and therefore if tree mortality occurs they can become carbon sources (Luyssaert et al, 2008).

Old-growth forests have accumulated carbon for centuries. There is the possibility that this carbon could be released back into the atmosphere if the forests are disturbed. The exclusion of old-growth forests from the Kyoto Protocol was based on the carbon neutral hypothesis. However, there is an increasing international understanding about the importance of forests and the impact deforestation has on the climate. Luyssaert et al (2008) stress the need for old-growth forests to be protected by the international community.

Apart from increasing understanding about the ability of old-growth forests to capture and store carbon dioxide, Luyssaert et al (2008) have highlighted potential issues with schemes of reforestation. Plantations with trees of similar ages can be potentially dangerous for the environment because if fungal attacks occur, the trees die and there is no secondary canopy to continue uptake of carbon dioxide. Therefore carbon dioxide release would exceed carbon dioxide uptake and the plantation would become a carbon dioxide source.

This study completely changes the understanding of old-growth forests and therefore further research should be applied to this area. Research should focus in particular on the impacts of planting even-aged plantations to mitigate climate change. There needs to be an increased emphasis on protecting these valuable ecosystems on an international scale. 

Luyssaert, S., E. Detlef Schulze, A. Börner, A. Knohl, D. Hessenmöller, B. E. Law, P. Ciais, and J. Grace (2008) 'Old-growth forests as global carbon sinks' Nature, 455, pp.213-215

doi:10.1038/nature07276

Old-growth forests as global carbon sinks