The European forests are exhausted. Deforestation rates and higher vulnerability to natural disturbances such as fires, storms and insects, and the following declining volume increment of trees, are leading to carbon sink saturation in forest biomass. The carbon sink services provided by European forests were predicted to be functional for decades, but since 2005 there have been signs of sink saturation, states the international team of scientists led by former EFI''s Assistant Director Gert-Jan Nabuurs, currently from Alterra, Wageningen, in their recent paper in Nature Climate Change. They looked at forest inventories for the whole European area and found that since 2005 there has been a decline in the rate of tree volume increase, and therefore also in sink capacity. This was calculated using the average annual volume of forest increment minus the average annual volume of harvest and other losses of trees.
The authors suggest that a few conditions may explain this. As European forests are increasingly mature, they are dominated by older trees. This condition, combined with reduced nitrogen deposition from the atmosphere and decreased summer air humidity due to climate change, can explain the lower growth in forest volume that curbs the carbon sink. In addition, urban sprawl and infrastructure expansion are driving deforestation rates, even if only on a modest scale, with effects on the sink strength. Finally, evidence shows that the older European forests are more susceptible to damage caused by natural disturbances, leading to release of carbon into the atmosphere.
The authors conclude that although managed European forests are closer to capacity than previously thought, changes in management practices can improve volume growth and slow down saturation of the carbon sink. While there is no one solution that would be suitable for all, the authors point out that a change in the way we value our forests is needed. A more regional spread of forest functions over Europe may be needed; where there are regions with enhanced conservation, and regions where the production of wood receives more attention, with enhanced rejuvenation.
"Countries should realise that a carbon sink in the forest biomass cannot be sustained forever. Only through a locally adapted management a balance can be found between a sink in the forest, and a continuous flow of wood products and biomass for bioenergy. A carbon sink is only one function of the forest, and should be valued with respect to other services and products, sometimes going beyond the boundaries of the forest sector. This calls for better coordination of policies beyond those that deal directly with forest and forest management", says Nabuurs.
The paper "First signs of carbon sink saturation in European forest biomass" by Gert-Jan Nabuurs, Marcus Lindner, Pieter J. Verkerk, Katja Gunia, Paola Deda, Roman Michalak and Giacomo Grassi has been published in Nature Climate Change. The research has been partly carried out in the connection of the COST Action Echoes, as well as EU funded projects MOTIVE, Trees4Future, Volante and GHG-Europe.