title: Revaluing unmanaged forests for climate change mitigation.
abstract: BACKGROUND
  Unmanaged or old-growth forests are of paramount importance for carbon sequestration and thus for the mitigation of climate change among further implications, e.g. biodiversity aspects. Still, the importance of those forests for climate change mitigation compared to managed forests is under controversial debate. We evaluate the adequacy of referring to CO2 flux measurements alone and include external impacts on growth (nitrogen immissions, increasing temperatures, CO2 enrichment, changed precipitation patterns) for an evaluation of central European forests in this context.
  
  
  RESULTS
  We deduce that the use of CO2 flux measurements alone does not allow conclusions on a superiority of unmanaged to managed forests for mitigation goals. This is based on the critical consideration of uncertainties and the application of system boundaries. Furthermore, the consideration of wood products for material and energetic substitution obviously overrules the mitigation potential of unmanaged forests. Moreover, impacts of nitrogen immissions, CO2 enrichment of the atmosphere, increasing temperatures and changed precipitation patterns obviously lead to a meaningful increase in growth, even in forests of higher age.
  
  
  CONCLUSIONS
  An impact of unmanaged forests on climate change mitigation cannot be valued by CO2 flux measurements alone. Further research is needed on cause and effect relationships between management practices and carbon stocks in different compartments of forest ecosystems in order to account for human-induced changes. Unexpected growth rates in old-growth forests - managed or not - can obviously be related to external impacts and additionally to management impacts. This should lead to the reconsideration of forest management strategies.
authors.affiliation: Johann Heinrich von Thuenen-Institute (vTI), Federal Research Institute for Rural Areas, Forestry and Fisheries, Institute for World Forestry, Leuschnerstr, 91, 21031, Hamburg, Germany. joachim.krug@vti.bund.de.
authors.forename: Joachim
authors.initials: J
authors.lastname: Krug
authors.forename: Michael
authors.initials: M
authors.lastname: Koehl
authors.forename: Dierk
authors.initials: D
authors.lastname: Kownatzki