Arbejdsrapport fra Miljøstyrelsen nr. 10/2005 – Effekten af statsskovenes overgang til naturnær drift på kulstofbinding i skovene

Effekten af statsskovenes overgang til naturnær drift på kulstofbinding i skovene

Summary and conclusions

The objective of this project was to quantify the change in carbon stocks in Danish state forests under conversion to nature-based forest management and to estimate the monitoring costs associated with the cap for the first commitment period of 183.000 t CO₂/yr.

We calculated the rate of carbon sequestration using scenarios for traditional (business as usual) and nature-based forest management. The conversion to nature-based forest management over the next 100 years was characterized by 9 different management options. The scenarios projected state forest stand data from 2004 over the next 100 years using models for harvesting and growth and carbon stocks in tree biomass was subsequently calculated using various conversion factors.

The results showed large temporal variability in carbon stocks in state forests over the next 100 years. The uneven age class distribution for stands leads to significant biomass accumulation and thus net carbon sequestration over the next 20-40 years. This situation is followed by a period with alternating modest carbon sequestration and modest carbon loss. Larger windstorm incidents may shortly affect these temporal dynamics, but have little influence on carbon sequestration during the whole period.

Mean carbon sequestration in tree biomass over the next 100 years is estimated at 0,091 mio. t CO₂/yr for the whole converted state forest area. This is less than previously estimated in analyses that did not account for the actual age class distribution. During the period 2008 – 2012 state forests may therefore alone account for the cap of 0,183 mio. t CO₂/yr. However, during later periods net losses of carbon will occur from time to time.

The scenario for nature-based forest management probably leads to slightly less carbon sequestration compared to the business as usual scenario. Positive effects on the rate of carbon sequestration (prolonged rotation period, more dead wood, non-intervention forests) are offset by negative effects (increased harvesting of conifers, less intensive regeneration measures, natural succession and open areas).

An estimation of the total carbon sequestration potential in state forests needs to also address possible changes in soil carbon stocks. The necessary knowledge for this is not available for all management options within nature-based forest management.

As guidelines for monitoring are not yet settled internationally it is difficult to plan monitoring and evaluate associated costs. Assuming that it is not necessary to identify single areas, national forest statistics can monitor carbon sequestration. However, it may be necessary to supplement the current sampling of Danish forests with a soil inventory. Furthermore, conversion factors such as biomass expansion factors need to be documented or verified for Danish conditions.