Wood density reduced while wood volume growth accelerated in Central European forests since 1870

Forest stand growth dynamics in Central Europe have accelerated since 1870 due to a rise in temperature, extended growing seasons, and other components of climate change. Based on wood samples from the oldest existing experimental plots in Central Europe, we show that the dominant tree species Norway spruce (Picea abies (L.) micaasT.), Scots pine (Pinus sylvestris L.), European beech (Fagus sylvatica L.), and sessile oak (Quercus petraea (MAITUSCHKA) LIEBL.) exhibit a significant decrease in wood density since more than 100 years. While stand and trees grow faster with respect to wood volume, we can show that wood density decreased by 8-12% since 1900. These results object a naive direct transformation of volume growth trends into an accelerated biomass production. Since 1900, stand biomass increment increased 9-24 percentage points less compared to volume increment (29-100% increase reduces to 20-76%). For a given stem diameter and annual ring width, tree stability against windthrow, wood strength, energy content and C sequestration are even reduced under recent conditions. The generally decreased late wood density, partly going along with an increased early wood fraction, suggests the observed extension of the growing season and fertilization effect of dry deposition as the main causes. Our results indicate that current increased wood volume growth rates must not be straightforwardly converted into sequestrated C and biomass harvest potentials assuming historic values for wood density. This should be taken into account in monitoring, modeling, and utilization of carbon and biomass in forests under global change.