Limited sink but large storage: Biomass dynamics in naturally developing beech (Fagus sylvatica) and oak (Quercus robur, Quercus petraea) forests of north‐western Germany

Abstract

Abstract Currently, the dynamics underlying the storage and acquisition of biomass, and thus carbon, in naturally developing forests are under debate. A better understanding of the biomass dynamics of forests is needed to clarify the role played by naturally developing forests in the mitigation of climate change. Long‐term monitoring data from unmanaged strict forest reserves (SFRs) in north‐western Germany were used to analyse the biomass dynamics of pure beech, mixed beech and mixed oak forests. A complete balance of above‐ground woody biomass (biomass) and growth, density‐dependent and ‐independent mortality, as well as deadwood decay was derived. Density‐independent mortality served as a proxy for disturbance severity. After a time since abandonment (TSA) of 50 years, the average biomass ranged between 334 t/ha in mixed oak and 478 t/ha in pure beech stands. The net change in biomass was positive in all forest types. Density‐independent mortality and decay rates were much lower than the growth rates. Pure beech forests reached higher levels of biomass, a higher net change in biomass, and more growth than either of the mixed forest types. Biomass increased linearly with TSA in pure beech stands but followed an asymptotic course in the mixed forests. In the latter, the net change in biomass and growth were consistent with a unimodal development pattern. The development of biomass could not be explained by the ageing of the tree communities. Synthesis. We hypothesized that the observed biomass dynamics are a result of the interaction between resource supply within a limited growing space and the resource‐use efficiency of the tree stand in conjunction with disturbances. The still‐linear increase in the biomass of pure beech forests was assumed to reflect the high resource‐use efficiency of beech, especially its use of light. The above‐ground capacity of naturally developing broadleaved forests to store and acquire carbon is substantial. Accordingly, allowing broadleaved forests to develop naturally can contribute substantially to carbon storage and sequestration. However, our study also suggests that the above‐ground carbon sink decreases after several decades.