Influence of Species Distribution, Site Conditions, and Model Structure on Carbon Stock and Sequestration Estimates in German Forests

Forests play a central role in climate change mitigation through carbon sequestration and storage, yet spatial estimates remain highly uncertain due to variability in species composition, site conditions, and model representations of ecosystem processes. An example is presented as carbon stocks and sequestration rates are estimated for all forests in Germany using the process-based LandscapeDNDC model. This demonstrates the dependence on dominant tree species considering beech (Fagus sylvatica), oak (Quercus spp.), spruce (Picea abies), and pine (Pinus sylvestris), as well site conditions. Regarding the latter, the spatial resolution of 10 × 10 km enables to evaluation the role of soil fertility and water storage as well as precipitation and temperature effects.

The example also illustrates that the model is not able to represent the large carbon losses that have occurred during and after the extreme dry years 2018/2019 and neglects potential legacy effects. Therefore, it is suggested relating tree mortality and tree water deficit (TWD). Physically determined thresholds as well as theoretical concepts are available to principally derive the probability of mortality in cohort-based forest models. The new module provides a physically-based mechanism that very much depends on species-specific traits such as sapwood longevity and rooting intensity, but is robust against uncertainties in soil texture initialization. Furthermore, it is shown that it can easily be evaluated with micro-dendrometer data which are increasingly available.