Mapping the next forest generation – the potential of national forest inventory data for identifying regeneration gaps

In light of global change and forest disturbances, there is an increasing recognition of the importance of forest regeneration to ensure future generations of trees. However, despite the importance of forest regeneration, there is a lack in spatial information on the current availability of trees in the seedling and sapling stage. In this study, we aimed to evaluate the potential to predict species-specific forest regeneration densities using regeneration data typically recorded within National Forest Inventories (NFIs). We then calculated three indicators for regeneration quantity and quality to locate potential gaps of regeneration under a changing climate. We successfully calibrated regeneration density models for 22 tree species using generalised additive models (GAMs) using regeneration density data from the 2012 German NFI and 44 environmental predictors. Subsequently, the models were used to create regeneration density maps for the German forest area at high spatial resolution (1 ha). Regeneration gaps were evaluated in terms of low total density (less than 1,000 ha-1), low species richness (≤2 species) and a high proportion (≥75%) of regeneration at high future cultivation risk. Our results indicate gaps in terms of total regeneration density and species richness for 13.4% and 47.1% of the forest area of Germany, respectively. A lack of climate-adapted species was found for 25.2%, exemplarily assessed for the Bavarian forest area. Along this example, we show how such results can be used to identify areas that require additional silvicultural intervention in order to increase the resilience of future forests. Our study highlights the potential of NFI data, particularly that on forest regeneration, and demonstrates the applicability of regeneration indicator maps for forest management and policymakers in times of change.