Nature-based solutions for drought resilient forests

Climatic extremes are the new normal for large parts of the world. Even temperate places, such as northern Bavaria, have experienced an abundance of extreme weather with significant impacts on the local ecology. Heavy torrential rainstorms are observed more often, while simultaneously, the precipitation required for a healthy ecology does not occur for increasing periods. These droughts, in combination with anthropogenic influences, have severely weakened the vitality of vast stretches of forest in northern Bavaria. In consequence, secondary pests were able to cause wide spread tree mortality. This indicates the need for innovative water management strategies to increase the resilience of forest ecosystems with regard to an increased occurrence of droughts.

This study aims at exploring the potential of nature-based solutions to increase the infiltration of surface runoff in forest in order to increase the plant-available soil moisture and therefore the drought resilience during dry periods. Specifically two measures are investigated, which alter the micro-topography of the forest floor. These are:

• “Dead-wood” left in the forest after timber-harvest and aligned along slopes to act as flow barriers during runoff events and,
• Small-scale basins of shallow depth that mimic the natural topography of the forest floor and act as retention basins.

To be able to understand and evaluate the effectiveness of the nature-based solutions and investigate the relevant hydrological and hydrodynamic processes, a small, forested slope in Northern Bavaria was modeled using an innovative coupling of the 2-dimensional hydrodynamic TELEMAC Model with Green & Ampt infiltration. In preparation of setting up the model, state-of-the-art drone LiDAR measurements were used to produce a high-resolution (10 cm resolution) Digital Elevation Model of the area. This enabled us to set up the model with a high enough resolution to capture and simulate the micro-topographic changes of the measures. We simulated various scenarios representing different implementations of the nature-based solutions and used the change of runoff coefficient as compared to the current state simulations as a measure of efficiency. In general, our findings show a clear link between the implementation of the measures and decreased runoff coefficient. While the aligning of dead wood along the slope reduced the runoff coefficient more as compared to a random distribution of dead-wood, the shallow retention-basins showed a significantly higher impact on the runoff coefficient. However, it is likely that the distribution of soil types, vegetation and soil animal activity are very crucial because they significantly affect the infiltration and therefore the efficiency of these measures for drought resilience. Theses aspects were not considered. Altogether, the results of this study should be considered as qualitative as compared to quantitative, due to the simplifications done, especially with regard to the soil and infiltration processes.

 

Keywords: Drought; Forest ecosystem; TELEMAC; Greene & Ampt; LiDAR