Climate Change and Reduction of Soil Organic Layer Thickness Increases Drought Risk in Mountain Forests of the Northern Alps

ABSTRACT Background On ca. 10% of the forested sites in the Bavarian Alps (Germany) mountain, forest soils (so‐called folic histosols) are characterized by thick organic layers overlying poorly weathered calcareous parent material. Aim These organic layers play a critical role in releasing water and nutrients for vegetation, yet they are also sensitive to climate change, with warming‐induced increases in respiration potentially reducing their thickness and water‐holding capacity. Methods We used the process‐based water balance model LWF‐Brook90 to simulate soil water availability for 117 sites represented by georeferenced folic histosol profiles and to study the effect of different climate change scenarios (Representative Concentration Pathway [RCP] 2.6, 4.5, and 8.5) and humus loss (0%–75%) on the water balance. Specifically, we used high‐resolution climate data and HYPROP‐based Mualem–van Genuchten parameters to study potential changes in soil water retention and drought risk. Results Simulating the effects of changes in precipitation and temperature in combination with organic layer degradation and the resultant loss of water‐holding capacity led to significant increases in predicted transpiration deficits. A 50% reduction in organic layer thickness resulted in a distinct decrease of plant‐available moisture even under the optimistic RCP 2.6 scenario. Under severe climate change (scenario RCP 8.5) and 75% organic layer loss, water availability shifted up to two levels on a seven‐part relative scale of moisture regime. Although such extreme levels (>50%) of organic layer loss are unlikely under closed canopy conditions, they may occur following stand‐replacing disturbance events such as large‐scale windthrows or bark‐beetle outbreaks, which are expected to increase under climate change. Drought‐induced reductions of vegetation productivity may inhibit litter production and the replenishment of organic layers. Conclusion Our findings highlight the vulnerability of sites dominated by thick organic layers to climate‐driven water stress. By the end of the century, even under an optimistic scenario (no humus loss, RCP 4.5), the site water balance class on folic histosols is projected to decline by one class for the drier sites (75th percentile). Preventing a downward spiral of mountain forests triggered by climate change requires a holistic silvicultural concept for preserving folic histosols.