Abstract Key message Soil-related hydrological processes must be considered alongside climate-based indices to reliably identify physiological drought stress in beech forests. Abstract Extreme summer droughts have increasingly damaged beech forests in central European mountain regions. In these environments, topography and soil hydrological properties are key determinants of soil moisture dynamics, but their relative controls are still not fully understood. Here, we investigated soil moisture dynamics in two beech stands in the Harz Mountains: one with vital trees on a downslope, and the other with damaged trees on an upslope. We hypothesized that low soil water retention at each slope position limits tree vitality during drought, while topography modulates the extent of tree damage through lateral flow under limited precipitation. Both sites shared a comparable soil profile characterized by three distinguishable soil water controlling zones: a two-part top debris layer (0–40 cm) with (i) the organic-rich Ah horizon in the upper part and (ii) a layer underneath with high contents of soil fine fraction ( 40 cm) with high porosity. Using hourly measurements of soil temperature, volumetric water content (VWC), and matric potential (Ψ s ), along with indices of relative extractable water (REW) and evapotranspiration (ET), we quantified soil moisture dynamics within the controlling zones across the slope positions. All parameters indicated distinct soil water deficits, with the upslope site experiencing a relatively pronounced soil drying. During the most severe drought, when REW and VWC fell below 0.4% and 15%, respectively, ET declined sharply on the upslope but was temporality sustained at the downslope by short rain events that replenished its soil water capacity. This relatively enhanced drought resistance is possibly due to lateral subsurface water flow, augmenting soil moisture in downslope positions. Soil texture at the different controlling zones strongly influenced water retention and redistribution, explaining the greater vulnerability of upslope trees, where lateral recharge was assumed to be weak. Our findings thus suggest that beech resilience to drought is possibly enhanced by slope-mediated lateral flow, while stands located on deep coarse debris layers lacking such hydrological recharge likely face increasing drought risk. With frequent and intense summer droughts projected for the Harz Mountains, beech forests on upslope positions are particularly threatened. Forest management should therefore integrate soil hydrological processes with climate projections and prioritize site-specific regeneration strategies by slope position using drought-tolerant species.
Klinge et al. (Sat,) studied this question.