Buffering effects of groupwise European beech (Fagus sylvatica L.) admixtures on forest microclimate and Humipedon conditions: a spatial analysis along beech-spruce/pine transects

Tree‑species composition strongly influences forest‑soil properties, a relationship that becomes critical during forest conversion and soil‑fertility considerations. We employed a transect‑based research design to quantify the spatial effects of 83‑year‑old European beech (Fagus sylvatica L.) “Green‑Eye” (GE) groups on site conditions in the Central European Uplands, focusing on moderately sandy, acidic soils. We also conducted ground vegetation inventories (species composition, cover, and Ellenberg indicator values) to infer site quality with respect to nutrient status, soil acidity, light regime, and moisture availability. The GE act as facilitators and generate a dual buffering effect: (1) a microclimatic buffer that moderates temperature extremes and soil moisture, and (2) a pH buffer that reduces soil acidity in the upper Humipedon (the organic‑organomineral surface horizons). Compared with adjacent coniferous stands (CS) of Norway spruce (Picea abies (L.) H. Karst.) and Scots pine (Pinus sylvestris L.), the GE showed a 28% higher Humipedon moisture during the growing season (37% higher in the leaf‑less period) and up to 20% higher pH in the forest floor (8% higher in the upper mineral soil). The spatial influence of the GE extended 2–6 m beyond the crown projection, while a pronounced litter shadow reached up to 24 m into the surrounding CS, creating a gradient of altered Humipedon properties that spills over into the coniferous forests. The “base‑pump” effect (supply of base cations from the European beech litter) also improves forest‑floor conditions well beyond the beech canopy. To validate the findings, we compiled a systematic dataset of tree‑species impacts on soil properties from 579 acidic‑soil forest stands across Central Europe. The broader analysis confirmed the GE effects and suggests that, on moderately sandy, acidic soils where beech groups have persisted for decades, European beech can serve as a modest natural restoration tool by accelerating organic‑matter decomposition and enhancing the biochemical status of the Humipedon. Overall, the study provides evidence for the long-term success of beech-driven forest conversion, demonstrating the delivery of key ecosystem services such as microclimate regulation and maintenance of soil fertility through the combined buffering of near-ground microclimatic conditions and soil pH. Graphical abstract