Wetter winters amplify temperature effects on fine root growth and increase tree growth in north-eastern European beech forests

Winters are predicted to become wetter in large parts of the temperate zone over the next century. Additionally, warmer temperatures will shift precipitation from snow towards rain, reducing soil insulation. To better understand the effect of altered winter precipitation on temperate forests, we conducted a factorial snow exclusion and rain addition experiment in European beech forests across the species´ north-eastern distribution range. Minirhizotrons, dendrometers, Plant Root Simulator (PRS) probes and teabags were used for analysing root growth, tree growth, nutrient availability and decomposition along gradients of winter precipitation and winter soil temperature, achieved by natural differences between sites and experimental manipulation. The study shows that the well-known positive effect of warmer soils on net fine root growth was stronger under wetter winter conditions. Basal area increment was promoted by 2.6 % per mm long-term precipitation, likely due to deep soil water recharge. NO₃-N and SO₄-S availability increased with wetter winters, but these effects did not persist into the growing season. Winter decomposition of organic material with low C:N ratio increased by 2 % per 100 mm of winter precipitation. Prolonged changes in the decomposition rates into summer suggest a microbial community shift in response to wetter winters. These biogeochemical changes, however, had only minor secondary effects on the growth variables. Our results provide evidence that winter precipitation plays a key role in temperate forest ecology. Wetter winters may increase the ability of European beech to cope with climate change, especially by buffering the impact of summer droughts and by supporting root growth if soils get warmer.