Future changes in water availability: Insights from a long-term monitoring of soil moisture under two tree species

Abstract. Vegetation interacts with both soil moisture and atmospheric conditions, contributing to water flow partitioning at the land surface. Therefore, both climate and land cover changes impact water resource availability. This study aimed to determine the differential effects of climate change on the soil water regime of two common Central European forest types: Norway spruce (Picea abies L.) and European beech (Fagus sylvatica L.) stands. A unique dataset, including 22 years (2000–2021) of measured soil water potentials, was used with a bucket-type soil water balance model to investigate differences in evapotranspiration and groundwater recharge both between the forest types and across years. While long-term column-averaged pressure head indicated drier soil at the spruce site overall, this was driven by the wettest years in the dataset. Seasonal and interannual variability of meteorological conditions drove complex but robust differences in flow partitioning between the forest types. Higher snow interception by spruce (27 mm season^-1) resulted in drier soil below the spruce canopy in the cold season. Higher transpiration by beech (70 mm season^-1) led to increasingly drier soils over the warm seasons. Low summer precipitation inputs exacerbated soil drying under beech as compared to spruce. Estimated summer recharge was lower under beech (25 mm season^-1) due to its lower transpiration. The difference was more pronounced (over 40 mm season^-1) during wetter summers. These suggest that expected trends in regional climate and forest species composition may interact to produce a disproportionate shift of recharge from the summer to the winter season.