Divergent water balance trajectories under two dominant tree species in montane forest catchment shifting from energy- to water-limitation

Abstract. Vegetation interacts with both soil moisture and atmospheric conditions, contributing to water flow partitioning at the land surface. Therefore, changes in both climate and land cover with vegetation affect the availability of water resources. This study aimed to determine the differential effects of climate change on the soil water regime of two common Central European montane forest types: Norway spruce (Picea abies L.) and European beech (Fagus sylvatica L.). 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. Results revealed an accelerating transition from a fully energy-limited state towards water-limitation, with evidence of strict water-limitation in recent outlier years, unprecedented in this system. While long-term column-averaged pressure heads 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 between the flow partitioning of the two forest types, which diverged further with increasing water-limitation. Higher snow interception by spruce (27 mm per season) resulted in drier soil below the spruce canopy in the cold season. Higher transpiration by beech (100 mm per season) led to increasingly drier soils over the warm seasons causing lower ground water recharge (34 mm per season). Low summer precipitation inputs exacerbated soil drying under beech more than under to spruce. 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.