Deep soil water as a dominant source for spruce water uptake in a subalpine forest: evidence from multi-year isotope data

- Norway spruce (Picea abies) dominates many European mountain forests, yet their seasonal water uptake strategies in high-elevation mono-specific natural stands remain poorly understood. We quantified contributions of shallow (0-10 cm) and deep (50-70 cm) soil layers to tree water uptake over three consecutive growing seasons (2020-2022) using stable water isotopes and Bayesian mixing analysis. - Contrary to the prevailing view of spruce as a shallow-rooted species relying primarily on water from the upper 10-20 cm of soil, our results showed more than 50% water uptake from deeper soil (50-70 cm), with deeper soil contributions crossing 80% in 2020. - During the dry and warm summer of 2022, positive soil recharge and elevated atmospheric demand increased evapotranspiration, with spruce trees taking up recently infiltrated rainfall from different soil depths, including >50% uptake from deeper layers. - Spruce water uptake shifted from cold-season-recharged soil water early in the growing season to warm-season precipitation in late summer. The timing of this shift in mid-summer can be explained by soil water recharge from recent rainfall infiltrated into the entire soil profile. This reliance on summer precipitation increases vulnerability of mono-specific spruce stands to more frequent droughts and heat waves under future climate change.