Summary

• Climate warming will increase the drought exposure of many forests world-wide. It is not well understood how trees adapt their hydraulic architecture to a long-term decrease in water availability.
• We examined 23 traits characterizing the hydraulic architecture and growth rate of branches and the dependent foliage of mature European beech (Fagus sylvatica) trees along a precipitation gradient (855–594 mm yr⁻¹) on uniform soil. A main goal was to identify traits that are associated with xylem efficiency, safety and growth.
• Our data demonstrate for the first time a linear increase in embolism resistance with climatic aridity (by 10%) across populations within a species. Simultaneously, vessel diameter declined by 7% and pit membrane thickness (T_m) increased by 15%. Although specific conductivity did not change, leaf-specific conductivity declined by 40% with decreasing precipitation. Of eight plant traits commonly associated with embolism resistance, only vessel density in combination with pathway redundancy and T_m were related.
• We did not confirm the widely assumed trade-off between xylem safety and efficiency but obtained evidence in support of a positive relationship between hydraulic efficiency and growth. We conclude that the branch hydraulic system of beech has a distinct adaptive potential to respond to a precipitation reduction as a result of the environmental control of embolism resistance.