Significance Sound knowledge of the limits of tree hydraulic function is crucial to understand drought-induced tree mortality, although the mechanisms are difficult to study in adult trees. We explored the dynamic changes that the tree hydraulic system undergoes during the progression of a severe drought in dying adults of Norway spruce. Xylem pressure and hydraulic conductance showed sudden, nonlinear declines that commenced at surprisingly low levels of xylem embolism and caused a rapid collapse of the tree’s hydraulic system and tree death. These observations provide striking field-based evidence for hydraulic failure in adult conifers that are on the trajectory to drought-induced mortality. The nonlinear temporal dynamic of dehydration suggests a higher mortality risk than previously assumed for this major European tree species.