Repeated summer drought changes the radial xylem sap flow profile in mature Norway spruce but not in European beech

Water consumption of trees is one of the most important processes connected to their survival under ongoing climate change and extreme events such as drought. Radial profiles of xylem sap flow density are an integral component to quantify the water transport for the level of an individual tree and that of ecosystems. However, knowledge of such radial profiles, in particular under stress, is very scarce. Here we show the radial profile of the xylem sap flow density in mature European beech (Fagus sylvatica L) and Norway spruce (Picea abies (L) Karst.) under repeated summer drought induced by throughfall exclusion (TE) and subsequent recovery compared to untreated control trees (CO). We measured xylem sap flow density (udaily in L dm-2 d-1) down to 8 cm sapwood depth at breast height using two different approaches, a thermal dissipation system and the heat field defor-mation method. In beech, repeated throughfall exclusion did not affect the radial xylem sap flow profile. However, in spruce, udaily was strongly reduced across the profile under repeated drought, changing the profile from a linear to a logarithmic regression. Even two years after drought release, the xylem sap flow profile did not fully recover in TE spruce. The reduction of udaily along the radial profile was accompanied by a reduction of the leaf area in TE spruce by c. 50%, while sapwood depth remained constant. The reduction of the xylem sap flow density along the profile reduced the calculated water consumption of TE spruce trees by more than 33% compared to CO trees, also after drought release. The impact of stressors such as repeated drought on the xylem sap flow density across the radial profile and its consequences for trees' and stands' water consumption needs to be addressed in more detail to minimize uncertainties in quantifying ecosystem water cycles.