Daily stem water deficit of Norway spruce and European beech in intra- and interspecific neighborhood under heavy drought

High-resolution measurements of stem radius variations provide information about the tree water status with changing climate conditions by swelling and shrinking due to the reduction of xylem water potential and to the exceedance of leaf transpiration over root water uptake. The aim of this study was to analyze daily stem radius variations of Norway spruce and European beech in intra- and interspecific neighborhood. The experimental plots are part of a rainfall exclusion experiment. These variations are species-specific, i.e. spruces have a higher phloem thickness and higher amplitudes during a day than beeches. The amplitudes were significantly higher at the rainfall exclusion plots, but the amplitudes of spruces decreased above 27°C with increasing drought due to reduced transpiration rates and exhausted soil water reserves. The shrinking amplitude was observed for spruces in intraspecific neighborhood from a soil volumetric water content of 0.21 m³ m⁻³. In interspecific neighborhood, a shrinking amplitude for spruces could not be observed and revealed a lesser tree water deficit than in intraspecific neighborhood. Beeches showed minor differences with a higher tree water deficit in interspecific neighborhood. Consequently, stem radius variations give insights into a tree's water supply, which could help to understand changes in tree growth.