Water relations of drought-stressed temperate trees benefit from short drought-intermitting rainfall events

Decreasing amounts of precipitation and more frequent dry periods will challenge temperate European forests in the future. During such dry periods, short drought-intermitting rainfall events might be the only renewing water source for trees. We investigated the effects of short drought-intermitting rainfall events on the water relations of mature individuals of six different tree species in a near-natural temperate forest during the exceptionally dry summer of 2015. We found the trees to strongly respond to short drought-intermitting rainfall events: maximum daily sap flow recovered already at precipitation amounts of 1.5 mm by up to 20%, and tree water deficit (TWD) and midday shoot water potential (Psi(midday)) improved by up to 60% from rainfall amounts of > 4.5 mm. We speculate that a mix of foliar and root water uptake as well as relaxation of canopy VPD and thus stomatal water control lead to the observed recoveries. Hydraulic conductivity was found to partly explain the differences in TWD recovery among species. Duration of the rainfall-facilitated recovery of tree water relations was on average 3 days in the coniferous species and Q. petraea but distinctly longer in C. betulus and F. sylvatica for which it reached 9 days. These results show that drought-intermitting short rainfall events strongly facilitate the relaxation of water stress in temperate tree species during drought events and possibly contribute as such to the ability of temperate trees to withstand longer periods of drought.