Critical limits of soil water availability (CL-SWA) for forest trees – an approach based on plant water status

Due to climate change, heat waves and drought are ex­ pected to increase in frequency and intensity in Central Europe. Thus, assessments of critical constraints of water supply in forest trees are needed to develop adequate for­ est adaptation measures. We present a novel ‘critical limit’ approach to soil water availability (SWA) for the major central European forest tree species based on the physio­ logical plant water status. In regards to the conductivity of trees’ xylem, three thresholds of pre-dawn water potential (ψ wp) were chosen, referring to (i) slight conductivity loss (10 %), (ii) critical conductivity loss (50 %) and (iii) com­ plete conductivity loss (> 90 %). In times of drought, predawn water potential relates to the soil water potential at the lowest soil depth in which plant’s root system is able to deplete water resources; the ‘effective rooting depth’ (ERD). The critical limit of soil water availability (CL-SWA) represents the proportion of plant-available water within the variable effective rooting depth (ERD) that meets both the critical soil water potential at the lower limit of the ERD and the critical plant water status. The CL-SWA-ap­ proach can be implemented in water budget models like BROOK90.