Forest under stress: responses and recovery of water relations across scales during the compound drought in summer 2022

Abstract

The summer of 2022 was characterized by compound soil and atmospheric drought (CSAD; low soil water content, SWC, and high vapor pressure deficit, VPD) in Central Europe, threatening forest carbon sequestration and water balance, yet linked ecosystem‐ and tree‐level hydraulic responses remain poorly resolved. We assessed the response and recovery of water relations of a beech‐dominated montane mixed deciduous forest in Switzerland (CH‐Lae) to the 2022 CSAD through a series of ecosystem‐scale (evapotranspiration, ET; canopy conductance, Gs) as well as tree‐scale measurements, including leaf‐level (leaf water potential, LWP; stomatal conductance, gs), stem‐level (nighttime stem rehydration, NSR) and root‐level (root water uptake depth, RWU depth) variables. During peak CSAD, ET and Gs declined by up to 50%, driven primarily by low SWC. Midday and predawn LWP declined significantly. Beech and maple accessed deeper water (> 70 cm), maintaining NSR and gs during CSAD and fully recovering thereafter; spruce and fir relied on shallow water (< 30 cm), showing stronger NSR reductions but still full recovery post‐CSAD. Our findings demonstrate that species‐specific hydraulic traits drive forest‐scale water dynamics under CSAD conditions. Integrating these insights into forest adaptation strategies will be critical for sustaining forests under intensifying compound droughts in the future.