Isohydric species show earlier drought-induced declines in stem water content, rehydration, sap flow, and growth than anisohydric species

Abstract

O_LIIsohydric species reduce water potential fluctuations through more stringent stomatal regulation. It is unclear whether isohydric behavior leads to smaller reversible diurnal stem shrinkage (i.e. greater nocturnal stem rehydration) during drought and a lower drought sensitivity of radial growth compared to anisohydric behavior. C_LIO_LIWith synchronous high-resolution sap flow and dendrometer measurements in mature anisohydric European beech and isohydric Douglas fir trees in pure and mixed stands, we quantified declines in stem water content (SWC), stem rehydration, sap flow and radial growth during soil dry-down and determined the critical soil moisture levels (expressed as Relative Extractable Water, REW) and elapsed desiccation time until 30-90% reductions in these traits. C_LIO_LISap flow and growth started to decline in both species at REW [~]0.6, preceding declines in stem rehydration. Water-spending Douglas fir approached 50% drops in SWC, sap flow, stem rehydration and growth during soil dry-down faster than beech, indicating higher drought sensitivity. In mixture, both species reached these reduction levels later than in monoculture, suggesting positive mixing effects on the species drought resistance. C_LIO_LIOur findings demonstrate that SWC, sap flow and radial growth decrease earlier than nocturnal stem rehydration, with isohydric and anisohydric species exhibiting different timelines of physiological downregulation during soil dry-down. C_LI