Drought-exposed trees regulate transpiration through stomatal closure, but residual water loss continues, potentially reducing turgor in photosynthetic and cambial tissues and thus growth. While isohydric species reduce water potential fluctuations through more stringent stomatal regulation, it is unclear whether this strategy also leads to improved diurnal rehydration of stem tissues and a lower drought sensitivity of radial growth compared to anisohydric species. With synchronous high-resolution sap flow and dendrometer measurements in mature anisohydric European beech ( Fagus sylvatica ) and isohydric Douglas fir ( Pseudotsuga menziesii ) trees in pure and mixed stands, we related progressive soil moisture (expressed as relative extractable water, REW) declines to coordinated changes in stem water content, stem rehydration, sap flow (as a proxy for transpiration) and radial growth during the severe 2022 drought. We defined phases of REW (high, moderate, and low) and examined how these shifts propagated into stem-level drought responses by quantifying REW thresholds and elapsed dehydration time until 10–90% reductions. Water-demanding Douglas fir depleted soil moisture faster and approached equivalent reductions in sap flow, stem water content, stem rehydration and growth earlier than beech, indicating higher drought sensitivity. Sap flow started to decline in both species already at high REW (∼0.8–0.6), preceding declines in growth and stem rehydration. In the mixture, both species generally took longer to reach these reduction levels than in monocultures, suggesting positive mixing effects on the species’ drought resistance. Our findings demonstrate that declining sap flow during soil drying triggers sequential declines in stem water content, radial growth and ultimately nocturnal stem rehydration, revealing a coordinated progression of drought responses that differ between species. Overall, these results indicate that species-specific water use strategies and stand structure, rather than degree of isohydry alone, govern the timing and propagation of drought effects within the soil-plant system.
Paligi et al. (Sat,) studied this question.