Linking sap flow and tree water deficit in an unmanaged, mixed beech forest during the summer drought 2022

Temperate mixed forests are currently experiencing severe drought conditions and face increased risk of degradation. However, it remains unclear how critical tree physiological functions such as sap flow density (SFD) and tree water deficit (TWD, defined as reversible stem shrinkage when water is depleted), respond to extreme environmental conditions and how they interact under dry conditions. We monitored SFD and TWD of three co-occurring European tree species (Fagus sylvatica, Fraxinus excelsior and Acer pseudoplatanus) in dry conditions, using high temporal resolution sap flow, dendrometer, and environmental measurements. Species-specific SFD responses to soil drying did not differ significantly, while TWD was significantly higher in F. excelsior. Inter-specific differences in wood anatomy and water use strategies did not consistently explain these responses. TWD and SFD responded both to soil moisture content (SWC) during wet (SWC ≥ 0.2) and dry (SWC < 0.2) phases, with SFD responding more strongly. There was a significant correlation for TWD and vapour pressure deficit (VPD) only in the wet phase, and for SFD and VPD only in the dry phase. During the dry phase, the incoming PPFD significantly correlated with SFD in all species, and with TWD only in F. sylvatica and F. excelsior. TWD negatively responded to SFD, showing hysteresis effects from which a decreasing sigmoidal phase along the soil drying gradient was observed. The nonlinear correlations between TWD and SFD may result from a time lag between the two variables, and their different sensitivities to SWC and VPD under different drought intensities. We conclude that, under drought stress, TWD cannot be used as a proxy for SFD or vice versa.