Quercus robur and Ulmus laevis water use patterns differ significantly under drought conditions and high vapor pressure deficit in the active floodplain of the lower middle Elbe.

Temperate hardwood floodplain forests (HFF) are highly heterogeneous and productive ecosystems threatened by anthropogenic influence and effects of global warming. Quercus robur (oaks) and Ulmus laevis (elms) are acknowledged in literature as the two highest the highest and second highest aboveground carbon biomass stores along the lower middle Elbe floodplain. Both species are adapted to the hydrological fluctuations of floodplain soils. However, in Central Europe, these hydrological fluctuations  are threatened by the IPCC (2022) expected increase of streamflow drought, soil moisture drought and lower groundwater levels, hindering key ecosystem services provided by HFF. Thus, we wanted to assess the water use patterns of both species under water limiting conditions and high vapor pressure deficit (VPD).

The study was conducted during the vegetation period of 2020 in the active floodplain of the Elbe. To understand the influence of soil texture in the soil water dynamics, two sites were selected, a sandy site located in the  high sand embankments and a loamy site, representing the low positioned sites of the floodplains. Sap flow was measured in 5 trees per species per site, using heat-ratio method devices. Additionally, 3 soil profiles per site were instrumented with volumetric water content and water tension sensors in defined depths up to 1.60 meters below ground. One week in June was selected to represent high soil water availability and one in August with less soil water availability, both periods shared similar VPD.

Both species show different reactions to soil type and water availability. Elms kept higher mean daytime sap velocity than oaks even under low water availability (~50% higher). Nonetheless, a steep decrease was recorded for the elms during August in sandy soils, what could be evidence of loss of conductivity due to cavitation. In both, the loamy and the sandy site, oaks had significantly lower mean daytime sap flow velocity than elms (E.g. in loamy soils: 13cm/h and 6cm/h, for elms and oaks respectively).  Intraspecific variability was observed for the oaks when the influence of the soil texture was considered. The oak reduced sap velocity in sandy soils significantly by approximately 50% compared to loamy soils. This indicates higher sensitivity of this species to soil texture and associated soil water potential. Furthermore, to understand the impact of soil texture on tree water use, the Jarvis model was applied. In the sandy site, under drought, the model was not able to explain the reduction in sap velocity considering potential evapotranspiration, thus under this condition soil water potential plays a stronger role in sap velocity regulation.

These results provide insights to the function that different adaptations by species and the influence of site-specific abiotic conditions could have over increased drought periods, providing information that may increment the success of restoration efforts of this ecosystem.