Transpiration patterns and water use strategies of beech and oak trees along a hillslope

Abstract

The role of landscape topography in mediating subsurface water availability and ultimately tree transpiration is still poorly understood. To assess how hillslope position affects tree water use, we coupled sap velocity with xylem isotope measurements in a temperate beech-oak forest along a hillslope transect in Luxembourg. We generally observed greater sap velocities at the upslope locations in trees from average-sized trees, suggesting the presence of more suited growing conditions. We found a lower difference in sap velocity among hillslope positions for larger trees, likely due to the exploitation of deeper and more persistent water sources and the larger canopy light interception. Beech trees exploited a shallower and seasonally less persistent water source than oak trees, due to the shallower root system than oak trees. The different water exploitation strategy could also explain the stronger stomatal sensitivity of beech to vapour pressure deficit compared to oak trees. Xylem isotopic composition was seasonally variable at all locations, mainly reflecting the contribution of variable soil water sources and suggesting that groundwater did not contribute, or only marginally contributed, to tree transpiration. Overall, our results suggest that trees along the hillslope mainly rely on water stored in the unsaturated zone and that seasonally shallow groundwater table may not necessarily subsidize water uptake for species that do not tolerate anoxic conditions. Contrary to previous studies, at our site, we did not find higher sap velocity downslope as the subsurface hillslope structure promotes vertical water flux over lateral redistribution in the vadose zone.

Sapwood and heartwood are not isolated compartments: Consequences for isotope ecohydrology

“…Additionally, during the dormant season, δ 18 O and δ 2 H showed the heaviest values of the studied period (Figures 2 and 3). The occurrence of the heavy isotopic composition in xylem water during leafless periods was previously reported in other studies (Martín-G omez et al, 2017;Oerter et al, 2019;Phillips & Ehleringer, 1995) and also observed in the study area in 2019 (Fabiani et al, 2021). This observation has been explained by the evaporation through the bark that occurs over the dormant season when sap flow is ceased (Martín-G omez et al, 2017;Oerter et al, 2019;Phillips & Ehleringer, 1995).…”

Comparison of Rainfall Partitioning and Estimation of the Utilisation of Available Water in a Monoculture Beech Forest and a Mixed Beech-Oak-Linden Forest

“…Fan et al [61] describe an average rooting depth of 5.23 m for oak and 0.8 m for beech. With such a distributed root system, beech mainly utilised the near-surface soil water, i.e., the effective precipitation, as is described by Fabiani et al [32]. On the other hand, oak could use the effective precipitation and, also, the groundwater, which its deep roots were able to reach [62].…”

“…Stands of European beech and sessile oak exhibit different patterns of transpiration due to their root system type, stomatal regulation in response to environmental variables, leaf area index or forest management [30,31]. A recent study indicated that oak in the mixed forest transpired more water than beech, especially in the dry periods [32]. It was most likely due to different rooting patterns.…”

Exploring tracer information in a small stream to improve parameter identifiability and enhance the process interpretation in transient storage models

“…7) also suggested that the transient storage at our site became less important in controlling solute transport with increasing discharge. The decrease in A TS and T sto with the increasing discharge has been argued to indicate an increase in groundwater gradients toward the stream channel with a consequent decrease in the hyporheic zone at different study sites (Morrice et al, 1997;Fabian et al, 2011). However, the observed groundwater gradients at the study site exclude the presence of significant hyporheic exchange during the three simulated tracer experiments.…”

“…The studied stream reach (49 • 49 38 N, 5 • 47 44 E) is located in western Luxembourg, downstream of the Weierbach experimental catchment (Hissler et al, 2021;Fabiani et al, 2022). The stream channel is unvegetated with a slope of 6 % and consists of deposited colluvium material and fragmented schists (up to 50 cm depth) with local outcrops of fractured slate bedrock in the streambed.…”