Which water do beech and chestnut trees prefer? Insights from a stable isotope approach in a small pre-alpine catchment

Abstract

*Corresponding author, email: diego.todini@iusspavia.it Stable isotopes are tracers used for the investigation of water flow paths, the quantification of the relative contribution of water sources to stream runoff or root water uptake, as well as to determine the spatial and temporal origin of water exploited by plants for transpiration. Previous studies have shown that uncertainties associated to samplings, soil and plant water extraction methods and the spatial variability in the isotopic composition of the water sources in a catchment can hamper the understanding of water cycling and the interactions between soil and plants.In this work, we used isotopic data collected during four growing seasons in a small forested catchment in the Italian pre-Alps to i) investigate the spatial and temporal variability of the isotopic composition of the sampled water sources, ii) determine the seasonal origin of the water sources, and iii) quantify the contribution of soil water to the root water uptake of beech and chestnut trees. The ecohydrological monitoring took place in the 2.4-ha Ressi catchment. Elevations are comprised between 598 and 721 m a.s.l., and the climate is temperate humid. The catchment is covered by a deciduous forest, with beech, chestnut, hazel and maple as the main tree species.Water samples for isotopic analysis were taken monthly from bulk precipitation, approximately bi-weekly from stream water, shallow groundwater and soil water by two suction lysimeter cups in the riparian zone. Plant water and bulk soil water samples were extracted by cryogenic vacuum distillation bi-weekly during summer. All water samples were analysed by laser spectroscopy, except plant water that was analysed by mass spectrometry. Results show that stream water, groundwater and soil water extracted by suction lysimeters were isotopically similar to precipitation, and on average they had an autumn or spring origin. Bulk soil water obtained by cryogenic vacuum distillation showed an evaporation signature, particularly on the hillslope sites where soil moisture was lower. At greater depths, bulk soil water extracted by cryogenic vacuum distillation was slightly less evaporated and less enriched in heavy isotopes compared to soil water extracted from shallow layers. Plant water was more similar to soil water extracted by cryogenic vacuum distillation, and mainly had a summer origin. Riparian trees tended to take up more water from shallow soil layers, whereas hillslope trees had a slightly larger preference for deep bulk soil water, particularly during the driest months.Our results suggest that, in the study area, trees likely use more bulk soil water than the mobile soil water, groundwater and stream water, and the preference for more shallow or deep soil water is likely due to the different rooting depth of the vegetation in the hillslope and the riparian zone. Keywords: stable isotopes; soil water; plant water; forested catchment; cryogenic vacuum distillation.