ABSTRACT:

Water stable isotope datasets associated with Lee et al., "Water sources for coastal forest trees correctly identified through centrifugation of xylem water from sapwood cores but not cryogenic vacuum extraction."

Identifying the water sources plants use is central to understanding potential shifts in water cycling with climate change. Studies have shown that the hydrogen isotope composition (δD) and oxygen isotope composition (δ18O) of xylem waters are sensitive to the water extraction method used, potentially obscuring potential water sources. Here, we report consistent differences in the δD and δ18O compositions of xylem water extracted via cryogenic vacuum distillation (CVD) relative to centrifugation for Douglas-firs and Tanoaks in an old-growth forest in Mendocino county, California across wet and dry seasons. On average, centrifuge extracted samples are more enriched, and the offset in δD between centrifuge and CVD extracted samples is 24‰ for Douglas fir and 15‰ for Tanoak. The effect on δ18O is smaller as a fraction of the mean measurement values, but is still significant for Douglas fir at 0.85‰ and potentially significant for Tanoak at 0.42‰. The xylem water composition of CVD extracted water before and after centrifugation was similar, with the exception of a 5.4‰ offset in δD for Douglas fir, suggesting that centrifugation removes a small but isotopically distinct volume of water from the xylem. Differences in the magnitude of isotopic offset between Douglas fir and Tanoak suggests that species differences in xylem structure may underlie the source of the offset. When compared with water sources at the extensively characterized field site, the centrifuge extracted xylem water δD and δ18O align well with recent rainfall during the wet season and lysimeter extracted moisture from the bedrock root-zone during the dry season. In contrast, xylem water and soil moisture extracted via CVD show similarities in both δD and δ18O, but neither align with rainfall, lysimeter samples, or an evaporative enrichment signal of rainfall for the site. We conclude that CVD extracted xylem is offset from source water in both δD and δ18O and centrifugation of xylem provides a more accurate method for identifying plant water sources. We recommend testing multiple extraction methods to assess potential isotopic differences between methods. This comparative approach allows for understanding method specific biases in δD and δ18O values. Further, when selecting a method for xylem water extraction, researchers should account for species-specific xylem structure which could yield isotopically distinct water pools.

ABSTRACT:

The datasets in this resource are associated with: King, Evan Robyn. Tracing water through a forest root zone. Diss. 2023
Abstract: Recent critical zone studies have highlighted the important role that unsaturated weathered bedrock plays in the storage of plant-available water, particularly during dry periods when incoming precipitation is limited. Unlike for soils, our knowledge of unsaturated water flowpaths within weathered bedrock, which may extend many meters into the subsurface before reaching the water table, remains relatively unknown. In this study, we employed water stable isotopes to trace the fate of waters entering a steep, weathered bedrock-dominated hillslope in Northern California. We used a subsurface vadose zone monitoring system (VMS) that contains sets of flexible sensors and samplers within inclined sleeves to sample waters at discrete intervals down to 16.6 m depth to fresh bedrock. Additionally, we sampled several other water fluxes and reservoirs at the hillslope, including storm samples and tightly-held matrix waters. Previous studies at the site revealed a dynamic, seasonally wetting and drying subsurface in response to a Mediterranean-type climate of long, dry summers and cool, wet winters. Dynamic storage estimates and drilling campaigns show that roots may extend to 16 m depth and likely play a role in the transmission of waters to groundwater and stream. We report the results of a tracer experiment, whereby a deuterated-water tracer was injected into the hillslope in May 2019 to simulate the last large storm of the wet season. We sampled waters transiting the unsaturated zone and monitored precipitation inputs for the three years following the tracer application to confidently detect the signal of the tracer to 4.7 m depth, with tracer signals sustained at a single depth interval for up to 21 months. We propose that mixing between dynamic and nondynamic waters within the weathered bedrock zone allows the persistence of the tracer signal through several dry seasons. We compare VMS-extracted waters with cryogenically extracted waters to show that isotopically distinct pools may exist within the hillslope. Finally, we explored how rooting depth may influence tracer transmission by simulating flow in the upper 10 m of our hillslope in HYDRUS-1D. We find that rooting depth may determine the extent to which the tracer is mixed with a nondynamic reservoir and the proportion of tracer that is extracted via transpiration.

ABSTRACT:

Water retention curve datasets for soils at the Rivendell hillslope at the Eel River Critical Zone Observatory measured via HYPROP.

ABSTRACT:

Water retention curve datasets for soils at the Rivendell hillslope at the Eel River Critical Zone Observatory measured via HYPROP.

ABSTRACT:

Bedrock weathering regulates nutrient mobilization, water storage, and soil production. Relative to the mobile soil layer, little is known about the relationship between topography and bedrock weathering. Here, we identify a common pattern of weathering and water storage across a sequence of three ridges and valleys in the sedimentary Great Valley Sequence in Northern California that share a tectonic and climate history. Deep drilling, downhole logging, and characterization of chemistry and porosity reveal two weathering fronts. At ridgetops, the elevation of each front relative to the channel increases with hillslope length. The shallower front is approximately 7 m deep at the ridge of all three hillslopes and marks the onset of pervasive fracturing and oxidation of pyrite and organic carbon. A deeper weathering front marks the onset extent of open fractures and discoloration. This front is 11 m deep under two ridges of similar ridge-valley spacing, but 17.5 m deep under a ridge with nearly twice the ridge-valley spacing. In all three hillslopes, closed fractures in otherwise unweathered bedrock are found under ridges to at least the elevation of the adjacent channels. Neutron probe surveys reveal that seasonally dynamic moisture is stored to approximately the same depth as the shallow weathering front. Under the channels that bound our study hillslopes, the two weathering fronts coincide and occur within centimeters of the ground surface. Our findings provide evidence for feedbacks between erosion and weathering in mountainous landscapes that result in systematic subsurface structuring and water routing.