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.