Beyond two water worlds: dynamic transpiration sourcing in a mixed-species boreal forest

The water budget of a forested catchment comprises inflows, storage pools, and outflows, each with a specific stable isotopic signature. The isotopic signature is often set by summer vs. winter precipitation events, where the subsequent use of these water sources can then be traced. Streamflow, one of the major outflows, is critically important for water supplies and flooding risk. Transpiration, a second major outflow, describes the evaporation of water vapor from within tree leaves; it reduces streamflow and is mechanistically associated with biomass production. The water sources used for streamflow and transpiration can be so isotopically distinct that they have been considered to represent two “water worlds,” with distinct controls and perhaps little mixing between them. Here we describe, on a daily time-step, the contributions of water sources used for transpiration between two species that commonly occur across the Eurasian boreal zone and have close relatives in North America. Norway spruce, which is shallow-rooted, was compared to Scots pine, which roots more deeply, and both were compared to stream water. We made these measurements in 2017, a typical summer, and compared them to 2018, a year of historic drought. Pine and spruce used distinctly different water sources. After the drought ended in 2018, the spruce switched to exclusively recent summer rainfall. Pine also switched sources, but less completely, consistent with its deeper root distributions. Streamwater was derived from residual water, with a greater representation of winter precipitation. These results support the notion that transpiration and streamwater are derived from different sources, while further dividing the transpiration between spruce and pine. They suggest modified predictions of streamflow and forest production, especially in response to extreme weather events. Models of boreal forest transpiration should be tested against these observations to determine how well they describe this water-source differentiation.