Water sources for woody shrubs on hillslopes: An investigation using isotopic and sapflow methods

Abstract

Shrub encroachment into grasslands is a globally occurring process, but resulting changes to catchment water flows and stores are not yet well understood. We examined seasonal patterns of water sources for regenerating woody shrubs in seasonally dry montane farmland. We compared stable isotope ratios of shrub stem water to soil water pools at various depths within the vadose zone, groundwater, and stream water. We compared the results of these water source analysis with riparian plant water uptake measured using sap flux sensors and some potential drivers of sap flow rates (soil moisture and meteorological data). The stable isotope data indicated that the shrubs derived the majority of their water from shallow soil (top 10 cm), during the summer growing season. Sapflow measurements provided support for the isotope results, as riparian shrubs showed a springtime peak in water use and reduced sapflow rates during late summer driven by intermittent drying of shallow soil layers. Analysis of groundwater and streamwater indicated that these water sources were isotopically separated from plant and shallow soil water and fed primarily by precipitation falling during winter months. Our results indicate that transpiration of groundwater that may otherwise have fed streamflow is likely to be minor during early stages of shrub regeneration. This is important because shrubs are costly to remove and also have measureable benefits for sediment retention and soil fertility on montane farms.

Does transpiration from invasive stream side willows dominate low‐flow conditions? An investigation using hydrometric and isotopic methods in a headwater catchment

“…Those studies showed that trees can use tightly bound soil water, especially during dry periods. At our study sites, analysis of stable dual isotopes (δ 18 O, δ 2 H) in water indicated water usage by individual willows in many cases diverged from stream, groundwater, and springtime near‐stream suction lysimeter samples for the stretch from Melrose to Pawseys and resembled dryer soil water values taken concurrently from a hillslope in nearby Lang's Gully (Dudley et al, ). This is a striking result, because our working hypothesis was that the main water source for stream‐side willows is stream water, especially because willows are partly situated in‐stream.…”

“…(b) Seasonal and temporal variation in stable isotope ratio for six individual willow trees. Soil water data derived from hillslope samples taken in nearby Lang's Gully (Dudley et al, ) is provided for comparison…”

“…Willow xylem water samples, however, mainly showed an evaporation signal by falling below the LMWL. Because there was insufficient soil moisture in streamside soils to provide a suction lysimeter sample during dry summer months, we have presented soil moisture data collected from dry hillslopes in Lang's Gully (Dudley et al, .) in Figure , for comparison with xylem samples.…”

“…Suction lysimeters and flushes in willow tree areas were used to sample the near‐surface water component. In addition, soil isotope samples collected from Langs Gully during 2015 and 2016 were used in comparison analysis (see Dudley, Marttila, Graham, Evison, & Srinivasan, for more details). All samples were stored in darkness at +4 °C until analysis.…”

“…Melrose to Pawseys and resembled dryer soil water values taken concurrently from a hillslope in nearby Lang's Gully (Dudley et al, 2017). This is a striking result, because our working hypothesis was that the main water source for stream-side willows is stream water, especially because willows are partly situated in-stream.…”