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.