Finding soil water sources for invasive riparian plants using stable isotopes

Background/Question/Methods
Invasive plant species grow abundantly in New Jersey’s riparian environments and have an impact on water availability and ecosystem structure, where they are often able to outcompete and replace native plant species. Several studies have linked successful competition in plant communities to root function and effective water sourcing, but little comparable research has been done in the Northeastern United States. Japanese knotweed (Fallopia japonica), phragmites (Phragmites australis) and multiflora rose (Rosa multiflora) are three non-native plant species that are common to the area. This study investigates the water sources within the vertical soil profile for each of these species in order to understand how they compete for water resources under various hydrologic regimes.

Samples were collected in the early, middle and late summer 2014 from two sites in northern New Jersey where the three non-native plant species grow together. Sampled material included plant stems, roots, river water, and soil at selected depths in the profile. Rain water was also sampled over the course of the field season. All samples were analyzed for δ²H_VSMOW and δ¹⁸O_VSMOW. Several mixing models, including IsoSource software, were used to calculate the proportions of plant tissue water coming from each of the soil depths. Physiological measurements, including transpiration rates and leaf water potential, were also taken from randomly selected plants of each species.

Results/Conclusions
Preliminary results suggest that Japanese Knotweed transpired water from shallow soil in the early summer, and accessed increasingly deeper sources later in the season. Phragmites used primarily shallow soil water throughout the summer, with an exception being dry periods during which shallow water is unavailable. Multiflora rose sourced water from shallow soil in May and from increasingly deeper depths in later months, similarly to Japanese knotweed. Results of this research are also useful for finding the effect that these plant species have on freshwater that would normally enter New Jersey’s reservoir system.