Vertical distribution of soil water use and maintenance of stomatal conductance in the invasive exotic plant Lepidium latifolium in the riparian zone of the Walker River

Some invasive plants, such as Lepidium latifolium, are known to have extensive root systems which hypothetically allow them to access ground water and gain a competitive advantage over native forbs and grasses. These deeply rooted invasives typically have higher consumptive water use than native species, and because of this, the availability of water in riparian areas may be affected by deep rooting invaders. This study tested the hypothesis that L. latifolium gains a competitive advantage through a deep root system and has a substantial root mass that uses water from deeper saturated zones in riparian areas. To test this hypothesis competition and stomatal conductance experiments were done using L. latifolium and Elymus trachycaulus, a native slender wheatgrass, grown in barrels subjected to various soil moisture regimes. To further compare root uptake of water as a function of depth, stable isotope analysis was performed on samples of L. latifolium collected from wild populations. For these experiments, samples of L. latifolium and associated soils were collected three times throughout the growing season at multiple locations. Xylem water was extracted from the L. latifolium plants and soil water was extracted from associated soil samples. The waters were then analyzed for stable isotopic ratios of 2H and 1H. The resulting ratios of 2H to 1H from the vegetation were then compared to those from the soil samples. Results of the barrel experiments demonstrated that L. latifolium was able to widely distribute its roots and utilize the artificially maintained water table to maintain high transpiration (as indicated by stomatal conductance rates) throughout the growing season under drought conditions. The presence of L. latifolium growing with E. trachycaulus in the mixed species treatment, however, did not cause a statistically significant reduction in the biomass of E. trachycaulus. At all three field sites L. latifolium used shallow water sources early in the growing season and deeper water sources later in the growing season. Use of water from deeper sources correlated with a decrease in moisture of shallow soils. This study found that L. latifolium does have a deep root system that extracts water throughout the soil profile in barrel experiments and field surveys. However, in the barrel competition experiments the presence of L. latifolium did not have significant negative effects on the growth of E. trachycaulus in the planting densities used.