Giant reed (Arundo donax L.), is an invasive, perennial grass found throughout the Rio Grande basin. It is considered a threat to water availability in the Rio Grande Basin by transpiring large amounts of water, reducing river flows and potentially impacting groundwater recharge. Our research is directed at understanding the ecohydrological controls and other drivers of giant reed productivity and water use in order to understand its impact on the water cycle and the potential restriction to its distribution. We tested whether soil moisture and other related features, such as small-scale topography or distance from the river, limited the transpiration and biomass of giant reed in riparian ecosystems. Four permanent transects (70~125 meters long) were established perpendicular to the water course. In nine evenly-spaced non-destructive plots, we measured transpiration of the second fully-expanded leaves using a gas exchange system (LI-COR 6400). In five evenly-spaced destructive plots (1 m²), aboveground shoots were harvested for dry weight, belowground biomass was sampled from the soil surface to 100 cm at 20 cm increments, and soil moisture was measured at 10 cm depth increments from 0-160 cm with a capacitance probe (Diviner 2000).

Results/Conclusions

We observed that soil texture, water and leaf nitrogen content were the major drivers of variation in productivity and performance of giant reed, such as transpiration, aboveground biomass, belowground biomass, stem high and stem density. Moreover, the transpiration inside meander bends differed from outside meander bends. In addition, leaf carbon isotope ratios increased with distance from river, which indicated increasing water use efficiency likely in response to declining soil moisture. We will determine if water is redistributed laterally across the floodplain within interconnected rhizomes. Results from our research efforts will determine the impact of giant reed on the Rio Grande River and potential constraints on its future distribution.