Persistence of floodwater sustains riparian plant transpiration during drought: A case study of Arundo donax on the Rio Grande

Background/Question/Methods

Arundo donax L. is a tall perennial grass that has invaded and come to dominate many riparian zones throughout the Southwestern US. Arundo forms large, dense stands and transpires large amounts of water to support its high rate of growth and is, therefore, expected to have a major impact on riparian zone hydrology. Our study aimed to assess to what extent Arundo uses shallow groundwater, how Arundo transpiration varies across riparian zones, the causes of this variation, and whether Arundo water use affects groundwater-river water interaction. We used natural-abundance stable isotopes of oxygen (d¹⁸O) and hydrogen (d²H) to determine the water sources of Arundo along four approximately 100-meter transects perpendicular to the Rio Grande River in Southwest Texas. Surface soil, river water, groundwater, precipitation and rhizome samples were collected every month in both summer 2010 and summer 2011, which coincided with a major flood that saturated soils in the first year followed by extreme drought in the second year. Leaf gas exchange was measured every month in summer 2011, and was related to simultaneously measured soil moisture and groundwater depth gradients. 

Results/Conclusions

The isotope ratio of rhizome water was consistent with shallow soil moisture uptake, and with previous observations of a relatively shallow, fibrous root system. Floodwater from July 2010 persisted in the soil for at least a year despite a severe drought, and became the dominant water source for Arundo. The alluvial water table in our site was shallow (< 6 m) and subject to changes in river level. However, groundwater seemed not to be an important source for Arundo, so long as the soil moisture was sufficient. Arundo was found to close its stomata in response to increasing vapor pressure deficit (VPD), causing declining transpiration rate and increasing leaf d¹³C composition. Spatial variability of Arundo transpiration rate was not associated with any gradients in soil moisture, depth to groundwater, or distance from the river. Therefore, even though individual plants grew in wetter or drier locations, it did not affect Arundo transpiration rate.