CLIMATIC AND WATER-USE VARIABILITY IMPACTS ON RIPARIAN PHREATOPHYTES IN THE GREAT PLAINS

Riparian corridors with phreatophytes exist in subhumid to semiarid grassland areas of the Great Plains of the U.S. This riparian zone vegetation is often dominated by native phreatophytes, such as cottonwood (Populus spp.) and willow (Salix spp.), but has increasingly transitioned to the invasive species salt cedar (Tamerix spp.) in some areas. The native phreatophyte communities have been sustained in riparian zones by ground-water levels that have maintained baseflows in the accompanying streams. Although the native phreatophytes have survived through great climatic variability, increased water use over the last several decades, especially from ground-water pumping, has imposed conditions that have decreased baseflow in streams and lowered water tables. This has led to a spread of phreatophytes into former parts of stream channels and, more recently, to increased stress on native phreatophytes. When the water table drops below the depths of native phreatophyte roots for substantial periods, these phreatophytes are severely stressed and, depending on lithology and other factors, can die. Subsequent periods of high runoff have allowed the spread of the invasive species salt cedar through deposition of seeds during floods. Salt cedar have been able to encroach into stream channels and replace native phreatophytes in many riparian zones. Continued declines in ground-water levels from pumping have caused the die-off of native phreatophytes to extend into the transition zone between semiarid and subhumid climatic regions. This has occurred even where the average ground-water level in the alluvial aquifer between wet and dry periods would provide baseflow to the stream. However, the combined climatic and water-use variability can produce conditions during dry periods in which the water table falls faster than native phreatophyte roots can follow. Examples of climatic and water-use variability impacts on phreatophytes will be discussed primarily for the Arkansas River in Kansas. Future increases in climatic variability caused by global climate change will exacerbate the stress on native phreatophytes in the Great Plains and potentially favor replacement by non-native species.