Restoration of riparian meadows degraded by livestock grazing: above- and belowground responses

Riparian meadows in the western US are highly productive areas within the riparian corridor that have been extensively utilized for livestock grazing and that are often degraded. This study aimed to increase our understanding of both the functional responses and restoration potential of mesic meadow systems in central Nevada, USA. Nitrogen addition and clipping treatments were used to examine grazing effects, while aeration and revegetation were included to evaluate potential restoration treatments. Belowground response was determined by examining gravimetric soil water content and rooting depth and activity. Aboveground response was determined for two key mesic meadow species, Carex nebrascensis and Poa pratensis, by quantifying gas exchange and water relations. Large yearly and seasonal differences among water tables largely determined the rooting activity and depth in these mesic meadows and influenced treatment responses. Little rooting activity occurred within or at the surface of the water table and rooting depth increased as water table level lowered during the growing season. In general, nitrogen addition decreased rooting activity and depth relative to controls. It resulted in less negative water potentials and photosynthetic rates that were higher early in the growing season, but lower later in the growing season. The effects of nitrogen addition could be attributed to accelerated phenology and earlier senescence. Clipping resulted in less negative plant water potentials in Carex and Poa, and Poa had higher photosynthetic rates immediately after clipping but only for certain dates and comparisons. Aeration increased rooting activity and depth where there was no confounding effect of water table. Also, predawn and midday water potentials were generally less negative for Carex and Poa in aerated plots. Establishment on the revegetation plots was dominated by annual and early seral species and, consequently, these plots exhibited reduced rooting depth and activity early in the growing season and high root turnover. Results indicate that water table depth and its effects on soil water are dominant factors in determining the functional processes and recovery potentials of these riparian meadows. Clipping had minimal effects on above- and belowground responses, perhaps because clipping was performed late in the growing season after plants had begun to senescence. Nitrogen addition decreased rooting activity and altered phenology, indicating deleterious effects. Aeration was effective at overcoming some of the negative affects of overgrazing and may be an effective restoration treatment. However, revegetation may have limited potential due to establishment constraints of the dominant species.