Soil disturbance, flood management, and riparian woody plant establishment in the Rio Grande floodplain

Abstract

The exotic saltcedar (Tamarix ramosissima) has become the dominant woody plant in many riparian systems in the southwestern United States. We evaluated the effects of saltcedar clearing and overbank river flooding on recruitment of cottonwood (Populus fremontii) and saltcedar vegetation in the Rio Grande floodplain of central New Mexico, USA in 1993 and 1994. Overbank flooding coincided with the natural river hydrograph. After two seasons of growth, cleared areas supported more 1993 cohort cotton-woods and saltcedar than uncleared controlled areas. There were no differences between cleared and uncleared areas for the 1994 cohort after one growing season. Although there was a 2-week difference in the date of peak river flows between 1993 and 1994, similar seedling densities of both species were recorded for both years. The descending limb of the 1993 hydrograph was more gradual than the 1994 hydrograph, however, resulting in a higher survival rate of 1993 seedlings. Soil texture, soil salinity, elevation, and soil moisture data were collected from all plots and compared with seedling densities for each seedling sample date. Elevation and soil moisture indices were the most influential variables on seedling density. Decreasing soil moisture variables resulted in consistent seedling mortality for all species in cleared and uncleared plots for both cohorts. Secondary channels developed as a result of vegetative clearing where natural river fluvial processes provided topographic relief and sediment deposition for seedling establishment. Greater densities of cottonwoods and coyote willows (Salix exigua) were found on these deposits compared with remaining portions of cleared plots. Saltcedar clearing in conjunction with peak river flows in late May or early June encourages recruitment of native riparian plants, particularly along sand deposits created as a result of secondary channel development. Receding flows correlated with a receding water-table level of about 2 cm/day enhances native seedling densities relative to saltcedar.