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

Abstract

The exotic s-,J.Itccdar (Tamarix rcmto.s'issio~a) 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 cottonwoods 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 lot 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 sahcedar.

Seedling competition between native cottonwood and exotic saltcedar: implications for restoration

“…Historically, flows of the Rio Grande River were characterized by an annual hydrograph typical of many river systems in western North America , with peak flows occurring in late May and June following snowmelt in upstream mountainous regions (Crawford et al 1993). However, due to damming of the Rio Grande at several places, such flow regimes have been disrupted and the natural hydrology of the river has been altered (Taylor et al 1999).…”

“…Plots for intraspecific competition in cottonwoods contained 4, 10, 20, 30, and 40 plants/ 0.25 m 2 . These densities were based on observations of natural plant communities in the area and previous studies (Taylor et al 1999;Sprenger et al 2002;Bhattacharjee et al 2006). To achieve the above densities, we selected areas that closely matched the required plant composition and established plots with sufficient plants of each species.…”

“…Saltcedar has been known to invade areas where young cottonwoods or other native vegetation are absent (Sher et al 2000). To reestablish stands of cottonwoods, germination sites must be initially created mechanically by removing existing vegetation followed by artificial flooding (Taylor et al 1999;Sprenger et al 2002;Bhattacharjee et al 2006). Several other restoration techniques such as planting of poles or seed-bearing branches along receding water levels in restoration sites following flooding have been reported to yield increased cottonwood recruitment.…”

“…Therefore, for successful restoration of such areas, it is often recommended to induce conditions that mimic the natural ecosystem processes (Attiwill 1994;Grumbine 1994;Hobbs and Norton 1996;Fule et al 1997). In natural riparian areas, overbank flooding after spring snowmelt results in scouring the substrate, removing competing vegetation, and potentially creating ''seed beds'' for native vegetation to regenerate (Taylor et al 1999;Bhattacharjee et al 2006). Restoration often becomes difficult if exotics respond to natural disturbances by establishing as successfully as natives (Sprenger et al 2002).…”

Historical Analysis of Riparian Vegetation Change in Response to Shifting Management Objectives on the Middle Rio Grande

“…Young seedlings can germinate within 2-3 days and are supported by consistent water availability in the following years [69]. Along the Rio Grande, cottonwood forest establishment can occur within three years of an overbank flooding event, usually located in areas moistened specifically due to flooding events [69][70][71]. Conversion to cottonwood forest also occurred from disturbance/canal (188 ha), active agriculture (236 ha), and shrubland (208 ha).…”