Controlled flooding and staged drawdown for restoration of native cottonwoods in the Middle Rio Grande Valley, New Mexico, USA

Alteration of natural hydrologic regimes of most rivers in the southwestern United States has led to degradation of riparian habitats. Most areas historically covered by Rio Grande cottonwood [Populus deltoides Marshall subsp. wislizenii (Wats.) Eckenw.] have been replaced by exotic saltcedar (Tamarix chinensis Lour.). Following an earlier study in the Middle Rio Grande Valley, New Mexico that evaluated faster staged water drawdowns to restore riparian habitat, we evaluated slower rates, 2 cm/day and 5 cm/day (starting depth = 30 cm) to determine if cottonwood seedling density could be increased. During the period of spring flood recession of the Rio Grande, we placed seed-bearing branches of cottonwood in experimental basins and applied the drawdown treatments. Following the end of drawdowns, we conducted the first vegetation sampling to determine cottonwood and saltcedar seedling densities in the area. We also conducted a mid-season, an end-season, and an over-winter vegetation sampling to observe changes in seedling densities over time. Saltcedar and cottonwood seedling densities did not differ statistically between the drawdown treatments. However, survival of cottonwood seedlings during the first growing season in the 2 cm/day drawdown was greater than in the 5 cm/day drawdown. Greater seedling survival in the slower drawdown was likely due to increased soil moisture levels in that treatment, corresponding to a more gradual descending limb of the historical hydrograph. Use of a slow water drawdown (2 cm/day; about 20 days duration) synchronized with natural seed rain of cottonwoods will result in high first season densities of this important riparian species.