Timing of Managed Seasonal Drawdowns to Promote the Establishment of Wetland Plant Species on the Tule Lake National Wildlife Refuge, California

Timing of seasonal drawdowns influenced the plant response on Hovey Point within the 3-year study. Early drawdowns occurring on 1 April and 24 April promoted the establishment of undesirable and invasive plant species, while later drawdowns occurring on 17 May, 9 June, and 2 July, promoted more plant species beneficial to waterfowl as food or cover. Species diversity and percent cover were low during the first year, but increased in later years. Areas that received additional moisture, either from water seepage on eastern portions of the site or within small depressions that held water for a longer period of time, established more desirable plant species. Slow drawdowns on fields of the refuge between 17 May and 2 July would probably promote the most desirable plant community. Intermittent wet-dry saturated treatments to soil cores removed from drawdown zones indicated that at least 50% of species, and greater than 80% of species in all but two zones, frequently found on the field in 1998 and 1999 (identified in ³50% of the survey plots) were identified. This method may allow the prediction of plant species that would be frequently found in other restoration sites on the refuge. Additional species not common within drawdown zones were also found in seed bank treatments; however, not all species found on site were present in the treatments. Increased moisture levels in soil samples seemed to inhibit the germination of Avena fatua and Bromus tectorum, both invasive plants. Maintaining saturated soil conditions could serve as a potential control for additional invasive plant species found on the refuge. Rorippa columbiae, a U.S. Fish and Wildlife Service Species of Concern, was identified on site. It occurred most frequently along the perimeter between the 9 June and 2 July drawdown zones, although it was also found along the perimeter of the zones drawdown on 17 May and 9 June. This species was historically located within the refuge and its occurrence on the study site may represent a re-establishment of the original population. This plant appeared most frequently in areas that received slow, late seasonal drawdowns that had few competitive species present.