Restoration of an urban wetland mitigation bank from invasion by Chinese Tallow

Background/Question/Methods

Chinese Tallow (Triadica sebifera) is an invasive species that has over 60% canopy cover in some sections of Greens Bayou Wetland Mitigation Bank (GBWMB) in Harris County, TX. Wetland mitigation bank certification requires that invasive plant species be < 5% of the canopy cover. To comply with this requirement an herbicide control program was initiated by Harris County Flood Control District (HCFCD) and Harris County Soil & Water Conservation District with the following research questions: What is the extent and spatial distribution of Chinese Tallow invasion within GBWMB and Harris County? Of four EPA approved herbicides (Garlon, Rodeo, Clearcast, & Renovate3) for use in wetlands: Which is the most effective in controlling Chinese Tallow? Finally, can canopy cover of Chinese Tallow be reduced to  < 5% using a single herbicide alone? We used Maximum Entropy modeling with Landsat Thematic Mapper spectral reflectance, and 2006 derived land cover, soil and climatic variables, and ~230 field locations to predict the probable occurrence and thus spatial extent of Chinese Tallow invasion in Harris County. We tested the effectiveness of the 4 herbicides and a control in a 5 treatment X 6 replications ANOVA design using thirty 10-m X 10-m field plots of Chinese Tallow within GBWMB. We acquired before-treatment, fall season leaf off, and after-treatment terrestrial laser scans (TLS) of a replications X 5 treatments design subset of the original 30 plots. We then used the TLS to estimate canopy cover in each stand.

Results/Conclusions

After 2 years of study, we found that the herbicide Garlon had the greatest efficacy and mortality ratings on Chinese Tallow relative to the other three herbicides. Tallow had 53% cover across all of HCFCD’s properties in Harris County and 16% within Harris County itself based on the field survey data.  Spatial modeling indicated that 9% of both GBWMB and Harris County are occupied by Chinese Tallow at 70 to 100 % probability of occurrence. Analysis of TLS data showed that Tallow was at least 60% of the canopy cover before herbicide treatment in the control study. Tallow distribution was primarily related to highly productive low elevation wetlands on alkaline fine sandy loam soils. Until Tallow is eradicated from sites outside of GBWMB, HCFCD will have to maintain control programs of Chinese Tallow.