OOS 41-4
Tamarix removal in the context of restoration
Tamarisk is one of the most common woody plant species in riparian areas of the arid and semi-arid western United States. Intensive efforts to manage the species have occurred throughout the affected region since the 1950s, but there have been very few studies that quantitatively assessed control, non-target ecological impacts and economic costs of common management strategies. In 2009, four sites in the Arkansas River watershed of southeastern Colorado were established to investigate the effectiveness of tamarisk removal methods, and their impacts on passive understory re-vegetation. Primary treatment plots (aerial imazapyr applications, mulching, excavation, untreated controls) were installed the first year, and secondary treatments (releases of tamarisk leaf beetles (Diorhabda carinulata), and individual plant treatments (IPTs) of imazapyr and triclopyr) were installed in 2010. Within each treatment plot multi-scale, nested sampling plots were randomly located to annually monitor tamarisk survival and understory vegetation recruitment and re-establishment.
Results/Conclusions
Results three years after treatment indicated that all primary removal methods resulted in 80-90% tamarisk mortality, but that whole tree extraction caused higher mortality than aerial imazapyr applications or mulching. Tamarisk mortality from secondary treatments was slightly higher from IPT using imazapyr compared to triclopyr applications or beetle defoliation. However, cost analyses indicated that biomass removal followed by releases of biological control agents was more cost effective than either aerial or targeted herbicide treatments. Aerial imazapyr applications resulted in a depauperate plant community dominated by kochia (Bassia scoparia), which has continued to dominate these sites five years after treatment. Plant species richness and diversity was slightly higher in plots where mechanically removed tamarisk was treated with D. carinulata beetles compared to those that received IPT herbicide applications.
Our research provides further evidence for the effectiveness of controlling tamarisk and similar woody, invasive species (e.g., Elaeagnus angustifolia (Russian olive)) using integrated, targeted strategies that do not detrimentally effect extant desirable plant communities. While short-term costs for these treatments were higher, over even a three year timeframe the ecological benefits of their reduced non-target impacts outweighed economic costs. Overall, plant community re-vegetation patterns appeared to be more strongly driven by ongoing drought than by any tamarisk removal methods. Results from this study strongly suggest that in addition to managing tamarisk it is critical to address general underlying causes of site degradation – particularly in arid and semi-arid habitats – in order to ensure desirable passive re-vegetation and promote ecological restoration.