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Most studies assessing vegetation response following control of invasive Tamarix trees along southwestern U.S. rivers have been small in scale (e.g., river reach), or at a regional scale but with poor spatial-temporal replication, and most have not included testing the effects of a now widely used biological control. We monitored plant composition following Tamarix control along hydrologic, soil, and climatic gradients in 244 treated and 172 reference sites across six U.S. states. This represents the largest comprehensive assessment to date on the vegetation response to the four most common Tamarix control treatments. Biocontrol by a defoliating beetle (treatment 1) reduced the abundance of Tamarix less than active removal by mechanically using hand and chain-saws (2), heavy machinery (3) or burning (4). Tamarix abundance also decreased with lower temperatures, higher precipitation, and follow-up treatments for Tamarix resprouting. Native cover generally increased over time in active Tamarix removal sites, however, the increases observed were small and was not consistently increased by active revegetation. Overall, native cover was correlated to permanent stream flow, lower grazing pressure, lower soil salinity and temperatures, and higher precipitation. Species diversity also increased where Tamarix was removed. However, Tamarix treatments, especially those generating the highest disturbance (burning and heavy machinery), also often promoted secondary invasions of exotic forbs. The abundance of hydrophytic species was much lower in treated than in reference sites, suggesting that management of southwestern U.S. rivers has focused too much on weed control, overlooking restoration of fluvial processes that provide habitat for hydrophytic and floodplain vegetation. These results can help inform future management of Tamarix-infested rivers to restore hydrogeomorphic processes, increase native biodiversity and reduce abundance of noxious species. [ABSTRACT FROM AUTHOR]

The northern tamarisk beetle ( Diorhabda carinulata ) was released in 2001 as a biocontrol agent for Tamarix spp., an invasive tree that dominates riparian ecosystems throughout the southwestern United States. The factors that influence its effectiveness at controlling Tamarix , and the effects of control on plant communities, are not well known. Here we report patterns of Tamarix dieback, mortality, and vegetation composition at ten of the early D. carinulata release sites in western Colorado. Across the ten release sites, 265 permanently marked Tamarix trees were measured over a six year period (2008–2014). Vegetation composition and woody debris adjacent to each of these trees were measured annually for four years (2011–2014). We examined relationships between site factors (soil properties, hydrology, and land use history), Tamarix dieback, and vegetation composition. Tamarix mortality was observed at seven of ten sites, where it ranged from 15% to 56% after six years. Overall, Tamarix crown cover decreased by more than half (54%) while crown volume decreased by 63% in the first two years of the study. Neither total plant cover nor fallen woody debris increased under Tamarix trees over the last four years of the study. Combined cover of classified noxious weeds and other non-native species was greater than native plant cover at eight of ten sites. D. carinulata proved to be effective in controlling the Tamarix invasion locally. However, the high cover of noxious weeds will continue to be a management problem, with or without Tamarix control by the northern tamarisk beetle. [ABSTRACT FROM AUTHOR]

Control of invasive species within ecosystems may induce secondary invasions of non-target invaders replacing the first alien. We used four plant species listed as noxious by local authorities in riparian systems to discern whether 1) the severity of these secondary invasions was related to the control method applied to the first alien; and 2) which species that were secondary invaders persisted over time. In a collaborative study by 16 research institutions, we monitored plant species composition following control of non-native Tamarix trees along southwestern U.S. rivers using defoliation by an introduced biocontrol beetle, and three physical removal methods: mechanical using saws, heavy machinery, and burning in 244 treated and 79 untreated sites across six U.S. states. Physical removal favored secondary invasions immediately after Tamarix removal (0–3 yrs.), while in the biocontrol treatment, secondary invasions manifested later (> 5 yrs.). Within this general trend, the response of weeds to control was idiosyncratic; dependent on treatment type and invader. Two annual tumbleweeds that only reproduce by seed ( Bassia scoparia and Salsola tragus ) peaked immediately after physical Tamarix removal and persisted over time, even after herbicide application. Acroptilon repens , a perennial forb that vigorously reproduces by rhizomes, and Bromus tectorum , a very frequent annual grass before removal that only reproduces by seed, were most successful at biocontrol sites, and progressively spread as the canopy layer opened. These results demonstrate that strategies to control Tamarix affect secondary invasions differently among species and that time since disturbance is an important, generally overlooked, factor affecting response. [ABSTRACT FROM AUTHOR]