Vegetation response to invasive Tamarix control in southwestern U.S. rivers: a collaborative study including 416 sites

Abstract

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.

Semiochemicals to enhance herbivory by Diorhabda carinulata aggregations in saltcedar (Tamarix spp.) infestations

“…We believe these results report a novel use of semiochemicals, to attract and retain beneficial defoliating insects and thereby yield greater defoliation of targeted plants for pest management purposes. This effect of increased biological control efficacy could have broad ecological implications, considering Tamarix biological control is the method most resilient to secondary invasion of other noxious weeds …”

“…However, this tolerance can be overwhelmed by the application of semiochemicals, resulting in a greater than expected reduction in canopy volume. Plants exposed to 2 years of defoliation commonly experience a 25–33% reduction in canopy . Our control plants exposed to lower beetle densities corroborate these results, with a reduction in plant canopy volume of 30% after 2 years of D. carinulata activity.…”

“…Two confounding variables were apparent through this study. D. carinulata are commonly known to avoid previously defoliated plants and the larvae are mobile, with the ability to move away from defoliated plants . Because plants treated with the semiochemicals were subjected to more intensive feeding by D. carinulata (Fig.…”

Longer term effects of biological control on tamarisk evapotranspiration and carbon dioxide exchange

“…Previous research suggests that the impacts of biological control on tamarisk canopy dieback and mortality do not vary predictably with environmental variables across drainages in the Western United States. (González et al, ; Henry et al, ; Hultine et al, ; Kennard et al, ). Furthermore, genetic adaptation may also play a role in determining mortality; local genetic adaptations have been found to make some populations of tamarisk more susceptible to dieback from beetle herbivory (Long et al, ).…”

Short‐term geomorphological and riparian vegetation responses to a 40‐year flood on a braided, dryland river

“…The great response of S. tragus to the flood was not surprising, as this species has been reported to colonize recently disturbed sites following efforts to remove Tamarix from riparian systems (González et al, 2017a(González et al, , 2017b. S. tragus is a tumbleweed, an annual forb that reproduces only by seed (Friesen, Beckie, Warwick, & Acker, 2009).…”