Data from: Vegetation response to control of invasive Tamarix in southwestern US rivers: a collaborative study including 416 sites

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.