Many riparian systems across the Southwest United States have been highly altered by extensive invasion by non-native tamarisk (Tamarix spp.). Along many rivers of the arid Southwest, much of the native woody riparian vegetation has been replaced by dense monocultures of tamarisk, creating novel ecosystems associated with varying changes in composition and abundance of native plants and animal, fire regimes, hydrology, fluvial geomorphic processes, soil chemistry, and other biophysical factors. The widespread distribution of tamarisk in dense monocultures or in mixed stands is reflected in the number of described vegetation alliances and associations in which tamarisk is important element. There have been a large number of efforts to remove tamarisk by mechanical or chemical means, with widely variable levels of success and failure. More recently, a new agent of change has been added to these systems through the introduction of a biological control agent, the tamarisk beetle (Diorhabda spp.), which is now spreading throughout the Southwest. We are studying the effects of the tamarisk beetle on the expected trajectories of these novel riparian ecosystems, and working with restoration practitioners to develop and implement restoration strategies, as needed, to help direct these systems towards a more desirable state.
Results/Conclusions
While there are numerous potential benefits to tamarisk suppression, short-term negative consequences are also possible, such as altered channel hydraulics and canopy defoliation during nesting season for species such as the endangered southwestern willow flycatcher. In preparation for anticipated impacts following beetle colonization, we developed a holistic restoration framework to promote recovery of native riparian habitat and biodiversity. Pivotal to this process is an ecohydrological assessment that identifies restoration sites based on consideration of natural and anthropogenic factors that, together, influence restoration opportunitiies: flood-scour dynamics, vegetation composition and structure, surface- and groundwater availability, soil texture and salinity, wildfire potential, habitat suitability, and land-use activities. Using case studies from the Virgin River (Nevada) and Gila River (Arizona), we will discuss how the various sources of data were synthesized to produce restoration plans that highlight both those areas best suited for active restoration and those in which passive, process-based restoration and natural revegetation processes are expected to create new riparian systems that provide the ecosystem services desired by local stakeholders. Monitoring and adaptive management approaches will be essential for long-term success given the uncertainties posed by changing climate and by potential co-evolution of the beetle, tamarisk, and other species in these systems.