Plant community shifts over time in Tamarix removal sites with and without biological control

Background/Question/Methods

Active removal of invasive Tamarix (tamarisk, salt cedar) is frequently a focus of restoration of riparian ecosystems in the American West, but many question whether this emphasis is appropriate, particularly in the context of biological control by the saltcedar leaf beetle, Diorhabdaspp. There is some concern as to whether removal of the trees by this and other methods actually improves biological diversity and ecosystem function. Vegetation monitoring data of more than 60 riparian restoration sites that track both change over time and relative to sites where no restoration was done (i.e. BACI design) was used to assess the effectiveness and impact of biological control relative to other approaches. 

Results/Conclusions

Most striking was the variability between regions and even among restoration projects within regions; at one end of the spectrum there were sites with rapid re-colonization of native species, while at the other end there were sites where no other trees are likely to grow, and/or secondary invasion of other weeds occurred. The latter condition was consistently found at the sites where aerial application of herbicide was applied. Management approaches that removed above ground biomass of exotic trees had the most dramatic and positive effects on the overall community structure, however monitoring over time revealed similar trajectories for native recovery in sites with only biological control. Putting all these outcomes in a larger framework of long term restoration goals is critical for any type of evaluation of whether Tamarix removal generally, or biological control in particular, is “good” or “bad”. Such evaluation is critical for informing management decisions going forward, for Tamarix and for any species similarly associated with dramatic changes in the landscape.