Secondary invasions of noxious weeds associated with control of invasive Tamarix are frequent, idiosyncratic and persistent

Authors: SHER, ANNA - University Of Denver; ANDERSON, ROBERT - University Of Denver; BAY, ROBIN - University Of Denver; BEAN, DANIEL - Colorado Department Of Agriculture; BISSONNETE, GABRIEL - Bureau Of Land Management; COOPER, DAVID - Colorado State University; DOHERNWEND, KARA - Rim To Rim Restoration; EICHHORST, KIM - University Of New Mexico; EL WAER, HISHAM - University Of Denver; KENNARD, DEBORAH - Colorado State University; HARMS-WEISSINGER, REBECCA - National Park Service; HENRY, ANNIE - University Of Denver; MAKARICK, LORI - National Park Service; Ostoja, Steven; REYNOLDS, LINSAY - Us Geological Survey (USGS); ROBINSON, WRIGHT - Grand County; SHAFROTH, PATRICK - Us Geological Survey (USGS); TABACCHI, ERIC - University Of Toulouse; GONZALES, EDUARDO - University Of Denver

Submitted to: Biological Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2017
Publication Date: 7/11/2017
Citation: Sher, A.A., Anderson, R.M., Bay, R., Bean, D.W., Bissonnete, G.J., Cooper, D.J., Dohernwend, K., Eichhorst, K.D., El Waer, H., Kennard, D.K., Harms-Weissinger, R., Henry, A., Makarick, L.J., Ostoja, S.M., Reynolds, L.V., Robinson, W.W., Shafroth, P.B., Tabacchi, E., Gonzales, E. 2017. Secondary invasions of noxious weeds associated with control of invasive Tamarix are frequent, idiosyncratic and persistent. Biological Conservation. 213(Part A):106-114. https://doi.org/10.1016/j.biocon.2017.06.043.
DOI: https://doi.org/10.1016/j.biocon.2017.06.043

Interpretive Summary: Tamarisk, also known as salt cedar is a group of nonnative trees and shrubs that contain several highly invasive species especially along rivers and streams of the western United States. In the past two decades a great deal of attention has been given to control and remove tamarisk including mechanical removal, chemical applications and biological control with a defoliating beetle. However, concern has arisen that these individual or in some cases combined management approaches might lead to increases of secondary invasive species. This study took data from numerous projects at 244 treated sites and 79 untreated sites to see if secondary weeds or invasive species increased after tamarisk control/removal. In short, it was found that methods to control or remove tamarisk that resulted in high site disturbance did favor secondary invasion shortly after the treatment but that tamarisk control treatments with lower site disturbance delayed the occurrence of secondary invasive weeds. This study shows that management of tamarisk can result in dominance of other invasive or noxious weeds. Further, if native species dominance is deemed desirable, follow up management actions or site restoration may be required.

Technical Abstract: 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.