Germination and growth of native and invasive plants on soil associated with biological control of Tamarisk (Tamarix spp.)

Authors: SHERRY, REBECCA - University Of Oklahoma; SHAFROTH, PATRICK - Us Geological Survey (USGS); BELNAP, JAVNE - Us Geological Survey (USGS); Ostoja, Steven; REED, SASHA - Us Geological Survey (USGS)

Submitted to: Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2016
Publication Date: 12/1/2016
Citation: Sherry, R.A., Shafroth, P., Belnap, J., Ostoja, S.M., Reed, S.C. 2016. Germination and growth of native and invasive plants on soil associated with biological control of Tamarisk (Tamarix spp.). Invasive Plant Science and Management. 9(4):290-307. doi: 10.1614/IPSM-D-16-00034.1.

Interpretive Summary: Tamarisk or salt cedar is a group of nonnative trees and shrubs that contain several highly invasive species especially along rivers and streams of the southwestern United States. In the past two decades a great deal of attention has been given to control and remove tamarisk including biological control with a defoliating beetle. The effects of the defoliating beetle, for example the production of large quantities of frass (insect excrement) has the potential to change the quality of soils which could differentially influence the plant community response following tamarisk defoliation – an example of indirect effects. Other tamarisk treatments also have the potential to influence plant-soil dynamics following treatments, for example the production of wood chips should the tamarisk be masticated. We wanted to understand the role of tamarisk control treatments on the soil and native and nonnative plant responses. The responses were variable but growth of all species was enhanced with increased tamarisk litter, soil leachate and fertilization but the response was greater for the nonnative species used in the study. And while the natives have the potential to respond positively with the byproducts of tamarisk treatments, these benefits were reduced in the face of competition with nonnative species.

Technical Abstract: Introductions of biocontrol beetles (tamarisk beetles) are causing dieback of exotic tamarisk in riparian zones across the western U.S., yet factors that determine plant communities that follow tamarisk dieback are poorly understood. Tamarisk-dominated soils are generally higher in nutrients, organic matter, and salts than nearby soils, and these soil attributes may influence the trajectory of community change. To assess physical and chemical drivers of plant colonization after beetle-induced tamarisk dieback, we conducted separate germination and growth experiments using soil and litter collected beneath defoliated tamarisk trees. Focal species were two common native (red threeawn, sand dropseed) and two common invasive exotic plants (Russian knapweed, downy brome), planted alone and in combination. Nutrient, salinity, wood chip, and litter manipulations examined how tamarisk litter affects the growth of other species in a context of riparian zone management. Tamarisk litter, tamarisk litter leachate, and fertilization with inorganic nutrients increased growth in all species, but the effect was larger on the exotic plants. Salinity of 4 dS/m benefitted Russian knapweed, which also showed the largest positive responses to added nutrients. Litter and wood chips generally delayed and decreased germination; however, a thinner layer of wood chips increased growth slightly. Time to germination was lengthened by most treatments for natives, was not affected in exotic Russian knapweed, and was sometimes decreased in downy brome. As natives showed only small positive responses to litter and fertilization and large negative responses to competition, Russian knapweed and downy brome are likely to perform better than these two native species following tamarisk dieback.