Introductions of biocontrol beetles (tamarisk beetles) are causing dieback of exotic tamarisk in riparian zones across the western United States, 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 might 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−1 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. Because 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.
Nomenclature: Downy brome, Bromus tectorum L.; Russian knapweed, Acroptilon repens (L.) DC.; tamarisk, Tamarix spp.; red threeawn, Aristida purpurea Nutt. var. longiseta (Steud.) Vasey; sand dropseed, Sporobolus cryptandrus (Torr.) Gray.
Management Implications: Following control of tamarisk, riparian land managers often seek to establish a native plant community, but secondary weed invasions can thwart such efforts. Soil conditions are among the factors that determine plant community composition following tamarisk die-back after biocontrol by tamarisk beetles. Soils that have been dominated by tamarisk for decades generally have a thick layer of tamarisk leaf litter and are higher in nutrients, organic matter, and salts than nearby soils. Mechanical removal of dead or dying tamarisk can leave behind wood chips or debris. Greenhouse experiments showed that such soils are not harmful to two native species, red threeawn and sand dropseed, but generally had positive effects on the growth of two common invasive exotic plants, Russian knapweed and downy brome, when plants were grown alone or in competition. Russian knapweed even showed a slight positive growth response to small increases in salinity. A thick layer of tamarisk wood chips was detrimental to plant germination, but a thin layer of wood chips increased growth slightly.
Russian knapweed and downy brome are likely to do well after tamarisk dieback where soils are enriched in nutrients from tamarisk litter and beetle frass, so managers should be prepared for secondary invasion by these species following tamarisk removal if they occur nearby. Further experimentation should elucidate when a thin layer of wood chips might benefit plant growth. However, soil conditions vary from site to site. Soil testing and assessment of which native species grow nearby could suggest desirable native species to plant to facilitate es