The exotic shrub red sesbania is an increasingly problematic weed in riparian and wetland ecosystems of California. Current control methods focus on manual removal, followed by herbicide application. Although this method effectively removes mature stands, the control is temporary because the presence of a large seed bank results in rapid germination and growth of new seedlings. We measured the density of seed banks beneath stands of varying densities and evaluated the potential of tarping and inundation for control of red sesbania seed banks. As expected, the abundance of viable red sesbania seeds in the soil was significantly greater beneath high-density stands than it was beneath low-density stands. Results for inundation and tarping experiments were mixed. Sustained inundation significantly decreased survivorship of germinated seeds compared with the control, as well as causing a statistically significant reduction in germination. Seven months after tarping, during the fall/winter growing season, there was no significant effect on red sesbania seedling abundance, stump resprout abundance, or height. Germination in the laboratory was significantly reduced by extended exposure to temperatures of 60 C, although lower temperatures did not reduce germination. Red sesbania appears to be resilient to tarping as a control method, at least in the settings studied.

Nomenclature: Red sesbania, Sesbania punicea (Cav.) Benth. SEBPU.

Management Implications: Inundation was the more-successful seed-bank control treatment studied because 2 mo of sustained inundation reduced Sesbania punicea seedling survivorship and caused a minor reduction in germination. However, this treatment did not eliminate the seed bank. It is possible that the surviving seeds and seedlings of S. punicea would be able to reestablish populations in inundated areas; thus, follow-up control methods may still be required. Whether the required duration of treatment is realistic in the field will depend upon the flood regime at potential sites and other management objectives, such as recreation and provision of wildlife habitat. Inundation would likely be most effective when S. punicea seedlings are small.

Tarping during the fall/winter growing season was not an effective control method for S. punicea. However, solarization (the practice of applying clear plastic film to maximize solar heating of the soil) could potentially be effective during hotter periods. Solarization during periods of average daily high temperatures warmer than 35 C could potentially achieve the 60 C soil temperatures we found were needed to reduce germination. Implementation of solarization is labor intensive, materials are expensive, and plastic sheeting must be monitored for holes, and patching performed if sheeting is damaged. Land managers will have to evaluate the labor costs of solarization compared with conventional biomass removal and herbicide treatments. Long-term control of S. punicea will be difficult because of its dense seed banks. Although complete eradication is unlikely, targeting small, isolated populations in the upstream portion of watersheds using conventional treatments may be the most-effective approach for minimizing downstream impacts.

Local abiotic and biotic conditions can alter the strength of exotic species impacts. To better understand the effects of exotic species on invaded ecosystems and to prioritize management efforts, it is important that exotic species impacts are put in local environmental context. We studied how differences in plant community composition, photosynthetically active radiation (PAR), and available soil N associated with Russian olive presence are conditioned by local environmental variation within a western U.S. riparian ecosystem. In four sites along the South Fork of the Republican River in Colorado, we established 200 pairs of plots (underneath and apart from Russian olive) to measure the effects of invasion across the ecosystem. We used a series of a priori mixed models to identify environmental variables that altered the effects of Russian olive. For all response variables, models that included the interaction of environmental characteristics, such as presence/absence of an existing cottonwood canopy, with the presence/absence of Russian olive canopy were stronger candidate models than those that just included Russian olive canopy presence as a factor. Compared with reference plots outside of Russian olive canopy, plots underneath Russian olive had higher relative exotic cover (exotic/total cover), lower perennial C4 grass cover, and higher perennial forb cover. These effects were reduced, however, in the presence of a cottonwood canopy. As expected, Russian olive was associated with reduced PAR and increased N, but these effects were reduced under cottonwood canopy. Our results demonstrate that local abiotic and biotic environmental factors condition the effects of Russian olive within a heterogeneous riparian ecosystem and suggest that management efforts should be focused in open areas where Russian olive impacts are strongest.

Nomenclature: Cottonwood, Populus deltoides W. Bartram ex Marshall ssp. monilifera (Aiton) Eckenwalder; Russian olive, Elaeagnus angustifolia L.

Management Implications: Understanding how invader impacts vary across the landscape is crucial to developing more efficient management strategies. Current strategies primarily treat landscapes as homogeneous; therefore, management efforts are often inefficient because they do not preferentially target the areas of most concern. Using a more targeted management approach, where early intervention is applied only to areas of the landscape likely to experience strong invader impacts, would be much more cost effective. Based on our results, we suggest that ideally Russian olive should be removed from all habitat types because it is associated with an increase in soil N and proportional exotic plant cover. However, when management funding is limited, we suggest prioritizing control efforts on locations where it is growing in the absence of a cottonwood canopy. In these areas, Russian olive has the largest impact on soil N and proportional exotic cover. Since Russian olive appears to cause an “invasional meltdown” by facilitating the invasion of other exotic species, particularly in open canopies, research on effects of removal is warranted to see if it reverses Russian olive impacts. If the increased soil N associated with Russian olive presence persists after the tree's removal, secondary invasion after the disturbance from the removal process is likely. Additionally, favoring cottonwood establishment over Russian olive can be accomplished by promoting flood disturbance by avoiding channel stabilization (i.e., by riprap) and construction.

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

Managing invasive winter annual grasses on noncrop and rangeland remains a constant challenge throughout many regions of the United States. Currently, there are limited management options for controlling winter annual grasses that work consistently, provide multiple years of control, and do not injure desirable plant communities. Imazapic has been one of the most widely used herbicides for downy brome control on rangeland; however, control with imazapic has been inconsistent beyond the application year and perennial grass injury is not uncommon. Indaziflam, a new herbicide mode of action for rangeland weed management, has shown promise in providing long-term downy brome control. A greenhouse study was conducted to compare pre-emergence activity of imazapic and indaziflam on six invasive winter annual grasses: downy brome, cereal or feral rye, jointed goatgrass, Japanese brome, medusahead, and ventenata. For both herbicides, seven rates were used to develop dose-response curves for each species. Log-logistic regression was conducted to determine the herbicide dose required to reduce biomass by 50% (GR₅₀ values). Indaziflam was significantly more active across all species compared to imazapic, with the exception of jointed goatgrass. Comparing all species, the GR₅₀ values for imazapic were on average 12 times higher than indaziflam. Japanese brome was the most sensitive to both herbicides, whereas jointed goatgrass and feral rye were the most difficult winter annual grasses to control with indaziflam and imazapic, respectively. This research provides evidence of a potential new mode of action for land managers to control the major invasive winter annual grasses.

Nomenclature: Imazapic; indaziflam; cereal or feral rye, Secale cereale L.; downy brome, Bromus tectorum L.; Japanese brome, Bromus japonicus Thunb. ex Murr.; jointed goatgrass, Aegilops cylindrica Host.; medusahead, Taeniatherum caput-medusae (L.) Nevski; ventenata, Ventenata dubia (Leers) Coss. in Dur.

Management Implications: Invasive winter annual grasses pose a major threat to native plant communities in the United States. The life cycle of these species increases their invasiveness because few native species behave as winter annuals, providing a niche for invasive annual grasses to exploit moisture and nutrients when most desirable perennial plants are dormant. Although downy brome alone infests over 22 million ha of U.S. rangeland, there are five other invasive winter annual grasses that cause significant economic and ecological impacts: feral rye, Japanese brome, jointed goatgrass, medusahead, and ventenata.

Currently, acetolactate synthase- (ALS) inhibiting herbicides such as imazapic and rimsulfuron are used for selective winter annual grass control, whereas nonselective herbicides like glyphosate are also recommended for dormant season applications (late fall or early spring). Unfortunately, none of these herbicides provide consistent control beyond 1 yr after treatment (YAT), resulting in rapid reinvasion of treated areas via the soil seed bank. Indaziflam (Bayer CropScience), a cellulose biosynthesis-inhibiting herbicide, is a new mode of action for invasive winter annual grass management. Previous field research demonstrated that indaziflam provided excellent downy brome and feral rye control 2 and 3 yr after treatment compared to imazapic. Two applications of indaziflam over a 5-yr period could substantially reduce or possibly eliminate the winter annual grass seed from the soil seed bank. The objective of this study was to evaluate indaziflam's potential to control other problematic invasi