Abstract Invasive grasses threaten native grassland ecosystems by altering soil processes and outcompeting native species, yet the mechanisms driving their competitive advantage remain unclear. Understanding how soil microbiomes and nutrient availability influence competition is essential for improving restoration strategies. We conducted three studies to separate density effects, soil microbiome dependence, and direct interspecific competition to better understand how these factors may impact changes to grassland ecosystems. Across the studies we compared the growth and competitive interactions of little bluestem [ Schizachyrium scoparium (Michx.) Nash], a native North American C₄ grass, and yellow bluestem [also King Ranch bluestem; Bothriochloa ischaemum (L.) Keng var. songarica ], an introduced C₄ grass from Afro-Eurasia, in three greenhouse experiments. In an evaluation of intraspecific competition across a range of densities (1 to 16 plants per pot), S. scoparium reached peak biomass at intermediate density (13.9 g per pot with 8 plants), while B. ischaemum achieved a much higher mass (22.7 g per pot) at the same density, indicating greater tolerance to crowding. Soil sterilization reduced biomass by 95% for S. scoparium and 89% for B. ischaemum , confirming the dependence of these species on soil microbiota. In mixtures, B. ischaemum produced more biomass than S. scoparium at all proportions. The relative competitive intensity (RCI)—a measure of the effect of competition relative to growth in isolation—was significantly greater for S. scoparium than for B. ischaemum , indicating stronger suppression of the native species. Differences in root morphology, with B. ischaemum exhibiting finer roots (0.35 ± 0.04 mm) than S. scoparium (0.63 ± 0.30 mm), may enhance nutrient foraging efficiency and contribute to the former’s competitive dominance. These results suggest that B. ischaemum ’s success is driven by traits that promote resource acquisition and tolerance to crowding in low-nutrient soils.
Abstract Wild parsnip ( Pastinaca sativa L.) is an invasive perennial that invades tallgrass prairies. The presence of native forbs prevents broadcast applications of herbicides that control P. sativa , as desirable native forbs are sensitive to herbicides used. Florpyrauxifen-benzyl may control P. sativa at low rates. This in combination with native forbs’ tolerance to florpyrauxifen-benzyl may allow broadcast applications. Research was conducted to evaluate the effectiveness of florpyrauxifen-benzyl applications on P. sativa and to determine what rate eliminates 95% of P. sativa and whether native forbs tolerate broadcast applications. Initial experiments found P. sativa was sensitive to florpyrauxifen-benzyl at rates as low as 3.7 g ai ha −1 at 60 d after treatment. Further experiments found 1.7 g ha −1 reduced P. sativa populations by 95%. Concurrent studies in P. sativa invaded tallgrass prairies confirmed florpyrauxifen-benzyl alone (14.7 g ha −1 ) reduced density of P. sativa plants by >85% at 1 yr after treatment (1 YAT) and that reduced rate (11 g ha −1 ) mixed with imazapic (35 g ae ha −1 ) was equally effective. These treatments reduced native forb cover 1 YAT (up to 25% compared with nontreated controls); however, the impact varied among forb species. Of the eight common native forbs, three were reduced by florpyrauxifen-benzyl alone 1 YAT. Reductions in cover in two of these species sensitive to the higher rate of florpyrauxifen-benzyl were not observed 1 YAT when reduced rates of florpyrauxifen-benzyl combined with imazapic were applied. Florpyrauxifen-benzyl was effective at controlling P. sativa at low rates (0.9 to 3.7 g ha −1 ), and while some native forbs were tolerant to spring broadcast applications, others were sensitive to higher rates (11 to 14.7 g ha −1 ). Further research is needed to quantify whether rates that provide acceptable P. sativa control are safe to native forbs in infested tallgrass prairies.
Abstract More than 50 yr of research activities have documented efficacy of chemical controls for monoecious and dioecious biotypes of hydrilla [ Hydrilla verticillata (L. f.) Royle] in the United States. Recently, a genetically dissimilar biotype of H. verticillata (“clade C”) was discovered in the northeastern United States. The response of three H. verticillata biotypes to eight aquatic herbicides with distinct active ingredients was simultaneously evaluated through a greenhouse-based herbicide trial. Planted apical fragments of monoecious, dioecious, and clade C H. verticillata biotypes were exposed to two concentrations of the following treatments: endothall, copper, diquat, florpyrauxifen-benzyl, and flumioxazin with 24 or 72 h of exposure, and penoxsulam, fluridone, and topramazone with 1,080 h (45 d) of static exposure. Posttreatment plant symptomology was visually evaluated and aboveground biomass was harvested 6 wk after the completed herbicide exposure period. Data were analyzed using mixed-effects models comparing response by treatment, biotype, and their interaction. Estimated visual injury of H. verticillata differed by treatment (P < 0.0001), and a significant treatment by biotype interaction (P < 0.0001) revealed some variation in visual treatment response among biotypes. Where differences in visual response to treatment by biotype occurred, clade C H. verticillata generally exhibited more injury than other biotypes. The reduction of H. verticillata biomass differed by treatment (P < 0.0001) but did not differ between biotype. Highly effective treatments (>90% reduction) against H. verticillata included endothall (2,500 µg ai L −1 [72 h]; 5,000 µg ai L −1 [24 and 72 h]), diquat (370 µg ai L −1 ppm [24 and 72 h]), and flumioxazin (200 µg ai L −1 [24 and 72 h]). This research demonstrates that all three U.S. H. verticillata biotypes respond similarly to herbicides when tested on a small scale with minimal environmental influence.
Wingleaf primrose-willow (Ludwigia decurrens Walter) is a pervasive rice (Oryza sativa L.) weed native or naturalized to South America, the Caribbean, and the southeastern United States. While it was first noted in California in 2011 and provided with the highest pest rating "A" by the California Department of Food and Agriculture, enforcing eradication or quarantine was relatively disregarded, and it did not appear to spread significantly past its initial infestation area. Here, we report the spread within four counties (Butte, Placer, Shasta, and Tehama) of California, for which it has been found up to 184 km away from the initial infestation, and the expansion of the initial infestation within Butte County. As L. decurrens is already a problematic rice weed in parts of Asia, Africa, India, and the United States, monitoring new infestations is pivotal to prevent negative economic and environmental consequences to California and the United States.
Herbicides are a critical tool for controlling aggressive invasive species, especially during large-scale restoration efforts, but their persistence in soils can hinder the establishment and success of desirable vegetation. Soil residence times differ among herbicides, and plant species exhibit variable tolerance to residues, causing difficulty in determining appropriate timelines for revegetation activities following herbicide application. Imazapyr is a herbicide commonly used to control highly aggressive species such as giant reed (Arundo donax L.), with reported soil residence time ranging from 3 to 12 mo. However, we observed herbicide damage on foundational tree species installed more than 2 yr after imazapyr was applied to control expansive A. donax monocultures as part of a large-scale riparian forest restoration project. We monitored signs of herbicide damage and growth for willow (Salix) and cottonwood (Populus spp.) individuals installed from cuttings to evaluate the relative susceptibility to and differential impacts of imazapyr residues on trees established using this standard propagation technique. Individuals from all species exhibited signs of herbicide damage, and the height of damaged individuals was significantly lower than that of healthy individuals across species. Red willow (Salix laevigata Bebb) had the greatest percentage of damaged individuals (27%), but the smallest reduction in height due to herbicide damage (-13%). The frequency of herbicide damage was similar among Populus species (<= 13%), but the fastest-growing species, black cottonwood (Populus trichocarpa Torr. & A. Gray), experienced a much greater decline in growth (-67%) compared with Fremont cottonwood (Populus fremontii S. Watson) (-32%) and Fremont & times; black cottonwood (P. & times; parryi Sarg.) (-31%). The study found that imazapyr, when used to control invasive monocultural species, can have lingering, previously undocumented effects on non-target riparian vegetation. Despite these negative impacts, imazapyr is still one of the most effective treatment options for some aggressive invaders. Thus, practitioners should be aware of the potential long-term effects of imazapyr and carefully consider species selection and planting schedule during revegetation.
