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.
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.
Prioritizing invasive species is crucial for managing invasions and mitigating impacts on biodiversity globally, yet most efforts are concentrated in the Global North, with significant gaps in the Global South. In the absence of region-specific assessments, land managers in the Global South often rely on broad global priority frameworks, whose relevance at smaller spatial scales remains uncertain. Robust prioritization frameworks should incorporate multiple dimensions of species distribution and impact in addition to spatial scale. To help fill this gap, we used complementary approaches in this study: herbarium-based and expert assessments to evaluate the distribution and impact of invasive plant species in a biodiversity hotspot in India. We scored species across different dimensions of impact, and generated a composite score that was used to rank and prioritize the invasive species. To understand the concordance of rankings across scales, we compared our regional rankings with national and global rankings for these species. We identified 79 invasive plant species in the Northern Western Ghats and Konkan region, which revealed a 10-fold difference in their spatial extent. Expert assessments indicated that only few species had high scores across the different distribution and impact dimensions, while most had low to moderate scores. The results show that distribution-related dimensions were correlated, but impact-associated dimensions remained independent, highlighting the need to include both in prioritization frameworks. Although most regional species were included in national and global invasive species lists, their priority rankings differed at these different scales. Overall, our study underscores the importance of integrating both distribution and impact dimensions for species prioritization and highlights the importance of local-scale assessments for effective invasive species management.
Maintenance control is a strategy in aquatic plant management whereby invasive populations are maintained at low densities through frequent monitoring and targeted intervention. This is a promising solution for giant salvinia (Salvinia molesta Mitchell) management in the southeastern United States, where single management tools are not effective. It also has potential to serve as a framework for integrated pest management approaches, which have been historically difficult to evaluate in natural systems. Field experiments were conducted at three sites in the southern United States using a factorial design that combined herbicide treatment thresholds (5%, 25%, 50%, 100% cover) and introduction of the biological control agent Cyrtobagous salviniae. Plots were monitored monthly for 6 mo over two growing seasons. Lower thresholds maintained lower seasonal mean cover (14.6 +/- 3.7% and 13.7 +/- 3.7% at 5% and 25% thresholds, compared with 27.6 +/- 3.7% and 32.8 +/- 3.7% at 50% and 100% thresholds). The lowest threshold required more frequent applications but did not increase cumulative herbicide volume. Addition of the weevil biocontrol agent reduced cover by 7% and decreased herbicide use by 37%, demonstrating that maintenance control can quantify biocontrol efficacy even when control is incomplete. With maintenance control, lower intervention thresholds maintained S. molesta cover below roughly 15% without increasing herbicide inputs. These results not only support implementation of maintenance control for S. molesta, but more broadly demonstrate how maintenance control provides a tractable framework for evaluating integrated strategies in systems where traditional thresholds are difficult to define.
