Invasive plants’ ability to extend their range depends upon their local environment and both positive and negative interactions with native species. Interactions between invasive and native plants may be indirectly linked to the soil fungal community, which may enhance or suppress invasion through mutualism or parasitism. Many invasive plants preferentially select fungal communities or change soil chemistry to gain a competitive advantage, and such changes can remain even after the invader is removed, known as legacy effects. Linaria vulgaris is an invasive forb that is aggressive in the Western U.S. but is non-aggressive in the Midwestern U.S. We evaluated the relationship between soil abiotic properties, nitrogen (N) enrichment, arbuscular mycorrhizal fungal (AMF) community composition and L. vulgaris invasion in aggressive (CO) and non-aggressive (IL) populations. We collected soil from uninvaded and invaded sites in Gothic, CO, and near Chicago, IL, and sequenced AMF community composition in each site. Using the same soil, we grew L. vulgaris and native species for 120 days, with half of pots receiving N-fertilization, and harvested biomass. We also injected a 15N-labelled tracer in pots and analyzed plant tissue for 15N enrichment and N uptake rates (NUR). In CO soil, L. vulgaris rhizomes sprouted more in invaded soil, whereas in IL soil, L. vulgaris only sprouted in uninvaded soil. N-fertilization had no impact on biomass and NUR did not differ significantly between any treatment. AMF communities differed between the two sites but were not significantly influenced by invasion history. Our results suggest that L. vulgaris leaves legacy effects, but that these effects are different between aggressive and non-aggressive populations. Legacy effects may facilitate reinvasion in CO, but we did not find conclusive evidence of legacy effects in IL, and differences between the sites could be shaped by endemic AMF communities.

Yellow toadflax (Linaria vulgaris Mill.; Scrophulariaceae) is an invasive herbaceous perennial weed of agricultural and natural habitats throughout North America. In pastures or native rangelands, use of biological control is an attractive option, particularly if the agent can be established quickly. Rhinusa pilosa (Gyllenhal) (Coleoptera: Curculionidae), a stem-galling weevil, was first released in Canada in 2014 to evaluate its potential to control L. vulgaris. Rhinusa pilosa requires young, vigorously growing shoots to establish. Ability to estimate when adequate shoots will be available could inform release timing, thus improving establishment success. There is currently no growing degree-day (GDD) model for L. vulgaris. Our main objective was to develop a GDD model for the emergence of L. vulgaris shoots and discuss the utility of such a model in relation to the establishment of R. pilosa in Nova Scotia. Four sites containing five randomly placed 1-m2 quadrats were monitored for the emergence of L. vulgaris shoots twice weekly in spring to summer 2017 and 2018 by recording number of shoots and shoots with flower buds. A GDD (Tbase 2 C) model for shoot emergence of L. vulgaris was developed and validated using independent shoot emergence data. Shoots emerged in the spring between 124 and 244 GDD with 90% of all shoots emerged between 681 and 1,117 GDD. Model estimation for the initiation of shoot emergence was 74 GDD, with 10%, 50%, and 90% shoot emergence estimated to occur at 179, 409, and 811 GDD, respectively. Rhinusa pilosa adults were released in 2016 (three sites) and 2017 (one site), and number of shoots with galls was recorded. Galls were observed in all three sites in 2016 and in three of the four sites in 2017, with none found in 2018. Timing of release and soil moisture are discussed as factors affecting establishment of R. pilosa in Nova Scotia.

Invasive populations of Dalmation toadflax [Linaria dalmatica (L.) Mill.] and yellow toadflax (Linaria vulgaris Mill.) are widespread throughout the Intermountain West, where gene flow between these nonnative species is producing vigorous and fertile hybrids. These hybrid toadflax populations are less responsive to herbicides than either parent species, and biocontrol agents routinely released on L. dalmatica and L. vulgaris often fail to establish on hybrid hosts. Early detection of hybrid Linaria populations is therefore essential for effective management, but resources are limited for scouting large expanses of range and wildland. We used species distribution modeling to identify environmentally suitable areas for these invasive Linaria taxa in Montana, Wyoming, and Colorado. Areas suitable for hybrid Linaria establishment were estimated using two different modeling approaches: first, based on known hybrid occurrence and associated environmental conditions, and second, based on zones environmentally suitable for co-occurrence of the parent species. This also allowed comparison of different model outputs, especially relevant when modeling emerging invasives, such as novel hybrids, with minimal occurrence data. Combining the two model outputs identified areas at greatest risk of hybrid Linaria invasion, including parts of north-central Montana, where model estimates indicate the hybrid may spread without prior co-invasion of the parents. Potential hybrid hot spots were also identified in western Montana; northwestern, northeastern, and southeastern Wyoming; and the Western Slope and Front Range of Colorado. Despite relatively few confirmed occurrences of hybrid populations to date, our results indicate that extensive spread of hybrid populations is possible within the studied area. Model-based maps of potential Linaria distributions will allow area weed managers to direct limited resources more effectively for locating and controlling these invaders.

Different growth rates of young seedlings (genets) and plants grown from root pieces (ramets) of yellow toadflax could influence their respective competitive ability and their susceptibility to management techniques. Shoot production was similar for genets and ramets (approximately 10 shoots were produced 12 or 13 wk after transplanting or cotyledon appearance, respectively), but the rate of shoot biomass accumulation was higher for genets than for ramets. Genets consistently produced more underground shoots than ramets. Replanted underground shoots separated from their roots were able to produce new shoots and roots. Rate of elongation for roots 0.5 to 1.5 mm in diameter was higher for ramets than for genets, but their shoot production potential was the same. Root pieces from genets did not have the ability to produce daughter shoots until 3 wk after cotyledon appearance. This indicates that very young genets would be more susceptible than older genets or ramets to management control systems.

Land managers commonly assume that nonindigenous plant species (NIS) are rapidly increasing in population size in all environments in which they occur. In fact, these plant species have differing levels of invasiveness depending on environment. A method was developed that quantifies invasiveness of a plant population based on annual changes in plant density and area occupied, within a series of permanently placed 1 m2 (10.76 ft2) monitoring plots. An invasiveness index (I) was calculated from the change in proportion of cells occupied and the proportions of cells that had growth rates > 1 and < 1; the possible value is restricted from −4 to +4. The method was tested on populations of yellow toadflax over 6 yr on a total of six populations within three distinct environments (Ridge, Valley, and Forest). Invasiveness values were different between environments. The Ridge populations had the highest mean level of invasiveness (I = 0.31), followed by the Valley (I = 0.26), and then the Forest (I = −0.90). Invasiveness also varied by year. The highest annual value of invasiveness was at the Ridge (I = 1.77) and the lowest was at the Forest (I = −1.90), both in 2005. Values of invasiveness were correlated (Pearson's correlation coefficient = 0.82) with the traditional calculations of population growth rate, but our method provides an enhanced measure of invasiveness because it includes information on both change in population area and density. This research shows that populations of yellow toadflax are not equally invasive in different environments or through time, although consistent patterns can be observed. The method presented and tested was implemented in approximately three person-days per year at less than $500 per year, and can be used to quantify the invasiveness of plant populations and thus allow land managers to prioritize the most invasive populations for management.