S. Luke Flory

Invasive shrub distribution varies with distance to roads and stand age in eastern deciduous forests in Indiana, USA

We documented the relationship between densities of invasive exotic shrubs, distance to road, and successional age of the forest in 14 forested sites throughout central and southern Indiana. Roadways are increasingly abundant, human-made features that can be conduits for the spread of invasive exotic plants in a number of ecosystems. Little is known, however, about the role of roads in eastern deciduous forest ecosystems where road density is high. Further, it is not known whether the distribution of exotic plants along roads depends on the successional age of the forest. In this study, densities of four of seven exotic shrub species declined with increasing distance to the nearest road across all successional ages. Greater densities of exotic shrubs were found in young and mid-successional forests than mature forests. However, there was no interaction between distance to road and forest age, suggesting that the role of roads in the invasion process does not change across forest successional ages. We outline several potential mechanisms that may drive patterns of shrub distribution along roadside edges as a guide for future research.

Pathogen accumulation and long-term dynamics of plant invasions

Summary 1. The diversity of pathogens on highly abundant introduced hosts has been positively correlated with time since introduction, geographical range of the introduced species and diversity of invaded habitats. However, little is known about the ecological effects of pathogen accumulation on nonnative invasive plants. 2. Pathogen accumulation on invasive plant species may result from ecological processes such as high plant densities, expanding geographical ranges and pathogen dispersal from the native range, or evolutionary mechanisms such as host range shifts and adaptation of native pathogens to invasive species. 3. Over time pathogen accumulation may cause decline in the density and distribution of invasive plants and facilitate recovery of native species. Alternatively, pathogens might build up on invasive species and then spill back onto co-occurring native species, further exacerbating the effects of invasions. 4. Synthesis. Research efforts should focus on determining the long-term outcomes of pathogen accumulation on invasive species. Such research will require multifaceted approaches including comparative studies of diverse invasive species and habitats, experimental manipulations of hosts and pathogens in nature and controlled environments, and predictive models of host-pathogen interactions within an invasion framework. Results of this research will improve our understanding and ability to predict the outcomes of biological invasions.

Experimental evidence for indirect facilitation among invasive plants

Summary Facilitation among species may promote non-native plant invasions through alteration of environmental conditions, enemies or mutualists. However, the role of non-trophic indirect facilitation in invasions has rarely been examined. We used a long-term field experiment to test for indirect facilitation by invasions of Microstegium vimineum (stiltgrass) on a secondary invasion of Alliaria petiolata (garlic mustard) by introducing Alliaria seed into replicated plots previously invaded experimentally by Microstegium. Alliaria more readily colonized control plots without Microstegium but produced almost seven times more biomass and nearly four times as many siliques per plant in Microstegium-invaded plots. Improved performance of Alliaria in Microstegium-invaded plots compared to control plots overwhelmed differences in total number of plants such that, on average, invaded plots contained 327% greater total Alliaria biomass and 234% more total siliques compared to control plots. The facilitation of Alliaria in Microstegium-invaded plots was associated with an 85% reduction in the biomass of resident species at the peak of the growing season and significantly greater light availability in Microstegium-invaded than control plots early in the growing season. Synthesis. Our results demonstrate that an initial plant invasion associated with suppression of resident species and increased resource availability can facilitate a secondary plant invasion. Such positive interactions among species with similar habitat requirements, but offset phenologies, may exacerbate invasions and their impacts on native ecosystems.

Competitive context alters plant–soil feedback in an experimental woodland community

Recent findings on feedback between plants and soil microbial communities have improved our understanding of mechanisms underlying the success and consequences of invasions. However, additional studies to test for feedback in the presence and absence of interspecific competition, which may alter the strength or direction of feedbacks, are needed. We tested for soil microbial feedback in communities of the invasive grass Microstegium vimineum and commonly co-occurring native plant species. To incorporate competitive context, we used a factorial design with three plant treatments (M. vimineum alone, M. vimineum with the native plant community, and the native community without M. vimineum) and two soil inoculum treatments (experimentally invaded and uninvaded soil). When competing with M. vimineum, native communities were 27% more productive in invaded than uninvaded soil. In contrast, soil type did not significantly affect M. vimineum biomass or fecundity. At the community level, these results indicate a net negative soil microbial feedback when native plants and M. vimineum are grown in competitive mixture, but not when they are grown separately. Since positive, not negative, feedback is associated with dominance and invasion, our findings do not support plant–soil feedback as a driver of invasion in this species. Our results do show that the importance of soil feedback can change with competitive context. Such context-dependency implies that soil feedback may change when competitive interactions between natives and invading species shift as invasions progress.

Experimental Light Treatments Affect Invasion Success and the Impact of Microstegium vimineum on the Resident Community

Abstract In forests of the eastern United States, exotic plant invasions can reduce native plant diversity and change ecosystem processes. Many invasive plants colonize forest edge habitat, but little is known about the ability of the exotic annual grass Microstegium vimineum to invade edges compared to interior forest. In particular, the extent of invasion and impact on resident plants might change along the environmental gradient from edge to interior sites. A common garden study was used to test how Microstegium invasion affected six forest understory graminoid species across three experimental light treatments. Contamination of the Microstegium seed stock with seed of the dominant native perennial grass Dichanthelium clandestinum provided the opportunity to study the interaction between the two species across the light treatments. The light treatments shifted the competitive balance between Microstegium and Dichanthelium, with Dichanthelium dominating in full sun, Microstegium dominating in full shade, and the two species producing more similar biomass in part shade. Invasion of resident communities by these two species resulted in a significant reduction in resident community biomass in part shade but not in full sun or full shade. Three of the six resident graminoid species produced significantly less biomass under invaded conditions. Successful management of Microstegium will require that land managers monitor partly shaded and fully shaded forests to locate invading populations. Invasions should be treated quickly to minimize impacts on resident community species.

Exotic Grass Invasion Reduces Survival of Amblyomma americanum and Dermacentor variabilis Ticks (Acari: Ixodidae)

Abstract Exotic plants often invade areas of high human activity, such as along trails, roads, and forest edges, and in disturbed riparian areas. These same habitat types are also favored by ticks. This convergence suggests that habitat modifications caused by exotic plant invasions may mediate disease vector habitat quality, indirectly affecting human disease risk at the local spatial scale. We tested the hypothesis that experimental invasions of Japanese stiltgrass, Microstegium vimineum (Trin.) A. Camus, alter soil surface microclimate conditions, thereby reducing habitat quality for ticks. Microstegium is an exotic annual grass that is highly invasive throughout the eastern United States where the vector ticks Amblyomma americanum (Linnaeus) and Dermancentor variabilis (Say) occur. Ticks (n = 100 per species) were introduced into experimentally invaded and native vegetation control plots (n = 5 per treatment). D. variabilis mortality rate increased 173% and A. americanum mortality rate increased 70% in the invaded plots relative to those in control plots. Microstegium invasion also resulted in a 13.8% increase in temperature and an 18.8% decrease in humidity, which are known to increase tick mortality. We predict that areas invaded by Microstegium will have lower densities of host-seeking ticks and therefore reduced human disease risk. Our results emphasize the role of invasive species in mediating disease vector populations, the unpredictable consequences of biological invasions, and the need for integrative management strategies that can simultaneous address exotic plant invasions and vector-borne disease.

Invasive Microstegium populations consistently outperform native range populations across diverse environments

Plant species introduced into novel ranges may become invasive due to evolutionary change, phenotypic plasticity, or other biotic or abiotic mechanisms. Evolution of introduced populations could be the result of founder effects, drift, hybridization, or adaptation to local conditions, which could enhance the invasiveness of introduced species. However, understanding whether the success of invading populations is due to genetic differences between native and introduced populations may be obscured by origin x environment interactions. That is, studies conducted under a limited set of environmental conditions may show inconsistent results if native or introduced populations are differentially adapted to specific conditions. We tested for genetic differences between native and introduced populations, and for origin x environment interactions, between native (China) and introduced (U.S.) populations of the invasive annual grass Microstegium vimineum (stiltgrass) across 22 common gardens spanning a wide range of habitats and environmental conditions. On average, introduced populations produced 46% greater biomass and had 7.4% greater survival, and outperformed native range populations in every common garden. However, we found no evidence that introduced Microstegium exhibited greater phenotypic plasticity than native populations. Biomass of Microstegium was positively correlated with light and resident community richness and biomass across the common gardens. However, these relationships were equivalent for native and introduced populations, suggesting that the greater mean performance of introduced populations is not due to unequal responses to specific environmental parameters. Our data on performance of invasive and native populations suggest that post-introduction evolutionary changes may have enhanced the invasive potential of this species. Further, the ability of Microstegium to survive and grow across the wide variety of environmental conditions demonstrates that few habitats are immune to invasion.

Leaf Blight Disease on the Invasive Grass Microstegium vimineum Caused by a Bipolaris sp.

In 2009, a previously undescribed disease was found on the nonnative invasive annual grass Microstegium vimineum (Japanese stiltgrass). Diseased plants exhibited foliar lesions, wilting, and in some cases, death of entire plants. We identified the causal agent as a Bipolaris sp. similar to B. zeicola. We observed spores and associated structures characteristic of Bipolaris spp. growing from leaf lesions on field collected plants. Pure cultures of the fungus were made and spore suspensions were applied to laboratory-reared M. vimineum seedlings in growth chamber and greenhouse experiments. Initial symptoms appeared on seedlings in the growth chamber experiment within 72 h of inoculation, and seedlings exhibited characteristic lesions within 10 days. The fungus was reisolated from lesions, and the internal transcribed spacer (ITS) region was sequenced to confirm its identity. In the greenhouse experiment, inoculated plants displayed characteristic lesions, and relatively greater spore loads increased disease incidence. Disease reduced seed head production by 40% compared to controls. This is the first report of a Bipolaris sp. causing disease on invasive M. vimineum. Following further analysis, including assays with co-occurring native species, this Bipolaris sp. may be considered as a biocontrol agent for invasive M. vimineum.

Assessing the Invasion Risk of Eucalyptus in the United States Using the Australian Weed Risk Assessment

Many agricultural species have undergone selection for traits that are consistent with those that increase the probability that a species will become invasive. However, the risk of invasion may be accurately predicted for the majority of plant species tested using the Australian Weed Risk Assessment (WRA). This system has been tested in multiple climates and geographies and, on average, correctly identifies 90% of the major plant invaders as having high invasion risk, and 70% of the noninvaders as having low risk. We used this tool to evaluate the invasion risk of 38 Eucalyptus taxa currently being tested and cultivated in the USA for pulp, biofuel, and other purposes. We predict 15 taxa to have low risk of invasion, 14 taxa to have high risk, and 9 taxa to require further information. In addition to a history of naturalization and invasiveness elsewhere, the traits that significantly contribute to a high invasion risk conclusion include having prolific seed production and a short generation time. Selection against these traits should reduce the probability that eucalypts cultivated in the USA will become invasive threats to natural areas and agricultural systems.

Experimental approaches for evaluating the invasion risk of biofuel crops

There is growing concern that non-native plants cultivated for bioenergy production might escape and result in harmful invasions in natural areas. Literature-derived assessment tools used to evaluate invasion risk are beneficial for screening, but cannot be used to assess novel cultivars or genotypes. Experimental approaches are needed to help quantify invasion risk but protocols for such tools are lacking. We review current methods for evaluating invasion risk and make recommendations for incremental tests from small-scale experiments to widespread, controlled introductions. First, local experiments should be performed to identify conditions that are favorable for germination, survival, and growth of candidate biofuel crops. Subsequently, experimental introductions in semi-natural areas can be used to assess factors important for establishment and performance such as disturbance, founder population size, and timing of introduction across variable habitats. Finally, to fully characterize invasion risk, experimental introductions should be conducted across the expected geographic range of cultivation over multiple years. Any field-based testing should be accompanied by safeguards and monitoring for early detection of spread. Despite the costs of conducting experimental tests of invasion risk, empirical screening will greatly improve our ability to determine if the benefits of a proposed biofuel species outweigh the projected risks of invasions.