Stacey A. Leicht-Young

A comparison of seed banks across a sand dune successional gradient at Lake Michigan dunes (Indiana, USA)

In habitats where disturbance is frequent, seed banks are important for the regeneration of vegetation. Sand dune systems are dynamic habitats in which sand movement provides intermittent disturbance. As succession proceeds from bare sand to forest, the disturbance decreases. At Indiana Dunes National Lakeshore, we examined the seed banks of three habitat types across a successional gradient: foredunes, secondary dunes, and oak savanna. There were differences among the types of species that germinated from each of the habitats. The mean seed bank density increased across the successional gradient by habitat, from 376 to 433 to 968 seeds m−2, but with foredune and secondary dune seed bank densities being significantly lower than the savanna seed bank density. The number of seeds germinated was significantly correlated with soil organic carbon, demonstrating for this primary successional sequence that seed density increases with stage and age. The seed bank had much lower species richness than that of the aboveground vegetation across all habitats. Among sites within a habitat type, the similarity of species germinated from the seed banks was very low, illustrating the variability of the seed bank even in similar habitat types. These results suggest that restoration of these habitats cannot rely on seed banks alone.

Are temperate mature forests buffered from invasive lianas

Abstract Mature and old-growth forests are often thought to be buffered against invasive species due to low levels of light and infrequent disturbance. Lianas (woody vines) and other climbing plants are also known to exhibit lower densities in older forests. As part of a larger survey of the lianas of the southern Lake Michigan region in mature and old-growth forests, the level of infestation by invasive lianas was evaluated. The only invasive liana detected in these surveys was Celastrus orbiculatus Thunb. (Celastraceae). Although this species had only attached to trees and reached the canopy in a few instances, it was present in 30% of transects surveyed, mostly as a component of the ground layer. Transects with C. orbiculatus had higher levels of soil potassium and higher liana richness than transects without. In contrast, transects with the native C. scandens had higher pH, sand content, and soil magnesium and lower organic matter compared to transects where it was absent. Celastrus orbiculatus appears to be a generalist liana since it often occurs with native lianas. Celastrus orbiculatus poses a substantial threat to mature forests as it will persist in the understory until a canopy gap or other disturbance provides the light and supports necessary for it to ascend to the canopy and damage tree species. As a result, these forests should be monitored by land managers so that C. orbiculatus eradication can occur while invasions are at low densities and restricted to the ground layer.

Effects of an Invasive Plant Species, Celastrus orbiculatus, on Soil Composition and Processes

Abstract Celastrus orbiculatus is a non-native, invasive liana that was introduced to the United States in the 1860s and has spread rapidly throughout the Northeast. Several attributes contribute to the invasiveness of C. orbiculatus, including tolerance to a wide range of light levels and habitat types. We compared soil characteristics in seven sets of adjacent, paired plots, spanning a range of habitats and soil types, with and without C. orbiculatus. The paired plots were similar other than the presence or absence of Celastrus. Plots with C. orbiculatus had significantly higher soil pH, potassium, calcium and magnesium levels. Furthermore, nitrogen mineralization and litter decomposition rates were higher in plots with C. orbiculatus. Phosphorus levels were not significantly different between the paired plots. The results of this study contribute to the growing body of research of the effects of invasive species on ecosystem processes.

Distinguishing native (Celastrus scandens L.) and invasive (C. orbiculatus Thunb.) bittersweet species using morphological characteristics.

Abstract Celastrus orbiculatus is an invasive liana in the Eastern United States. Its native congener, C. scandens, is less common and declining in the Northeast. The correct identification of these two species is often difficult because of their similar vegetative characteristics. Using morphological characteristics of both species growing naturally along a sand dune/forest ecotone, we built models for use in discriminating between the species, given a suite of leaf and fruit traits. We confirmed that the two species can be discriminated effectively using fruit characters, notably fruit volume and seed number. Several leaf traits, such as length-to-width ratio and leaf apex length can also discriminate between the species, but without the same predictive reliability of fruit traits. In addition, we determined that at leaf out in the spring the leaves of the two species were folded differently in the bud allowing them to be successfully discriminated in the early spring. Land managers could use this information to differentiate between the two species in the field and thereby control for the invasive C. orbiculatus, while preserving remaining populations of C. scandens.

Effect of removal of hesperis matronalis (Dame's rocket) on species cover of forest understory vegetation in NW indiana

Abstract Exotic invasive plant species differ in their effects on indigenous vegetation as evidenced by research evaluating community response to their removal. We used a removal approach to quantify the response of a mesic woodland to the removal versus retention of an invasive plant, Hesperis matronalis (dames rocket) from paired treatment plots over 3 y. Cover of H. matronalis did not differ between control and treatment plots prior to removal, declined in the removal plots and remained significantly lower in cover compared to the control plots. Removal did not significantly affect species richness and species diversity (evenness, Shannon and Simpson) at the plot scale, but did result in increased species richness overall in the removal plots in the last sampling year when compared to control plots. Non-metric multidimensional scaling ordination analysis indicated a significant compositional change in the spring plant composition of plots over the 3 y, reflecting an increase in exotic woody species. Exotic woody plants, especially Rosa multiflora and Euonymus alatus, increased in cover in response to H. matronalis removal. In the 3 y, neither native nor exotic forbs, nor native woody plants responded to the removal of H. matronalis in a statistically significant manner. The increasing cover of woody invasive plants in response to the removal of H. matronalis has important management implications for restoration of degraded communities.

Fifteen woody species with potential for invasiveness in New England

In a recent paper (Herron et al. 2007), we published a statistical model for predicting invasiveness of exotic woody plant species in New England. A dataset was compiled of 28 biological/ecological characters for each of 248 exotic trees, shrubs, and lianas currently available in the nursery trade in northeastern North America. We applied a Bayesian hierarchical analysis to explore 1) what set of plant traits confer invasive success on non-native plants in New England, and 2) which currently non-invasive species share important traits with the 33 woody species listed by the Invasive Plant Atlas of New England (Mehrhoff et al. 2003). Using this approach we were able to compare non-native species that are known invaders to non-native species that are not currently considered invasive in New England. One benefit of the hierarchical approach is that we could measure the responses of distinct growth forms, a crucial advantage not only for more robust predictions, but also for determining whether different invasion strategies are employed by each woody growth form (tree, shrub, liana). RHODORA, Vol. 110, No. 943, pp. 345–353, 2008 E Copyright 2008 by the New England Botanical Club

Encroachment of Oriental Bittersweet into Pitcher's Thistle Habitat

ABSTRACT: Common invasive species and rare endemic species can grow and interact at the ecotone between forested and non-forested dune habitats. To investigate these interactions, a comparison of the proximity and community associates of a sympatric invasive (Celastrus orbiculatus; oriental bittersweet) and native (C. scandens; American bittersweet) liana species to federally threatened Cirsium pitcheri (Pitchers thistle) in the dunes habitats of Lake Michigan was conducted. Overall, the density of the invasive liana species was significantly greater in proximity to C. pitcheri than the native species. On the basis of composition, the three focal species occurred in both foredune and blowout habitats. The plant communities associated with the three focal species overlapped in ordination space, but there were significant differences in composition. The ability of C. orbiculatus to rapidly grow and change the ecological dynamics of invasion sites adds an additional threat to the successional habitats of C. pitcheri.

Competitive Interactions of Garlic Mustard (Alliaria petiolata) and Damesrocket (Hesperis matronalis)

Abstract Competitive interactions between native plants and nonnative, invasive plant species have been extensively studied; however, within degraded landscapes, the effect of interspecific interactions among invasive plants is less explored. We investigated a competitive interaction between two sympatric, invasive mustard species that have similar life history strategies and growth forms: garlic mustard and damesrocket. Greenhouse experiments using a full range of reciprocal density ratios were conducted to investigate interspecific competition. Garlic mustard had a negative effect on the final biomass, number of leaves, and relative growth rate in height of damesrocket. Survival of damesrocket was not negatively affected by interspecific competition with garlic mustard; however, garlic mustard showed higher mortality because of intraspecific competition. These results indicated that although garlic mustard has been observed to be the dominant species in this landscape, it may not completely outcompete damesrocket in all situations. Studies of invasive species in competition are important in degraded landscapes because this is the common situation in many natural areas. Nomenclature: Damesrocket, Hesperis matronalis L.; garlic mustard, Alliaria petiolata (Bieb.) Cavara & Grande. Management Implications: In degraded habitats, nonnative, invasive plant species not only compete against the resident, native plant species but also compete against the other invasive species present. To take the first steps in understanding the dynamics of invasive plant competition, we conducted a greenhouse study in which we grew two species in competition that regularly occur together in northwest Indiana: garlic mustard [Alliaria petiolata (Bieb.) Cavara & Grande] and damesrocket (Hesperis matronalis L.). These two species are closely related (both are in the Brassicaceae [mustard] family) and share similar life histories (both are biennials). These species were grown both alone and together in a wide range of densities to determine which was the better competitor. Final aboveground biomass, final leaf number, height growth rate, and mortality were all monitored. At the end of the study, it was determined that when garlic mustard was grown in mixture with damesrocket, it had a negative effect on the aboveground biomass, final leaf number, and height growth rate of damesrocket. Garlic mustard, however, had greater mortality when grown in mixture with damesrocket because of intraspecific competition. Thus, based on this study, it is unclear which species would become dominant in areas where they coexist. Other factors, such as availability of light and moisture as well as differences in phenology, are most likely important in the final outcome and need to be researched further in field studies. The ability of damesrocket to survive in competition with garlic mustard indicates that despite the domination by garlic mustard in the northeastern U.S. landscape, damesrocket can compete with this species, at least in the context presented in the greenhouse study. Thus, it is possible that in certain scenarios, damesrocket may prove to be a threat similar to garlic mustard.

Mislabeling of an Invasive Vine (Celastrus orbiculatus) as a Native Congener (C. scandens) in Horticulture

The horticultural industry is an important source of invasive ornamental plant species, which is part of the motivation for an increased emphasis on using native alternatives. We were interested in the possibility that plants marketed in the midwestern United States as the native Celastrus scandens, or American bittersweet, were actually the difficult-to-distinguish invasive Celastrus orbiculatus (oriental bittersweet) or hybrids of the two species. We used nuclear microsatellite DNA loci to compare the genetic identities of 34 plants from 11 vendors with reference plants from wild populations of known species identity. We found that 18 samples (53%) were mislabeled, and 7 of the 11 vendors sold mislabeled plants. Mislabeled plants were more likely to be purchased through Internet or phone order shipments and were significantly less expensive than accurately labeled plants. Vendors marketed mislabeled plants under five different cultivar names, as well as unnamed strains. Additionally, the most common native cultivar, ‘Autumn Revolution,’ displays reproductive characteristics that diverge from the typical C. scandens, which could be of some concern. The lower price and abundance of mislabeled invasive plants introduces incentives for consumers to unknowingly contribute to the spread of C. orbiculatus. Revealing the potential sources of C. orbiculatus is critical for controlling further spread of the invasive vine and limiting its impact on C. scandens populations. Nomenclature American bittersweet, Celastrus scandens L. oriental bittersweet, Celastrus orbiculatus Thunb

Observation of mycorrhizal colonization in roots in natural populations of Celastrus orbiculatus Thunb.

Abstract This paper documents the colonization of the roots of Celastrus orbiculatus by endomycorrhizal fungi in field-collected specimens in New England. The presence of endomycorrhizal fungi in the roots of C. orbiculatus in its introduced range may assist this species in its successful colonization of new habitats.