Cynthia D. Huebner

Housing is positively associated with invasive exotic plant species richness in New England, USA.

Understanding the factors related to invasive exotic species distributions at broad spatial scales has important theoretical and management implications, because biological invasions are detrimental to many ecosystem functions and processes. Housing development facilitates invasions by disturbing land cover, introducing nonnative landscaping plants, and facilitating dispersal of propagules along roads. To evaluate relationships between housing and the distribution of invasive exotic plants, we asked (1) how strongly is housing associated with the spatial distribution of invasive exotic plants compared to other anthropogenic and environmental factors; (2) what type of housing pattern is related to the richness of invasive exotic plants; and (3) do invasive plants represent ecological traits associated with specific housing patterns? Using two types of regression analysis (best subset analysis and hierarchical partitioning analysis), we found that invasive exotic plant richness was equally or more strongly related to housing variables than to other human (e.g., mean income and roads) and environmental (e.g., topography and forest cover) variables at the county level across New England. Richness of invasive exotic plants was positively related to area of wildland-urban interface (WUI), low-density residential areas, change in number of housing units between 1940 and 2000, mean income, plant productivity (NDVI), and altitudinal range and rainfall; it was negatively related to forest area and connectivity. Plant life history traits were not strongly related to housing patterns. We expect the number of invasive exotic plants to increase as a result of future housing growth and suggest that housing development be considered a primary factor in plans to manage and monitor invasive exotic plant species.

Invasibility of mature and 15-year-old deciduous forests by exotic plants

High species richness, resource availability and disturbance are community characteristics associated with forest invasibility. We categorized commonly measured community variables, including species composition, topography, and landscape features, within both mature and 15-year-old clearcuts in West Virginia, USA. We evaluated the importance of each variable for predicting the degree of forest invasion by early-establishing exotic invasive plants. Biotic variables, including overall richness (excluding exotic invasive species) and mutually exclusive native and exotic non-invasive species richness, were the strongest indicators of invasibility. Sites that were located on northeast-facing slopes, more mesic conditions, or in clearcuts were more likely to be invaded by exotic invasive plants. Invasion of clearcut sites was more dependent on available microsites (e.g., lower solar radiation, northeast-facing slopes, and lower elevations) within each site than on the condition of the surrounding landscape, whereas invasion into the mature forests was dependent more on the surrounding landscape (e.g., proximity to paved roads). Our results indicate that exotic invasive plant species in our study area respond similarly as other plant species to resource availability and that competitive interactions are relatively unimportant. Current invasion into this landscape is more likely to be a passive reaction to site conditions instead of a driver of change.

Genetic Structure of the Invasive Tree Ailanthus altissima in Eastern United States Cities

Ailanthus altissima is an invasive tree from Asia. It now occurs in most US states, and although primarily an urban weed, it has become a problem in forested areas especially in the eastern states. Little is known about its genetic structure. We explore its naturalized gene pool from 28 populations, mostly of the eastern US where infestations are especially severe. Five microsatellite markers were used to examine presumed neutral genetic variation. Results show a gene pool that is moderately diverse and sexually active and has significant but small genetic differences among populations and little correspondence between geographic and genetic distance. These findings are consistent with a model of multiple introductions followed by high rates of gene exchange between cities and regions. We propose movement along road and railway systems as the chief mode of range expansion.

The distribution of a non-native (Rosa multiflora) and native (Kalmia latifolia) shrub in mature closed-canopy forests across soil fertility gradients.

Background and aimsA soil fertility gradient, ranging from infertile to highly fertile soils, may define whether or not a plant will establish and spread at a site. We evaluated whether or not such a fertility gradient exists for Rosa multiflora Thunb., a nonnative invasive shrub, and Kalmia latifolia L., a native problem shrub, in closed-canopy forests of the eastern U.S.MethodsWe sampled soil and vegetation at the regional scale, along four randomly located 1-km transects in 70+ year-old undisturbed forests in each of three national forests in Ohio, Pennsylvania and West Virginia. We also sampled soil, vegetation and leaf tissue at the local scale, from ten individual shrubs of each species in each national forest.ResultsRegional analyses showed a significant fertility gradient with Ohio being the most fertile and West Virginia the least. Soil fertility was associated with pH (most acidic in West Virginia and least acidic in Ohio) and elevation (highest in West Virginia and lowest in Ohio). At the local level, R. multiflora was associated with soil Ca:Al ratios greater than 0.5, and K. latifolia was associated with Ca:Al ratios less than 0.3. Rosa multiflora foliage contained higher concentrations of Ca, Mg, and K than K. latifolia, while K. latifolia foliage contained higher concentrations of Mn and Zn.ConclusionsOur research documents the importance of soil fertility as a predictor of the establishment of invasive and expansive shrubs. This study further shows that R. multiflora can establish and spread across a broader range of soil conditions than K. latifolia.

Seed longevity and dormancy state suggest management strategies for garlic mustard ( Alliaria petiolata ) and Japanese stiltgrass ( Microstegium vimineum ) in deciduous forest sites

An effective management plan for invasive herb populations must consider the potential for regeneration from the soil seedbank. To test this potential, we examined two species, Japanese stiltgrass and garlic mustard, at deciduous forest sites in southeastern Ohio. Seeds were buried in nylon mesh bags and recovered at regular intervals over 24 mo. Recovered seeds were tested for germination and viability. Burial was replicated on north- and south-facing slopes to test for environmental control of dormancy state. Stiltgrass seeds experienced severe mortality in the soil, rarely surviving the full 24 mo. Stiltgrass showed fractional germination in the lab ranging from 86% to 89% of viable seeds in late spring (the season of natural seedling emergence) to complete nongermination in winter. Most garlic mustard seeds survived through the experimental period (82% and 88% survival across 24 mo) with consistently low mortality (0% to 13%) unrelated to season. Slope aspect had no significant effect on survival or dormancy state in either species. Extrapolation of garlic mustard mortality implies that reproduction would need to be suppressed for a substantial period (perhaps >10 yr) to ensure eradication of a population. In stiltgrass, rapid seed mortality suggests that control can be achieved in 2 to 4 yr. Nomenclature: Garlic mustard, Alliaria petiolata (Bieb.) Cavara and Grande; Japanese stiltgrass, Microstegium vimineum (Trin) A. Camus.

Seed longevity and dormancy state in a disturbance-dependent forest herb, Ageratina altissima

Does seed dormancy allow disturbance-oriented forest herbs such as Ageratina altissima to persist in heterogeneous natural communities? To document seed longevity and dormancy state, Ageratina seeds were buried in nylon mesh bags in second-growth forest stands in south-eastern Ohio, USA. Bags were recovered at 2-month intervals, and seeds were tested for viability and germinability in the lab. Live seed numbers declined rapidly, with seed banks exhausted in an estimated 33–37 months. Seeds showed a strong dormancy polymorphism, with 71–84% of live seeds germinable between March and July, the season of natural seedling emergence. At other dates, most seeds appeared to be in a state of induced dormancy, allowing little (0–21%) germination. The slope aspect of the burial site, an important factor controlling aboveground vegetation, had no effect on seed longevity or dormancy condition. Dormancy in Ageratina appears to be adapted to allow opportunistic germination in late spring–early summer, but to prevent germination in less benign periods. Seed longevity is shorter than the natural frequency of gaps in mesophytic forest. We conclude that a long-term dispersal/dormancy trade-off is probably an oversimplification of the spatial ecology of this species. The primary function of dormancy appears to be short-term tracking of seasonal variation.

Invasion ecology: still in the ‘establishment stage’ 50 years after Elton

Fifty Years of Invasion Ecology: The Legacy of Charles Elton is a summary of presentations given at the Centre for Invasion Biology Symposium held at Stellenbosch University in South Africa, 2008. Richardson’s book flows logically from a focus on invasive species basic knowledge to a focus on new directions of invasion science and, briefly, to how this research might relate to management and policy development. This book contains a series of chapters that acknowledge and question Elton’s place in invasion ecology as well as question the legitimacy of invasion ecology as a science 50 years beyond Elton’s 1958 publication of The Ecology of Invasions by Animals and Plants. Landscape ecologists interested in invasive species will appreciate this evaluation of invasion research by scientists, managers, and a philosopher. Kitching gives the reader a historical peak at Elton’s understated personality and under-appreciated intellectual impact. Simberloff credits the Scientific Committee on Problems of the Environment (SCOPE) invasions programme (1982) and subsequent publications for inciting interest in invasion ecology. Carlton’s summary of ocean invasions addresses Elton’s concerns about the oyster industry, stocking of predatory fish, and the introduction of earthworms, all of which are indeed prophetic. Chew and Hamilton’s chapter on nativeness effectively expresses the idea that invasion ecologists are still in the stages of standardizing units (i.e., nativeness, novelty, or diversity) in order to effectively measure change over space and time. They point out conundrums, such as if a species evolves to its invasiveness in a new environment, is it not now native to the environment in which it evolved? These chapters and the chapter by Hobbs and Richardson, which compares the solution-based restoration ecology with the problem-based invasion ecology, introduce the theme that invasion ecology is inherently problem-driven and value-based. The next sections of the book summarize current trends and overarching findings in invasion ecology research with the strongest chapter by Pyšek and Hulme, who evaluate the current distribution of invasive species noted by Elton in 1958 using data compiled in the Delivering Alien Invasive Species Inventories for Europe (DAISIE). They also discuss the use of DAISIE in the Assessing Large Scale Risks for Biodiversity using Tested Methods (ALARM) invasive species risk assessment project, and summarize macroecological invasion patterns found in ALARM-related publications. The largest section of the book summarizes specific invasion ecology topics linked to traditional ecological disciplines, including dispersal ecology, system resiliency and diversity, plant–microbe interactions, mutualisms, evolutionary ecology, reproductive ecology, and invasive species impacts. Tsoar et al. skillfully evaluate movement ecology using the Egyptian C. D. Huebner (&) USDA Forest Service Northern Research Station, Morgantown, WV 26505, USA e-mail: chuebner@fs.fed.us

Testing Basic Ecological, Evolutionary, and Biogeographical Principles Using Invasive Species

Sax et al. argue in Species Invasions: Insights into Ecology, Evolution, and Biogeography that species invasion is an on-going experiment and a research tool with which to test fundamental ecology, evolution, and biogeography tenets.