Naturally diverse plant communities do not resist invasion by the strong competitor, Microstegium vimineum.

PREMISE: Theory predicts and empirical studies have shown that ecologically manipulated communities with high species diversity are resistant to invasion, but do these predictions and results hold true when applied to highly competitive invaders in natural communities? Few studies of diversity-mediated invasion resistance have measured both invasion resistance and invader impact in the same study. METHODS: We used a two-year field experiment to test: (1) diversity-mediated competitive resistance to patch expansion by the grass, Microstegium vimineum; and (2) the competitive effect of M. vimineum on resident plant diversity. We examined responses of M. vimineum to two native plant density-reduction treatments that had opposite effects on species diversity: (1) reducing species richness via the removal of rare species; and (2) reducing dominance by reducing the density of the dominant resident species. We examined the effects of M. vimineum reduction by pre-emergent herbicide on resident diversity in the second year of the study. RESULTS: Neither rare species removal nor dominant species reduction significantly increased M. vimineum density (relative growth rate). The pre-emergent herbicide dramatically reduced M. vimineum in year 2 of the study, but not most resident plants, which were perennials and indirectly benefited from the herbicide at a more productive site, presumably due to reduced competition from M. vimineum. CONCLUSIONS: Diversity-mediated resistance did not effectively deter invasion by a highly competitive invader. In the case of M. vimineum and at more productive sites, it would appear that nearly complete removal of this invader is necessary to preserve plant species diversity.

Litter, Plant Competition, and Ecosystem Dynamics: A Theoretical Perspective.

AbstractCommunity structure depends jointly on species' responses to, and effects on, environmental factors. Many such factors, including detritus, are studied in ecosystem ecology. Detritus in terrestrial ecosystems is dominated by plant litter (nonliving organic material), which, in addition to its role in material cycling, can act as a niche factor modulating interactions among plants. Litter thus links traditional community and ecosystem processes, which are often studied separately. We explore this connection using population dynamics models of two plant species and a litter pool. We first find conditions determining the outcome of interactions between these species, highlighting the role that litter plays and the role of broader ecosystem parameters, such as decomposition rate. Species trade-offs in tolerance to direct competition and litter-based interference competition allow for coexistence, provided the litter-tolerant species produces more litter at the population level; otherwise, priority effects may result. When species coexist, litter-mediated interactions between plants disrupt the traditional relationship between biomass accumulation and decomposition. Increasing decomposition rate may have no effect on standing litter density and, in some cases, may even increase litter load. These results illustrate how ecosystem variables can influence community outcomes that then feed back to influence the ecosystem.

First report of Curvularia intermedia Boedijn causing leaf blight disease on Microstegium vimineum in China.

Stiltgrass [Microstegium vimineum (Trin.) A. Camus], is an annual C4 grass of Asiatic origin whose native range includes India, Pakistan, Nepal, China, Korea, and Japan (Cole et al 2004). In China, it is mainly distributed south of the Yangtze River, and is one of the most important weeds in autumn-maturing dryland crops, orchards, tea gardens, and plantations. With its high shade tolerance, M. vimineum also invades forest understories and crowds out the local vegetation (Warren et al. 2011). From June to August 2019, a leaf disease was observed causing severe defoliation of stiltgrass on the roadside of Sun Yat-sen Mausoleum in Nanjing City, Jiangsu Province, China (32.045964°N, 118.840064°E). Yellow or yellow-brown necrotic spots were observed on leaf tips and margins of the lower canopy, which later expanded to the entire leaf and progressed up the plant. Disease incidence was approximately 75-85% in August. Thirty symptomatic leaves were collected, and tissue samples (5 × 5 mm) were surface disinfected with 75% ethanol for 30 s, 0.02% NaClO for 30 s, 75% ethanol for 30 s, and washed twice with sterile water. Disinfected tissues were placed on potato dextrose agar (PDA) and incubated at 28°C for 5 days. Twenty-seven morphologically similar isolates were obtained from the leaves and purified by single-spore culturing for further study. Colonies on PDA were 70 to 85 mm in diameter after 4 to 5 days, initially white becoming gray-green with flocculent aerial mycelia. Conidiophores were solitary or clustered, 85 to 139 µm long × 5 to 8 µm wide (n = 50), and conidia were obclavate to ellipsoid or spindle shaped, brown, and measured 28 to 37 µm long × 13 to 18 µm wide (n = 50) with three false dissepiments. All characteristics were consistent with the morphology of Curvularia intermedia Boedijn (Sivanesan 1987). The rDNA internal transcribed spacer (ITS), glyceraldehyde-3-phosphate dehydrogenase (GPDH) and translation elongation factor (TEF) of a representative isolate (JSNJ-2019) were amplified using primers ITS1/ITS4, GPD1/GPD2 and EF1-983F/EF1-2218R (Manamgoda et al. 2014). The ITS sequence of JSNJ-2019 (GenBank: MZ613310) showed 99.83% (582/583bp) identity with C. intermedia (GenBank: MF370184 and GU073102); the GPDH sequence (GenBank: MZ701795) showed 99.66% (581/583bp) identity with C. intermedia (GenBank: LT715828) and the TEF sequence (Genbank: OM282974) showed 99.77% (864/866bp) identity with C. intermedia (GenBank: MF370186). Phylogenetic analysis based on the TEF sequences using Maximum-Likelihood and Bayesian methods placed JSNJ-2019 in the same clade with reference strain C. intermedia B19. The isolate was deposited in China Centre for Type Culture Collection (CCTCC) (Isolate code: CCTCC AF 2022041). For the pathogenicity assay, ten healthy M. vimineum plants grown in plastic pots (five to six leaf stage) were sprayed with 20ml conidial suspension (5×104 spores /ml); another ten healthy plants sprayed with sterile water served as controls. All inoculated and control plants were covered with transparent polyethylene bags immediately and were maintained in a greenhouse at 28±1℃. The transparent polyethylene bags were removed after 24 hours. The pathogenicity test was repeated three times. Five days post-inoculation, inoculated plants showed leaf blight symptoms as observed in the field, whereas no disease symptoms was observed on control plants. Reisolations were performed from inoculated plants, and the reisolated pathogen was confirmed as C. intermedia inter based on morphological and PCR assay (Konstantinova et al. 2002). No pathogens were isolated from control plants. Host range tests showed, C. intermedia JSNJ-2019 was pathogenic on corn, wheat, sorghum, barnyardgrass, crabgrass, green foxtail, Chinese sprangletop, cynodon, cogongrass, goosegrass, purslane and bedstraw and non-pathogenic on barley, rice, oat, cotton, bean, peanuts, rapeseed, tobacco and tea. These findings suggest C. intermedia could be used as a biocontrol agent against invasive M. vimineum and farmland weeds. However, application of C. intermedia as a bioherbicide should be limited to insensitive crop growing areas.

First Report of Macruropyxis fraxini Causing Rust Disease on Microstegium vimineum in China.

Stiltgrass (Microstegium vimineum (Trin.) A. Camus) is an annual Poaceae weed with a broad native range throughout East Asia. Stiltgrass is an invasive grass that is distributed in more than 15 provinces in China, posing a major threat to native biodiversity and restoration efforts in introduced areas. Stiltgrass often forms dense near-monocultures in forest understories and riparian areas where it disrupts forest succession, nitrogen cycling, and alters native communities (Stricker et al. 2016). In August 2018, M. vimineum with rust disease were observed near the roadside (26.759482 °E, 114.283519 °N) in Jinggangshan City, Jiangxi Province, China. Diseased plants were observed at a 2 × 10 m shady location with lesions on leaves and stems, disease incidence was over 90% (n=100). Sixty disease samples were collected to confirm the pathogen. Early symptoms on the upper leaf surfaces consisted of rust pustules, which were circular, subcircular to irregular, orange to dark-orange, crust-like, and granular. At later stages, lesions coalesced, spreading all over the plant, causing severe defoliation. Uredinia were predominantly formed on the upper surface of leaves and young stems but rarely also found on the abaxial leaf surface, exposed, yellow to yellow-orange, and 0.2-0.5 mm in diameter, occasionally reaching 0.9-1 mm, surrounded by purple lesions (n=30) (Olympus SZX7). Telia were predominantly formed on the lower surface of leaves, stems, exposed, chestnut-brown to dark-brown. Urediospores were nearly spherical, oval or obovate, light yellow, 18-23 µm × 20-26 µm, cell wall is about 2-2.5 µm (n=200) (ZEISS AXIO Imager. M2). Teliospores were ellipsoid, 37-55 µm × 25-36 µm, 2-celled, inner wall brown, 4-5.5 µm thick, outer wall hyaline, smooth, germ pores 2-4 per cell; pedicels were hyaline, composed of cell walls with loss of cytoplasm, 4.5-6.5 µm wide, and up to 160 µm long (n=200) (ZEISS AXIO Imager. M2). Pycniospores, aeciospores and basidiospores were not observed in this study. The telial morphology features were consistent with those reported of M. fraxini, but uredinial stages were not observed in these studies (Azbukina 1974; Jung et al. 2020). Genomic DNA was extracted from a representative specimen (JGS-1) and was characterized by PCR amplification and sequencing of 28S rDNA using the primer pair NS1 and NS4 (Aime 2006). The 1094-bp sequence (Genbank: ON739170) shared 99.18% nucleotide identity with M. fraxini (Genbank: KP858144). The internal transcribed spacer (ITS) region was sequenced by rust fungal primer pairs ITS4rust and ITS5u (Pfunder et al. 2001). The 564-bp sequence (Genbank: ON739169) shared 99.12% nucleotide identity with M. fraxini (Genbank: KP858145), which was consistent with the morphological features observed. To complete Koch's postulates, plants were inoculated by brushing a urediniospore suspension (1 ×105 spores/ml) onto the leaves, placed in a plant growth chamber (25℃, 8 h/d of dark, 30℃, 16 h/d of light, 8000 lux of light intensity, RH ≥ 90%). Urediniospores were formed on the leaf surface 7 to 10 days after inoculation, and all infected plants showed symptoms similar to those observed in the field, along with spores, whereas the control plants remained symptomless. Host range tests showed that rice, wheat, barley, sorghum, maize, cotton, peanut and rape were resistant to M. fraxini but soybean and peas were susceptible. More research is needed to determine whether this pathogen can be a biocontrol agent for stiltgrass, such as exploring the potential impact of this rust pathogen, expanding host range tests, and finding its alternate hosts. To the best of our knowledge, this is the first report of rust disease on stiltgrass caused by M. fraxini in China.

Long-term trends indicate that invasive plants are pervasive and increasing in eastern national parks.

While invasive plant distributions are relatively well known in the eastern United States, temporal changes in species distributions and interactions among species have received little attention. Managers are therefore left to make management decisions without knowing which species pose the greatest threats based on their ability to spread, persist and outcompete other invasive species. To fill this gap, we used the U.S. National Park Service's Inventory and Monitoring Program data collected from over 1,400 permanent forest plots spanning 12 yr and covering 39 eastern national parks to analyze invasive plant trends. We analyzed trends in abundance at multiple scales, including plot frequency, quadrat frequency, and average quadrat cover. We examined trends overall, by functional group, and by species. We detected considerably more increasing than decreasing trends in invasive plant abundance. In fact, 80% of the parks in our study had at least one significant increasing trend in invasive abundance over time. Where detected, significant negative trends tended to be herbaceous or graminoid species. However, these declines were often countered by roughly equivalent increases in invasive shrubs over the same time period, and we only detected overall declines in invasive abundance in two parks in our study. Present in over 30% of plots and responsible for the steepest and greatest number of significant increases, Japanese stiltgrass (Microstegium vimineum) was the most aggressive invader in our study and is a high management priority. Invasive shrubs, especially Japanese barberry (Berberis thunbergii), Japanese honeysuckle (Lonicera japonica), multiflora rose (Rosa multiflora), and wineberry (Rubus phoenicolasius), also increased across multiple parks, and sometimes at the expense of Japanese stiltgrass. Given the added risks to human health from tick-borne diseases, invasive shrubs are a high management priority. While these findings provide critical information to managers for species prioritization, they also demonstrate the incredible management challenge that invasive plants pose in protected areas, particularly since we documented few overall declines in invasive abundance. As parks work to overcome deferred maintenance of infrastructure, our findings suggest that deferred management of natural resources, particularly invasive species, requires similar attention and long-term commitment to reverse these widespread increasing invasive trends.

Predators in the plant-soil feedback loop: aboveground plant-associated predators may alter the outcome of plant-soil interactions.

Plant-soil feedback (PSF) can structure plant communities, promoting coexistence (negative PSF) or monodominance (positive PSF). At higher trophic levels, predators can alter plant community structure by re-allocating resources within habitats. When predator and plant species are spatially associated, predators may alter the outcome of PSF. Here, I explore the influence of plant-associated predators on PSF using a generalised cellular automaton model that tracks nutrients, plants, herbivores and predators. I explore key contingencies in plant-predator associations such as whether predators associate with live vs. senesced vegetation. Results indicate that plant-associated predators shift PSF to favour the host plant when predators colonise live vegetation, but the outcome of PSF will depend upon plant dispersal distance when predators colonise dead vegetation. I apply the model to two spider-associated invasive plants, finding that spider predators should shift PSF dynamics in a way that inhibits invasion by one forest invader, but exacerbates invasion by another.

Impacts of an invasive plant are fundamentally altered by a co-occurring forest disturbance.

Invasive species frequently co-occur with other disturbances, which can impact the same ecosystem functions as the invader. Yet invasion studies rarely control for the presence of these other disturbances, although their overlapping effects may influence the direction and magnitude of impacts attributed to the invader alone. Here, we ask whether controlling for the presence of a co-occurring disturbance, as well as the time since disturbance, yields different values of an invader's ecosystem effects than when these factors remain unaddressed. We used a chronosequence of six forest stands at a single site: five logged stands that each contained paired invaded-uninvaded plots of the forest understory invasive grass Microstegium vimineum, as well as one unlogged and uninvaded control stand. By controlling for the presence of both logging and invasion, we untangled the effects of each through time. We found that the co-occurring disturbance of logging can dramatically alter the measured effects of M. vimineum by amplifying, dampening, negating, or entirely reversing the direction of the invader's impacts. During its period of peak impact, logging amplified the invader's positive effect on the size of the soil microbial biomass pool by 24%, reduced the invader's positive effect on soil water holding capacity by 5%, negated the invader's positive effect on the particulate organic matter carbon pool (from a 9% increase to no significant effect), and reversed the direction of the invader's impact on net nitrogen mineralization rate from a 51% increase to a 52% decrease. Furthermore, the influence of logging on the invader's impacts was not static, but dynamic through time. The results from our site therefore demonstrate that failure to account for the impacts of a co-occurring disturbance, as well as the time since disturbance, can result in flawed inference about the nature of an invader's effects. Future research should determine how widespread such flawed inference might be among other invasive species and across different environmental contexts. To help guide such research, we describe a general framework for disentangling the overlapping effects of invasions and co-occurring disturbances through time.

Emergence and accumulation of novel pathogens suppress an invasive species.

Emerging pathogens are a growing threat to human health, agriculture and the diversity of ecological communities but may also help control problematic species. Here we investigated the diversity, distribution and consequences of emerging fungal pathogens infecting an aggressive invasive grass that is rapidly colonising habitats throughout the eastern USA. We document the recent emergence and accumulation over time of diverse pathogens that are members of a single fungal genus and represent multiple, recently described or undescribed species. We also show that experimental suppression of these pathogens increased host performance in the field, demonstrating the negative effects of emerging pathogens on invasive plants. Our results suggest that invasive species can facilitate pathogen emergence and amplification, raising concerns about movement of pathogens among agricultural, horticultural, and wild grasses. However, one possible benefit of pathogen accumulation is suppression of aggressive invaders over the long term, potentially abating their negative impacts on native communities.

Assessment of Puccinia polliniicola as a potential biological control agent for Microstegium vimineum.

BACKGROUND: Microstegium vimineum (Trin.) A. Camus, commonly called stiltgrass, is a dominant weed in the United States and China. Although a lot of control approaches have been attempted, an economic, effective and practical measure has not been available to control the weed so far. RESULTS: A serious rust disease of Microstegium vimineum was observed in three regions of Wenzhou city in China, from 2019 to 2021, with a disease incidence ranging from 82% to 97%. Typical rust disease symptoms on Microstegium vimineum were prominently visible during the early monsoon season (June-July), with chlorotic spots on the leaf surfaces. The morphological characterization of the strain WZ-1 which was isolated from the diseased leaves was consistent with Puccinia polliniicola. The virulence tests showed that the average disease index of Microstegium vimineum plants could reach 35% at 10 days post-inoculation. The host specificity of Puccinia polliniicola was tested on 64 plant species from 12 families and it did not cause any diseased symptoms on 24 major crops and 36 weeds, but slightly infected four gramineous weeds, Arthraxon hispidus, Polypogon fugax, Cynodon dactylon, and Microstegium ciliatum. However, newly-produced urediniospores were not observed on the slightly infected plants. The urediniospores of strain WZ-1 infected the Microstegium vimineum leaves by two main approaches: mycelium or appressorium invaded the stoma; and mycelium or appressorium directly invaded intercellular spaces. Field experiments showed that the rust disease naturally prevailed among Microstegium vimineum populations, causing severe rust disease symptoms on the leaf surface. The rust epidemic effectively controlled all of the target plants in the closed plot where the rust was released. CONCLUSION: Puccinia polliniicola strain WZ-1 has great potential to be used as a classical biological control agent against Microstegium vimineum. © 2024 Society of Chemical Industry.

Aboveground versus soil-mediated effects of an invasive grass on fire-dependent forbs in an oak woodland.

Most work on plant competition intensity in general has focused on how aboveground and belowground competition for resources between plants changes with soil resource availability. In contrast, much work on the competitive effects of non-native invasive species on native species has focused on other mechanisms (e.g., allelopathy and microbial changes) and has largely ignored how these effects interact with mechanisms of resource competition along productivity gradients. We examined aboveground effects of an invasive grass, Microstegium vimineum, along with soil differences between invaded and non-invaded areas on two native perennial herbs at a productive and an unproductive oak woodland site in north Mississippi, USA. We transplanted 32 individuals each of Helianthus silphioides and Potentilla simplex from uninvaded areas into natural patches dominated by M. vimineum at each of the sites. Each transplant was randomly assigned to a pot with either native soil or soil from around M. vimineum roots. Aboveground competition was manipulated by securing M. vimineum shoots in a non-shading position around the transplant. We monitored survival of all transplants weekly in the growing seasons of 2020 and 2021. Transplant survival of H. silphioides was lowest in M. vimineum soil at the more productive site when M. vimineum was not pinned back. Transplant survival of P. simplex was lower at the more productive site but was mostly unresponsive to pinning and soil treatments. Synthesis. Our results suggest that soil-mediated legacy effects of an invader may reduce some native species' ability to compete for light at productive sites.

Woody plant secondary chemicals increase in response to abundant deer and arrival of invasive plants in suburban forests.

Plants in suburban forests of eastern North America face the dual stressors of high white-tailed deer density and invasion by nonindigenous plants. Chronic deer herbivory combined with strong competition from invasive plants could alter a plant's stress- and defense-related secondary chemistry, especially for long-lived juvenile trees in the understory, but this has not been studied. We measured foliar total antioxidants, phenolics, and flavonoids in juveniles of two native trees, Fraxinus pennsylvanica (green ash) and Fagus grandifolia (American beech), growing in six forests in the suburban landscape of central New Jersey, USA. The trees grew in experimental plots subjected for 2.5 years to factorial treatments of deer access/exclosure × addition/no addition of the nonindigenous invasive grass Microstegium vimineum (Japanese stiltgrass). As other hypothesized drivers of plant secondary chemistry, we also measured nonstiltgrass herb layer cover, light levels, and water availability. Univariate mixed model analysis of the deer and stiltgrass effects and multivariate structural equation modeling (SEM) of all variables showed that both greater stiltgrass cover and greater deer pressure induced antioxidants, phenolics, and flavonoids, with some variation between species. Deer were generally the stronger factor, and stiltgrass effects were most apparent at high stiltgrass density. SEM also revealed that soil dryness directly increased the chemicals; deer had additional positive, but indirect, effects via influence on the soil; in beech photosynthetically active radiation (PAR) positively affected flavonoids; and herb layer cover had no effect. Juvenile trees' chemical defense/stress responses to deer and invasive plants can be protective, but also could have a physiological cost, with negative consequences for recruitment to the canopy. Ecological implications for species and their communities will depend on costs and benefits of stress/defense chemistry in the specific environmental context, particularly with respect to invasive plant competitiveness, extent of invasion, local deer density, and deer browse preferences.

Invasive grass (Microstegium vimineum) indirectly benefits spider community by subsidizing available prey.

Invasive plant species cause a suite of direct, negative ecological impacts, but subsequent, indirect effects are more complex and difficult to detect. Where identified, indirect effects to other taxa can be wide-ranging and include ecological benefits in certain habitats or locations.Here, we simultaneously examine the direct and indirect effects of a common, invasive grass species (Microstegium vimineum) on the invertebrate communities of understory deciduous forests in the eastern United States. To do this, we use two complementary analytic approaches to compare invaded and reference plots: (a) community composition analysis of understory arthropod taxa and (b) analysis of isotopic carbon and nitrogen ratios of a representative predatory spider species.Invaded plots contained a significantly greater abundance of nearly all taxa, including predators, herbivores, and detritivores. Spider communities contained over seven times more individuals and exhibited greater species diversity and richness in invaded plots.Surprisingly, however, the abundant invertebrate community is not nutritionally supported by the invasive plant, despite 100% ground cover of M. vimineum. Instead, spider isotopic carbon ratios showed that the invertebrate prey community found within invaded plots was deriving energy from the plant tissue of C3 plants and not the prevalent, aboveground M. vimineum. Synthesis and applications. We demonstrate that invasive M. vimineum can create non-nutritional ecological benefits for some invertebrate taxa, with potential impacts to the nutritional dynamics of invertebrate-vertebrate food webs. These positive impacts, however, may be restricted to habitats that experience high levels of ungulate herbivory or reduced vegetative structural complexity. Our results highlight the importance of fully understanding taxon- and habitat-specific effects of invading plant species when prioritizing invasive species removal or management efforts.

Site conditions are more important than abundance for explaining plant invasion impacts on soil nitrogen cycling.

Invasive plant species can alter critical ecosystem processes including nitrogen transformations, but it is often difficult to anticipate where in an invaded landscape, these effects will occur. Our predictive ability lags because we lack a framework for understanding the multiple pathways through which environmental conditions mediate invader impacts. Here, we present a framework using structural equation modeling to evaluate the impact of an invasive grass, Microstegium vimineum (M.v.), on nitrogen cycling based on a series of invaded sites that varied in invader biomass and non-M.v. understory biomass, tree basal area, light availability, and soil conditions. Unlike previous studies, we did not find an overall pattern of elevated nitrate concentrations or higher nitrification rates in M.v.-invaded areas. We found that reference plot conditions mediated differences in mineralization between paired invaded and reference plots at each site through indirect (via M.v. biomass), direct, and interactive pathways; however, the strongest pathways were independent of M.v. biomass. For example, sites with low reference soil nitrate and high non-M.v. understory biomass tended to have faster mineralization at 5-15 cm in invaded plots. These findings suggest that more attention to reference conditions is needed to understand the impact of invasive species on soil nitrogen cycling and other ecosystem processes and that the greatest impacts will not necessarily be where the invader is most abundant.

Understory upheaval: factors influencing Japanese stiltgrass invasion in forestlands of Tennessee, United States.

BACKGROUND: Invasions by non-native plants contribute to loss of ecosystem biodiversity and productivity, modification of biogeochemical cycles, and inhibition of natural regeneration of native species. Japanese stiltgrass (Microstegium vimineum (Trin.) A. Campus) is one of the most prevalent invasive grasses in the forestlands of Tennessee, United States. We measured the extent of invasion, identified potential factors affecting invasion, and quantified the relative importance of each factor. We analyzed field data collected by the Forest Inventory and Analysis Program of the U.S. Forest Service to measure the extent of invasion from 2005 to 2011 and identified potential factors affecting invasion during this period using boosted regression trees. RESULTS: Our results indicated that presence of Japanese stiltgrass on sampled plots increased 50% (from 269 to 404 plots) during the time period. The probability of invasion was correlated with one landscape condition (elevation) (20.5%) and five forest features (including tree species diversity, basal area, stand age, site productivity, and natural regeneration) (79.5%). Boosted regression trees identified the most influential (highly correlated) variables as tree species diversity (30.7%), basal area (22.9%), elevation (20.5%), and stand age (16.7%). Our results suggest that Japanese stiltgrass is likely to continue its invasion in Tennessee forests. CONCLUSIONS: The present model, in addition to correlating the probability of Japanese stiltgrass invasions with current climatic conditions and landscape attributes, could aid in the on-going development of control strategies for confronting Japanese stiltgrass invasions by identifying vulnerable areas that might emerge as a result of likely changes in climatic conditions and land use patterns.

Effects of white-tailed deer and invasive plants on the herb layer of suburban forests.

Lack of hunting and predators and proximity to human communities make suburban forests prone to high deer abundance and non-native plant invasions. I investigated these likely drivers of community structure in the herb layers of six suburban forests in one region of New Jersey, USA. In 223 plots I assessed the herb layer response to 2.5 years with or without deer fencing and the early stage of invasion from seed additions of Microstegium vimineum, an invasive, annual grass. Non-native plants and herbaceous native plants were affected very little by fencing or M. vimineum invasion. In contrast, across all forests the combination of deer access and M. vimineum addition had a strongly negative effect on woody native percent cover. Forests differed in overall fencing effects on woody natives; their cover was greater in fenced plots in just three forests, suggesting greater deer pressure in those forests during the experiment. The early invasion by M. vimineum was greatest in two of these same forests, but was not influenced by fencing. Multi-group structural equation modelling compared two groups of forests that differed in vegetation abundance and other characteristics. It paralleled the results above and also showed no negative influence of non-native cover on native cover, even in the forests where non-native cover was greater. It identified a positive effect of light level on herb layer plants in the forests with less vegetation, and also revealed a positive effect of soil water potential (SWP) on non-native plants in the forests with more vegetation, which had higher SWP. These suburban forests within a common region varied widely in native and non-native herb layer abundance, the early success of M. vimineum invasion and the herb layer's response to early invasion and protection from deer.

Response of an understory plant community to elevated [CO2 ] depends on differential responses of dominant invasive species and is mediated by soil water availability.

• Rising atmospheric CO2 concentrations are likely to have direct effects on terrestrial ecosystems. Here, we describe effects of elevated concentrations of CO2 on an understory plant community in terms of production and community composition. • In 2001 and 2002 total and species-specific above-ground net primary productivity (ANPP) were estimated by harvesting above-ground biomass within an understory community receiving ambient [CO2 ] and elevated [CO2 ] at Oak Ridge National Laboratory's free-air carbon dioxide enrichment (FACE) facility. • During a wet year, community composition differed between plots receiving ambient [CO2 ] and elevated [CO2 ], but total ANPP did not differ. By contrast, during a drier year, community composition did not differ, but total ANPP was greater in elevated than ambient [CO2 ] plots. These patterns were driven by the response of two codominant species, Lonicera japonica and Microstegium vimineum, both considered invasive species in the south-eastern United States. The ANPP of L. japonica was consistently greater under elevated [CO2 ], whereas the response of M. vimineum to CO2 enrichment differed between years and mediated total community response. • These data suggest that community and species responses to a future, CO2 -enriched atmosphere may be mediated by other environmental factors and will depend on individual species responses.