Adaptive constraints at the range edge of a widespread and expanding invasive plant.

Identifying the factors that facilitate and limit invasive species' range expansion has both practical and theoretical importance, especially at the range edges. Here, we used reciprocal common garden experiments spanning the North/South and East/West range that include the North American core, intermediate and range edges of the globally invasive plant, Johnsongrass (Sorghum halepense) to investigate the interplay of climate, biotic interactions (i.e. competition) and patterns of adaptation. Our results suggest that the rapid range expansion of Johnsongrass into diverse environments across wide geographies occurred largely without local adaptation, but that further range expansion may be restricted by a fitness trade-off that limits population growth at the range edge. Interestingly, plant competition strongly dampened Johnsongrass growth but did not change the rank order performance of populations within a garden, though this varied among gardens (climates). Our findings highlight the importance of including the range edge when studying the range dynamics of invasive species, especially as we try to understand how invasive species will respond to accelerating global changes.

Hybridization in agricultural weeds: A review from ecological, evolutionary, and management perspectives.

Agricultural weeds frequently hybridize with each other or with related crop species. Some hybrid weeds exhibit heterosis (hybrid vigor), which may be stabilized through mechanisms like genome duplication or vegetative reproduction. Even when heterosis is not stabilized, hybridization events diversify weed gene pools and often enable adaptive introgression. Consequently, hybridization may promote weed evolution and exacerbate weed-crop competition. However, hybridization does not always increase weediness. Even when viable and fertile, hybrid weeds sometimes prove unsuccessful in crop fields. This review provides an overview of weed hybridization and its management implications. We describe intrinsic and extrinsic factors that influence hybrid fitness in agroecosystems. We also survey the rapidly growing literature on crop-weed hybridization and the link between hybridization and invasiveness. These topics are increasingly relevant in this era of genetic tools for crop improvement, intensive and simplified cropping systems, and globalized trade. The review concludes with suggested research priorities, including hybridization in the context of climate change, plant-insect interactions, and redesigned weed management programs. From a weed management perspective, hybridization is one of many reasons that researchers and land managers must diversify their weed control toolkits.

Phylogenetic relatedness can influence cover crop-based weed suppression.

Cover crops are plants grown to provide regulating, supporting, and cultural ecosystem services in managed environments. In agricultural systems, weed suppression services from cover crops can be an important tool to promote sustainability as reliance on herbicides and tillage for weed management has caused pollution, biodiversity loss, and human health issues. However, to effectively use weed suppression services from cover crops, farmers must carefully select species that fit within their rotations and suppress their problematic weeds. Understanding how the relatedness between cover crops and weeds affects their interactions will help farmers select cover crops for targeted weed management. The phylogenetic distance between species reflects their relatedness and was studied through a series of field experiments that compared weed suppression in winter and summer cover crops with tilled controls. This study demonstrates that cover crops can reduce up to 99% of weed biomass and alter weed community structure by suppressing phylogenetically related weed species. Results also suggest that cover crop planting season can influence weed community structure since only overwintering treatments affected the phylogenetic distance of weed communities. In an applied context, these results help develop cover crop-based weed management systems, demonstrating that problematic weeds can be managed by selecting phylogenetically related cover crop species. More broadly, this study provides a framework for evaluating weed communities through a phylogenetic perspective, which provides new insight into plant interactions in agriculture.

Nitrogen Deposition Shifts Grassland Communities Through Directly Increasing Dominance of Graminoids: A 3-Year Case Study From the Qinghai-Tibetan Plateau.

Nitrogen (N) deposition has been increasing for decades and has profoundly influenced the structure and function of grassland ecosystems in many regions of the world. However, the impact of N deposition on alpine grasslands is less well documented. We conducted a 3-year field experiment to determine the effects of N deposition on plant species richness, composition, and community productivity in an alpine meadow of the Qinghai-Tibetan Plateau of China. We found that 3 years of N deposition had a profound effect on these plant community parameters. Increasing N rates increased the dominance of graminoids and reduced the presence of non-graminoids. Species richness was inversely associated with aboveground biomass. The shift in plant species and functional group composition was largely responsible for the increase in productivity associated with N deposition. Climatic factors also interacted with N addition to influence productivity. Our findings suggest that short-term N deposition could increase the productivity of alpine meadows through shifts in composition toward a graminoid-dominated community. Longer-term studies are needed to determine if shifts in composition and increased productivity will be maintained. Future work must also evaluate whether decreasing plant diversity will impair the long-term stability and function of sensitive alpine grasslands.

Biomass allocation of Vincetoxicum rossicum and V. nigrum in contrasting competitive environments.

PREMISE: Understanding how drought and biomass allocation patterns influence competitive ability can help identify traits related to invasiveness and guide management. Vincetoxicum nigrum and V. rossicum are increasingly problematic herbaceous perennial vines in the northeastern United States and southeastern Canada. METHODS: Using a greenhouse experiment, we investigated how biomass allocation and competition intensity of Vincetoxicum spp. responded to four competitive regimes at two levels of soil water availability in the presence of conspecific or congeneric neighbors. RESULTS: Soil moisture was the most important influence on growth and biomass allocation. Vincetoxicum nigrum had a greater capacity for growth and reproduction than V. rossicum, especially under drought. Drought reduced the probability of reproduction for V. rossicum. Vincetoxicum rossicum had a higher root-to-shoot ratio than V. nigrum under adequate soil moisture. This difference more than doubled under drought. Under interspecific competition, V. nigrum maximized its biomass, while V. rossicum limited aboveground growth and reproduction. Root-only competition increased shoot and root biomass relative to shoot-only competition. The effects of root and shoot competition were additive under interspecific competition, but interacted under intraspecific competition (negative interaction under drought and positive interaction under sufficient soil moisture). CONCLUSIONS: Management strategies targeting mixed populations of V. rossicum and V. nigrum are most important under ample water availability. Under drought conditions, strategies focused on V. nigrum should effectively limit Vincetoxicum growth and seed reproduction. Phenotypic plasticity and the positive competition intensity associated with drought in monocultures may contribute to drought resistance in these invasive species.

Eco-physiological processes are more sensitive to simulated N deposition in leguminous forbs than non-leguminous forbs in an alpine meadow of the Qinghai-Tibetan Plateau.

Increased nitrogen (N) deposition can affect ecosystem processes and thus influence plant eco-physiological processes in grasslands. However, how N deposition affects eco-physiological processes of leguminous and non-leguminous forbs in alpine grasslands is understudied. A long-term field experiment using a range of simulated N deposition rates (0, 8, 24, 40, 56, and 72 kg N ha-1 year-1) was established to examine the effects of N deposition on various eco-physiological parameters in leguminous and non-leguminous forbs in an alpine meadow of the Qinghai-Tibetan Plateau. We found that the responses of leguminous and non-leguminous forbs to simulated N deposition varied. Net photosynthetic rate of leguminous and non-leguminous forbs exhibited different response patterns, but chronic increases in simulated N deposition rates may lead to negative effects in both functional groups. Neither functional group responded differently in aboveground biomass under the highest N addition level (72 kg N ha-1 year-1) compared to the control. Differences in aboveground biomass of leguminous forbs were observed at intermediate N levels. Short-term simulated N deposition significantly promoted N uptake of both functional groups. In leguminous forbs, simulated N deposition affected net photosynthetic rates (PN) and aboveground biomass (AGB) mainly via stomatal conductance (gs), water use efficiency (WUE), and plant N uptake. In non-leguminous forbs, simulated N deposition affected PN and AGB mainly through WUE and plant N uptake. Our findings suggest that leguminous and non-leguminous forbs have differential response mechanisms to N deposition, and compared with non-leguminous forbs, leguminous forbs are more sensitive to continuing increased N deposition. The obvious decline trend in photosynthetic capacity in leguminous forbs is likely to exacerbate the already divergent ecological processes between leguminous and non-leguminous forbs. More importantly, these changes are likely to alter the future composition, function, and stability of alpine meadow ecosystems.

A regional assessment of white-tailed deer effects on plant invasion.

Herbivores can profoundly influence plant species assembly, including plant invasion, and resulting community composition. Population increases of native herbivores, e.g. white-tailed deer (Odocoileus virginianus), combined with burgeoning plant invasions raise concerns for native plant diversity and forest regeneration. While individual researchers typically test for the impact of deer on plant invasion at a few sites, the overarching influence of deer on plant invasion across regional scales is unclear. We tested the effects of deer on the abundance and diversity of introduced and native herbaceous and woody plants across 23 white-tailed deer research sites distributed across the east-central and north-eastern USA and representing a wide range of deer densities and invasive plant abundance and identity. Deer access/exclusion or deer population density did not affect introduced plant richness or community-level abundance. Native and total plant species richness, abundance (cover and stem density) and Shannon diversity were lower in deer-access vs. deer-exclusion plots. Among deer-access plots, native species richness, native and total cover, and Shannon diversity (cover) declined as deer density increased. Deer access increased the proportion of introduced species cover (but not of species richness or stem density). As deer density increased, the proportion of introduced species richness, cover and stem density all increased. Because absolute abundance of introduced plants was unaffected by deer, the increase in proportion of introduced plant abundance is likely an indirect effect of deer reducing native cover. Indicator species analysis revealed that deer access favoured three introduced plant species, including Alliaria petiolata and Microstegium vimineum, as well as four native plant species. In contrast, deer exclusion favoured three introduced plant species, including Lonicera japonica and Rosa multiflora, and 15 native plant species. Overall, native deer reduced community diversity, lowering native plant richness and abundance, and benefited certain invasive plants, suggesting pervasive impacts of this keystone herbivore on plant community composition and ecosystem services in native forests across broad swathes of the eastern USA.

Predicting invasions of Wedelia trilobata (L.) Hitchc. with Maxent and GARP models.

Wedelia trilobata (L.) Hitchc., an ornamental groundcover plant introduced to areas around the world from Central America, has become invasive in many regions. To increase understanding of its geographic distribution and potential extent of spread, two presence-only niche-based modeling approaches (Maxent and GARP) were employed to create models based on occurrence records from its: (1) native range only and (2) full range (native and invasive). Models were then projected globally to identify areas vulnerable to W. trilobata invasion. W. trilobata prefers hot and humid environments and can occur in areas with different environmental conditions than experienced in its native range. Based on native and full occurrence points, GARP and Maxent models produced consistent distributional maps of W. trilobata, although Maxent model results were more conservative. When used to estimate the global invasive distribution of the species, both modeling approaches projected the species to occur in Africa. The GARP full model succeeded in predicting the known occurrences in Australia, while the other models failed to identify favorable habitats in this region. Given the rapid spread of W. trilobata and the serious risk of this species poses to local ecosystems, practical strategies to prevent the establishment and expansion of this species should be sought.

Deer browsing delays succession by altering aboveground vegetation and belowground seed banks.

Soil seed bank composition is important to the recovery of natural and semi-natural areas from disturbance and serves as a safeguard against environmental catastrophe. White-tailed deer (Odocoileus virginianus) populations have increased dramatically in eastern North America over the past century and can have strong impacts on aboveground vegetation, but their impacts on seed bank dynamics are less known. To document the long-term effects of deer browsing on plant successional dynamics, we studied the impacts of deer on both aboveground vegetation and seed bank composition in plant communities following agricultural abandonment. In 2005, we established six 15 × 15 m fenced enclosures and paired open plots in recently followed agricultural fields near Ithaca, NY, USA. In late October of each of six years (2005-2010), we collected soil from each plot and conducted seed germination cycles in a greenhouse to document seed bank composition. These data were compared to measurements of aboveground plant cover (2005-2008) and tree density (2005-2012). The impacts of deer browsing on aboveground vegetation were severe and immediate, resulting in significantly more bare soil, reduced plant biomass, reduced recruitment of woody species, and relatively fewer native species. These impacts persisted throughout the experiment. The impacts of browsing were even stronger on seed bank dynamics. Browsing resulted in significantly decreased overall species richness (but higher diversity), reduced seed bank abundance, relatively more short-lived species (annuals and biennials), and fewer native species. Both seed bank richness and the relative abundance of annuals/biennials were mirrored in the aboveground vegetation. Thus, deer browsing has long-term and potentially reinforcing impacts on secondary succession, slowing succession by selectively consuming native perennials and woody species and favoring the persistence of short-lived, introduced species that continually recruit from an altered seed bank.

Does polyembryony confer a competitive advantage to the invasive perennial vine Vincetoxicum rossicum (Apocynaceae)?

Determining which traits may allow some introduced plant species to become invasive in their new environment continues to be a key question in invasion biology. Vincetoxicum rossicum is an invasive, perennial vine colonizing natural and seminatural habitats primarily in the northeastern United States and southeastern Canada. More than half its seeds exhibit polyembryony, a relatively uncommon condition in which a single seed produces multiple seedlings. For evaluating the potential consequences of polyembryony on invasiveness, V. rossicum plants derived from seeds of three embryonic classes-singlets, doublets, and triplets (one, two, and three seedlings per seed, respectively)-were paired in all combinations intraspecifically and with the co-occurring native herbs Solidago canadensis and Asclepias syriaca in a greenhouse study. Vincetoxicum rossicum biomass was 25-55% greater and follicle production 55-100% greater under intraspecific competition compared with interspecific competition. However, within a competitive environment, follicle production varied little. Regardless of competitive environment, V. rossicum originating from seeds with a greater number of embryos typically performed no better than plants arising from seed with fewer embryos (singlets = doublets = triplets)-except intraspecifically where doublets outperformed singlets, and with S. canadensis where triplets outperformed singlets. Our findings suggest that overall performance and fitness of V. rossicum is higher in monocultures than in mixed stands and that its ability to invade new habitats may not be attributable to the production of polyembryonic seeds.

Survival and performance of the invasive vine Vincetoxicum rossicum (Apocynaceae) from seeds of different embryo number under two light environments.

The nonnative vine Vincetoxicum rossicum threatens several ecosystems in the Lower Great Lakes Basin of North America. One feature that may contribute to its invasiveness is the production of some seeds with multiple embryos (polyembryony), which may be beneficial as a bet-hedging strategy in variable environments. However, lower seed reserves per embryo in polyembryonic seeds may entail costs in low-light environments. The effect of seed from three embryonic classes (1, 2, or 3 embryos/seed) on V. rossicum survival and growth was studied under two forest understory light environments: full canopy (shade) or canopy gaps (light) in New York state. Two seedling cohorts were planted, in May 2004 and in May 2005. The survival and growth of seedlings was monitored biweekly for two (2005 cohort) or three (2004 cohort) seasons. For both cohorts, plants grown in canopy shade had reduced survival and growth compared with those grown in gaps. Contrary to expectations, seed embryo number had no effect on the final height, survival, or dry mass of plants in either habitat. Our results suggest that any fitness advantage provided by polyembryony may be habitat (light) dependent and not a general trait that affords V. rossicum a benefit in all habitats colonized.

Decoupling of light intensity effects on the growth and development of C3 and C4 weed species through sucrose supplementation.

Light availability has a profound effect on plant growth and development. One of the ways to study the effects of light intensity on plant growth and development without the confounding problem of photosynthate availability is sucrose injection/supplementation. A greenhouse experiment was conducted to evaluate the effects of light levels (0% and 75% shade) and sucrose injection (distilled water or 150 g sucrose l(-1)) on three weed species: redroot pigweed (Amaranthus retroflexus L., C4), lambsquarters (Chenopodium album L., C3) and velvetleaf (Abutilon theophrasti Medic., C3). The average total sucrose uptake was 7.6 and 5.9 g per plant for 0% and 75% shading, respectively, representing 47% of the average total weed dry weight. Plants injected with sucrose had greater dry weights and shoot-to-root ratios under both light levels. In spite of sucrose supplementation the reduction in dry matter due to shading was greater for roots and reproductive structures than vegetative shoot tissues, indicating light level regulation of morphological changes resulting in changed C allocation that are independent of photosynthate availability. Dry weights of plants injected with sucrose under 75% shading were not different from distilled water-injected unshaded plants. However, both sucrose-injected and control plants, regardless of their photosynthetic pathways, underwent similar changes in allocation of dry matter and morphology due to shading, suggesting that these effects are strictly due to light intensity and not related to photosynthate availability.

Impact of a fungal pathogen, Colletotrichum coccodes on growth and competitive ability of Abutilon theophrasti.

The combined effects of planting density and of different inoculation frequencies ofColletotrichum coccodes (Wallr.) Hughes on growth and competitive performance of Abutilon theophrasti Medik. were studied using a soybean-A. theophrasti target-neighbour design in a controlled environment. In both trials of the experiment, A. theophrasti inoculated at the highest planting density (four plants per ppt) suffered significantly greater reductions in height (41%) than did A. theophrasti at the lowest density (one plant per pot) (7%). Above-ground biomass and leaf area reductions, however were significantly greater at the highest density for only one of the trials. Soybean plants grown with inoculated A. theophrasti at the two highest planting densities had a significantly greater above-ground biomass (61%) and leaf area (68%) than did plants grown with uninoculated A. theophrasti at the same densities. By contrast, at the two lower densities, soybean above-ground biomass and leaf area were not increased significantly fallowing inoculation. Either one or two C. coccodes inoculations caused the greatest reductions in A. theophrasti growth compared with uninoculated plants. Conversely, three applications of the fungus generally resulted in less severe disease symptoms and resulted in the smallest decreases in A. theophrasti growth. Induced systemic resistance following two inoculations might have played an important role in limiting disease. However, the significantly greater biomass and height of A. theophrasti plants subjected to the triple C. caicudes treatment, compared with plants receiving either one or two inoculations in one of the trials, provides some evidence of a possible compensatory response in .4. theophrasti. The relevance of these findings for biological weed control is examined.