Higher Intensities of Mulching-Head Treatments Limit the Response of a Target Invasive Shrub (Lonicera maackii) and Promote Herbaceous-Layer Species Diversity

• Many invasive shrubs leaf out earlier and senesce later than native plants. • Amur honeysuckle (AH), an invasive understory shrub, can modify forest phenometrics. • VENµS time series detected phenological differences from AH invasion. • Early spring is optimal to distinguish AH from native vegetation. • Phenology-based distribution modeling achieved moderate accuracy. Many invasive shrubs in the eastern deciduous forests of the United States use the temporal niche before and after the native tree canopy leaf-on period (leafing out prior to most native species and retaining leaves after most natives senesce) to establish in the light-limited environment of the understory. To support an increased understanding of invasive shrub species’ ecology and distribution patterns and inform better management plans, this key phenological difference needs to be characterized in detail. Here we leveraged the high-resolution observations from the French-Israel VENµS mission to examine the phenological characteristics of a widespread invasive shrub species—Amur honeysuckle (AH; Lonicera maackii (Rupr.) Herder)—compared to native deciduous trees in Robinson Forest, Kentucky. VENµS offered daily superspectral (12 narrow bands) observations at 4 m resolution in a limited number of global sites, providing us with crucial data for the analysis. We identified three forest communities with respect to AH presence through field surveys ( i.e., uninvaded forest stands, forest stands with AH understory, and AH shrub thickets) and compared their VENµS-derived spectral signatures and time series of vegetation indices. In 2023, AH shrub thickets greened up one month earlier than uninvaded forest stands (mid-March vs. mid-April). AH leaf growth advanced into full green before the canopy tree greenup started in early April, marking an optimal window for isolating areas with AH understory from the uninvaded forest using remote sensing. Based on the phenological differences identified, we predicted the distribution of AH in the study area using a two-date differencing model and a spectral mixture analysis. Our detailed findings using VENµS data offer insights into the temporal dynamics of invasive shrubs and native trees in a typical eastern deciduous forest. While our prediction of the AH distribution was confounded by the presence of native early greening and/or evergreen understory plants at a few locations, it was still moderately accurate (overall accuracy ∼ 70 %) and its abundance estimates agreed with observations in forest stands with minimal native understory growth. Moving forward, high-resolution remote sensing observations combined with a phenology-based approach will likely support more precise monitoring and management of invasive understory plants in native forest ecosystems.

Invasive shrubs in forest understories threaten biodiversity and forest regeneration in the eastern United States. Controlling these extensive monotypic shrub thickets is a protracted process that slows the restoration of degraded forest land. Invasive shrub removal can be accelerated by using forestry mulching heads, but evidence from the western United States indicates that mulching heads can promote exotic species establishment and mulch deposition can reduce native plant species abundance. We compared the effectiveness of the mulching head and the “cut-stump” method for controlling the invasive shrub Amur honeysuckle (Lonicera maackii), as well as their impacts on native plant community recovery, in mixed-hardwood forests of Indiana. After two growing seasons, mulching head treatment resulted in greater L. maackii regrowth and regeneration. The recovery of native plant abundance and diversity following shrub removal did not differ between the two methods. However, mulch deposition was associated with increased abundance of garlic mustard (Alliaria petiolata), an invasive forb. Increasing mulching head treatment depth reduced L. maackii regrowth, but additional study is needed to determine how it affects plant community responses. The mulching head is a promising technique for invasive shrub control and investigating tradeoffs between reducing landscape-scale propagule pressure and increased local establishment will further inform its utility.

Recent studies have shown that complex species interactions can regulate above- and belowground processes in terrestrial systems. Ungulate herbivory and invasive species are known to have strong effects on plant communities in some systems, but their impacts on soil biota and belowground processes are lesser known. Growing evidence suggests white-tailed deer (Odocoileus virginianus) and invasive plants facilitate increased abundance of exotic earthworms in temperate forests of the eastern United States. We conducted an experimental study that manipulated deer access and the presence of an invasive understory shrub in an eastern deciduous forest of southwestern Ohio, USA, from 2013 to 2017. Earthworm density and biomass, and standing litter biomass were measured in five paired deer access and exclosure plots, each with a split-plot removal of Amur honeysuckle (Lonicera maackii). Earthworm density declined in response to the experimental exclusion of deer, with earthworm density decreasing over time in the deer exclosure plots relative to deer access plots. Deer exclusion produced greater variation in earthworm species composition relative to access plots. Multivariate analyses indicated that larger earthworms in the genus Lumbricus were associated with deer exclosure plots, while smaller endogeic species were ubiquitous in both treatments. Standing litter biomass decreased over time in the deer-access plots. In contrast, honeysuckle removal had little effect on earthworm density and standing litter biomass. There was an interaction between deer and honeysuckle treatments on earthworm biomass, with honeysuckle removal reducing earthworm biomass when deer were excluded. Our results demonstrate strong effects of herbivores on invasive earthworms and ecosystem processes, but indicate a weaker influence of invasive shrubs. Further, our findings suggest that the effects of deer overabundance in forest ecosystems are potentially reversible with long-term intervention.

Ungulate browse and invasive plants exert pressure on plant communities and alter the physical and chemical properties of soils, but little is known about their effects on litter-dwelling arthropods. In particular, ants (Formicidae) are ubiquitous in temperate forests and are sensitive to changes in habitat structure and resources. As ants play many functional roles, changes to ant communities may lead to changes in ecosystem processes. We conducted a long-term experiment that controlled white-tailed deer (Odocoileus virginianus) access and presence of an invasive understory shrub in deciduous forests located in southwestern Ohio, USA from 2011 to 2017. Several leaf-litter ant community responses and litter biomass were measured in five paired deer access and exclosure plots, each with a split-plot removal of Amur honeysuckle (Lonicera maackii). Ant abundance and species richness increased with time in deer exclosures, but not in deer access plots. Honeysuckle removal reduced abundance and richness of ants. There were additive effects of deer and honeysuckle on ant richness, and interactive effects of deer and honeysuckle on ant abundance. Deer exclusion reduced variation in ant composition relative to access plots. There was little evidence that treatments directly influenced species diversity of ants. However, all ant measures were positively related to litter biomass, which was greater in deer exclosures relative to access plots. Our results indicate strong indirect effects of herbivores and honeysuckle on litter-dwelling ants, mediated through changes in litter biomass and likely vegetation structure, which may alter ant-mediated ecosystem processes.

ABSTRACT: Non-native invasive plants can negatively affect the abundance and survival of native plant species and alter ecosystem function. Amur honeysuckle (Lonicera maackii) is an invasive shrub that is an increasingly onerous problem for forest managers in eastern North America. We examined site-specific characteristics related to the presence and abundance of Amur honeysuckle, and other non-native invasive plants, in 15 second growth forests in central Kentucky. Individual remnants were characterized by: (1) frequency and cover of non-native invasive and native shrubs, (2) overstory tree basal area, (3) forest floor litter depth, (4) species composition of leaf litter, and (5) soil characteristics. Of the measured variables, the only statistically significant relationships we discovered were between Amur honeysuckle and characteristics of the forest floor. We found significant negative relationships between Amur honeysuckle presence and both forest floor litter depth (P = 0.01) and the percent of oak ...

White-tailed deer (Odocoileus virginianus) (hereafter, ‘deer’) and invasive plants affect forest understories, but few studies have investigated their interaction. We investigated long-term (11-yr) effects of excluding deer and removing the invasive shrub, Amur honeysuckle [Lonicera maackii(Rupr.) Herder], on plants in southwest Ohio. Deer exclusion enhanced tree seedling richness and density, but reduced annual and bare ground cover. Vine density was reduced byLonicera, especially where deer were excluded. Seedlings of several tree species, the invasive shrub, burning bush [Euonymus alatus(Thunb.) Siebold], and the invasive vine, winter creeper [Euonymous fortunei(Turcz.) Hand.-Maz.], were indicator species of the deer exclosure byLoniceraremoval treatment combination. The effect of deer on cover of native species depended onLoniceratreatment: where shrubs were removed, cover was higher where deer were excluded, but where shrubs were present, cover was higher where deer had access. We attribute these interactions to the reduced growth of, and shading by,Lonicerawhere deer had access and browsed this invasive shrub. Some of these effects were evident in the first 6 yr, but are now larger. Other effects were not evident in the earlier evaluation. These findings inform management of areas with high densities of deer and invasive shrubs palatable to deer. Control of only invasive shrubs will reduce native cover and not improve tree regeneration. Managing only deer will increase woody plants but reduce native cover. Management of both stressors is needed to promote tree regeneration and restoration of plant communities.

University campuses represent human-managed and relatively protected ecosystems. However, anthropogenic activities such as gardening and landscaping contribute to the introduction and proliferation of alien tree and shrub species. The present study assesses the diversity and distribution of invasive and non-invasive alien trees and shrubs across undisturbed areas and human-impacted areas of Mahatma Gandhi University (MGU) campus, Kerala, India, and compares the findings with other Indian university campuses. Twenty-three alien trees belonging to 22 genera and 12 families were recorded along with 40 alien shrubs representing 30 genera and 22 families. These included two invasive trees, six invasive shrubs, 21 non-invasive trees, and 34 non-invasive shrubs. The human-impacted zone exhibited the highest alien species richness and taxonomic diversity, particularly among non-invasive trees and shrubs. These results suggest that anthropogenic disturbance significantly promotes the richness and proliferation of both invasive and non-invasive alien trees and shrubs, with invasive shrubs demonstrating the ability to establish even in relatively undisturbed habitats. A comparative analysis with other Indian university campuses revealed that alien trees and shrubs constitute a significant proportion (40–55%) of the recorded woody flora, with invasives (comprising 10–35% of the alien pool) consistently posing a greater ecological risk. The findings highlight the necessity of systematic monitoring and management strategies to mitigate the spread of invasive alien trees and shrubs and safeguard campus biodiversity.

Invasion by non-native species is a growing threat to ecosystems and economies. In eastern North American woodlands, invasive exotic shrubs are displacing native plants and transforming understory communities. The displacement of native plants by invasive non-natives may reduce the resources available to higher trophic levels by supporting diminished arthropod communities. Despite a conceptual understanding of the potential for invasive shrubs to transform forest communities, little research has focused on characterizing the arthropod communities they support or their impact on higher trophic levels in eastern North American forests. To address this gap, we compared arthropod diversity, abundance, and community composition on a highly invasive shrub in southwestern Ohio, Amur honeysuckle (Lonicera maackii, Maxim., Dipscales: Caprifoliaceae), to a related, ecologically similar, native shrub, blackhaw (Viburnum prunifolium, Linnaeus, Dipscales: Adoxaceae), over a growing season. Relative to the native shrub, we found that L. maackii hosted a depauperate arthropod community overall, with about 25% fewer individuals and 28% fewer species than its native counterpart, V. prunifolium-primarily driven by a smaller herbivore community on L. maackii. In contrast, the abundance and richness of predatory arthropods were similar between the native and invasive shrubs. The arthropod communities on the native and exotic plants broadly overlapped in ordination space; however, community composition varied significantly, albeit modestly. These findings support the prediction that invasive plants support depauperate arthropod communities and provide reduced ecosystem services.

Evaluating Composition and Conservation Value of Roadside Plant Communities in a Grassland Biome.

Summary Though co‐occurrence of invasive plant species is common, few studies have compared the community and ecosystem impacts of invaders when they occur alone and when they co‐occur. Prioritization of invasive species management efforts requires sufficient knowledge of impacts – both among individual invasive species and among different sets of co‐occurring invaders – to target resources towards management of sites expected to undergo the largest change. Here, we observed differences in above‐ and below‐ground impacts of two invasive woody shrubs, Lonicera maackii and Ligustrum sinense, among plots containing both shrubs (mixed), each species singly or lacking both species (control). We found additive and non‐additive effects of these co‐occurring invasives on plant communities and soil processes. Mixed plots contained two times more subdominant invasive plant species than L. maackii or L. sinense plots. Compared to control plots, mixed plots had three times the potential activity of β‐glucosidase, a carbon‐degrading extracellular soil enzyme. L. maackii plots and mixed plots had less acidic soils, while L. sinense plots had higher soil moisture than control plot soils. Differences in soil properties among plots explained plant‐ and ground‐dwelling arthropod community composition as well as the potential microbial function in soils. Synthesis and applications. Our study highlights the importance of explicitly studying the impacts of co‐occurring invasive plant species singly and together. Though Lonicera maackii and Ligustrum sinense have similar effects on ecosystem structure and function when growing alone, our data show that two functionally similar invaders can have non‐additive impacts on ecosystems. These results suggest that sites with both species should be prioritized for invasive plant management over sites containing only one of these species. Furthermore, this study provides a valuable template for future studies exploring how and when invasion by co‐occurring species alters above‐ and below‐ground function in ecosystems with different traits.

Amur honeysuckle (Lonicera maackii) is an exotic invasive shrub that is rapidly expanding into forests of eastern North America. This species forms a dense forest understory, alters tree regeneration, negatively affects herb-layer biodiversity, and alters ecosystem function. In a second-growth forest in central Kentucky, we examined the timing and production of leaf litter and compared litter chemistry, decay rates, and microbial community colonization of Amur honeysuckle to that of two native trees, white ash (Fraxinus americana) and hickory (Carya spp.). The distribution of Amur honeysuckle was clumped, allowing us to compare differences in decomposition under and away from Amur honeysuckle shrubs. Amur honeysuckle leaf litter had significantly higher nitrogen, lower C:N, and lower lignin than the other species, and decomposition rates were greater than 5× faster. Despite the much higher rate of Amur honeysuckle decomposition compared with the native species (p < 0.0001), decomposition of all species was significantly slower (p = 0.0489) in sites located under Amur honeysuckle shrubs. Nitrogen concentration increased through time in decomposing Amur honeysuckle litter; however, total mass of N rapidly declined. We found the initial microbial community on leaf litter of Amur honeysuckle was distinct from two native species and although all microbial communities changed through time, the microbial community of Amur honeysuckle remained distinct from native communities. In summary, a distinct microbial community that may originate on Amur honeysuckle leaves prior to senescence could explain the rapid decay rates.

I surveyed the distribution of invasive (non-native) shrubs in Kentucky, along with their properties, effects, and control measures. Kentucky's floral, physical geography, and climate are representative of a much broader region of the US, and the state has a large number of counties with available data. Invasive shrub species richness increases with county population and appears to be underreported in most of the state. The most widespread of the 68 species reported were Rosa multiflora (Multiflora Rose), Elaeagnus umbellate (Autumn Olive), Morus alba (Mulberry), Lonicera maackii (Amur Honeysuckle), Albizia julibrissin (Mimosa), Euonymus alatus, and Ligustrum sinense (Common Chinese Privet). An additional 17 were identified as ones that may become widespread in the future. Families with the largest number of species were Rosaceae, Caprifoliaceae, and Oleaceae. Most shrubs originated in eastern Asia, were introduced in the 19th century, have animal-dispersed fruit, reproduce vegetatively, are at l...

Soil chemistry and microbial community functional responses to invasive shrub removal in mixed hardwood forests

The biology of headwater streams is intimately linked to that of the surrounding terrestrial environment through organic matter subsidies. Lonicera maackii, an invasive shrub that is becoming abundant in headwater stream riparian areas, deposits substantial quantities of organic matter into the aquatic system. This organic material has allelopathic effects on terrestrial plants and insects, and a growing body of work suggests strong connections between L. maackii invasion and aquatic biota. Lonicera maackii deposits fruit and flowers in quantities and timings that are unique, and we tested the hypothesis that these subsidies would negatively affect survival and growth of laboratory-cultured Hyalella azteca and field-collected Anthopotamus verticis and Allocapnia spp. Invertebrates were exposed to a gradient of fruit (reference sediment + 0, 0.31, 0.62, 1.25, or 2.5 g dry mass [DM]) and flower (reference sediment + 0, 0.30, 0.60, 1.2, or 2.4 g DM) biomass in laboratory and field sediment exposure tests. Hyalella azteca survival was significantly reduced by exposure to L. maackii fruit in the laboratory and in the field exposures, and a negative effect was observed for A. verticis (p< 0.05). Lonicera maackii flower biomass was associated with negative effects on survival of H. azteca in the field and laboratory exposures and of A. verticis in the laboratory exposure. During the laboratory exposures, dissolved O2 (DO) and pH were <2 mg/L and 5.5, respectively. In the field exposures, DO and pH were comparable to stream conditions during fruit exposures, declining significantly with increasing flower biomass. Our results suggest that L. maackii fruit and flowers, novel subsidies in these systems, can negatively affect benthic organism survival and growth. Research focused on verifying this novel subsidy hypothesis for L. maackii and other species could enhance our understanding of invasion biology and terrestrial–aquatic linkages.

The success of annual migrations for songbirds is greatly affected by habitat quality at stopover sites, particularly in relation to food needed for rapid refueling. The abundance and nutritional quality of important food resources may be linked to the presence of deciduous shrub species that provide seasonal fruits in the fall and support insects in the spring. The objective of this study was to determine whether migrating songbirds benefit from resources provided by native or invasive fruit-bearing shrubs found at 2 bird-banding stations in Rochester, NY. We conducted nutritional analyses (energy density, fat content, total soluble solids) on the fruits of common shrub species at the study sites, monitored removal of the fall fruits of focal native and invasive shrub species in the field, and measured the abundance of midges—a common insect resource for migrating songbirds— supported by the focal shrub species in the spring. The highest fat content and energy densities were found in fruits of native shrubs, ranging from 6.57 to 48.72% fat and 18.83 to 28.68 kJ/g of energy. All invasive fruits had ≤0.99% fat and ≤17.17 kJ/g of energy. We also found a significant positive correlation between fat and energy content of the fruits. Native dogwood fruits were consumed by migrating songbirds at higher rates than invasive fruits over the fall migration period. However, there was no clear pattern of midge abundance between native and invasive shrub species during the spring migration period. Our results suggest that fruits of native shrubs are of greater nutritional value to migrating songbirds than the fruits of invasive shrubs during fall migration, which is supported by the higher removal rates by songbirds of native dogwood fruits than fruits of the 4 other invasive fruit species. This finding suggests that removal of invasive fruit-bearing shrubs or plants will not negatively impact migrating birds when high-quality native fruit-bearing shrubs are available. However, additional study on the relative value of these shrubs in the spring and over multiple seasons is needed to provide insight into their overall value for birds during annual migrations.