David L. Gorchov

Forest floor plant community response to experimental control of the invasive biennial, Alliaria petiolata (garlic mustard)1

Abstract The impact of invasive plant species on native plants is largely assumed to be negative, but supporting evidence is sparse. A common control method of non-native plants is herbicide application, but little is known about the effects of these chemicals on non-target plant populations, or differences in these populations before and after control measures are taken. We examined the response of the forest floor plant community to herbicide-mediated reduction of Alliaria petiolata in an old-growth and a second-growth forest stand in Hueston Woods State Park, Preble and Butler Counties, OH. Fifty 1 × 1 m plots were established in each stand, and 25 plots per stand were treated with Round-up© each November 2000–2004, which reduced cover of adult Alliaria petiolata but did not suppress recruitment. Percent cover of herbs and woody plants ≤0.85 m tall was assessed in May and June, 2000–2005. To determine compositional differences between sprayed and unsprayed plots in each stand we ordinated plots based on peak cover of each species using nonmetric multidimensional scaling, tested for differences in community composition with a multiple response permutation procedure, and compared total cover of growth forms with Kruskal-Wallis tests. Five years of Alliaria petiolata control only modestly affected the forest floor vegetation. Neither species richness nor diversity differed significantly between sprayed and unsprayed treatments in any year of the study. Community composition differed each year between stands, but treatments differed significantly only in 2002 (in the second-growth stand), and marginally in 2004 and 2005 (in the old-growth stand). Treatment affected cover of some growth forms during the study, but only in some years: in the second-growth stand sprayed plots had significantly greater cover of spring perennials and graminoids in 2003 and marginally lower cover of annuals in 2005; in the old-growth stand sprayed plots had marginally more spring perennials in 2005. Wintergreen species, particularly the exotic annual Stellaria media, had lower cover in sprayed plots in the old-growth stand in 2005. We attribute the compositional differences we observed in the forest floor community to competitive impacts of Alliaria petiolata, but suggest that effects were modest due to the persistence of rosettes in the sprayed plots.

White-tailed deer (Odocoileus virginianus) disperse seeds of the invasive shrub, Amur Honeysuckle (Lonicera maackii).

ABSTRACT: White-tailed deer (Odocoileus virginianus Zimmerman) are known to disperse seeds of a hybrid complex of invasive honeysuckle shrubs in northeastern United States. We investigated whether they also disperse seeds of Amur honeysuckle, Lonicera maackii, a problematic invasive shrub in the Midwestern and eastern U.S. We found that deer ingest ripe fruit and void intact seeds of L. maackii. Based on tetrazolium tests, most (68%) of these voided seeds are viable, but the proportion viable was significantly lower than that for seeds taken directly from ripe fruits. White-tailed deer are potentially important in the long-distance dispersal of this invasive shrub.

Source-sink dynamics within a plant population: the impact of substrate and herbivory on palm demography

Site factors have frequently been shown to affect survival, growth, and reproduction in plant populations. The source-sink concept proposed by Pulliam is one way of integrating this spatial demographic variation into population models. Source-sink models describe a population where propagules from “source” habitats sustain less productive “sink” areas. We adapted this concept to model the population dynamics of the understory palm Chamaedorea radicalis on two substrates, rock outcrops and forest floor. In our model, sources and sinks correspond to fine-scale demographic structure within the population, rather than spatially discrete subpopulations as described in the Pulliam model. We constructed a stage-structured population matrix model that integrates the site-specific demography of individuals across two habitats types that are linked by migration. We then parameterized this model with field data from C. radicalis. To address whether observed differences in palm demography between rock outcrops and the forest floor were due to natural variation between microsites or due to differences in browsing intensity from free range livestock, we parameterized separate models based on the substrate-specific demography of protected, non-browsed palms and of palms exposed to burro browse. Results showed that herbivory reduced survival and fecundity on the forest floor, which in the absence of seed migration resulted in a projected decline of forest floor palms (sinks). However with seed dispersal, palms persisted and total population growth (both substrates) was projected to be positive, indicating that seed dispersal from non-browsed palms on rock outcrops (sources) was sufficient to sustain C. radicalis on the forest floor.

Invasion of an Exotic Shrub into Forested Stands in an Agricultural Matrix

Abstract We investigated the relative importance of stand and landscape characteristics in the invasion of a nonnative shrub, Amur honeysuckle, in 40 woodlots in an agricultural matrix in southwest Ohio. We quantified stand characteristics that could influence invasibility, the intrinsic susceptibility of an area to invasion, including woodlot size, perimeter-to-area ratio, tree basal area, and stand age. At the landscape scale we included factors that potentially influence propagule rain (the contribution of seeds from individuals established outside the focal area), including the land cover and road density in a 1,500-m buffer around each woodlot, as well as the extent to which the perimeter was forested at two points in the past, and latitude (based on an apparent south-to-north invasion in this region). Based on stepwise regression, we determined that honeysuckle cover was determined primarily by landscape parameters, particularly the percent of the buffer comprised of cropland. Woodlots surrounded by more cropland had less honeysuckle cover, which we attribute to paucity of nearby seed sources and/or minimal movement of seed-dispersing animals. From these findings, we argue that impediments to propagule rain are more important in shaping the invasion of this exotic shrub than are characteristics of the woodlots themselves, i.e., community invasibility. Nomenclature: Amur honeysuckle, Lonicera maackii (Rupr.) Herder. Management Implications: Understanding the factors that make forests susceptible to invasion can inform management strategies, including identifying forest stands at greatest risk of invasion and formulating steps that can be taken to minimize invasion risk. We investigated what stand and landscape characteristics best explained the cover of the invasive shrub Lonicera maackii (Amur honeysuckle), in a landscape it recently invaded, consisting of woodlots in an agricultural matrix in southwest Ohio. We found that cover of this shrub was best explained by landscape characteristics, rather than by stand characteristics, such as age or basal area. Specifically, the percentage of the 1,500-m buffer around the woodlot that was comprised of cropland, as opposed to pasture, forest, and other land-cover types, was the best predictor of honeysuckle cover. Woodlots surrounded by more cropland had lower cover, which we think indicates more recent colonization. Thus cropland impedes honeysuckle invasion, either by providing a buffer free of seed sources (fruiting shrubs), or a land cover that is unlikely to be crossed by animals dispersing seeds from more distant sources. These findings suggest that woodlots surrounded by cropland, and perhaps by other shrub-free land covers, are at lower risk of invasion by animal-dispersed nonnative plants, and that active management of buffers around forest stands will reduce invasion risk.

Long-Distance Dispersal and Diffusion in the Invasion of Lonicera maackii

Abstract To investigate the relative importance of long-distance dispersal vs. diffusion in the invasion of a nonnative plant, we used age structure to infer the contribution to recruitment of external propagule rain vs. within-population reproduction. We quantified the age structure of 14 populations of Amur honeysuckle in a landscape where it recently invaded, in Darke County, OH. We sampled the largest honeysuckle individuals in each population (woodlots), and aged these by counting annual rings in stem cross sections. Individuals in the oldest four 1-yr age classes are assumed to be from external recruitment, given the minimum age at which shrubs reproduce. We used these recruitment rates to model external recruitment over the next 5 yr and used observed age structures to estimate total recruitment. We used the difference between total and external recruitment to infer the rate of internal recruitment. Our findings indicate that recruitment from within the population is of about the same magnitude as immigration in the fifth to seventh year after population establishment, but by years 8 to 9 internal recruitment dominates. At the landscape scale, the temporal-spatial pattern of population establishment supports a stratified dispersal model, with the earliest populations establishing in widely spaced woodlots, about 4 km from existing populations, and these serving as “nascent foci” for diffusion to nearby woodlots. Understanding the relative importance of long-distance dispersal vs. diffusion will inform management, e.g., whether it is more effective to scout for isolated shrubs or remove reproducing shrubs at the edge of invaded areas. Nomenclature: Amur honeysuckle, Lonicera maackii (Rupr.) Herder. Management Implications: The relative importance of diffusion (expanding front) vs. long-distance dispersal can inform management of invasive species. If diffusion dominates, it will be most efficient to monitor for, and eradicate, new patches on the edges of existing patches. If long-distance dispersal dominates, one should scout for, and eradicate, initial colonists in previously uncolonized patches (Moody and Mack 1988). Furthermore, removing reproductive individuals at the edge of the current range would slow the spread of diffusing populations, but have little effect if long-distance dispersal dominates. We assessed the relative importance of diffusion vs. long-distance dispersal in the invasion of Amur honeysuckle [Lonicera maackii (Rupr.) Herder] in forested patches in an agricultural landscape in southwest Ohio. We examined the age structures of woodlot populations, used these to estimate the importance of immigration vs. within-population recruitment, and examined the temporal-spatial pattern of population initiation. Our findings indicate that long-distance dispersal dominates early in the invasion, and new populations grow slowly, until the original colonists begin reproducing. Thus, efforts to slow the spread of L. maackii should involve scouting for colonists in woodlots up to 4–5 km from existing populations. Fortunately, this is feasible, as even small honeysuckle shrubs are easily spotted during early spring and late fall, when native deciduous woody plants are not in leaf. These searches could be as infrequent as every 3 yr, given the lack of reproduction in the youngest age classes.

Patterns of Coarse Woody Debris in Hardwood Forests across a Chronosequence of Ash Mortality Due to the Emerald Ash Borer (Agrilus planipennis)

ABSTRACT: The invasive emerald ash borer (Agrilus planipennis) (EAB) is causing widespread ash (Fraxinus spp.) mortality in 25 U.S. states and two Canadian provinces. We investigated the impact of EAB on coarse woody debris (CWD) volume across 24 sites in western and central Ohio, USA, representing a chronosequence of ash mortality, quantified by the year ash mortality reached 25% (Year25%Dead). CWD volume averaged 60.36 m3/ha, and was positively associated with total (live plus dead) ash basal area (BA), but was not greater in sites where ash death occurred earlier. For the volume of CWD in the first, least-rotted, decay class, stepwise regression revealed that both ash BA and Year25%Dead were significant predictors; sites with more ash BA and where ash mortality had occurred earlier had more class 1 CWD. Additionally, class 1 CWD in those early mortality sites was primarily (87%) ash, compared to 40% ash in sites with more recent ash mortality. This large influx of CWD, particularly ash CWD, combined with future inputs from ash that are still standing, will elevate CWD volume in the near future, especially in sites with greater ash basal area.

The effect of emerald ash borer-caused tree mortality on the invasive shrub Amur honeysuckle and their combined effects on tree and shrub seedlings

Invasive insects and plants are major threats to the health and viability of North American forests. Emerald ash borer (Agrilus planipennis) (EAB) may cause extensive changes to forest composition due to rapid ash (Fraxinus spp.) mortality. Invasive shrubs like Amur honeysuckle (Lonicera maackii) may benefit from EAB and have negative effects on woody seedlings. We predict that ash mortality has positive effects on seedling abundance, recruitment, and survival, but that these effects are influenced by L. maackii basal area and/or cover. We sampled 16 sites, representing a chronosequence of ash mortality throughout western Ohio. We tested whether L. maackii growth and fecundity varied in relation to ash decline. We also investigated effects of ash decline, stand basal area (BA), L. maackii BA and percent cover on woody seedling abundance, recruitment, and survival using linear mixed models evaluated with Akaike’s Information Criterion. These same responses were also investigated for four seedling groups: L. maackii, invasive plants (excluding L. maackii), shade tolerant natives, and shade intolerant natives. We found a significant positive relationship between ash decline and L. maackii BA growth. Lower seedling species richness corresponded with greater L. maackii BA and better ash condition. Greater L. maackii BA was also associated with lower seedling abundance and recruitment, as well as abundance and recruitment of shade-tolerant species, and recruitment of shade-intolerant species. Sites with poorer ash condition and greater L. maackii BA had more L. maackii seedlings. These findings indicate that the negative effects of L. maackii are more important to future forest composition than ash decline; however ash decline increases L. maackii growth, hence exacerbating the effects of this invasive shrub.

White-tailed deer browse on an invasive shrub with extended leaf phenology meets assumptions of an apparent competition hypothesis

We investigated whether white-tailed deer browse on the invasive shrub, Amur honeysuckle, supports key assumptions of the hypothesis that invasives with extended leaf phenology (ELP) impact natives via “apparent competition,” elevating the impact of a shared herbivore. We found that deer browse on honeysuckle was particularly high in early spring, a time of protein scarcity. The leafy twigs of honeysuckle in early spring were higher in protein than leafless twigs of native trees. We estimated that consumption of honeysuckle twigs accounted for 14-47% of the annual food consumption of deer in the study site. These findings show that the hypothesis of apparent competition is compelling.

Are Native Tree Seedlings Facilitated by an Invasive Shrub Where White-Tailed Deer Are Abundant?

ABSTRACT: Positive interactions (facilitation) between plant species have been documented, particularly in stressful environments. We investigated whether an invasive shrub enhances growth or survival of native tree seedlings in forests where white-tailed deer (Odocoileus virginianus) are abundant. Seedlings of four tree species were planted under, or 0.5 m outside of, the canopy of Amur honeysuckle (Lonicera maackii) shrubs in plots unfenced or fenced to exclude deer in two stands in southwestern Ohio, USA, and monitored for 1 y. One species succumbed to transplant shock, but browse was extensive on the other three species in unfenced plots. Facilitation was evident on sugar maple (Acer saccharum), as seedlings under shrub cover had higher survival and final leaf count in unfenced plots, but not where deer were excluded. However, there was no significant facilitation by L. maackii of seedling growth or survival for white oak (Quercus alba) or shagbark hickory (Carya ovata), indicating this shrubs branches do not deter deer browse on highly preferred species or the effect is too modest to emerge in a one-year study. In at least some cases, the negative effects of L. maackii on tree seedlings that have been documented previously are offset by mitigation of the negative effects of deer herbivory. This suggests that where browsing impacts are problematic, deer abundance should be reduced before this, and perhaps other invasive shrubs, are managed.

Introduction to the Special Issue: Ungulates and invasive species: quantifying impacts and understanding interactions

High densities of white-tailed deer negatively impact ecosystems in parts of North America, many of which are also impacted by invasive plants. Whether plant invasions are a result of high deer population, and how deer and invasives interact in their impacts, are not well understood. This motivated a colloquium at the 2016 Botanical Society of America meeting. Nine of those presentations were expanded for this special issue of AoB PLANTS, “Interactions between white-tailed deer and invasive plants in North American forests.” This Introduction to this issue highlights the context of the problems and summarizes and synthesizes insights from these papers.