Thinking Locally for Urban Forest Restoration: A Simple Method Links Exotic Species Invasion to Local Landscape Structure

Restoring urban forests often involves eradicating exotic species and diligently guarding against future invasions. Understanding how landscape structure contributes to the distribution of exotic species may inform these management efforts. To date, the distribution of exotic species in forested patches has been correlated with the type of development surrounding the patch, with those surrounded by agricultural or urban development often more highly invaded. Yet, previous studies have categorized land use types and have not examined more local-scale changes in land use. These local changes may be particularly important in urban areas where forested patches are immediately surrounded by diverse land use types. Our study examined how two key aspects of landscape structure, patch size and adjacent land use, may influence patterns of exotic species invasion of riparian buffers within Raleigh and Cary, North Carolina, United States. We found that large patch size alone, in our case, wide riparian buffers, does not protect against exotic species invasion. Patches surrounded by higher canopy-cover landscapes (e.g., forests and older residential developments with mature canopy) were more likely to be invaded than those surrounded by less canopy cover (e.g., shopping malls and other commercial development). We attribute these results, in part, to increased pressure from exotic propagules from adjacent forests. When restoring urban forests, attention should be paid to local land use to better plan for successful, long-term eradication of exotic species.     

Springtime in the city: exotic shrubs promote earlier greenup in urban forests

Despite the widespread recognition that urban areas are frequently dominated by exotic and invasive plants, the consequences of these changes in community structure have not been explicitly considered as an explanation for the pattern of advanced leaf phenology, or early greenup, reported in many urban areas. As such, we evaluated two hypotheses that could account for advanced greenup in forests along an urban to rural gradient: advanced phenology within individual species or differences in woody plant community. We monitored the spring leafing phenology of Aesculus glabra (Ohio buckeye), Lonicera maackii (Amur honeysuckle), and Acer negundo (box elder) in 11 forests spanning an urban to rural gradient in central Ohio, USA. From February to April 2006, we monitored these species, recorded woody plant composition, and documented daily minimum and maximum temperatures at each site. We found a weak but general trend of advanced phenology within species in more urban landscapes. Monthly average minimum temperatures were higher with increasing urbanization while monthly average maximum temperatures were similar across the urban to rural gradient. We also found evidence for shifts in woody plant communities along the urbanization gradient, mainly driven by the abundance of L. maackii, an invasive exotic species, in the more urban forests. Because L. maackii leafs out weeks earlier than native woody species and is very abundant in urban forests, we suggest that the invasion of forests by this species can generate earlier greenup of urban forests.     

Importance of scale,land‐use,and stream network properties for riparian plant communities along an urban gradient

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Contrasting habitat use of morphologically similar bat species with differing conservation status in south‐eastern Australia

The east‐coast free‐tailed bat Mormopterus norfolkensis Gray, 1839 is a threatened insectivorous bat that is poorly known and as such conservation management strategies are only broadly prescribed. Insectivorous bats that use human‐modified landscapes are often adapted to foraging in open microhabitats. However, few studies have explored whether open‐adapted bats select landscapes with more of these microhabitat features. We compared three morphologically similar and sympatric, molossid bats (genus Mormopterus) with different conservation status in terms of their association with vegetation, climate, landform and land‐use attributes at landscape and local habitat element scales. We predicted that these species would use similar landscape types, with semi‐cleared and low density urban landscapes used more than forested and heavily cleared landscapes. Additionally, we explored which environmental variables best explained the occurrence of each species by constructing post‐hoc models and habitat suitability maps. Contrary to predictions, we found that the three species varied in their habitat use with no one landscape type used more extensively than other types. Overall, M. norfolkensis was more likely to occur in low‐lying, non‐urban, riparian habitats with little vegetation cover. Mormopterus species 2 occupied similar habitats, but was more tolerant of urban landscapes. In contrast, Mormopterus species 4 occurred more often in cleared than forested landscapes, particularly dry landscapes with little vegetation cover. The extensive use of coastal floodplains by the threatened M. norfolkensis is significant because these habitats are under increasing pressure from human land‐uses and the predicted increase in urbanization is likely to further reduce the amount of suitable habitat.     

Removal of invasive shrubs reduces exotic earthworm populations

Invasive species are a leading threat to native ecosystems, and research regarding their effective control is at the forefront of applied ecology. Exotic facilitation has been credited with advancing the success of several aggressive invasive species. Here, we suggest using the knowledge of exotic facilitations to control invasive earthworm populations. In northern hardwood forests, the invasive shrubs Rhamnus cathartica (buckthorn) and Lonicera x bella (honeysuckle) produce high quality leaf litter, and their abundance is positively correlated with exotic earthworms, which increase nutrient cycling rates. We performed an invasive plant removal experiment in two northern hardwood forest stands, one dominated by buckthorn and the other by honeysuckle. Removal of invasive shrubs reduced exotic earthworm populations by roughly 50% for the following 3 years. By targeting invasive species that are part of positive feedback loops, land managers can multiply the positive effects of invasive species removal.     

Foliar production and decomposition rates in urban forests invaded by the exotic invasive shrub, <Emphasis Type="Italic">Lonicera maackii</Emphasis>

Exotic invasive shrubs can form dense monocultures in forest understories, which can have cascading effects on ecosystem structure and function. Amur honeysuckle, an exotic shrub that forms dense canopies in eastern forests, has the potential to alter plant community structure and ecosystem functions, such as primary production and decomposition. The goal of this study was to examine foliar productivity and leaf litter decomposition in forests invaded by Amur honeysuckle (Lonicera maackii) and to determine the extent to which the presence of this dominant exotic species may alter ecosystem function in these forests. We found that forests invaded by Amur honeysuckle had 16 times greater honeysuckle foliar biomass and 1.5 times lower total foliar biomass than forests of equivalent tree basal area, but having few honeysuckle shrubs. This suggests that productivity of native tree and shrub species may be reduced where honeysuckle density is high. Additionally, honeysuckle litter decayed four times faster and released nitrogen more rapidly than sugar maple litter, and sugar maple litter decayed 19% faster in forests invaded by Amur honeysuckle. These findings suggest that forests invaded by Amur honeysuckle may exhibit lower rates of organic matter accrual and less nitrogen retention in the forest floor. Since honeysuckle leaves develop in early spring before those of other shrubs or trees in the area, the rapid release of nitrogen from honeysuckle litter that we measured in early spring is timed to benefit this invasive species. The temporally coincident phenologies of nitrogen release during decomposition with the foliar growth needs of this shrub indicates that a potential positive feedback loop may exist between these processes that promotes continued growth and dominance of honeysuckle shrubs in these forested systems.     

Spreading messages about invasives

Although control of invasive species remains a common part of ecological restoration efforts, there is a growing dialogue within scientific and conservation communities regarding positive influences of invaders and potential negative consequences of their removal. As one example, a recent Diversity & Distributions article cautions that removal of exotic and invasive honeysuckle (Lonicera spp.) may negatively affect populations of frugivorous birds and, therefore, may have undesirable ecological outcomes. In response, I share several insights from research in my lab on bird‐honeysuckle interactions that show how honeysuckle disproportionately impacts birds of conservation concern and acts as an ecological trap even for generalist species. Although there is a real need to fully consider both positive and negative consequences of invasive species, if such research is not placed within the proper ecological context, we risk sending distorted or mixed messages to managers.     

Logging,exotic plant invasions,and native plant reassembly in a lowland tropical rain forest

Habitat modification and biological invasions are key drivers of global environmental change. However, the extent and impact of exotic plant invasions in modified tropical landscapes remain poorly understood. We examined whether logging drives exotic plant invasions and whether their combined influences alter understory plant community composition in lowland rain forests in Borneo. We tested the relationship between understory communities and local‐ and landscape‐scale logging intensity, using leaf area index (LAI) and aboveground biomass (AGB) data from 192 plots across a logging‐intensity gradient from primary to repeatedly logged forests. Overall, we found relatively low levels of exotic plant invasions, despite an intensive logging history. Exotic species were more speciose, had greater cover, and more biomass in sites with more local‐scale canopy loss. Surprisingly, though, exotic species invasion was not related to either landscape‐scale canopy loss or road configuration. Moreover, logging and invasion did not seem to be acting synergistically on native plant composition, except that seedlings of the canopy‐dominant Dipterocarpaceae family were less abundant in areas with higher exotic plant biomass. Current low levels of invasion, and limited association with native understory community change, suggest there is a window of opportunity to manage invasive impacts. We caution about potential lag effects and the possibly severe negative impacts of exotic plant invasions on the long‐term quality of tropical forest, particularly where agricultural plantations function as permanent seed sources for recurrent dispersal along logging roads. We therefore urge prioritization of strategic management plans to counter the growing threat of exotic plant invasions in modified tropical landscapes.     

A multi-scale assessment of the occurrence of exotic plants on the Olympic Peninsula,Washington

Abstract. Exotic plants were surveyed in 208 plots within the Dungeness and Hoh river watersheds on the Olympic Peninsula, Washington, USA. Landscape patch types included uplands (clearcuts, young and mature forests) and riparian zones (cobble bars, shrub patches, riparian forests, and alder flats). Patterns of exotic plant invasions were assessed between watersheds, between riparian and upland areas, among patch types, and within clearcuts. 52 exotic plant species were encountered, accounting for 23% of the flora in each watershed. In both watersheds, exotic species richness was approximately 33% greater in riparian zones than in uplands, and mean number and cover of exotic species were > 50% greater in riparian zones than in uplands. Among landscape patch types, richness and mean number and cover of exotics was highest in young riparian patches, intermediate in clearcuts and riparian forests, and lowest in young and mature forests. The exception to this was Hoh alder flats, which had the highest mean cover of exotic plants. Cover of exotic plants peaked in uplands 3 to 7 yr after clearcutting, then decreased with increased canopy closure. Disturbance type and time since disturbance were major factors influencing invasibility. Landscape patch size, position within watershed (distance from patch to human population centers, major highway, or river mouth), and environmental variables (slope, aspect, and elevation) were not important indicators of landscape patch invasibility. Riparian zones facilitated movement of exotic plants through landscapes, but did not appear to act as sources of exotic plants for undisturbed upland areas.     

Urban-associated habitat alteration promotes brood parasitism of Acadian Flycatchers

ABSTRACT.   High rates of brood parasitism are generally associated with agricultural landscapes, but recent evidence suggests that urbanization may also increase the likelihood of brood parasitism. I evaluated the extent to which brood parasitism by Brown-headed Cowbirds ( Molothrus ater ) was explained by differences in (1) body size of adult hosts, presumably relating to the ability to defend nest from cowbirds, (2) nest placement in substrate and relative to habitat edges, (3) habitat structure surrounding nests, (4) host density, (5) cowbird abundance, both absolute and relative to host numbers, (6) landscape composition, and (7) Julian date. From 2001 to 2006, I monitored nest fate and measured vegetation characteristics surrounding nests of Acadian Flycatcher ( Empidonax virescens ) breeding in mature riparian forests in central Ohio, USA. The likelihood that a nest would be parasitized was best explained by the number of understory stems surrounding the nest and, to a lesser extent, by the amount of urbanization in the surrounding 1-km-radius landscape. Parasitized nests were surrounded by 1.6 times more stems and nearly twice the amount of urbanization than nonparasitized nests. Numbers of understory stems were positively associated with increasing urbanization, primarily due to invasion of urban forests by Amur honeysuckle ( Lonicera maackii ). Thus, urban-associated changes in habitat characteristics around nests may be important contributors to the greater vulnerability of urban nests to brood parasitism than nests in more rural landscapes. This pattern suggests that ecological restoration, such as removing exotic shrubs, may be an effective strategy to ameliorate certain negative consequences of urbanization near wooded reserves.     

Inferring relationships between native plant diversity and Lonicera japonica in upland forests in north Mississippi,USA

Question: Do anthropogenic disturbances interact with local environmental factors to increase the abundance and frequency of invasive species, which in turn exerts a negative effect on native biodiversity? Location: Mature Quercus‐Carya and Quercus‐Carya‐Pinus (oak‐hickory‐pine) forests in north Mississippi, USA. Methods: We used partial correlation and factor analysis to investigate relationships between native ground cover plant species richness and composition, percent cover of Lonicera japonica, and local and landscape‐level environmental variables and disturbance patterns in mature upland forests. We directly measured vegetation and environmental variables within 34 sampling subplots and quantified the amount of tree cover surrounding our plots using digital color aerial photography. Results: Simple bivariate correlations revealed that high species richness and a high proportion of herbs were associated with low Lonicera japonica cover, moist and sandy uncompacted soils, low disturbance in the surrounding landscape, and periodic prescribed burning. Partial correlations and factor analysis showed that once we accounted for the environmental factors, L japonica cover was the least important predictor of composition and among the least important predictors of species richness. Hence, much of the negative correlation between native species diversity and this invasive species was explained by soil texture and local and landscape‐level land‐use practices. Conclusions: We conclude that negative correlations between the abundance of invasive species and native plant diversity can occur in landscapes with a gradient of human disturbance, regardless of whether there is any negative effect of invasive species on native species.     

Deposition of exotic bird-dispersed seeds into three habitats of a fragmented landscape in the northeastern United States

Dispersal by frugivorous birds facilitates invasion by many exotic plants. We measured the seed rain of ornithochorous native and exotic plants at three habitats of a fragmented landscape of the northeastern United States for 1 year. We studied maple-beech forests, old fields, and abandoned conifer plantations. Across all sites we collected 2,196 ornithochorous seeds, including seeds from six exotic species and 10 native species. The majority (90%) of collected seeds were from exotic species. Seed dispersal was broadly similar among habitats, though seed rain of exotic species was higher in old fields than forested habitats. Seed rain was not strongly influenced by artificial perches for most species. However, seeds of exotic species were more commonly found in traps under an artificial perch in old fields. Seed rains for the exotic Elaeagnus umbellata, Rhamnus cathartica, and Rosa multiflora were positively associated with local density of mature plants. Seed rain of R. cathartica was positively associated with abundance of seedlings but not saplings, suggesting that post-dispersal mortality was important. Seed dispersal of the exotic Lonicera spp. was high in all habitats, accounting for 66% of all seeds collected. With the exception of Lonicera spp., seed rain of common exotic invaders was affected by the abundance of seed sources, and these species might be effectively controlled by elimination of local fruiting plants. Fruits of Lonicera morrowii, which has extensively invaded our area, are apparently a common component in the diet of frugivores.     

Human activity and habitat type affect perceived predation risk in urban white‐footed mice (Peromyscus leucopus)

Predation risk is one of the largest costs associated with foraging in small mammals. Small mammals select microhabitat features such as tree and shrub canopy cover, woody debris and vegetative ground cover that can lower the risk of detection from predators and provide greater protection if discovered. Small mammals also increase foraging activity and decrease selection for cover when cloud cover increases and moon illumination is less. Often researchers assume small mammals in urban areas respond to these cues in the same manner as in natural areas, but these cues themselves are altered in urban zones. In this study, we investigated how Amur honeysuckle (Lonicera maackii) and coarse woody debris (CWD) affected giving‐up density (GUD) in white‐footed mice (Peromyscus leucopus). Each of three habitat treatments (open flood channel, the edge and interior of the honeysuckle patch) contained cover treatments with coarse woody debris present or absent. The six treatment combinations were compared to environmental variables (temperature, humidity and illumination) and habitat variables to test their effect on GUD. Peromyscus leucopus foraged to lower densities in areas with CWD present and also under the honeysuckle canopy, using this invasive shrub to decrease predation risk, potentially increasing survivability within this urban park. Increased human presence negatively affected foraging behavior across treatments. Human presence and light pollution significantly influenced P. leucopus, modifying their foraging behavior and demonstrating that both fine‐ and coarse‐scale urban factors can affect small mammals. Foraging increased as humidity increased, particularly under the honeysuckle canopy. Changes in illumination due to moonlight and cloud cover did not affect foraging behavior, suggesting urban light pollution may have altered behavioral responses to changes in light levels. Lonicera maackii seemed to facilitate foraging in P. leucopus, even though it adversely affects the plant community, suggesting that its impact may not be entirely negative.     

Do riparian reserves support dung beetle biodiversity and ecosystem services in oil palm-dominated tropical landscapes?

Agricultural expansion and intensification are major threats to global biodiversity, ecological functions, and ecosystem services. The rapid expansion of oil palm in forested tropical landscapes is of particular concern given their high biodiversity. Identifying management approaches that maintain native species and associated ecological processes within oil palm plantations is therefore a priority. Riparian reserves are strips of forest retained alongside rivers in cultivated areas, primarily for their positive hydrological impact. However, they can also support a range of forest‐dependent species or ecosystem services. We surveyed communities of dung beetles and measured dung removal activity in an oil palm‐dominated landscape in Sabah, Malaysian Borneo. The species richness, diversity, and functional group richness of dung beetles in riparian reserves were significantly higher than in oil palm, but lower than in adjacent logged forests. The community composition of the riparian reserves was more similar to logged forest than oil palm. Despite the pronounced differences in biodiversity, we did not find significant differences in dung removal rates among land uses. We also found no evidence that riparian reserves enhance dung removal rates within surrounding oil palm. These results contrast previous studies showing positive relationships between dung beetle species richness and dung removal in tropical forests. We found weak but significant positive relationships between riparian reserve width and dung beetle diversity, and between reserve vegetation complexity and dung beetle abundance, suggesting that these features may increase the conservation value of riparian reserves. Synthesis and applications: The similarity between riparian reserves and logged forest demonstrates that retaining riparian reserves increases biodiversity within oil palm landscapes. However, the lack of correlation between dung beetle community characteristics and dung removal highlights the need for further research into spatial variation in biodiversity–ecosystem function relationships and how the results of such studies are affected by methodological choices.     

Evidence of biotic resistance to exotic plant invasion in degraded Bornean forests

Intact tropical forests are generally considered to be resistant to invasions by exotic species, although the shrub Clidemia hirta (Melastomataceae) is highly invasive in tropical forests outside its native range. Release from natural enemies (e.g., herbivores and pathogens) contributes to C. hirta invasion success where native melastomes are absent, and here we examine the role of enemies when C. hirta co-occurs with native Melastomataceae species and associated herbivores and pathogens. We study 21 forest sites within agricultural landscapes in Sabah, Malaysian Borneo, recording herbivory rates in C. hirta and related native Melastoma spp. plants along two 100-m transects per site that varied in canopy cover. Overall, we found evidence of enemy release; C. hirta had significantly lower herbivory (median occurrence of herbivory per plant = 79% of leaves per plant; median intensity of herbivory per leaf = 6% of leaf area) than native melastomes (93% and 20%, respectively). Herbivory on C. hirta increased when closer to native Melastoma plants with high herbivory damage, and in more shaded locations, and was associated with fewer reproductive organs on C. hirta. This suggests host-sharing by specialist Melastomataceae herbivores is occurring and may explain why invasion success of C. hirta is lower on Borneo than at locations without related native species present. Thus, natural enemy populations may provide a “biological control service” to suppress invasions of exotic species (i.e., biotic resistance). However, lower herbivory pressures in more open canopy locations may make highly degraded forests within these landscapes more susceptible to invasion.     

The Importance of Soil Characteristics in Determining Survival of First‐Year Cottonwood Seedlings in Altered Riparian Habitats

Most major rivers in the southwestern United States have been hydrologically altered to meet human needs. Altered hydrological regimes have been associated with declines in native riparian forests. Today, many riparian areas have little or no regeneration of native riparian species and are now dominated by exotic Saltcedar (Tamarix chinensis Lour.). Success of riparian restoration efforts at least partially depends on the number of seedlings surviving the first growing season. Seedling survival is influenced by many abiotic and biotic factors including competition from other plants and available soil moisture, which is partially dependent on soil texture. In this study, we evaluated the relative importance of four soil categories (sandy loam, loam, silt, and clay), rate of soil moisture decline, salinity, beginning‐ and end‐season Saltcedar density, initial Cottonwood (Populus deltoides Marshall subsp. wislizenii (Wats.) Eckenw.) seedling density, percent vegetation cover by potential dominant competitors Pigweed (Amaranthus L.) and Barnyard grass (Echinochloa crusgalli L., Beauv.), and average total vegetation height to Cottonwood seedling survival. Factors influencing seedling survival differed among the four soil types. Rate of moisture decline was important in sandy soils, whereas vegetation height influenced seedling survival in loamy soils. Overall, models of seedling survival in all the four soil types indicated rate of moisture decline as the single most important variable influencing Cottonwood survival. High initial densities of Saltcedar were correlated to higher survival in Cottonwood seedlings. Therefore, it is important to identify soil texture and understand soil moisture decline rates when proposing riparian Cottonwood restoration.     

Potential interactions between invasive woody shrubs and the gypsy moth (Lymantria dispar), an invasive insect herbivore

As the range of the invasive and highly polyphagous gypsy moth (Lymantria dispar) expands, it increasingly overlaps with forest areas that have been subject to invasion by non-native shrubs. We explored the potential for interactions between these co-occurring invasions through a gypsy moth feeding trial using the following three highly invasive, exotic shrubs: honeysuckle (Lonicera maackii), privet (Ligustrum sinense) and burning bush (Euonymus alatus). We compared these with two native shrubs: spicebush (Lindera benzoin) and pawpaw (Asimina triloba). We fed gypsy moth caterpillars foliage exclusively from one of the five shrubs and measured their relative consumptive rate (RCR), relative growth rate (RGR), and development time (DT). The RCR of gypsy moth was strongly influenced by the species of foliage (F = 31.9; P < 0.0001) with little or no consumption of honeysuckle and privet. Caterpillar RGR was influenced by the shrub species (F = 66.2; P < 0.0001), and those caterpillars fed spicebush, honeysuckle or privet lost weight over the course of the assay. Caterpillar DT was also significantly (F = 11.79, P < 0.0001) influenced by the shrub species and those fed honeysuckle, privet and spicebush died prior to molting. Overall, our data suggest that honeysuckle, privet, and spicebush could benefit (indirectly) from the invasion of gypsy moth, while burning bush and pawpaw could be negatively impacted due to direct effects (herbivory). Similarly, invading gypsy moth populations could be sustained on a shrub layer of burning bush and pawpaw in the event of canopy defoliation. Further field and laboratory analysis is needed to clarify herbivore resistance of invasive shrubs, and to investigate the potential interactions among co-occurring insect and plant invasions.     

Assessing the impact of local habitat variables and landscape context on riparian birds in agricultural,urbanized, and native landscapes

Large tracts of natural habitat are being replaced by agriculture and urban sprawl in Mediterranean regions worldwide. We have limited knowledge about the effects of human activities on native species in these landscapes and which, if any, management practices might enhance the conservation of native biodiversity within them. Through a citizen volunteer bird-monitoring project, we compared bird abundance and species richness in northern Californian riparian zones surrounded by vineyards, urban areas, and natural areas. We assessed both local and landscape-level variables that may enhance native bird diversity in each land use type. We also demonstrate a new statistical approach, generalized estimating equations, to analyze highly variable data, such as that collected by volunteers. Avian abundance was highly correlated with both landscape context and local habitat variables, while avian richness was correlated with local habitat variables, specifically shrub richness, and percent of tree cover. In particular, shrub species richness has a strong positive correlation with riparian-preferring bird species. This suggests that active local management of riparian zones in human-dominated landscapes can increase our ability to retain native bird species in these areas.     

Spatial heterogeneity and scale‐dependent habitat selection for two sympatric raptors in mixed‐grass prairie

Sympatric predators are predicted to partition resources, especially under conditions of food limitation. Spatial heterogeneity that influences prey availability might play an important role in the scales at which potential competitors select habitat. We assessed potential mechanisms for coexistence by examining the role of heterogeneity in resource partitioning between sympatric raptors overwintering in the southern Great Plains. We conducted surveys for wintering Red‐tailed hawk (Buteo jamaicensis) and Northern Harrier (Circus cyanea) at two state wildlife management areas in Oklahoma, USA. We used information from repeated distance sampling to project use locations in a GIS. We applied resource selection functions to model habitat selection at three scales and analyzed for niche partitioning using the outlying mean index. Habitat selection of the two predators was mediated by spatial heterogeneity. The two predators demonstrated significant fine‐scale discrimination in habitat selection in homogeneous landscapes, but were more sympatric in heterogeneous landscapes. Red‐tailed hawk used a variety of cover types in heterogeneous landscapes but specialized on riparian forest in homogeneous landscapes. Northern Harrier specialized on upland grasslands in homogeneous landscapes but selected more cover types in heterogeneous landscapes. Our study supports the growing body of evidence that landscapes can affect animal behaviors. In the system we studied, larger patches of primary land cover types were associated with greater allopatry in habitat selection between two potentially competing predators. Heterogeneity within the scale of raptor home ranges was associated with greater sympatry in use and less specialization in land cover types selected.