Native Plant Abundance Research Articles

Different functional characteristics can explain different dimensions of plant invasion success

Abstract The success of invasive plant species can be evaluated using different dimensions, such as, range size, abundance and impact. These different dimensions do not always covary but are rarely separated, suggesting an urgency to disentangle the functional mechanisms behind them. A dataset of leaf traits and four dimensions of invasion success (i.e. range size, local abundance, impact on native plant abundance and impact on native plant diversity) were compiled for 395 non‐native plant species in the US and Europe. Associations among dimensions of invasion success and between leaf traits and dimensions were analysed with general linear models (LMs) and supplemented by phylogenetic generalized least square (PGLS) models, which control for the phylogenetic relatedness across species. The pair‐wise associations between most pairs of invasion dimensions were weak or neutral. The only exception was the association between impact on native plant abundance and impact on native plant diversity, which was strongly positive. Traits of species that have large range sizes were associated with a high metabolic rate; whereas, traits of species that were abundant or had a strong impact at the local scale were associated with low metabolic rate. In addition, traits of species with a large range size or having strong impacts on native plant abundance were associated with acquisitive strategies; whereas, traits of species with a high local abundance or strong impacts on native plant diversity were associated with conservative strategies. Synthesis. Different dimensions of invasion success were associated with different functional traits. Invasion success at the regional scale was related to traits that promote rapid colonization; whereas, invasion success at the local scale was related to traits that are potentially less preferred by herbivores. Some locally successful invaders even possessed traits that facilitate a high stress tolerance and conservative strategy, which were similar to locally abundant native species. Therefore, an ambiguous definition of ‘invasion success’ in mechanism‐related studies may produce inconsistent or even controversial conclusions, highlighting the importance of separately studying different dimensions of invasion success.

The Effects of Integrated Vegetation Management on Richness of Native Compatible Flowering Plants and abundance of Noncompatible Tree Species on a Right-of-Way in Central Pennsylvania, USA

We examined the effects of integrated vegetation management (IVM) and nonselective mechanical removal techniques (hand cutting and mowing) on the richness and abundance of native compatible flowering plants and noncompatible trees on an electric transmission line right-of-way in central Pennsylvania, USA. Our study focused on native flowering plants to help determine how different vegetation management techniques may affect native wildlife communities. We found no correlation between amount of herbicide applied and native flowering plant species richness or tree abundance. We found that the richness of native flowering plants did not differ between plots treated with an IVM herbicide approach and those that were mechanically treated (t = 1.06, df = 1, p = 0.31). However, mechanically treated plots had significantly higher abundance of trees than IVM plots (t = 3.10, df = 1, p = 0.009). We found that plots that were treated with herbicide mixtures that contained glyphosate in 2012 had lower native flowering plant species richness in 2016 than those treated with herbicide mixtures that did not contain glyphosate (t = −2.44, df = 1, p = 0.04). Our study indicates that long-term IVM approaches support native flowering plant species richness while limiting tree abundance under electric transmission line right-of-way. However, further study is needed to determine if the herbicide type and method (selective versus broadcast) of application affects species richness of native flowering plant communities.

Patterns of invasive plant abundance in disturbed versus undisturbed forests within three land types over 16 years

AbstractAimLong‐term monitoring of forest understorey species was used to describe changes in native and invasive exotic plant abundances over time within different disturbed or undisturbed forest types. This information was then used to determine the predictive invasion model (passenger/opportunist, driver or back‐seat driver).LocationCheat Ranger District of the Monongahela National Forest, West Virginia, USA.MethodsUnderstorey vegetation of mature and clear‐cut forests was sampled every 5–6 years for 16 years. Stands were stratified across three land types following a moisture gradient. Invasive plant richness, frequency and abundance were evaluated across land type and disturbance type with general linear mixed models. Change in richness, diversity and abundance of both native and invasive non‐native plants was evaluated as a measure of impact.ResultsThe mesic mature stands had the greatest invasive plant richness, frequency and abundance. The moderately mesic clear‐cut stands initially showed the greatest invasive plant richness, frequency and abundance, but over time these values became greater for the drier clear‐cuts. The mature forests showed no change in native species abundance in response to invasion. Clear‐cut‐drier stands showed a decrease, while the more mesic stands showed an increase in native plant richness, diversity and abundance in response to invasion. The drier clear‐cuts, with increasing invasions and negative native species impacts, were indicative of the back‐seat driver model. The hitchhiker model, a new term, described increasing plant invasions with no measurable impacts.Main ConclusionsThe drier clear‐cut stands exhibit a lack of biotic resistance to invasion, unlike the mesic clear‐cut stands. Increasing invasion in the mature forests suggests that a threshold may be reached that results in impacts on the native vegetation, but with no increase in native plant abundance to help alleviate these effects.

Remaining eucalypt trees may hamper woody plant regeneration in a neotropical savanna

When an area previously used for agricultural or sylvicultural purposes is abandoned, it may undergo natural regeneration. However, the outcome of such regeneration depends on a number of factors, including the presence of non-native invasive plants, and, when production forests such as eucalypt plantations are considered, remaining non-native trees may also affect the natural regeneration possibilities. We studied how remaining eucalypt trees affect woody species regeneration and invasive grasses in an abandoned Eucalyptus grandis plantation in a regenerating Brazilian savanna site. We sampled the woody vegetation and invasive grasses in 30 8 × 8-m plots and surveyed the eucalypt trees within these plots and in 3 m-wide strips around them. Eucalypts negatively affected the cover of the C4 invasive African grass Melinis minutiflora and the total abundance and basal area of native woody plants in general and of two dominant species, suggesting an overall negative effect on native plant regeneration notwithstanding the also negative effects on African grasses. Conversely, they had no effect on species richness, species diversity nor mean height of woody plant communities nor on the abundance of three other dominant native woody species. Interestingly, a low eucalypt abundance had small effects on native woody species but much stronger effects on the cover of M. minutiflora, possibly due to shading. Thus, our results indicate that remaining eucalypt trees may hamper the natural regeneration of native woody plants, but also suggest that maintaining a low density of eucalypt trees may be a cost-effective strategy, given the high costs associated with the complete removal of remaining trees and the difficulties in controlling invasions by African grasses.

Long-term grazing removal increased invasion and reduced native plant abundance and diversity in a sagebrush grassland

Effects of long-term grazing removal on vegetation are highly variable across ecosystems and grazing contexts. In some cases, long-term exclosures can be used to enhance vegetation production, plant diversity, or wildlife habitat. In other situations, exclosures can become more invaded by undesirable species, less drought-tolerant, or less diverse than grazed sites. Grasslands in North America’s semi-arid Great Plains prairies can tolerate large herbivore grazing and resist invasion, while cold desert shrublands of the Intermountain West are more invisible and sensitive to grazing. These major ecoregions intersect in northeast Wyoming, where a diverse ecotonal plant community supports a wide array of sensitive wildlife. The effects of grazing removal on vegetation and invasive species in this ecotone are poorly understood, despite clear implications for conservation and management. In 2016, we measured effects of long-term (>49 year-old) livestock exclosures on vegetation and soil properties in the ecotone. Compared to adjacent grazed sites, exclosures had 72% less foliar cover of drought-resistant, warm-season perennial grasses, fewer native species, and nearly three times as much invasive annual grass cover. Long-term livestock removal did not affect live sagebrush shrub cover, density or size, but led to taller herbaceous vegetation structure. Findings suggest that although long-term grazing removal may be useful for achieving taller structure, light to moderate levels of large herbivore grazing may be necessary to resist invasion and maintain ecosystem integrity in this ecotone. In the context of ongoing global change, it will be critical to maintain natural disturbance regimes such as grazing in ecosystems that evolved with disturbance. In these systems, a total lack of natural disturbance does not represent a reasonable target or reference state. Instead, management for a range of disturbance frequencies and intensities across the landscape will likely support high vegetation heterogeneity, which in turn will help conserve high biodiversity and ecosystem function.

A native annual forb locally excludes a closely related introduced species that co-occurs in oak-savanna habitat remnants.

Despite the ubiquity of introduced species, their long-term impacts on native plant abundance and diversity remain poorly understood. Coexistence theory offers a tool for advancing this understanding by providing a framework to link short-term individual measurements with long-term population dynamics by directly quantifying the niche and average fitness differences between species. We observed that a pair of closely related and functionally similar annual plants with different origins—native Plectritis congesta and introduced Valerianella locusta—co-occur at the community scale but rarely at the local scale of direct interaction. To test whether niche and/or fitness differences preclude local-scale long-term coexistence, we parameterized models of competitor dynamics with results from a controlled outdoor pot experiment, where we manipulated densities of each species. To evaluate the hypothesis that niche and fitness differences exhibit environmental dependency, leading to community-scale coexistence despite local competitive exclusion, we replicated this experiment with a water availability treatment to determine if this key limiting resource alters the long-term prediction. Water availability impacted population vital rates and intensities of intraspecific versus interspecific competition between P. congesta and V. locusta. Despite environmental influence on competition our model predicts that native P. congesta competitively excludes introduced V. locusta in direct competition across water availability conditions because of an absence of stabilizing niche differences combined with a difference in average fitness, although this advantage weakens in drier conditions. Further, field data demonstrated that P. congesta densities have a negative effect on V. locusta seed prediction. We conclude that native P. congesta limits abundances of introduced V. locusta at the direct-interaction scale, and we posit that V. locusta may rely on spatially dependent coexistence mechanisms to maintain coexistence at the site scale. In quantifying this competitive outcome our study demonstrates mechanistically how a native species may limit the abundance of an introduced invader.

Native plant abundance, diversity, and richness increases in prairie restoration with field inoculation density of native mycorrhizal amendments

Ecological restoration efforts can increase the diversity and function of degraded areas. However, current restoration practices cannot typically reestablish the full diversity and species composition of remnant plant communities. Restoration quality can be improved by reintroducing key organisms from the native plant microbiome. In particular, root symbionts called arbuscular mycorrhizal (AM) fungi are critical in shaping grassland communities, but are sensitive to anthropogenic disturbance, which may pose a problem for grassland restoration. Studies of mycorrhizal amendments include inoculation densities of 2–10,000 kg of inocula per hectare. These studies report variable results that may depend on inocula volume, composition, or nativeness. Here we test eight different densities of native AM fungal amendment, ranging from 0 to 8,192 kg/ha in a newly installed prairie restoration. We found that native plant establishment benefited from native mycorrhizal inocula, resulting in improvements in native plant abundance, richness, and community diversity. Moreover, the application of very low densities of native mycorrhizal inocula, as suggested on commercial mycorrhizal products, were ineffective, and higher concentrations were required to benefit native plant abundance and community diversity. These data suggest that higher densities of mycorrhizal amendment or perhaps alternative distribution methods may be required to maximize benefits of native mycorrhizal amendments in restoration practices.

Invasive plant Arundo donax alters habitat use by carnivores

Invasive plants can have significant negative interactions with native flora and fauna, often decreasing the abundance and diversity of native plants and invertebrate and vertebrate herbivores. Less is known, however, about how invasive plants influence higher-order consumers, such as carnivores. Arundo donax is a globally distributed invasive grass that forms dense monotypic stands in the habitats where it successfully establishes. This study investigated the influence of Arundo invasion on mammal assemblages in California’s Santa Clara River Valley. Specifically, we aimed to determine whether Arundo alters habitat use for carnivores, and if so, assess the possible mechanisms driving these differences. We used remote cameras to evaluate carnivore habitat use and model occupancy, and live traps and remote cameras to assay abundance of small mammals, common prey of these larger predators. We found that detections of all carnivores were significantly lower in Arundo-dominated habitat patches, suggesting decreased preference for Arundo habitat. This was reflected in the occupancy model, which found the highest probability of coyote and bobcat habitat use in native patches and lowest in Arundo; however, habitat did not seem to be as important of a predictor as in the pooled species results. Small mammal abundance was similar if not higher in Arundo, suggesting the possibility of Arundo acting as a refuge for prey species and altered predator–prey dynamics in Arundo-dominated patches. This study improves our knowledge of the often-complex ways that invasive plants, like Arundo, influence the ecology and behavior of faunal communities in invaded ecosystems.

Synergistic effects of wild ungulates and management intensification suppress native plants and promote exotics

In managed forest landscapes, conflicts among wood production, ungulates, native biodiversity, and exotic species are common. As humanity allocates more land as intensively managed plantations, these conflicts may become more severe. For instance, native ungulates have been implicated in the loss of native plant diversity, declines in timber revenues and the spread of exotic species in many forest systems, yet the synergistic effects of management and ungulate herbivory are not well understood.We hypothesized that herbicide and herbivore-induced suppression of native forage species promotes the release of exotic species in young forest plantations. Further, we expected herbivory and the retention of native forage via less intensive management would have negative consequences for reforestation objectives. To test this hypothesis, we conducted an experiment that manipulated management intensity (via herbicides) and access of two deer species to vegetation (via exclosures) on 28 operational Douglas-fir plantations of the Pacific Northwest, USA. We recorded yearly plant species cover estimates and tested the effects of herbivory and herbicides on native and exotic plants during the first six years of plantation establishment.Heavier herbicide treatments reduced the cover and diversity of native plants, and increased the cover of exotics over time, particularly that of exotic herbaceous species. Regardless of herbicide intensity, herbivores reduced the cover of native forage species. In a treatment that represents operational standards applied to over 2.5 million ha in the region, the suppression of native species by deer corresponded with an increase in exotic species cover by 23 percent, resulting in equitable abundance of native and exotic plants. As expected, Douglas-fir growth was suppressed when herbivores were present and vegetation was left untreated by herbicides, although the presence of deer promoted both crop-tree growth and the relative abundance of exotics in our most intensive treatment, presumably due to the added suppression of native competitors.Our findings suggest that wild ungulates amplify management-driven shifts toward exotic species in intensively managed forest plantations. Exotic plant forage status and adaptations to frequent disturbance seem to be key mechanisms for the synergistic effects we observed. Without herbicides, diverse assemblages of native species are buffered from herbivory and exotic species proliferation, with potential tradeoffs for timber production. Our results highlight the role of management intensity in modifying the interactions among exotic plants and native herbivores, contributing to a mechanistic understanding of the role of native biodiversity in regulating exotic species spread. Our data provide further support for growing evidence that synergistic effects of multiple drivers can facilitate exotic plant proliferation.

Vegetation change over seven years in the largest protected Pacific Northwest Bunchgrass Prairie remnant.

Temperate grasslands are one of the most altered ecosystems on Earth. Consequently, conservation of important characteristics of such ecosystems (e.g., biodiversity) is uncertain even within grasslands that have been protected. Invasion by non-native plants is considered a primary threat to intact grasslands. Here, we evaluated native and non-native vegetation composition change over seven years in the largest Pacific Northwest Bunchgrass remnant. We sampled 124 permanent plots across the Zumwalt Prairie Preserve (northeastern Oregon, USA) twice, seven years apart. With data collected from three grassland community types (xeric prairie, mesic prairie, old fields), we asked: (1) how has species composition changed over time; (2) which species showed the greatest changes in abundance; and (3) how did abundance of Ventenata dubia (the most abundant non-native species) relate to patterns of native and non-native plant abundance? Vegetation composition changed in all three plant communities. Ventenata dubia, an annual non-native grass: (1) became the third most dominant species across the study area; (2) was the only non-native that increased in abundance substantially in all three communities; and (3) was negatively related to native perennial forb cover. Relative cover of non-native species decreased in old fields concomitant with increases in native bunchgrass (Festuca idahoensis) and V. dubia cover. Increased cover of native perennial grasses and non-native annual grasses in old fields were associated with loss of bare ground, but not with reductions in non-native perennial grass cover. Native species dominated in the mesic prairie; however, non-native cover (particularly V. dubia) increased (mean cover increased from 3 to 10%) while mean native perennial forb cover decreased (from 30 to 25%) over time. Continued shifts towards non-native annual grass dominance coupled with potentially declining native perennial forbs, may challenge conservation efforts in one of the last large tracts of Pacific Northwest Bunchgrass Prairie.

Management of invasive Pinaceae is imperiled by the lack of invasive ungulate control: successful restoration requires multiple-species management

The magnitude and direction of an invasive species’ impact may be determined by its co-existence and interaction with other species. In Argentina, wild boar, red deer, and fallow deer are introduced mammals that have important negative impacts on ecosystems. In Patagonia, nonnative Pinaceae removal programs have been promoted since the 1980s to control exotic conifers and restore native vegetation. The consequences of removing exotic trees without continued management intervention may be unexpected due novel interactions between nonnative species. We evaluate habitat use of introduced ungulates, interactions with native and exotic plants, and their implications for restoration objectives in areas where pines have been removed. Exotic plant species richness and abundance was greater in clear-cuts than native forest or pine plantations. In contrast, richness and abundance of native plants was highest in native forests. Clear-cuts had the highest plant species diversity, primarily driven by high numbers of exotics, relative to native forests. Boar and deer were recorded in clear-cuts, native forests, and pine plantations, but deer were more frequently observed in clear-cuts. Removal of pines created a suitable habitat for deer. The herbivory pressure of deer, coupled with the presence and abundance of invasive plants, can prevent or hinder the establishment and development of native shrubs and trees that are required for the structure and function of the native ecosystem. Although the conditions in clear-cuts present complex challenges for natural regeneration, these sites could be an excellent opportunity for active restoration strategies, by excluding ungulates and controlling exotic plants to promote native species recovery.

Invasive shrub re-establishment following management has contrasting effects on biodiversity

Effective control of an invasive species is frequently used to infer positive outcomes for the broader ecosystem. In many situations, whether the removal of an invasive plant is of net benefit to biodiversity is poorly assessed. We undertook a 10-year study on the effects of invasive shrub management (bitou bush, Chrysanthemoides monilifera ssp. rotundata) on native flora and fauna in a eucalypt forest in south-eastern Australia. Bitou bush eradication is a management priority, yet the optimal control regime (combination of herbicide spray and fire) is difficult to implement, meaning managed sites have complex management histories that vary in effectiveness of control. Here we test the long-term response of common biodiversity indicators (species richness, abundance and diversity of native plants, birds, herpetofauna and small mammals) to both the management, and the post-management status of bitou bush (% cover). While average bitou bush cover decreased with management, bitou bush consistently occurred at around half of our managed sites despite control efforts. The relationship between biodiversity and bitou bush cover following management differed from positive, neutral or negative among species groups and indicators. Native plant cover was lower under higher levels of bitou bush cover, but the abundance of birds and small mammals were positively related to bitou bush cover. Evidence suggests that the successful control of an invader may not necessarily result in beneficial outcomes for all components of biodiversity.

Effects of Environmental Factors and Alien Plant Invasion on Native Floral Diversity in Mt. Manunggal, Cebu Island, Philippines

The status of native floral diversity in Mt. Manunggal, Cebu Island, the Philippines in relation to environmental factors and alien plant invasion has not been recorded and published. To assess factors influencing native plant communities in Mt. Manunggal, a vegetation survey of native and alien plants, as well as selected environmental variables at two sites (Site 1- invaded plot; Site 2- adjacent uninvaded plot) were measured, and anthropogenic disturbances observed using a t-test, regression analysis, and Pearson correlation. For each site, selected environmental parameters, as well as native and alien plant abundance and diversity, were determined last October-November, 2016. There was also a characterization of sites with respect to anthropogenic disturbances. Altitude (t= -14.11, p=0.000), soil temperature (t=2.68, p=0.014), and light intensity (t = -2.61; p=0.016) significantly differed between study sites due to the complex topography and geology of Mt. Manunggal, the variation in floral cover, and degree of anthropogenic activities. Among all environmental factors at both sites, altitude was highly positively correlated to alien plant abundance (r=0.709;p=0.000) at Site 1 while native plant abundance at Site 2 was negatively correlated with soil pH (r=-0.579;p=0.003).There was an apparent domination of alien plants in terms of species richness and abundance at both sites and this could have significantly affected native plant abundance. The strong correlation of environmental parameters to alien plant invasion could favor the continued dominance of alien plant species, resulting in a further decrease of native species.

Short-Term Vegetation Responses to Invasive Shrub Control Techniques for Amur Honeysuckle (Lonicera maackii [Rupr.] Herder)

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.

Effects of experimental harvesting of an invasive hybrid cattail on wetland structure and function

Invasive plants, such as the hybrid cattail Typha × glauca, can reduce biodiversity and alter the ability of wetlands to provide critical ecosystem services, including nutrient cycling and carbon storage. Several approaches have been used to reduce Typha dominance and restore invaded wetlands, but long‐term studies assessing benefits of these restoration efforts are limited. A previous study demonstrated that aboveground harvesting of Typha × glauca stems and litter reduced Typha dominance 2 years post‐treatment in a Great Lakes coastal wetland. In the current study, we extended monitoring of experimental aboveground Typha harvest to 4 years post‐treatment and added assessments of treatment effects on soil nutrients, carbon emissions, and microbial community composition. Aboveground harvest treatment resulted in a dramatic reduction in Typha litter cover that persisted for 4 years, increased soil temperature, and increased abundance of the native plant genus Carex. However, aboveground harvest treatment did not significantly reduce Typha abundance, nor did it have significant effects on soil nutrient concentrations, carbon fluxes, or the taxonomic composition of soil microbial communities. We did observe differences in bacterial community composition between plots based on time since Typha invasion, which may indicate some legacy effects of Typha invasion. At the scale of this experiment (4 × 4 m plots), our results indicate that a single aboveground removal of Typha × glauca is not sufficient to restore a heavily invaded freshwater wetland ecosystem, and that periodic harvesting of Typha stems and litter may be required to maintain native plant abundance.

Riverbanks as Battlegrounds: Why Does the Abundance of Native and Invasive Plants Vary?

The abundance of invasive alien plants (IAPs) can vary dramatically over small spatial scales for reasons that are often unclear. Understanding these could offer key insights for containing invasions, accepting that eradication is often no longer feasible. This study investigated determinants of IAP cover on riverbanks, a well-known hotspot of invasion, using Impatiens glandulifera, a prolific invader across the Northern Hemisphere, as a model species. Within this framework, we included the potential for dominant native vegetation cover, mediated by favourable environmental conditions, to resist invasion by I. glandulifera through negative association. Our analyses, using structural equation modelling, showed that I. glandulifera is more sensitive to environmental conditions than dominant native vegetation. High soil moisture was a key determinant of I. glandulifera cover, having negative effects across the riparian zone. Spatially, I. glandulifera and dominant native vegetation responded differently to environmental conditions. Sites with steeper banks had less dominant native vegetation at the water’s edge, potentially favouring I. glandulifera cover through reduced competition. In general, greater abundance of dominant native vegetation presented a more invasion-resistant community. Maintaining dominant native vegetation at high abundance is thus key to preventing large monospecific I. glandulifera stands from forming. Our findings highlight the opportunities to indirectly limit plant invasions in general via targeted environmental management and restoration, as well as understanding future risks from long-term environmental change.

Non-native plants affect generalist pollinator diet overlap and foraging behavior indirectly, via impacts on native plant abundance

Flowering invasive plants can have dramatic effects on the resource landscape available to pollinators. Because many pollinators exhibit behavioral plasticity in response to competitor or resource density, this in turn can result in impacts on ecological processes such as pollination and plant reproduction. We examine how interactions between five common generalist eusocial bees change across an invasion gradient by examining how bee abundance and diet overlap changed with variation in both invasive plant abundance and competitor abundance in a temperate oak-savannah ecosystem. Specifically we focus on the bumblebees Bombus bifarius, B. mixtus, B. melanopygus and B. vosnesenskii, as well as the non-native honeybee Apis mellifera, and their interactions with the native flowering plants Camassia quamash, Camassia liechtlinii, and the invasive shrub Cytisus scoparius. We further examine whether changes in pollinator visits to the invasive and two common native plants can explain changes in diet overlap. Abundance of the invasive plant and other common floral resources had strong impacts on focal bee abundance, with certain species more likely to be present at highly invaded sites. This may be because highly invaded sites tended to be embedded in forested landscapes where those bees are common. Diet overlap was most affected by abundance of a common native plant, rather than the invasive plant, with diet overlap increasing non-linearly with abundance of the native plant. Furthermore, Apis mellifera, did not appear to have direct competitive effects on native bumblebees in this habitat. However, visit patterns suggest that bees most abundant at highly invaded sites may compete for access to native resources. Thus the impacts of this invasive plant on our focal bee species may be primarily indirect, via its’ competitive effects on native plants.

Coarse Woody Debris as a Land Reclamation Amendment at an Oil Sands Mining Operation in Boreal Alberta, Canada

Coarse woody debris (CWD) is an important component of natural forests and is now being used in reclaiming oil sands land to control erosion, enhance diversity, and function as longer term storage of organic matter. However, the impact of woody debris on reclaimed ecosystems may vary depending on the amount applied and the soil it is applied to. We studied the impact on the plant community (including tree regeneration and understory plants) and soil properties of four levels of CWD cover (None, Low, Moderate, High) on two reclamation soils (forest floor-mineral mix and peat-mineral mix) and natural post-fire soils. Significant differences were observed among soil types in terms of the plant community and soil properties but fewer differences were attributable to CWD. However, overall native plant species’ diversity and abundance decreased with High CWD while cover of non-native species on the reclaimed soils greatly decreased with Low CWD. Natural seedling regeneration density of trembling aspen was unaffected by CWD on both reclamation soil types. The soil nutrient supply rates and soil moisture were significantly different among soil types but there were no differences among debris treatments while soil temperature decreased with CWD. Overall, a Low (up to 30% ground cover) woody debris application appears to be optimal for maintaining native plant species diversity and abundance while controlling undesirable plant species.

Functional trait variation predicts distribution of alien plant species across the light gradient in a temperate rainforest

Several interspecific studies have related functional traits to alien plant invasiveness. Few studies have explicitly addressed how intraspecific variation in functional traits influences alien plant distribution across environments. In an old-growth temperate rainforest, we evaluated alien plant distribution across the light gradient, its relationship with native plant diversity and abundance, and variation in functional traits related to light capture that could explain the ecological breadth of alien plant species across the light gradient. In addition, we estimated possible ecological and phylogenetic constraints to intraspecific variation in these functional traits. In sixty 8-m2 plots we recorded light availability together with richness and abundance of alien and native plant species. We measured in 14 alien plant species three functional traits (specific leaf area, chlorophyll content and root/shoot ratio) and calculated their coefficient of variation. We estimated ecological constraints by measuring correlations among traits that could limit functional responses to light. We assessed phylogenetic constraints by calculating the phylogenetic signal of those functional traits. Richness and abundance of alien plant species were positively associated with light availability, while abundance was negatively related to native plant species abundance. Dominance of alien species increased with light availability, but some of them were dominant even in the shade. Phenotypic variation of functional traits was positively associated with ecological breadth and dominance of alien plant species. We found little evidence of ecological or phylogenetic constraints on functional trait variation. Light availability and functional trait variation correlated with the distribution and dominance of alien plants in this temperate rainforest. Alien plant species showed considerable phenotypic variation in low light, which may facilitate colonization of the forest understory. This process would not be hindered by ecological or phylogenetic constraints. Intraspecific variation in functional traits can render insightful information on current and future alien plant distribution across environmental gradients.

Short-term responses to warming vary between native vs. exotic species and with latitude in an early successional plant community.

Climate change is expected to favor exotic plant species over native species, because exotics tend to have wider climatic tolerances and greater phenological plasticity, and also because climate change may intensify enemy release. Here, we examine direct effects of warming (+ 1.8°C above ambient) on plant abundance and phenology, as well as indirect effects of warming propagated through herbivores, in two heavily invaded plant communities in Michigan, USA, separated by approximately three degrees latitude. At the northern site, warming increased exotic plant abundance by 19% but decreased native plant abundance by 31%, indicating that exotic species may be favored in a warmer world. Warming also resulted in earlier spring green-up (1.65 ± 0.77days), earlier flowering (2.18 ± 0.92days), and greater damage by herbivores (twofold increase), affecting exotic and native species equally. Contrary to expectations, native and exotic plants experienced similar amounts of herbivory. Warming did not have strong ecological effects at the southern site, only resulting in a delay of flowering time by 2.42 ± 0.83days for both native and exotic species. Consistent with the enemy release hypothesis, exotic plants experienced less herbivory than native plants at the southern site. Herbivory was lower under warming for both exotic and native species at the southern site. Thus, climate warming may favor exotic over native plant species, but the response is likely to depend on additional environmental and individual species' traits.