Influence of woody invader control methods and seed availability on native and invasive species establishment in a Hawaiian forest

When invasive woody plants become dominant, they present an extreme challenge for restoration of native plant communities. Invasive Morella faya (fire tree) forms extensive, nearly monospecific stands in wet and mesic forests on the Island of Hawai’i. We used logging, girdling, and selective girdling over time (incremental girdling) to kill stands of M. faya at different rates, with the objective of identifying a method that best promotes native forest re-establishment. We hypothesized that rapid canopy opening by logging would lead to establishment of fast-growing, non-native invaders, but that slower death of M. faya by girdling or incremental girdling would increase the establishment by native plants adapted to partial shade conditions. After applying the M. faya treatments, seed banks, seed rain, and plant recruitment were monitored over 3 years. Different plant communities developed in response to the treatments. Increased light and nitrogen availability in the logged treatment were associated with invasion by non-native species. Native species, including the dominant native forest tree, (Metrosideros polymorpha) and tree fern (Cibotium glaucum), established most frequently in the girdle and incremental girdle treatments, but short-lived non-native species were more abundant than native species. A diverse native forest is unlikely to develop following any of the treatments due to seed limitation for many native species, but girdling and incremental girdling promoted natural establishment of major components of native Hawaiian forest. Girdling may be an effective general strategy for reestablishing native vegetation in areas dominated by woody plant invaders.     

Functional traits,growth patterns,and litter dynamics of invasive alien and co-occurring native shrub species of chir pine forest in the central Himalaya,India

Across the continents, plant invasion is identified as one of the main threats to ecosystem functioning and stability. The main objective of this research was to evaluate the differences in the functional traits between invasive alien (Ageratina adenophora (Spreng.) and Lantana camara L.) and native (Berberis asiatica Roxb. Ex DC., Pyracantha crenulata (D. Don.) M. Roemer and Rubus ellipticus Sm.) shrub species of chir pine (Pinus roxburghii Sarg.) forest in the central Himalaya. Three 0.5 hectare chir pine forest stands were selected and individuals of similar diameter were tagged for comparative studies of leaf traits, growth pattern, and biomass accumulation in structural organs of each invasive alien and native species. Our one-way ANOVA and Tukey’s post hoc test results showed that both the invasive alien species have significantly (p?<?0.05) higher SLA, LWC, total chlorophyll content, foliar nutrient (N and P), RGR, LMR, SMR, nutrient uptake, and nutrient use efficiencies than native species. Leaf litter decomposition rate and nutrient release were also significantly (p?<?0.05) higher in both the invasive alien species. Native species, R. ellipticus, shared some of the traits, such as leaf area, chlorophyll content, RGR, LAR, LMR, and nutrient uptake efficiency with invasive alien species. The majority of traits differed among invasive alien and native species, implying that the success of invasive alien species is best described by being functionally distinct from native species. These findings indicate that invasive alien species had advanced functional traits which may be playing an important role in a rapid spread in the central Himalaya.

     

A 10-year study of changes in forest vegetation on Silhouette island, Seychelles

A 10 year study of forest communities on Silhouette island, Seychelles demonstrates stability of forest composition in most areas over this time-scale. Areas with heavy invasion by alien species were found to be regenerating, particularly with the rapid loss of Clidemia hirta. This is attributed to the abundance of well-adapted native plants allowing competitive exclusion to take place, throughout competition for light. It was noted that invasive plant species tend to be unstable on the rocky slopes covered by native high forest. A high rate of tree fall and limited seed dispersal may reduce the impact of the invasive tree Paraserianthes falcataria in the future. Other species such as Cinnamomum verum and Psidium cattleianum may persist as major invaders due to wider seed dispersal.     

Invasive Plants can Inhibit Native Tree Seedlings: Testing Potential Allelopathic Mechanisms

The mechanisms by which invasive species affect native communities are not well resolved. For example, invasive plants may influence other species through competition, altered ecosystem processes, or other pathways. We investigated one potential mechanism by which invasive plants may harm native species, allelopathy. Specifically, we explored whether native tree species respond differently to potential allelopathic effects of two invasive plant species. We assessed the separate effects of Lolium arundinaceam (tall fescue) and Elaeagnus umbellata (autumn olive) on three common successional tree species: Acer saccharinum (silver maple), Populus deltoides (eastern cottonwood), and Platanus occidentalis (sycamore). Tall fescue and autumn olive are widely planted and highly invasive or persistent throughout North America where they often grow in forest edges, old fields, and other sites colonized by pioneering tree species. In an exploratory greenhouse experiment, we applied aqueous extracts derived from soil, leaf litter, or live leaves to native trees. We compared these treatments to a sterile water control and also to minced leaves leached in water, a common, but potentially less realistic method of testing for allelopathy. For all tree species, minced leaves from tall fescue reduced the probability that seedlings emerged, and minced leaves of autumn olive reduced the number of days to emergence. During other demographic stages, the three native tree species diverged in their responses to the invasive plants. Platanus occidentalis exhibited the widest range of responses, with reduced root biomass due to minced tissue from both invasive species, reduced days to emergence and marginally reduced survival from minced tall fescue, and reduced leaf biomass from tall fescue leaf litter. Populus deltoides appeared insensitive to most extracts, although survival was marginally increased with application of minced or fresh leaf extracts from autumn olive. In addition, minced tall fescue shortened the time to seedling emergence for Acer saccharinum, potentially a positive effect. Overall, results suggest that allelopathy may be one mechanism underlying the negative impacts of tall fescue and autumn olive on other plant species, but that effects can depend strongly upon the source of allelochemicals and the tree species examined.     

潮虫消耗木本植物凋落物的可选择性试验

近年来,越来越多的学者关注外来植物入侵对土壤生态功能的影响效应及其相应反馈机制的探索与研究,然而本地原生土壤生物群落对不同入侵程度下的外来植物种以及本地原生植物种之间是否存在消耗差异却尚不明了.通过等足目潮虫的选择性喂养试验来测试10个本地种、5个非入侵性外来种和5个强入侵性外来种之间的适口性差异,试图求证外来植物的入侵性是否与植物落叶被消耗率呈现必然联系.数据分析结果显示潮虫对本地种、非入侵性外来种和入侵性外来种的消耗并无显著差异;而潮虫对不同生活型下木本植物的取食却存在显著差异,即灌木消耗率显著高于乔木.其次,通过植物初始性状指标(包括木质素、纤维素、半纤维素、碳、氮含量)与相应消耗率的相关比较,消耗量总体上与植物凋落物的氮含量呈正相关关系(R2 =0.358).由此,研究结论强调植物落叶的降解速率并不一定与植物入侵性或入侵阶段呈绝对相互关连,但是氮含量,抑或各种形式的植物氮元素成分都可能在一定程度上参与并影响着外来植物的入侵进程.     

Invasive alien plants in China: diversity and ecological insights

China’s current invasive alien plant species were analyzed with regard to their floristic status, biological attributes and invasion status elsewhere. Most of the 270 species identified were annuals, followed by perennial herbs. Woody perennials made only about 10% of the species. The invasives were comprised of 59 families, the largest being Asteraceae, Poaceae, and Brassicaceae. The genera with most invasive species were Amaranthus, Ipomoea, and Solanum. Most of the species originated from the New World, notably from South America. About one-third of the species were serious invaders of natural habitats in countries other than China. The proportion of invasive alien plants in province floras ranged from 0.5 to 3.8%, absolute numbers from nine to 117 species per province. Density of invasive species was correlated positively with native species density at provincial scale. The results demonstrate that in China invasive plants are present throughout the country, with a particularly high species richness in the Southeast. The ecological diversity of invasive plants suggests wide ranging impacts which need to be assessed.     

Invasive litter, not an invasive insectivore, determines invertebrate communities in Hawaiian forests

In Hawaii, invasive plants have the ability to alter litter-based food chains because they often have litter traits that differ from native species. Additionally, abundant invasive predators, especially those representing new trophic levels, can reduce prey. The relative importance of these two processes on the litter invertebrate community in Hawaii is important, because they could affect the large number of endemic and endangered invertebrates. We determined the relative importance of litter resources, represented by leaf litter of two trees, an invasive nitrogen-fixer, Falcataria moluccana, and a native tree, Metrosideros polymorpha, and predation of an invasive terrestrial frog, Eleutherodactylus coqui, on leaf litter invertebrate abundance and composition. Principle component analysis revealed that F. moluccana litter creates an invertebrate community that greatly differs from that found in M. polymorpha litter. We found that F. moluccana increased the abundance of non-native fragmenters (Amphipoda and Isopoda) by 400% and non-native predaceous ants (Hymenoptera: Formicidae) by 200%. E. coqui had less effect on the litter invertebrate community; it reduced microbivores by 40% in F. moluccana and non-native ants by 30% across litter types. E. coqui stomach contents were similar in abundance and composition in both litter treatments, despite dramatic differences in the invertebrate community. Additionally, our results suggest that invertebrate community differences between litter types did not cascade to influence E. coqui growth or survivorship. In conclusion, it appears that an invasive nitrogen-fixing tree species has a greater influence on litter invertebrate community abundance and composition than the invasive predator, E. coqui.     

Restoring habitat for native and endemic plants through the introduction of a fungal pathogen to control the alien invasive tree <Emphasis Type="Italic">Miconia calvescens</Emphasis> in the island of Tahiti

Success of biological control programs is commonly assessed by studying the direct negative impacts of released agents on the target invasive species. Very few quantitative studies have focused on the indirect positive effects on native biodiversity. In this study, we monitored the response of the plant community (both native and alien species) in permanent plots located in four different sites in montane rainforests of the tropical island of Tahiti (South Pacific) severely invaded over decades by the alien invasive tree Miconia calvescens DC (Melastomataceae), after the release of a defoliating fungal pathogen Colletotrichum gloeosporioides f. sp. miconiae Killgore & L. Sugiyama. Results of five years of monitoring showed that total native and endemic species richness and plant cover increased in all sites and plots. Partial defoliation of miconia canopy trees (between 6% and 36%) led to significant recruitment of light-demanding pioneer species, but also to the appearance of some semi-shade and shade tolerant rare endemic species. Native ferns and angiosperms remained dominant (ca. 80%) in the forest understorey during the monitoring period. Colonization by a small number of alien plant species occurred in one permanent plot located at the lower elevation. We conclude that biological control may be considered a tool for partial habitat restoration and recovery of native and endemic species, but long-term monitoring is needed to confirm the stability and resilience of the “novel plant assemblage”.     

Exotic species as modifiers of ecosystem processes: Litter decomposition in native and invaded secondary forests of NW Argentina

Invasions of exotic tree species can cause profound changes in community composition and structure, and may even cause legacy effect on nutrient cycling via litter production. In this study, we compared leaf litter decomposition of two invasive exotic trees (Ligustrum lucidum and Morus sp.) and two dominant native trees (Cinnamomum porphyria and Cupania vernalis) in native and invaded (Ligustrum-dominated) forest stands in NW Argentina. We measured leaf attributes and environmental characteristics in invaded and native stands to isolate the effects of litter quality and habitat characteristics. Species differed in their decomposition rates and, as predicted by the different species colonization status (pioneer vs. late successional), exotic species decayed more rapidly than native ones. Invasion by L. lucidum modified environmental attributes by reducing soil humidity. Decomposition constants (k) tended to be slightly lower (−5%) for all species in invaded stands. High SLA, low tensile strength, and low C:N of Morus sp. distinguish this species from the native ones and explain its higher decomposition rate. Contrary to our expectations, L. lucidum leaf attributes were similar to those of native species. Decomposition rates also differed between the two exotic species (35% higher in Morus sp.), presumably due to leaf attributes and colonization status. Given the high decomposition rate of L. lucidum litter (more than 6 times that of natives) we expect an acceleration of nutrient circulation at ecosystem level in Ligustrum-dominated stands. This may occur in spite of the modified environmental conditions that are associated with L. lucidum invasion.     

Post-fire spread of alien plant species in a mixed broad-leaved forest of the Insubric region

How do tree species regenerate and which ecological conditions are required after forest fire in the Insubric region of the Alps? Are indigenous stand-forming tree species resistant over the invasion of alien plant species after such a disturbance? We addressed these questions in a case study in the Swiss canton of Ticino. In April 2006, a surface fire with severe intensity burnt a forest area of 55 ha on a south-facing slope (400-800 m.a.s.l.). The dominant trees in the investigated area were chestnut (Castanea sativa Mill.), beech (Fagus sylvatica L.) and deciduous oaks (Quercus spp.) mixed with tree species of intermediate height. Vegetation data were collected in May and August 2009 by systematic sampling. Resprouting of the survived trees and generative regeneration were analysed by counting vegetative shoots from sprouting stools, of seedling age, height and damage rate, respectively. Different vegetation structures related to low or high fire intensity were clearly visible three years after the forest fire, creating various habitats for both new invaders and seedlings of the stand-forming trees. The dominant chestnut was the only tree species that regenerated effectively by sprouting from stools. Seedlings of the stand-forming trees grew in high abundance under shadow conditions close to their mother trees which provided the seed source. In contrast, pioneer trees invaded patches where full light was available. Under such conditions the two main woody alien plant species, Ailanthus altissima and Robinia pseudoacacia, grew in high abundance. Due to the different ecological requirements of indigenous and alien tree seedlings, not any interaction between the two groups was detected.     

Facilitation and Inhibition of Seedlings of an Invasive Tree (Acer platanoides) by Different Tree Species in a Mountain Ecosystem

Facilitation is known to be an important process structuring natural plant communities. However, much less is known about its role in facilitating the invasion of ecosystems by non-native plant species. In this study we evaluated the effects of invasive (Acer platanoides) and native (Pseudotsuga menziesii) forest types on the performance of A. platanoides seedlings, and related these effects to structural and functional properties associated with the two forest types, in a native P. menziesii forest that is being invaded by A. platanoides. Acer platanoidesseedlings had higher densities, recruitment, and survival, and experienced less photoinhibition and water stress when beneath conspecific canopies than in the adjacent P. menziesii forest. Soil moisture and canopy cover were greater in the invaded patch than the native forest. There was no difference in soil fertility or understory light levels between locations. These demographic (i.e. seedling survival), physiological, and environmental differences appeared to be due to the effects of A. platanoides and P. menziesii trees. Thus, Acer trees appear to produce a more mesic environment by modifying the structure and phenology of the forest canopy and by altering the timing of transpirational water loss relative to P. menziesii. Environmental modification by invaders that lead to positive effects on conspecifics may help us to understand the dramatic success and lag periods of some invasive species     

Forest dynamics following eastern hemlock mortality in the southern Appalachians

Understanding changes in community composition caused by invasive species is critical for predicting effects on ecosystem function, particularly when the invasive threatens a foundation species. Here we focus on dynamics of forest structure, composition and microclimate, and how these interact in southern Appalachian riparian forests following invasion by hemlock woolly adelgid, HWA, Adelges tsugae. We measured and quantified changes in microclimate; canopy mortality; canopy and shrub growth; understory species composition; and the cover and diversity in riparian forests dominated by eastern hemlock Tsuga canadensis over a period of seven years. Treatments manipulated hemlock mortality either through invasion (HWA infested stands) or girdling (GDL) hemlock trees. Mortality was rapid, with 50% hemlock tree mortality occurring after six years of invasion, in contrast to more than 50% mortality in two years following girdling. Although 50% of hemlock trees were still alive five years after infestation, leaf area lost was similar to that of girdled trees. As such, overall responses over time (changes in light transmittance, growth, soil moisture) were identical to girdled stands with 100% mortality. Our results showed different growth responses of the canopy species, shrubs and ground layer, with the latter being substantially influenced by presence of the evergreen shrub, rhododendron Rhododendron maximum. Although ground layer richness in the infested and girdled stands increased by threefold, they did not approach levels recorded in hardwood forests without rhododendron. Increased growth of co‐occurring canopy trees occurred in the first few years following hemlock decline, with similar responses in both treatments. In contrast, growth of rhododendron continued to increase over time. By the end of the study it had a 2.6‐fold higher growth rate than expected, likely taking advantage of increased light available during leaf‐off periods of the deciduous species. Increased growth and dominance of rhododendron may be a major determinant of future responses in southern Appalachian ecosystems; however, our results suggest hemlock will be replaced by a mix of Acer, Betula, Fagus and Quercus canopy genera where establishment is not limited by rhododendron.     

Do Invasive Trees have a Hydraulic Advantage over Native Trees?

The hypothesis was tested that invasive trees have hydraulic traits that contribute to their invasive nature. Five pairs of co-occurring invasive and native trees, in mesic habitats, were selected: (1) Tamarix ramosissima and Salix amygdaloides; (2) Robinia pseudoacacia and Alnus rhombifolia (3) Schinus terebinthifolius and Myrica cerifera; (4) Ligustrum sinense and Acer negundo; and (5) Sapium sebiferum and Diospyros virginiana, respectively. Resistance to cavitation (the water potential [Ψ _x ] at 75% loss of hydraulic conductivity [Ψ₇₅]) was not consistently greater for invasive compared to native species (Ψ₇₅=−1.91 and −1.67 MPa, respectively). Xylem specific conductivity (K _s), a measure of xylem efficiency, was not different between native and invasive species (K _s = 3.50 and 3.70 kg s⁻¹ MPa⁻¹ m⁻¹, respectively). The lack of difference for resistance to cavitation among invasive and native species suggests that the sampled invaders are not more tolerant to water stress than co-occurring native species. Apparently the spread and invasive nature of the sampled species cannot be explained by hydraulic traits alone.     

Influence of litter removal and mineral soil disturbance on the spread of an invasive grass in a Central Hardwood forest

Soil and litter disturbances within Central Hardwood forests may facilitate exotic plant species invasion of interior forest areas. Microstegium vimineum is an annual exotic grass that has become common throughout the Southeastern United States. Three replicates of three different mineral soil and litter disturbance treatments, plus a control with no disturbance, were established on the leading edge of M. vimineum patches prior to seed fall. All patches were located in areas with similar forest canopy structure and slope in three Central Hardwood forest stands prior to seed fall. At the beginning of the following growing season, each individual M. vimineum seedling was mapped within the treatment plots. The mean number of M. vimineum individuals that established within each treatment did not differ significantly from the control. The distance at which 90% of the individuals had spread, and the overall mean distance spread were significantly farther for the litter removal treatment than the control. The farthest individual seedling from the boundary of existing patches in both the litter removal and the mineral soil disturbance and litter removal treatments were significantly farther than the control. The individuals that spread the farthest are of most concern due to the large number of viable seed that a single M. vimineum plant can produce. These results suggest that disturbance of the litter layer may increase the spread rate of M. vimineum and facilitate its invasion of new habitats, and that leaving litter layers intact may slow the spread of M. vimineum.     

Evaluating nurse plants for restoring native woody species to degraded subtropical woodlands

Harsh habitats dominated by invasive species are difficult to restore. Invasive grasses in arid environments slow succession toward more desired composition, yet grass removal exacerbates high light and temperature, making the use of “nurse plants” an appealing strategy. In this study of degraded subtropical woodlands dominated by alien grasses in Hawai'i, we evaluated whether individuals of two native (Dodonaea viscosa, Leptocophylla tameiameia) and one non‐native (Morella faya) woody species (1) act as natural nodes of recruitment for native woody species and (2) can be used to enhance survivorship of outplanted native woody species. To address these questions, we quantified the presence and persistence of seedlings naturally recruiting beneath adult nurse shrubs and compared survival and growth of experimentally outplanted seedlings of seven native woody species under the nurse species compared to intact and cleared alien‐grass plots. We found that the two native nurse shrubs recruit their own offspring, but do not act as establishment nodes for other species. Morella faya recruited even fewer seedlings than native shrubs. Thus, outplanting will be necessary to increase abundance and diversity of native woody species. Outplant survival was the highest under shrubs compared to away from them with few differences between nurse species. The worst habitat for native seedling survival and growth was within the unmanaged invasive grass matrix. Although the two native nurse species did not differentially affect outplant survival, D. viscosa is the most widespread and easily propagated and is thus more likely to be useful as an initial nurse species. The outplanted species showed variable responses to nurse habitats that we attribute to resource requirements resulting from their typical successional stage and nitrogen fixation capability.     

Beetle responses to artificial gaps in an oceanic island forest: implications for invasive tree management to conserve endemic species diversity

Natural forests are often replaced by invasive alien trees on isolated oceanic islands. Adequate eradication of invasive trees should be conducted with the goal of biodiversity conservation, because islands support many endemic organisms that depend on native forests. An invasive alien tree, Bischofia javanica Blume (Euphorbiaceae), has invaded and replaced natural forests on the oceanic Ogasawara (Bonin) Islands, Japan, in the northwestern Pacific Ocean. To determine how the removal of B. javanica trees affects insect diversity, we examined flying beetles captured using Malaise traps in B. javanica forests on Hahajima. The abundance, species density, and species composition of wood-boring beetles (Coleoptera: Cerambycidae, Elateridae, Mordellidae, and Scolytidae) were compared between closed-canopy sites and gaps created by girdling B. javanica trees in alien forests during two seasons (June–July and October–November 2005). Of the collected beetles, 75.8, 87.5, 90.0, and 0.0% of cerambycid, elaterid, mordellid, and scolytid beetle species, respectively, were endemic to the Ogasawara Islands. More cerambycid, elaterid, and mordellid individuals were captured in June–July than in October–November; the number of scolytid individuals did not differ between seasons. More cerambycid, elaterid, and scolytid individuals were captured in artificial gaps than on the closed-canopy forest floor. Although fewer mordellid individuals were captured in gaps, more endemic mordellids were captured in gaps. More cerambycid and scolytid species were captured in artificial gaps than in closed-canopy areas. The positive responses of beetles to artificial gaps suggest that the removal of B. javanica increases beetle diversity and the abundance of endemic beetles.     

Ecosystem Level Impacts of Invasive Acacia saligna in the South African Fynbos

Recent efforts to clear invasive plants from the fynbos of South Africa forces managers to think about how N₂‐fixing invasives have altered ecosystem processes and the implications of these changes for community development. This study investigated the changes in nitrogen (N) cycling regimes in fynbos with the invasion of Acacia saligna, the effects of clear‐cutting acacia stands on soil microclimate and N cycling, and how altered N resources affected the growth of a weedy grass species. Litterfall, litter quality, soil nutrient pools, and ion exchange resin (IER)‐available soil N were measured in uninvaded fynbos, intact acacia, and cleared acacia stands. In addition, a bioassay experiment was used to ascertain whether the changes in soil nutrient availability associated with acacia would enhance the success of a weedy grass species. Acacia plots had greater amounts of litterfall, which had higher concentrations of N. This led to larger quantities of organic matter, total N, and IER‐available N in the soil. Clearing acacia stands caused changes in soil moisture and temperature, but did not result in differences in IER‐available N. The alteration of N availability by acacias was shown to increase growth rates of the weedy grass Ehrharta calycina, suggesting that secondary invasions by nitrophilous weedy species may occur after clearing N₂‐fixing alien species in the fynbos. It is suggested that managers use controlled burns, the addition of mulch, and the addition of fynbos seed after clearing to lower the levels of available N in the soil and initiate the return of native vegetation.     

Patterns of Seed Dispersal and Dispersal Failure in a Hawaiian Dry Forest Having Only Introduced Birds

Dry forests are among the most endangered natural communities in the Hawaiian Islands. Most have been reduced to isolated trees and small forest fragments in which native tree species reproduce poorly. The replacement of native birds by introduced generalists may be contributing to dry forest decline through modification of seed dispersal patterns. To document seed dispersal by introduced birds, we conducted foraging observations on fleshy-fruited trees and measured seed rain under trees and in adjacent open areas for 1 year in a dry forest dominated by native trees. Although trees covered only 15.2 percent of the study area, 96.9 percent of the bird-dispersed seeds were deposited beneath them. The Japanese white-eye (Zosterops japonicus) was the principal dispersal agent. Among bird-dispersed seeds, those of the invasive tree Bocconia frutescens accounted for 75 percent of all seeds collected beneath trees (14.8 seeds/m²/yr) and the invasive shrub Lantana camara accounted for 17 percent. Although nearly 60 percent of the reserve's native woody species possess fleshy fruits, introduced birds rarely disperse their seeds. Native trees accounted for <8 percent of all bird-dispersed seeds and are consequently experiencing dispersal failure by falling directly under parent trees. Smaller-seeded non-native plants, in contrast, may be benefiting from dispersal by introduced birds. Current dispersal patterns suggest that these readily disseminated non-native plants may eventually replace the remaining native flora.     

Invasive plants in Minnesota are “joining the locals”: A trait‐based analysis

Questions

Predicting which newly arrived species will establish and become invasive is a problem that has long vexed researchers. In a study of cold temperate oak forest stands, we examined two contrasting hypotheses regarding plant functional traits to explain the success of certain non‐native species. Under the “join the locals” hypothesis, successful invaders are expected to share traits with resident species because they employ successful growth strategies under light‐limited understorey conditions. Instead, under the “try harder” hypothesis, successful invaders are expected to have traits different from native species in order to take advantage of unused niche space.

Location

Minnesota, USA.

Methods

We examined these two theories using 109 native and 11 non‐native plants in 68 oak forest stands. We focused on traits related to plant establishment and growth, including specific leaf area (SLA), leaf carbon‐to‐nitrogen ratio (C:N), wood density, plant maximum height, mycorrhizal type, seed mass and growth form. We compared traits of native and non‐native species using ordinations in multidimensional trait space and compared community‐weighted mean (CWM) trait values across sites.

Results

We found few differences between trait spaces occupied by native and non‐native species. Non‐native species occupied smaller areas of trait space than natives, yet were within that of the native species, indicating similar growth strategies. We observed a higher proportion of non‐native species in sites with higher native woody species CWM SLA and lower CWM C:N. Higher woody CWM SLA was observed in sites with higher soil pH, while lower CWM C:N was found in sites with higher light levels.

Conclusions

Non‐native plants in this system have functional traits similar to natives and are therefore “joining the locals.” However, non‐native plants may possess traits toward the acquisitive end of the native plant trait range, as evidenced by higher non‐native plant abundance in high‐resource environments.