Invasive Woody Species Research Articles

Foraging decisions with conservation consequences: Interaction between beavers and invasive tree species.

Herbivore species can either hinder or accelerate the invasion of woody species through selective utilization. Therefore, an exploration of foraging decisions can contribute to the understanding and forecasting of woody plant invasions. Despite the large distribution range and rapidly growing abundance of beaver species across the Northern Hemisphere, only a few studies focus on the interaction between beavers and invasive woody plants.We collected data on the woody plant supply and utilization at 20 study sites in Hungary, at two fixed distances from the water. The following parameters were registered: taxon, trunk diameter, type of utilization, and carving depth. Altogether 5401 units (trunks and thick branches) were identified individually. We developed a statistical protocol that uses a dual approach, combining whole‐database and transect‐level analyses to examine foraging strategy.Taxon, diameter, and distance from water all had a significant effect on foraging decisions. The order of preference for the four most abundant taxa was Populus spp. (softwood), Salix spp. (softwood), Fraxinus pennsylvanica (invasive hardwood), and Acer negundo (invasive hardwood). The diameter influenced the type of utilization, as units with greater diameter were rather carved or debarked than felled. According to the central‐place foraging strategy, the intensity of the foraging decreased with the distance from the water, while both the taxon and diameter selectivity increased. This suggests stronger modification of the woody vegetation directly along the waterbank, together with a weaker impact further from the water.In contrast to invasive trees, for which utilization occurred almost exclusively in the smallest diameter class, even the largest softwood trees were utilized by means of carving and debarking. This may lead to the gradual loss of softwoods or the transformation of them into shrubby forms. After the return of the beaver, mature stages of softwood stands and thus the structural heterogeneity of floodplain woody vegetation could be supported by the maintenance of sufficiently large active floodplains.The beaver accelerates the shift of the canopy layer's species composition toward invasive hardwood species, supporting the enemy release hypothesis. However, the long‐term impact will also depend on how plants respond to different types of utilization and on their ability to regenerate, which are still unexplored issues in this environment. Our results should be integrated with knowledge about factors influencing the competitiveness of the studied native and invasive woody species to support floodplain conservation and reconstruction.

Pathways towards the Sustainable Management of Woody Invasive Species: Understanding What Drives Land Users’ Decisions to Adopt and Use Land Management Practices

Sustainable land management (SLM) practices are key for achieving land degradation neutrality, but their continued implementation lag behind the progression of various forms of land degradation. While many scholars have assessed the drivers of SLM uptake for restoring land affected by desertification, drought, and floods (SDG 15.3 and partly SDG 2.4), little is known about the implication of SLM implementation on invasive alien species (IAS) management. This study aimed at understanding the challenges and proposing solutions for the uptake of SLMs with respect to the management of the invasive tree, Prosopis juliflora, in Baringo County, Kenya. Data were collected with semi-structured questionnaires, the responses were coded into themes, and c-coefficient tables were used to determine code linkages. Our results show that the availability of incentives is the main motivation for invasion management. Thus, management efforts have often focused on private parcels, while communally shared lands tended to be neglected despite their vulnerability to invasion. We conclude that sustainable IAS management lies at a landscape scale, and thus the national IAS management strategies should adopt a collective approach by empowering local actors to engage in SLM implementation.

Перспективность применения древесно-кустарниковых растений в озеленении города Краснодар

Nowadays, about half of the world’s population lives in cities. Green spaces, as well as the condition of landscaping facilities and the quality of woody and shrubby plants play an important role in creating a comfortable environment and improving the ecological conditions for people living in the city. The research purpose is to assess the current state of landscaping objects, the quality of woody and shrubby plants and the resistance of tree species to anthropogenic loads using Krasnodar as a case study. The following parks and squares became the research objects: Solnechny Ostrov (“Sunny Island”) Park, Park named after Maxim Gorky, Kuban Stadium Park, Chistyakovskaya Grove, Botanical Garden of Professor I.S. Kosenko, Marshal Zhukov’s Park, Druzhba Narodov Square, Mayskiy Square, Square named after L.G. Gatov, Vishnyakovsky Square, Square named after Ivan Kochubey, Festivalny Square, and Novorozhdestvenskiy Square. The research was carried out in 2016–2019. During the research, 27 options of studies were described, 2634 trees and shrubs, their 50 species and forms related to 2 groups, 21 families and 30 genera were studied. The plantations that occupy large areas of the landscaping sites were sampled for study. An assessment of the state of woody species in Krasnodar was carried out. The results are compared with those obtained by other authors. Among the studied species, 16 woody species meet the recommendations of Dyuval’-Stroyev M.R. and 27 do not meet them; no data is available for 7 species. Having analyzed the main characteristics of native and invasive woody and shrubby species used in landscaping of Krasnodar, we come to the conclusion that the main part of introduced species belongs to the strongly weakened category, while the native species are in the weakened category. At present, the conditions for tree growth in Krasnodar have changed: anthropogenic load has increased. In this regard, there is a need for further inventory of tree species and assessment of woody and shrubby plants to create a complete database of trees and shrubs that are most resistant to the current urban conditions. For citation: Primakov N.V. Prospects for Woody and Shrubby Plants in the Landscaping of Krasnodar. Lesnoy Zhurnal [Russian Forestry Journal], 2022, no. 1, pp. 98–109. DOI: 10.37482/0536-1036-2022-1-98-109

Sandy grasslands regeneration results of the conservation management on the Homoktövis Conservation Area in Budapest (2009 2021)

In Budapest, despite the density of the population, precious plant communities still remain in a lot of, mostly isolated fragments of habitats which,are outstanding in the diversity of species and are rare inhabitants of endemic species. On the examined territory, since 2006 long term reconstructions of the habitats have been ongoing which strive for the decrease of the invasive woody species on the territory and the insurance of the habitat of the sandy lawn, as well as the long term conservation of the fragments of the lawn and the creation of the natural sandy lawn. This activity is conducted by the volunteers of the MME and with the help of the students of educational institutions in parallel requesting for the permits of the Municipality of the Capital. During these interventions, the shoots were mechanically beat down and they were wiped dried as well. Besides these interventions, it was fundamental to preserve the sensitive, rare or legally protected species and to increase their volume as much as possible.

Low passive restoration potential following invasive woody species removal in oak woodlands

The restoration of oak woodlands often requires removal and management of woody invasive plants. This can trigger germination of the soil seed bank, which can alter restoration trajectories. In degraded deciduous woodlands generally, it is unclear whether the soil seed bank will contribute native plant material in sufficient quantities to help achieve restoration goals and allow practitioners to rely on passive restoration without supplemental seeding. To support restoration decision‐making around passive or active restoration in three Rhamnus cathartica‐invaded forest preserves in the Chicago region, we asked: (1) Does the soil seed bank differ from standing aboveground vegetation at reference and unrestored sites? and (2) Can the species richness, Shannon diversity, floristic quality (measured by abundance‐weighted mean coefficients of conservatism), or density of germinable seeds of native species in the soil seed bank be predicted by a site's restoration status (reference or unrestored)? We found that species composition differed significantly between aboveground vegetation and the soil seed bank at reference and unrestored sites, with a significant interaction between restoration status and location. Despite high variation among the three forest preserves, restoration status also predicted native species richness, diversity, floristic quality, and seed density in the soil seed bank, with unrestored sites significantly lower than reference sites in all measures. Results suggest that reintroduction of native seeds will be necessary to fully restore desired native plant communities in oak woodlands following the removal of invasive woody plants.

Spatial and functional niche overlap between invasive Ligustrum lucidum and native woody species in an urban shrine forest in Japan

Spatial and functional niche overlap between invasive Ligustrum lucidum and native woody species in an urban shrine forest in Japan

The successive trend of vegetation confirms the removal of non-indigenous woody species as an insufficient restoration action

Invasive woody species can substantially affect their environment and communities, enhancing the interest in conservation management. Non-indigenous dwarf pine (Pinus mugo) in the treeline ecotone of the Eastern High Sudetes Mountains and its experimental removal is the model example. We asked how species composition and diversity develop after it is clearcut, and what are the main drivers leading to alpine grasslands restoration. On the study site (Jeseniky Mts., the Czech Republic), three distinct habitats were distinguished: plots after clearcut, alpine grasslands, and dwarf pine plantation. Plant species composition was assessed by means of phytosociological releves together with selected plant traits and environmental factors. We compared temporal trends in species number, diversity, and plant traits among the habitats. Species richness and diversity has increased since the dwarf pine was clearcut. This trend was the same for all three types of habitats studied, although they were significantly different in spatial compositional heterogeneity and species structure. Species composition among study habitats differed and went through its own successive trend of community development during time as the consequence of species turnover. We revealed that light-demanding plant species preferring low nitrogen concentrations were associated with the successional trend after the dwarf pine was clearcut. Dwarf pine removal creates conditions for alpine grassland restoration, but restoration can take a long time because of the environmental conditions, dwarf pine legacy, and plant traits that determine the succession direction. We recommend implementing adaptive management with long-term monitoring followed by actions to correct the desired state.

Impact of <i>Lantana camara</i> Invasion on Herbaceous Species Diversity and Soil Physio-Chemical Properties in Eastern Ethiopia

Lantana camara L. is highly invasive woody species in Ethiopia that with serious social, economic and environmental impact. Examining the potential impact of this invasive species will be needed in planning better strategies. Thus, the study assessed the impact of L. camara invasion on herbaceous species diversity and soil physio-chemical properties in Eastern Ethiopia. To collect the herbaceous species data, 1m x1m plots were laid down in each 60 selected lantana individuals under and outside the cover in four directions. Total of 480 plots was laid down for herbaceous species data collection 240 plots in each under and outside the lantana camara. From each plot, a soil sample was taken from 0- 15cm depth after removing litter from the soil surface for Soil physiochemical analysis. There was higher species richness (49.69), species diversity (H'=1.7253), and evenness (E=1.1305) at outside the L. camara cover area than under L. camara cover. Soil texture had didn’t show a significance difference between under and outside lantana cover, but there was soil texture difference between sampling locations. Soil samples collected under the L. camara cover were high in Soil PH, Organic Matter, Total Nitrogen, and Organic Carbon (p<0.05) when compared to the adjacent outside L. camara cover except for available Phosphors. L. camara invasion has highly affected herbaceous species diversity and alters soil physiochemical properties of the area. So this study confirming its invasiveness and its negative impact on biodiversity appeal, proper control and management actions to be undertaken by government and different stakeholders.

A reintroduced ecosystem engineer species may exacerbate ongoing biological invasion: Selective foraging of the Eurasian beaver in floodplains

Species reintroductions aim to re-establish populations of locally extinct, often keystone species. As a consequence of the reintroduction and protection of the Eurasian beaver (Castor fiber), the species has recolonized the suitable habitats (including floodplains) across Europe in the past few decades. Floodplain habitats have already been significantly altered during the period when the beaver was listed as a locally extinct species. Nowadays, alluvial forests are being highly impacted by rapidly spreading invasive woody species. We examined the supply of woody species and their utilization by beaver in this changed environment, with a fine-scale method, at 20 sites, along 39 transects (Danube catchment area, Hungary). The invasive species (Acer negundo, Fraxinus pennsylvanica and Amorpha fruticosa) were utilized by beaver, but the utilization ratio of softwood genera (Salix and Populus) dominated by native species was usually higher. Based on the preference analysis, performed for the thin branches category, at least one of the softwood genera was significantly preferred along 17 transects, and they were never avoided. In parallel, at least one of the invasive woody species was significantly avoided along 17 transects, and preferred along only one transect. We conclude that beaver’s forage selection is capable of indirectly accelerating the spread of already present invasive species, a negative process from a nature conservation perspective. This adverse impact points to the need for increased attention on changes that can occur within the habitat of a reintroduced species, and on the possibility of unexpected conflicts among conservation targets.

Effect of Method and Time of Management on European Buckthorn (Rhamnus cathartica L.) Growth and Development in Minnesota

European buckthorn is an exotic problematic invasive woody species that has displaced native plant species in Minnesota woodlands. Buckthorn is also an overwintering host for oat crown rust and soybean aphids, which can cause significant crop yield losses. The overall goal of this study was to test multiple buckthorn control methods and examine the establishment of native plant species in colonized areas. Specific objectives were to 1) determine the effectiveness of buckthorn control methods when applied in different seasons, 2) monitor seedling recruitment and resprouting ability of buckthorn saplings following treatment, 3) monitor recruitment and survival of native plant species following treatment, and 4) characterize buckthorn carbohydrate fluctuations and considerations for timely and effective buckthorn management. Field experiments conducted for two years in two locations (Eagle Lake Regional Park and Battle Creek Regional Park, Minnesota, U.S.A), tested four buckthorn control treatments: 1) cutting only; 2) cutting+stump treatment with herbicide (triclopyr); 3) cutting+stump treatment with herbicide+burning, and 4) cutting+burning. Untreated controls were included in each experiment. Across management seasons, the cutting+stump treatment with herbicide resulted in higher seedling densities for buckthorn and other species the next season compared to cutting only without herbicide application. Spring management resulted in the lowest seedling density the next season for both buckthorn and other plant species, and spring control treatments that included herbicide and burning resulted in higher buckthorn and native species seedling densities than treatments without burning. Because seasonal total nonstructural carbohydrate levels in buckthorn crowns were highest in the fall season, we recommend applying systemic herbicides in the fall when carbohydrates are translocated for storage to facilitate herbicide translocation and efficacy. Our study shows that integrating multiple buckthorn control methods reduces buckthorn populations and increases native species diversity. For long-term control of buckthorn seedling establishment, follow-up treatments like applying foliar herbicide sprays can be used in addition to prescribed burning.

Responses of native and invasive woody seedlings to combined competition and drought are species-specific.

Woody species invasions are a major threat to native communities with intensified consequences during increased periods of summer drought as predicted for the future. Competition for growth-limiting nitrogen (N) between native and invasive tree species might represent a key mechanism underlying the invasion process, because soil water availability and N acquisition of plants are closely linked. To study whether the traits of invasive species provide an advantage over natives in Central Europe in the competition for N under drought, we conducted a greenhouse experiment. We analyzed the responses of three native (i.e., Fagus sylvatica L., Quercus robur L. and Pinus sylvestris L.) and two invasive woody species (i.e., Prunus serotina Ehrh. and Robinia pseudoacacia L.) to competition in terms of their organic and inorganic N acquisition, as well as allocation of N to N pools in the leaves and fine roots. In our study, competition resulted in reduced growth and changes in internal N pools in both native and invasive species mediated by the physiological characteristics of the target species, the competitor, as well as soil water supply. Nitrogen acquisition, however, was not affected by competition indicating that changes in growth and N pools were rather linked to the remobilization of stored N. Drought led to reduced N acquisition, growth and total soluble protein-N levels, while total soluble amino acid-N levels increased, most likely as osmoprotectants as an adaptation to the reduced water supply. Generally, the consequences of drought were enhanced with competition across all species. Comparing the invasive competitors, P. serotina was a greater threat to the native species than R. pseudoacacia. Furthermore, deciduous and coniferous native species affected the invasives differently, with the species-specific responses being mediated by soil water supply.

Impacts of experimental defoliation on native and invasive saplings: are native species more resilient to canopy disturbance?

Many non-native, invasive woody species in mesic forests of North America are both shade tolerant and more productive than their native counterparts, but their ability to tolerate disturbances remains unclear. In particular, complete defoliation associated with herbivory and extreme weather events may have larger impacts on invaders if natives maintain greater resource reserves to support regrowth. On the other hand, invaders may be more resilient to partial defoliation by means of upregulation of photosynthesis or may be better able to take advantage of canopy gaps to support refoliation. Across a light gradient, we measured radial growth, new leaf production, non-structural carbohydrates (NSCs), chlorophyll content and survival in response to varying levels of defoliation in saplings of two native and two invasive species that commonly co-occur in deciduous forests of Eastern North America. Individuals were subjected to one of the four leaf removal treatments: no-defoliation controls, 50% defoliation over three growing seasons, 100% defoliation over one growing season and 100% defoliation over two growing seasons. Contrary to our hypothesis, native and invasive species generally did not differ in defoliation responses, although invasive species experienced more pronounced decreases in leaf chlorophyll following full defoliation and native species' survival was more dependent on light availability. Radial growth progressively decreased with increasing defoliation intensity, and refoliation mass was largely a function of sapling size. Survival rates for half-defoliated saplings did not differ from controls (90% of saplings survived), but survival rates in fully defoliated individuals over one and two growing seasons were reduced to 45 and 15%, respectively. Surviving defoliated saplings generally maintained control NSC concentrations. Under high light, chlorophyll concentrations were higher in half-defoliated saplings compared with controls, which may suggest photosynthetic upregulation. Our results indicate that native and invasive species respond similarly to defoliation, despite the generally faster growth strategy of invaders.

Prioritization and Planning to Improve Urban Tree Health in the Chicago Region

The Chicago Region Trees Initiative (CRTI) has collected one of the largest data sets on urban forestry in the United States. This data informs where and how CRTI prioritizes its work and capacity building. This data has been incorporated into interactive online resources that communities and neighborhoods can access to help decision makers, landowners, and managers understand where and what to plant, the value of the urban forest, impacts of woody invasive species, heat island challenges, and where opportunities exist for oak ecosystem enhancement. This data helps CRTI and its partners to prioritize action.

Evaluating the role of phenology in managing urban invasions: A case study of Broussonetia papyrifera

Multidisciplinary approaches are required for the management of invasive woody species in urban areas. In this context, phenological studies are a useful tool to understand tree development and devise suitable management strategies under urban forestry conditions. The significant role of phenology in attributing competitive advantages to invasive alien species has also been long recognized by community ecologists. Therefore, phenological calendars of invasive species may help in their detection, management and mitigation. In the present study, an attempt has been made to understand the invasive potential of an emerging invasive tree of urban landscapes, Broussonetia papyrifera (paper mulberry; Moraceae), through its phenological assessment by using a standardized numerical scale, BBCH (Biologische Bundesanstalt, Bundessortenamt, CHemische Industrie). The tree is native to southeastern and eastern Asia and the Pacific Islands, and is rapidly spreading across various tropical and subtropical regions. The phenology of B. papyrifera was described in eight principal growth stages (PGSs) with two developmental cycles in a year, presented as primary (January-June) and secondary (June-November) flushes. The observations were further supported by the corresponding dates, photographs, meteorological data (air temperature, precipitation, and photoperiod), and climatic water balance of the study area. Results suggest that the occurrence of two developmental cycles, a prolonged reproductive period, strategy to attract frugivores with brightly colored pulpy fruits, and ability to survive under a wide temperature range help in the spread of B. papyrifera. The phenological scale provided in this study describes accurate and precise developmental stages of the tree that can be used to devise efficient management strategies for its control in urban areas. The information can also be exploited for assessing the climatic conditions required for its prevalence, predicting its future geographic range, and further research.

Different resource-use strategies of invasive and native woody species from a seasonally dry tropical forest under drought stress and recovery

Exotic plants in semiarid region have developed strategies for efficient use or capture of resources. They have become invasive and outperform native species. To understand which factors could explain the success of invasive woody species in a semiarid region, several physiological traits were analyzed in young plants of two invasive and two native species exposed to different water availability. Invasive plants showed low leaf construction cost, high phosphorus and nitrogen contents, reduced loss of instantaneous energy use efficiency, and smaller specific leaf area when compared to native species. This strategy led to a higher biomass gain and a high root/shoot ratio in both water treatments. After rehydration, invasive plants showed faster recovery and higher rates of CO2 assimilation. This resilience is fundamental for species in semiarid regions, and also increase uptake of nutrients. Maintaining a high photosynthetic rate, whenever there is water availability is a strategy that increases the performance of the species in relation to biomass gain. The low leaf construction cost and the fast recovery of the photosynthetic metabolism of invasive plants after limiting water resources explains the success of these species, and suggests that their potential may increase under prolonged and severe drought seasons.

Evaluating Regeneration in Heterobasidion Root Disease Infested Stands in the Lake States

Abstract Heterobasidion root disease (HRD) infection is becoming increasingly common in red pine plantations of the Lake States. Although stump treatment can reduce rates of overland transmission, infested stands continue to experience expansion of mortality pockets. Natural regeneration into HRD mortality pockets can assist with revegetation. The objective of this study was to determine what tree species naturally regenerate in red pine plantations impacted by HRD in Wisconsin and Michigan. We visited 49 pockets (in 31 stands) in Wisconsin and Michigan that had existing confirmed HRD infections during the fall of 2017. We surveyed the regeneration in the pocket and measured the species and basal area of overstory trees 10 m from the expanding edge of the pocket. Overall, pockets were regenerating with desirable species, some of which are susceptible to HRD. Additionally, woody invasive species were present in 39 percent of the pockets. We expect these pockets to generally fill with desirable species of regeneration, although long-term successful regeneration by susceptible species such as pines may be limited.

Predicting invasiveness of exotic woody species using a traits-based framework.

Identifying potentially invasive species and preventing their introduction and establishment are of critical importance in invasion ecology and land management. Although an extensive body of research has been dedicated to identifying traits that confer invasiveness, our current knowledge is still often inconclusive due to limitations in geographic extent and/or scope of traits analyzed. Here, using a comprehensive set of 45 traits, we performed a case study of invasive traits displayed by exotic woody plants in the United States (U.S.) by comparing 63 invasive and 794 non-invasive exotic woody plant species naturalized across the country. We found that invasive woody species often bear the following two key traits: vegetative reproduction and long-distance seed dispersal (via water, birds or mammals). Boosted classification tree models based on these traits accurately predicted species invasiveness (86% accuracy on average). Presented findings provide a generalized understanding of the relative importance of functional traits in identifying potentially invasive woody species in the U.S. The knowledge generated in this study can be used to improve current classification systems of non-native woody plants used by various U.S. governmental agencies and land managers.

UAV data as alternative to field sampling to map woody invasive species based on combined Sentinel-1 and Sentinel-2 data

Invasive plant species can pose major threats to biodiversity, ecosystem functioning and services. Satellite based remote sensing has evolved as an important technology to spatially map the occurrence of invasive species in space and time. With the new era of the Sentinel missions, Synthetic Aperture Radar (SAR) and multispectral data are now freely available and repeatedly acquired on a high spatial and temporal resolution for the entire globe. However, the high potential of such sensors for automatic mapping procedures cannot be fully harnessed without sufficient and appropriate reference data for model calibration. Reference data are commonly acquired in field surveys, which however, are often relatively expensive and affected by sampling and observer bias. Moreover, a direct transferability to the remote sensing perspective and scale is difficult. Accordingly, we firstly assess the potential of Unmanned Aerial Vehicles (UAV) for semi-automatic reference data acquisition on species cover of three woody invasive species Pinus radiata, Ulex europaeus and Acacia dealbata occurring in Chile. Secondly, we test the upscaling of the estimated species cover to the spatial scale of Sentinel-1 and Sentinel-2. The proposed workflow includes the visual sampling of respective canopies in UAV orthomosaics and the subsequent spatial extrapolations using MaxEnt with spectral (RGB, Hyperspectral), textural (2D) and canopy structural (3D) predictors derived from UAV-based photogrammetry. These UAV-based maps are then used to train random forest models with multitemporal Sentinel-1 and Sentinel-2 data to map the invasive species cover on large spatial scales. Our results show that the semi-automatic UAV-based mapping of the three invasive species results in accurate predictions. Depending on the predictor combination, the correlation was 0.70, 0.77 and 0.90 for Pinus radiatia, Ulex europaeus, Acacia dealbata, respectively. Among the three species, we observed clear differences in the model performance between the tested photogrammetric predictors and their combinations (spectral, 2D texture or 3D structure). For scaling up the UAV-based estimates to the satellite-scale, the Sentinel-2 data (multispectral) were more important than Sentinel-1 data (SAR). An independent validation revealed that the R2 of the upscaling accounted for 0.78 or higher for all species and RMSE lower than 12%. Our results hence demonstrate that UAV-based reference data acquisitions are a promising alternative to traditional field surveys if the target species are directly identifiable in the UAV data.

Characteristics of the soil seed bank of invasive and non-invasive plants in their native and alien distribution range

The ability to form persistent seed banks might contribute substantially to determine the invasion potential of alien plants in their new distribution ranges, given the role of seed banks as sources of propagules, genetic diversity, and in spreading the risk of germination failure over time. Using the largest seed bank dataset collated to date, comprising 14,293 records for 2566 species, we examined whether the type (transient vs persistent) and density of the seed banks of invasive species differ in their native (home) and alien (abroad) range, and whether these attributes differ among invasive and non-invasive congeners, at home and abroad. A lower probability of forming a persistent seed bank in the alien range was identified when analyzing data for 140 invasive species, although phylogenetic analyses run for 104 of those species did not confirm such differences. However, invasive woody species formed denser seed banks in the alien range, suggesting greater seed production and/or lower seed predation or mortality in the alien than native range. Interestingly, invasive species consistently showed a higher probability of forming persistent seed banks as well as denser seed banks than their non-invasive congeners in their native range, but not in their alien range. These findings provide the first quantitative evidence, based on a large number of species globally, of preadaptation with respect to species life-history traits resulting in the formation of a persistent seed bank in invasive species compared to their non-invasive congeners. The fact that both invasive and non-invasive congeners have similar probabilities of forming persistent seed banks abroad suggests that this might be an important attribute for the establishment of alien species in new ranges (naturalization phase), but not for their spread (invasion phase). Our findings also indicate that the characteristics of native seed banks should be an important component of risk assessments aimed at identifying species that are more likely to become invasive if introduced in new ranges.

Species-Specific Outcome in the Competition for Nitrogen Between Invasive and Native Tree Seedlings.

The outcome of competition for nitrogen (N) between native and invasive tree species is a major concern when considering increasing anthropogenic N deposition. Our study investigated whether three native (i.e., Fagus sylvatica, Quercus robur, and Pinus sylvestris) and two invasive woody species (i.e., Prunus serotina and Robinia pseudoacacia) showed different responses regarding morphological and physiological parameters (i.e., biomass and growth indices, inorganic vs. organic N acquisition strategies, and N allocation to N pools) depending on the identity of the competing species, and whether these responses were mediated by soil N availability. In a greenhouse experiment, tree seedlings were planted either single or in native-invasive competition at low and high soil N availability. We measured inorganic and organic N acquisition using 15N labeling, total biomass, growth indices, as well as total soluble amino acid-N and protein-N levels in the leaves and fine roots of the seedlings. Our results indicate that invasive species have a competitive advantage via high growth rates, whereas native species could avoid competition with invasives via their higher organic N acquisition suggesting a better access to organic soil N sources. Moreover, native species responded to competition with distinct species- and parameter-specific strategies that were partly mediated by soil N availability. Native tree seedlings in general showed a stronger response to invasive P. serotina than R. pseudoacacia, and their strategies to cope with competition reflect the different species’ life history strategies and physiological traits. Considering the responses of native and invasive species, our results suggest that specifically Q. robur seedlings have a competitive advantage over those of R. pseudoacacia but not P. serotina. Furthermore, native and invasive species show stronger responses to higher soil N availability under competition compared to when growing single. In conclusion, our study provides insights into the potential for niche differentiation between native and invasive species by using different N forms available in the soil, the combined effects of increased soil N availability and competition on tree seedling N nutrition, as well as the species-specific nature of competition between native and invasive tree seedlings which could be relevant for forest management strategies.