Alien Shrub Research Articles

×Sorbaronia ′Mitschurinii′—an overlooked alien shrub rapidly expands in temperate Scots pine forests

Abstract Biological invasions are one of the most important threats to biodiversity. However, spread mechanisms have been described in detail only for some widespread taxa. Here, we aim to characterize the unique phenomenon of ×Sorbaronia ′Mitschurinii′ (Rosaceae) spread into temperate Scots pine forests. We assessed the surroundings of an abandoned ×Sorbaronia ′Mitschurinii′ plantation, established in 1986 near Ośno Lubuskie (W Poland). Around the plantation we established 66 study plots in 30 m intervals along eight transects, in a systematic design. Using generalized linear mixed-effects models we analysed how the distance from the propagule source (i.e. abandoned plantation), maximum age of ×Sorbaronia shrubs assessed using dendrochronological techniques, and light availability determined the density and biomass of the studied plant. We found that its mean aboveground biomass in the plantation was 7.22 ± 0.66 Mg ha−1, and decreased with distance from the propagule source: 6.83 Mg ha−1 30 m from the abandoned plantation, 1.38 Mg ha−1 at a distance of 90 m, and 0.11 Mg ha−1 at a distance of 180 m, almost disappearing at further distances (but present at the maximum distance of 450 m). The density and aboveground biomass of ×Sorbaronia ′Mitschurinii′ increased with increasing maximal age of the studied plant and its light availability. The ×Sorbaronia ′Mitschurinii′ plantation acted as a strong propagule source, as it reached a density of 100 ind. 100 m−2 after five years, forming a dense shrub layer, capable of reproducing. We conclude that alien species planted at high density and left with no management control might transform into invasion hotspots under favourable circumstances. The study highlights the importance of scientific awareness of similar cases of overlooked invasion sources, as management to limit their spread is still possible.

High temperature can improve the performance of invasive plants by facilitating root growth.

The ever-increasing temperatures of the Anthropocene may facilitate plant invasions. To date, studies of temperature effects on alien plants have mainly focused on plant aboveground traits but ignored belowground roots, which may hinder our predictions of plant invasion risks under climate change. The temperature effects on the root growth dynamics of two alien shrubs, invasive Mimosa sepiaria and naturalized Corchorus capsulari, were studied through a 3D transparent growth system under five temperature treatments (18/13°C - 34/29°C), which cover the present and future warming scenario temperature distributions in China. We measured the dynamics of both root depth and width growth in response to temperature treatments across time (84 days). We also investigated the intraspecies and interspecies competitions of the two alien species along the temperature gradient by planting two individuals together. Both M. sepiaria and C. capsularis showed optimum shoot growth in the middle-temperature treatment, whereas M. sepiaria exhibited the fastest root growth under the highest temperature treatment (34/29°C), and the root growth of C. capsularis persistently decreased as temperature increased. Root depth growth was more susceptible than root width to neighbor competition pressure in both species. Compared to C. capsularis, M. sepiaria had relatively stronger intraspecies and interspecies competition advantages with increasing temperature, possibly because high temperature improved root growth. These results suggest that temperature increases can improve the performance of alien plants by facilitating the groth of their underground root widths and depths, which deserves serious concern in future invasion risk management. This article is protected by copyright. All rights reserved.

Reproductive Ecology of the Invasive Alien Shrub Pyracantha angustifolia in the Grassland Biome, South Africa.

Knowledge on reproductive traits of problematic invasive alien plants, such as the woody invasive shrub Pyracantha angustifolia of temperate Chinese origin, can help better manage invasive species. To determine factors contributing to its invasion, we investigated floral visitors and pollen loads, self-compatibility, seed set, seed rain, soil seed banks, and seed longevity in the soil. Generalist insects were recorded visiting flowers and all carried pollen loads of high purity (>70%). Floral visitor exclusion experiments showed that P. angustifolia can set seed (66%) without pollen vectors, although natural pollination resulted in higher fruit set (91%). Fruit count surveys and seed set showed an exponentially increased relationship between seed set and plant size with high natural seed yield (±2 million seeds m-2). Soil core samples revealed a high seed density of 46,400 ± (SE) 8934 m-2 under shrubs, decreasing with distance away from the shrub. Bowl traps stationed under trees and fences confirmed that seeds were efficiently dispersed by animals. Buried seeds survived for less than six months in the soil. Due to high seed production, self-compatibility augmented by generalist pollen vectors, and effective seed dispersal by local frugivores, it is difficult to manage the spread manually. Management of this species should focus on the short life span of seeds.

Fuel trait effects on flammability of native and invasive alien shrubs in coastal fynbos and thicket (Cape Floristic Region).

In June 2017, extreme fires along the southern Cape coast of South Africa burnt native fynbos and thicket vegetation and caused extensive damage to plantations and residential properties. Invasive alien plants (IAPs) occur commonly in the area and were thought to have changed the behaviour of these fires through their modification of fuel properties relative to that of native vegetation. This study experimentally compared various measures of flammability across groups of native and alien invasive shrub species in relation to their fuel traits. Live plant shoots of 30 species (10 species each of native fynbos, native thicket, and IAPs) were sampled to measure live fuel moisture, dry biomass, fuel bed porosity and the proportions of fine-, coarse- and dead fuels. These shoots were burnt experimentally, and flammability measured in terms of maximum temperature (combustibility), completeness of burn (consumability), and time-to-ignition (ignitability). Multiple regression models were used to assess the relationships between flammability responses and fuel traits, while the Kruskal-Wallis H test was used to establish if differences existed in flammability measures and fuel traits among the vegetation groups. Dry biomass significantly enhanced, while live fuel moisture significantly reduced, maximum temperature, whereas the proportion of fine fuels significantly increased completeness of burn. Unlike other similar studies, the proportion of dead fuels and fuel bed porosity were not retained by any of the models to account for variation in flammability. Species of fynbos and IAPs generally exhibited greater flammability in the form of higher completeness of burn and more rapid ignition than species of thicket. Little distinction in flammability and fuel traits could be made between species of fynbos and IAPs, except that fynbos species had a greater proportion of fine fuels. Thicket species had higher proportions of coarse fuels and greater dry biomass (~fuel loading) than species of fynbos and IAPs. Live fuel moisture did not differ among the vegetation groups, contrary to the literature often ascribing variation in flammability to fuel moisture differences. The fuel traits investigated only explained 21–53% of the variation in flammability and large variation was evident among species within vegetation groups suggesting that species-specific and in situ community-level investigations are warranted, particularly in regard fuel moisture and chemical contents.

Seed dispersal by frugivores and germination of the invasive alien shrub Pyracantha angustifolia (Franch.) C.K. Schneid. in Free State Province, South Africa

Seed dispersal by frugivores and germination of the invasive alien shrub Pyracantha angustifolia (Franch.) C.K. Schneid. in Free State Province, South Africa

‘The Best of Two Worlds’—Combining Classifier Fusion and Ecological Models to Map and Explain Landscape Invasion by an Alien Shrub

The spread of invasive alien species promotes ecosystem structure and functioning changes, with detrimental effects on native biodiversity and ecosystem services, raising challenges for local management authorities. Predictions of invasion dynamics derived from modeling tools are often spatially coarse and therefore unsuitable for guiding local management. Accurate information on the occurrence of invasive plants and on the main factors that promote their spread is critical to define successful control strategies. For addressing this challenge, we developed a dual framework combining satellite image classification with predictive ecological modeling. By combining data from georeferenced invaded areas with multispectral imagery with 10-meter resolution from Sentinel-2 satellites, a map of areas invaded by the woody invasive Acacia longifolia in a municipality of northern Portugal was devised. Classifier fusion techniques were implemented through which eight statistical and machine-learning algorithms were ensembled to produce accurate maps of invaded areas. Through a Random Forest (RF) model, these maps were then used to explore the factors driving the landscape-level abundance of A. longifolia. RF models were based on explanatory variables describing hypothesized environmental drivers, including climate, topography/geomorphology, soil properties, fire disturbance, landscape composition, linear structures, and landscape spatial configuration. Satellite-based maps synoptically described the spatial patterns of invaded areas, with classifications attaining high accuracy values (True Skill Statistic, TSS: 0.895, Area Under the Receiver Operating Curve, ROC: 0.988, Kappa: 0.857). The predictive RF models highlighted the primary role of climate, followed by landscape composition and configuration, as the most important drivers explaining the species abundance at the landscape level. Our innovative dual framework—combining image classification and predictive ecological modeling—can guide decision-making processes regarding effective management of invasions by prioritizing the invaded areas and tackling the primary environmental and anthropogenic drivers of the species’ abundance and spread.

Phenotypic plasticity may mediate habitat filtering in a forest edge community

A key process determining community assembly is habitat filtering, which reduces the range of trait values and thus leads to trait convergence. Habitat filtering is often assumed to involve a reduction in species richness, but such trait convergence could also result from phenotypic plasticity. By allowing more species in the community to show the ‘right' phenotype and pass the filter, phenotypic plasticity could maintain species richness despite trait convergence. If phenotypic plasticity plays this role, environmental changes should modify trait distribution but not species composition or richness. We tested this hypothesis in a forest edge community recently invaded by the alien shrub Rubus praecox. We measured functional traits beneath Rubus canopy (Rubus‐invaded community) and in adjacent microsites naturally free from Rubus (original community), comparing trait dispersion and community structure parameters. We also characterized the microenvironments to seek functional explanations for the observed phenotypic changes. The functional dispersion parameter FDis showed habitat filtering in the Rubus‐invaded community (trait convergence), while in the original community it did not differ from a null model. Species richness and composition did not differ between the two communities. Soil characteristics were similar in both communities but light availability was greatly reduced in the Rubus‐invaded community. A principal component analysis of community‐weighted means of functional traits showed that phenotypic changes between communities were mostly consistent with functional responses to shading, including an increase in specific leaf area and a reduction in flavonoid concentration in the Rubus‐invaded community. We verified the conditions necessary to infer that phenotypic plasticity mediates the outcome of habitat filtering driven by Rubus in the study system, i.e. plasticity would explain species persistence despite trait convergence. Phenotypic plasticity, often subsumed within intraspecific variation in community assembly studies, should be considered individually to improve our understanding of the causes and consequences of habitat filtering.

Can Sentinel-2 be used to detect invasive alien trees and shrubs in Savanna and Grassland Biomes?

The impact of alien plant invasions on ecosystems translates into major economic losses for nations across the globe. Freely available satellite imagery can be used to provide relevant data for effective management of alien plant invasions, especially for developing nations with limited budgets. However, detecting alien herbaceous and shrubby vegetation using freely available imagery has not been widely tested. The aim of this study was to detect invasive alien plants in the Savanna and Grassland Biomes of the uMngeni Catchment. The novelty of this study is the focus on not only alien trees, but also alien shrubs, which tend to be more sparsely spread and are harder to detect. Using the free Google Earth Engine Platform for repeatability, we explore the efficacy of data fusion of Sentinel-2, Sentinel-1 and landform data, various feature selection techniques and two powerful non-parametric classifiers. We found that feature selection techniques result in lower classification accuracies and are therefore not recommended for similar applications. Data fusion, such as combining Sentinel-2, Sentinel-1 and landform data, did not improve classification results. Support Vector Machine shows superiority over the Random Forest classifier. The final classifications indicate, with 80–90 % accuracy, that for 2019 about 17.5 % (736.15–763.75 km2) of the uMngeni Catchment was occupied by invasive alien trees and shrubs (e.g., gums, wattles, pines and Bugweed), either in the form of plantations or invasions. This translates to between 88.3 million and 173 million m3 of water per year in evapotranspiration losses, or 23 % or the current system yield. At a spatial resolution of 20 × 20 m, these outputs are useful for regional or local planning and prioritization. Furthermore, the repeatable approach of this study is a significant advance compared to past efforts that have produced publicly available datasets for the region, and could be employed by other data-scarce developing nations.

Controlling invasive alien shrub species, enhancing biodiversity and mitigating flood risk: A win–win–win situation in grazed floodplain plantations

The high nature conservation value of floodplain ecosystems is severely threatened by invasive alien species. Besides adversely affecting native biodiversity, these species also pose a major threat from a wider socio-ecological perspective (e.g. ‘roughness’ increases flood risk). Finding options to control dense shrub layers consisting of invasive alien species is therefore of high priority for multipurpose management. We studied cattle grazing impacts on the cover, composition and diversity of the herb and shrub layers in floodplain poplar plantations along the Tamiš river, Serbia. Non-grazed, moderately grazed, intensively grazed and resting place stands were sampled in five locations in three sampling points. Non-grazed stands had substantially higher cover of invasive alien shrub species (on average 65%) than moderately and intensively grazed stands, and resting places (5.17, 0.02 and 0.00%, respectively), but without considerable differences between the grazing intensity categories. The number of invasive alien species in the shrub layer decreased considerably from non-grazed to intensively grazed stands. Species composition in the herb layer changed from non-grazed to intensively grazed stands, while resting places differed substantially from the other categories. Total species richness, richness of native generalist herbaceous grassland species, and the cover of palatable grasses were the highest in moderately and intensively grazed stands. Our results suggest that cattle grazing in floodplains is effective at controlling invasive alien shrub species. Furthermore, continuous moderate or intensive grazing would contribute to multifunctional management of invaded floodplains by enhancing local biodiversity, reducing flood risk, and providing additional grazing areas for the local community.

Life history traits and host suitability of a gall-forming fly, Polymorphomyia basilica (Diptera: Tephritidae), for the biological control of Chromolaena odorata (Asteraceae) in South Africa

The tephritid Cecidochares connexa has been used to good effect as a biological control agent on the Asian/West African biotype of the invasive alien shrub Chromolaena odorata, but does not develop well on the different, southern African C. odorata biotype. A stem-galling tephritid fly, Polymorphomyia basilica, from the northern Caribbean islands, was considered as a potential biological control agent for C. odorata in South Africa. Life history traits and host range on 32 asteraceous plants were investigated in single-choice tests and using single pairs of adults in no-choice tests, under laboratory conditions. Positive biological characteristics of P. basilica include a high rate of increase, the production of several generations per year, long-lived and mobile adults, the ability of females to produce viable offspring without repeated mating, and the ability of adults to eclose from galls on dry stems. Use of a single pair of adults for no-choice tests proved to be efficient. Oviposition and larval development through to adulthood occurred on three other South American and on two South African species; for the latter, one was in the tribe Eupatorieae, closely related to C. odorata, and the other in the Astereae, less closely related to the weed, but both at a lower and slower rate. Females tended to retain their eggs under no-choice conditions in the presence of an unsuitable host, and to compensate by ovipositing at a higher rate when presented later with a C. odorata plant. The poor offspring survival on non-target plants tested in this study demonstrates the suitability of P. basilica for release in South Africa.

Similar Impacts of Alien and Native Tree Species on Understory Light Availability in a Temperate Forest

Research Highlights: We evaluated influence of alien and native trees and shrubs on stand leaf area index to basal area ratio, indicating that both groups provide similar amounts of foliage. Background and Objectives: Foliage traits determine tree species effect on understory light availability. Direct comparisons of understory light availability due to different foliage traits of tree species are conducted less often at the stand level. We hypothesized that light availability is driven by canopy leaf area, and alien species contribution to canopy foliage will be similar to native species due to analogous patterns of biomass allocation in tree species. Materials and Methods: We studied forests dominated by alien and native tree species in Wielkopolski National Park (Western Poland). We measured light availability using the LAI-2200 canopy analyzer (Li-Cor Inc., Lincoln, NE, USA) and we calculated leaf area index (LAI) in 170 stands using published models of foliage biomass and data on specific leaf area. Results: Our study confirmed an impact of LAI on light availability in the understory layer. Analyzing the proportion of contribution to stand LAI and basal area (BA) we found that most alien species did not differ in LAI to BA ratio from native species. The exception was Prunus serotina Ehrh., with a LAI to BA ratio higher than all native and alien trees. However, the highest LAI to BA ratios we found were for the alien shrub Cotoneaster lucidus Schltdl. and native shrubs of fertile broadleaved forests. Conclusions: Our study showed that alien species contribution to shading the understory is comparable to native species, with the exception of P. serotina due to its dominance in the higher shrub canopy strata where it exhibits different patterns of biomass allocation than native trees. Our study explained that invasive tree species impact on light availability in forest ecosystems is mainly mediated by the increased quantity of foliage, not by more effective LAI to BA ratio.

Interactions between two biological control agents and their target weed: a beetle, a bug and a cactus weed

ABSTRACTPereskia aculeata Miller (Cactaceae) is an invasive alien shrub introduced into South Africa from Brazil. The leaf-feeding beetle, Phenrica guerini Bechyne (Chrysomelidae), was released as a biological control agent in South Africa in 1991 followed by the stem-wilting bug, Catorhintha schaffneri Brailovsky & Garcia (Coreidae), in 2014. This study investigated the interactions between the two agents under laboratory conditions. Potted plants were exposed to one of four treatments: control (no agents), P. guerini only, C. schaffneri only and both species together. Four densities, ranging from 2 to 12 insects per plant were used. Cathorhitha schaffneri alone at low to moderate densities resulted in the same reduction in number of leaves and shoot length as when combine with P. guerini. At the highest density, C. schaffneri reduced the number of leaves significantly more than any treatment. Mortality of P. guerini was significantly higher than C. schaffneri at the highest density when in combination. The antagonistic interaction between P. guerini and C. schaffneri suggests that these agents should not be released together because this would impact negatively on the overall biocontrol programme against P. aculeata. It is recommended that C. schaffneri should be released at sites where P. guerini is not present. Extrapolation of laboratory-based studies into the field is often challenging, so mass-rearing and releases of P. guerini should continue until there is convincing proof that C. schaffneri alone is more effective than P. guerini in the field.

Facilitation of management plan development via spatial classification of areas invaded by alien invasive plant

Propagule supply and habitat suitability strongly influence the success of invasive alien plants. Thus, an invaded area is likely to have an adequate propagule supply, a suitable habitat, or both for species persistence. Based on this idea, we classified invaded areas into four categories as follows but with establishment still occurring in some cases: Class 1, adequate propagule supply and habitat suitability; Class 2, adequate propagule supply but limited habitat suitability; Class 3, limited propagule supply and adequate habitat suitability; and Class 4, mid- to low-level propagule supply and habitat suitability. We propose a framework for the classification of invaded areas into these four classes and present a case study in which this framework was applied. Classifying target areas in this manner could facilitate more efficient and practical management planning, thereby saving time and resources. We selected the alien shrub Leucaena leucocephala L. (Fabaceae) as a model species, which has invaded the Nakodo-jima Island in the Ogasawara Archipelago of Japan. We developed a species distribution model by incorporating proxy variables for propagule supply and habitat suitability as well as submodels for propagule supply or habitat suitability. Using these submodels, we estimated the levels of propagule supply and habitat suitability in each, and classified the current distribution range appropriately. Using these classifications, land managers could set priorities to concentrate their efforts to efficiently control target species.

Evaluation of factors that affect the spatial distribution of alien shrubs to monitor their invasion in a Pampa protected landscape in Tandil, Argentina

Invasive alien plants which replace native species represent a global threat that compromises biodiversity conservation by simplifying and homogenizing the landscape. The present study focuses on the problem of invasions in a mountainous area of the pampas (Tandil, Argentina) where several alien shrubs behave as invaders displacing a high diversity of native grassland. The aim of the study was to analyze, by means of geographical information system, the distribution of the most conspicuous alien shrub populations in relation to natural and anthropogenic factors in a protected area of landscape of the pampa region, in order to contribute to decision making and management based on knowledge. The selected working areas were those with spontaneous vegetation, conducting unsupervised classification of SPOT satellite images. A number of points in invaded areas were georeferenced in field to control the classification made by geographical information system. To measure the influence of the factors analyzed in the dispersal processes of the species spatial logistic regression techniques were used. Results showed that 20.40% of the study area was occupied by alien species of which 11.06% were tree plantations made by man and 9.34% corresponded to three alien shrub species distributed invading different landscape sections: Genista monspesullana occupied 307.85 ha, Rubus ulmifolius 237.82 ha and Spartium junceum 126.55 ha. In order to explain R. ulmifolius propagation all the analysed variables except quarries and pits were significant, but streets, roads and trails and stone fences had a stronger incidence. For S. junceum the significant explanatory variables were quarries and pits, altitude and slope. For G. monspesullana the explanatory variable was streets, roads and trails.

Invasion of the alien shrub Prunus laurocerasus in suburban deciduous forests: Effects on native vegetation and soil properties

Most invading alien plants affect native biodiversity and ecosystem functioning. In a field survey, we assessed the impact of the invasive shrub Prunus laurocerasus on the native vegetation and soil properties in suburban deciduous forests in the region of Basel, Switzerland. We installed four pairs of plots in patches of P. laurocerasus and in adjacent not invaded areas in each of twelve forest areas. Native species richness, Shannon-diversity and species composition of the ground vegetation and shrub layer were assessed in each plot. Furthermore, in each plot we measured physical and soil chemical characteristics, enzyme activities and the carbon source utilization pattern of the soil microbial community using Ecoplates™. The maximum age of P. laurocerasus in each plot was determined using tree ring analysis, indicating the time elapsed since the invasive plant has established. A lower native plant species richness in both the ground vegetation and shrub layer was observed in plots with presence of P. laurocerasus. A different species composition of the ground vegetation was also found among plots with and without the invasive shrub. Plots invaded by P. laurocerasus had a lower soil moisture content than control plots. The intensity, diversity and substrate richness of the carbon sources were increased in soil from invaded plots compared to soil in control plots. However, the chemical soil characteristics examined and the activities of enzymes were not influenced by the invasive plant. The effects of P. laurocerasus became more pronounced with the time elapsed since the invasive plant has established. Thus, the removal of young P. laurocerasus individuals would be an appropriate management practice for this invasive shrub species.

Состав и численность адвентивных и инвазивных кустарников и деревьев подлеска в лесопарках г. Екатеринбурга

Состав и численность адвентивных и инвазивных кустарников и деревьев подлеска в лесопарках г. Екатеринбурга

Invasive Rhus typhina invests more in height growth and traits associated with light acquisition than do native and non-invasive alien shrub species

The preferential investment in height growth and traits associated with light acquisition may provide insight into the invasiveness of Rhus typhina. Plant traits have been shown to be associated with invasiveness. Rhus typhina, a shrub or small tree native to North America, shows invasiveness in non-native habitats. Compared to the native shrub Vitex negundo var. heterophylla and the non-invasive alien shrub Amorpha fruticosa, R. typhina showed a high photosynthetic capacity and specific leaf area but a low-leaf nitrogen content, indicating that R. typhina gained high photosynthetic income with low-leaf constituent cost. The non-photochemical quenching of R. typhina was the highest among the three species, which can be linked to its capacity for photoprotection under high light conditions. Allometry analyses showed that R. typhina invested more in height growth than the other two species, pointing to its investment strategy in light acquisition. The low leaf nitrogen-to-phosphorus ratio may be associated with the fast growth of R. typhina. Additionally, R. typhina allocated more biomass to photosynthetic leaves than did the other two species. The preferential investment strategies and traits associated with light acquisition may help explain the invasiveness of R. typhina.

Alien plants in forest communities of the Middle Volga Region: dissemination and degree of naturalization

In forest communities of the Middle Volga forest-steppe zone (within Penza, Samara and Ulyanovsk Regions) there are 60 alien species of vascular plants. 25 species of adventives (42%) have naturalized in forests, 7 species are epecophytes (Cotoneaster lucidus , Oenothera biennis, O. rubricaulis, and O. villosa, Populus suaveolens, Ulmus minor, Xanthoxalis stricta) and 18 species are agriophytes (Acer negundo, Amelanchier spicata, Bidens frondosa, Caragana arborescens, Conyza canadensis, Echinocystis lobata, Heracleum sosnowskyi, Impatiens parviflora, Fraxinus lanceolata, F. pennsylvanica, Lonicera tatarica, Lupinus polyphyllus, Malus domestica, Parthenocissus inserta, Salix euxina, Sambucus racemosa, S. sibirica, Ulmus pumila). Transformer-types are arboreal - Acer negundo, Fraxinus lanceolata, F. pennsylvanica, Parthenocissus inserta, Ulmus pumila, as well as herbaceous plants - Bidens frondosa, Conyza canadensis, Echinocystis lobata, Heracleum sosnowskyi, Impatiens parviflora. The introduction of alien trees and shrubs in the forest can lead to dense thickets, disrupt underbrush resumption and main forest-forming species growth, as well as negatively affect the herbaceous layer. Most vulnerable to the introduction of adventive plants are roadsides and the places where forests are close to human settlements. The introduction of alien plants in forest communities is the result of ornithochory (33; 54%), anemochory (11 species; 18%), anthropochory (8 species; 13%) or a combination of various ways of introduction.

Interaction between invasive and potentially invasive shrub species does not influence relationships between their ecological success and distance from propagule sources

Although interactions between alien and native plant species are well studied, data on interactions between two co-existing alien species with respect to their invasibility are scarce. The aims of this study were (1) to evaluate three factors shaping abundance of the alien shrub species Cornus alternifolia: abundance of another alien (invasive) shrub species (Prunus serotina), type of tree stand (coniferous vs. broadleaved) and distance to propagule sources and (2) to assess the potential dispersal distance of the species studied. Densities of both species were assessed within 194 experimental plots (located in experimental plantations of trees) in Rogów Arboretum (Central Poland). P. serotina occurred on 79 and C. alternifolia on 33 of the 194 plots. The furthest distance of C. alternifolia from the propagule source was 338 m. C. alternifolia reached higher densities in coniferous than broadleaved tree stands. Density of C. alternifolia depended on tree stand type and distance from the propagule source, but did not depend on density of P. serotina. Density of C. alternifolia decreased with increasing distance from the propagule source; however, this relationship was modified by the type of tree stand: densities were lower in broadleaved than in coniferous stands. The presence of the invasive species seems to neither facilitate nor limit the dispersal distance of C. alternifolia, as these two species differ in shade tolerance. The study also provided the first information about C. alternifolia potential invasiveness, because earlier this species was noticed only as casually escaping from cultivation in Slovakia.

Dominance of an alien shrub Rhus typhina over a native shrub Vitex negundo var. heterophylla under variable water supply patterns.

Temporal heterogeneity of a resource supply can have a profound effect on the interactions between alien and native plant species and their potential invasiveness. Precipitation patterns may be variable and result in a higher heterogeneity of water supply with global climate change. In this study, an alien shrub species, Rhus typhina, introduced to China from North America and a native shrub species, Vitex negundo var. heterophylla, were grown in monoculture and mixed culture under different water supply regimes, with four levels of water supply frequencies but with a constant level of total supplied water. After 60 days of treatments, the alien species was found to be the superior competitor in the mixed culture and was unaffected by changes in the water supply pattern. The dominance of R. typhina was mainly owing to its greater biomass and effective modulation of leaf physiology. However, in the mixed culture, V. negundo var. heterophylla exhibited both leaf- and whole-plant-level acclimations, including higher leaf length to petiole length and root to shoot biomass ratios, and lower specific leaf weight and leaf length to leaf width ratio. Plant height of V. negundo var. heterophylla was comparable to that of R. typhina in the mixed culture, which is a strategy to escape shading. Although water treatments had little effect on most traits in both species, the possible influence of water regimes should not be neglected. Compared with high-frequency water supply treatments, more individuals of V. negundo var. heterophylla died in low-water-frequency treatments when in competition with R. typhina, which may lead to species turnover in the field. The authors recommended that caution should be exercised when introducing R. typhina to non-native areas in the context of global climate change.