Seed-bank dynamics of native and invasive Impatiens species during a five-year field experiment under various environmental conditions

A Modeling Framework to Frame a Biological Invasion: Impatiens glandulifera in North America

Biological invasions are a major component of global environmental change with severe ecological and economic consequences. Since eradicating biological invaders is costly and even futile in many cases, predicting the areas under risk to take preventive measures is crucial. Impatiens glandulifera is a very aggressive and prolific invasive species and has been expanding its invasive range all across the Northern hemisphere, primarily in Europe. Although it is currently spread in the east and west of North America (in Canada and USA), studies on its fate under climate change are quite limited compared to the vast literature in Europe. Hybrid models, which integrate multiple modeling approaches, are promising tools for making projections to identify the areas under invasion risk. We developed a hybrid and spatially explicit framework by utilizing MaxEnt, one of the most preferred species distribution modeling (SDM) methods, and we developed an agent-based model (ABM) with the statistical language R. We projected the I. glandulifera invasion in North America, for the 2020-2050 period, under the RCP 4.5 scenario. Our results showed a predominant northward progression of the invasive range alongside an aggressive expansion in both currently invaded areas and interior regions. Our projections will provide valuable insights for risk assessment before the potentially irreversible outcomes emerge, considering the severity of the current state of the invasion in Europe.

Effect of Water Deficit on Germination, Growth and Biochemical Responses of Four Potentially Invasive Ornamental Grass Species

Ornamental plant species introduced into new environments can exhibit an invasive potential and adaptability to abiotic stress factors. In this study, the drought stress responses of four potentially invasive ornamental grass species (Cymbopogon citratus, Cortaderia selloana, Pennisetum alopecuroides and P. setaceum) were analysed. Several seed germination parameters were determined under increasing polyethylene glycol (PEG 6000) concentrations. Additionally, plants in the vegetative stage were subjected to intermediate and severe water stress treatments for four weeks. All species registered high germination rates in control conditions (no stress treatment), even at high PEG concentrations, except C. citratus, which did not germinate at -1 MPa osmotic potential. Upon applying the water stress treatments, P. alopecuroides plants showed the highest tolerance, and C. citratus appeared the most susceptible to drought. Stress-induced changes in several biochemical markers (photosynthetic pigments, osmolytes, antioxidant compounds, root and shoot Na⁺ and K⁺ contents), highlighted different responses depending on the species and the stress treatments. Basically, drought tolerance seems to depend to a large extent on the active transport of Na⁺ and K⁺ cations to the aerial part of the plants, contributing to osmotic adjustment in all four species and, in the case of the most tolerant P. alopecuroides, on the increasing root K⁺ concentration under water deficit conditions. The study shows the invasive potential of all species, except C. citratus, in dry areas such as the Mediterranean region, especially in the current climate change scenario. Particular attention should be given to P. alopecuroides, which is widely commercialised in Europe as ornamental.

Changes in the composition of the soil seed bank of grassland after giant ragweed (Ambrosia trifida L.) invasion

Giant ragweed (Ambrosia trifida L.), an invasive weed, has an expanding distribution area and has recently started to spread in grasslands. This unusual event threatens grasslands worldwide. In this study, we aimed to evaluate the changes in the grassland soil seed banks caused by the giant ragweed invasion in Yili Valley, Xinjiang, China. Using the space-for-time substitution approach, we compared and quantified the soil seed bank communities in a grassland over eight years following giant ragweed invasion and after its removal. The results showed that the duration of invasion determined whether giant ragweed might pose a significant threat to the native seed bank community. Four years after the invasion, the in-site seed bank density of native community significantly decreased (30.44%), while the relative coverage of giant ragweed aboveground reached 83.75%. Furthermore, the species richness in the seed bank decreased significantly (12.36%), while the relative coverage of giant ragweed reached 100% six years after the invasion. Eight years after the invasion, the seed bank density and species richness of the native community decreased by 83.28% and 39.33%, respectively, whereby the seed banks tended to be homogeneous. After the removal of giant ragweed, the potential for regeneration was limited by the residual seed bank densities of the native community. Although the native seed bank density had increased significantly after three years of restoration, new growth was dominated by weedy species, rather than by the distinctive components of the grassland habitat. Our study clarifies the process by which giant ragweed causes damage to grasslands and serves as a reference for grassland restoration and management efforts.

Comparative Analysis of Tolerance to Salt Stress and Water Deficit in Two Invasive Weeds of the Genus Erigeron (Asteraceae)

Erigeron bonariensis and E. sumatrensis are two noxious weeds present in many parts of the world. Their tolerance to salinity and water deficit was analysed at the seed germination stage and during vegetative development. Seed germination was tested in solutions with different concentrations of NaCl and polyethylene glycol (PEG). Growth parameters, photosynthetic pigments, ion accumulation, and antioxidant mechanisms were analysed in plants that were subjected to increasing NaCl solutions, or severe water deficit by completely restricting irrigation. Seed germination was mostly affected by NaCl, but less by PEG in both species. E. bonariensis had a faster germination in all treatments and maintained a higher percentage of germination under the highest concentration of salt applied. Growth responses were similar in the two species, both being more affected by higher salt concentrations than by water deficit. The main differences in the responses of the two species to stress regard K⁺ and proline concentration. K⁺ in roots decreased under salt stress in E. sumatrensis, but remained constant in leaves, whereas in E. bonariensis increased in roots and leaves in salt-stressed plants. Proline concentration increased in all E. bonariensis plants under salt stress, but only in those under the highest salt concentration in E. sumatrensis. The results obtained indicate that the two species are relatively tolerant to water deficit and medium salinity but are susceptible to high NaCl concentrations.

Long-term seed burial reveals differences in the seed-banking strategies of naturalized and invasive alien herbs

Soil seed viability and germinability dynamics can have a major influence on the establishment and spread of plants introduced beyond their native distribution range. Yet, we lack information on how temporal variability in these traits could affect the invasion process. To address this issue, we conducted an 8-year seed burial experiment examining seed viability and germinability dynamics for 21 invasive and 38 naturalized herbs in the Czech Republic. Seeds of most naturalized and invasive species persisted in the soil for several years. However, naturalized herbs exhibited greater seed longevity, on average, than invasive ones. Phylogenetic logistic models showed that seed viability (but not germinability) dynamics were significantly related to the invasion status of the study species. Seed viability declined earlier and more sharply in invasive species, and the probability of finding viable seeds of invasive species by the end of the experiment was low. Our findings suggest that invasive herbs might take advantage of high seed viability in the years immediately after dispersal, while naturalized species benefit from extended seed viability over time. These differences, however, are not sufficiently strong to explain the invasiveness of the species examined.

Relating Invasibility and Invasiveness: Case Study of Impatiens parviflora

Data on alien species show that plant invasions are caused by a complex combination of characteristics of invasive species (invasiveness) and characteristics of invaded environment (invasibility). Impatiens parviflora is one of Europe’s top invasive species. The present study aimed to evaluate molecular diversity of populations of highly invasive in Lithuania I. parviflora by applying several DNA markers and relating genetic parameters to abiotic and biotic environment. For sampling, urban forests, riparian forests, and agrarian shrublands were selected. Three different DNA-based techniques, Simple Sequence Repeats (SSR), Inter Simple Sequence Repeat (ISSR) markers, and Random Amplified Polymorphic DNA (RAPD), were used for detecting genetic variation between 21 populations. All population individuals were monomorphic and homozygotic for four loci and heterozygotic for one locus by SSR analysis. Hierarchical analyses of molecular variance at ISSR and RAPD loci revealed significant differentiation of populations depending on geographic zones of the country. Bayesian Structure analyses of molecular data demonstrated existence of many genetic clusters and this finding is in support to multiple introduction of the species. The polymorphism extent at ISSR loci was positively correlated with the total coverage of herbaceous plant species. The coverage by I. parviflora was negatively correlated with the total number of herbaceous species and light in the sites. Our results indicate that Lithuanian sites with I. parviflora might be distinguished by high soil nutrient levels. According to the principal component analysis, the coverage by I. parviflora was a more important variable of populations compared to molecular data or parameters of abiotic environment. In conclusion, complexity of invasibility and invasiveness factors determine the variability of I. parviflora sites, including genetic traits, coverage of invasive species and conditions of environment that were significant and interrelated.

Persistent soil seed banks promote naturalisation and invasiveness in flowering plants

With globalisation facilitating the movement of plants and seeds beyond the native range, preventing potentially harmful introductions requires knowledge of what drives the successful establishment and spread of alien plants. Here, we examined global-scale relationships between naturalisation success (incidence and extent) and invasiveness, soil seed bank properties (type and densities) and key species traits (seed mass, seed dormancy and life form) for 2350 species of angiosperms. Naturalisation and invasiveness were strongly associated with the ability to form persistent (vs. transient) seed banks but relatively weakly with seed bank densities and other traits. Our findings suggest that seed bank persistence is a trait that better captures the ability to become naturalised and invasive compared to seed traits more widely available in trait databases. Knowledge of seed persistence can contribute to our ability to predict global naturalisation and invasiveness and to identify potentially invasive flowering plants before they are introduced.