Early bird catches the worm: germination as a critical step in plant invasion

Rain-based soil solarization for reducing the persistent seed banks of invasive plants in natural ecosystems - Acacia saligna as a model

BACKGROUND

A large persistent seed bank of invasive plants is a significant obstacle to restoration programs. Soil solarization was demonstrated to be an effective method for reducing the seed bank of Australian acacias. However, use of this method in natural habitats might be limited due to the requirement to moisten the soil by irrigation. This study examined the possibility of replacing irrigation by trapping the soil moisture caused by the most recent rainfall, i.e. rain-based soil solarization (RBS).

RESULTS

Exposure of Acacia saligna seeds to 57 °C at 20% soil moisture for 68 h resulted in almost complete loss of seed viability. Similarly, RBS treatment significantly reduced the viability of A. saligna seeds buried at a soil depth of 1-19 cm as well as seed density in the natural seed bank, and almost completely eliminated seedling emergence from natural seed banks of A. saligna and other environmental weeds.

CONCLUSION

Our results indicate that RBS is an effective method for reducing the seed bank of invasive plants in natural habitats located in various climate regions characterized by different soil types. This is the first demonstration of a successful application of RBS for soil disinfestation. © 2018 Society of Chemical Industry.

The role of fruit heteromorphism in the naturalization of Asteraceae

BACKGROUND AND AIMS

Fruit heteromorphism is considered to be a bet-hedging strategy to cope with spatially or temporally heterogeneous environments. The different behaviours of the fruit morphs of the same species might also be beneficial during naturalization, once the species has been introduced to a new range. Yet, no study to date has tested the association between fruit heteromorphism and global-scale naturalization success for a large set of plant species.

METHODS

We compiled two large datasets on fruit heteromorphism in Asteraceae. One dataset was on native species in Central Europe (n = 321) and the other was on species frequently planted as ornamentals (n = 584). Using phylogenetic linear and logistic regressions, we tested whether heteromorphic species are more likely to naturalize outside their native range, and in more regions of the world than monomorphic species. We also tested whether the effect of heteromorphism is modulated by life history and height of the species.

KEY RESULTS

We show that heteromorphic species were more likely to naturalize outside their native range. However, among the naturalized species, heteromorphic and monomorphic species did not differ in the number of world regions where they became naturalized. A short life span and tall stature both promoted naturalization success and, when life history and height were included in the models, the effect of fruit heteromorphism on the ability to naturalize became non-significant. Nevertheless, among tall plants, heteromorphic ornamental species were significantly more likely to become naturalized in general and in more regions than monomorphic species.

CONCLUSIONS

Our results provide evidence that in Asteraceae the production of heteromorphic fruits is associated with naturalization success. It appears, however, that not fruit heteromorphism per se, but a successful combination of other biological traits in fruit heteromorphic species, namely short life span and tall stature, contributes to their naturalization success.

Differential germination strategies of native and introduced populations of the invasive species Plantago virginica

Germination strategies are critically important for the survival, establishment and spread of plant species. Although many plant traits related to invasiveness have been broadly studied, the earliest part of the life cycle, germination, has received relatively little attention. Here, we compared the germination patterns between native (North America) and introduced (China) populations of Plantagovirginica for four consecutive years to examine whether there has been adaptive differentiation in germination traits and how these traits are related to local climatic conditions. We found that the introduced populations of P.virginica had significantly higher germination percentages and faster and shorter durations of germination than native populations. Critically, the native populations had a significantly larger proportion of seeds that stayed dormant in all four years, with only 60% of seeds germinating in year 1 (compared to >95% in introduced populations). These results demonstrate striking differences in germination strategies between native and introduced populations which may contribute to their successful invasion. Moreover, the germination strategy of P.virginica in their native range exhibited clear geographical variation across populations, with trends towards higher germination percentages at higher latitudes and lower annual mean temperatures and annual precipitation. In the introduced range, however, their germination strategies were more conserved, with less variation amongst populations, suggesting that P.virginica may have experienced strong selection for earlier life history characteristics. Our findings highlight the need to examine the role of rapid evolution of germination traits in facilitating plant invasion.

A research agenda for seed-trait functional ecology

Trait-based approaches have improved our understanding of plant evolution, community assembly and ecosystem functioning. A major challenge for the upcoming decades is to understand the functions and evolution of early life-history traits, across levels of organization and ecological strategies. Although a variety of seed traits are critical for dispersal, persistence, germination timing and seedling establishment, only seed mass has been considered systematically. Here we suggest broadening the range of morphological, physiological and biochemical seed traits to add new understanding on plant niches, population dynamics and community assembly. The diversity of seed traits and functions provides an important challenge that will require international collaboration in three areas of research. First, we present a conceptual framework for a seed ecological spectrum that builds upon current understanding of plant niches. We then lay the foundation for a seed-trait functional network, the establishment of which will underpin and facilitate trait-based inferences. Finally, we anticipate novel insights and challenges associated with incorporating diverse seed traits into predictive evolutionary ecology, community ecology and applied ecology. If the community invests in standardized seed-trait collection and the implementation of rigorous databases, major strides can be made at this exciting frontier of functional ecology.

Effect of cold stratification on seed germination in Solidago × niederederi (Asteraceae) and its parental species

In this study, we investigated the influence of cold stratification on seed germination in S. × niederederi, a hybrid between the North American S. canadensis and the European S. virgaurea, using fruit samples collected in 2016 in Poland. We aimed to test the hypothesis that the low temperature exposure decreases the final percentage and speed of seed germination in the hybrid and its parental species. For each species, sets of 100 achenes in three replications were mixed with dry sand and stored in Petri dishes in darkness for 12 weeks, at −18 °C and + 4 °C, and + 25 °C. The seeds were incubated for 21 d at room temperature (+25 °C), under the 12 h photoperiod (630 lx). We showed a lack of significant differences in: (i) the final percentage of germinated seeds of studied species stored at the same conditions, (ii) the final percentage of germinated seeds between the applied stratification conditions in the hybrid and its parental species, and (iii) the mean values of Timson’s index, mean germination time, and coefficient of velocity of germination between the stratification conditions in each species. The statistically significant inter-specific differences in the mean germination time parameter after the +25 °C treatment suggest that the seeds of S. × niederederi are able to germinate faster than the seeds of its parental species. However, to improve our knowledge of naturalization and invasion abilities of S. × niederederi by sexual reproduction, the seed germination and seedling survival of the hybrid should be tested in the field.

Factors influencing seedling emergence of three global invaders in greenhouses representing major eco-regions of the world

Successful germination and seedling emergence in new environments are crucial first steps in the life history of global plant invaders and thus play a key role in processes of range expansion. We examined the germination and seedling emergence success of three global plant invaders - Lupinus polyphyllus, Senecio inaequidens and Verbascum thapsus - in greenhouses and climate chambers under climate regimes corresponding to seven eco-regions. Seed materials were collected from one non-native population for L. polyphyllus and S. inaequidens, and from 12 populations for V. thapsus (six natives and six non-natives). Experimental climates had significant effects on species responses. No species germinated in the dry (humidity ≤ 50%) and cool (≤ 5 °C) experimental climates. But all species germinated and emerged in two moderately cool (12-19 °C) and in three warm (24-27 °C) experimental climates. In general, V. thapsus showed higher fitness than S. inaequidens and L. polyphyllus. The climate of the seed source region influenced responses of native and non-native populations of V. thapsus. Non-native populations of V. thapsus, originating from the warmer seed source, showed higher performance in warm experimental climates and lower performance in moderately cool experimental climates compared to native populations. Responses of V. thapsus populations were also related to precipitation of the seed source region in moderately dry experimental climates. The warm, semi-arid and humid experimental climates are suitable for the crucial first steps of invasion success for L. polyphyllus, S. inaequidens and V. thapsus. The species adaptation to its source region modified the responses of our studied plants under different experimental climates representing major eco-regions of the world.

Germination and Seedling Growth of Water Primroses: A Cross Experiment between Two Invaded Ranges with Contrasting Climates

Aquatic ecosystems are vulnerable to biological invasions, and will also be strongly impacted by climate change, including temperature increase. Understanding the colonization dynamics of aquatic invasive plant species is of high importance for preservation of native biodiversity. Many aquatic invasive plants rely on clonal reproduction to spread, but mixed reproductive modes are common. Under future climate changes, these species may favor a sexual reproductive mode. The aim of this study was to test the germination capacity and the seedling growth of two water primrose species, Ludwigia hexapetala and Ludwigia peploides, both invasive in Europe and in the United States. We performed a reciprocal transplant of seeds of L. hexapetala and L. peploides from two invasive ranges into experimental gardens characterized by Oceanic and Mediterranean-type climates. Our results showed that higher temperatures increased or maintained germination percentages and velocity, decreased survivorship of germinants, but increased their production of biomass. The origin of the seeds had low impact on L. hexapetala responses to temperature, but greatly influenced those of L. peploides. The invasiveness of water primroses in ranges with Oceanic climates might increase with temperature. The recruitment from seed banks by these species should be considered by managers to improve the conservation of native aquatic and wetland plant species.