Eszter Ruprecht

Does disturbance enhance the competitive effect of the invasive Solidago canadensis on the performance of two native grasses

The impact of invasive species on native plant communities can strongly depend on habitat disturbances. Thus, the joint study of invasion and disturbances are necessary to distinguish whether invasive species (1) are just ‘passengers’ of major environmental changes, (2) are the real cause (drivers) of native species decline, or (3) do disturbances and invasive species additively suppress native species (back-seat drivers). Therefore, we experimentally explored both the single and additive effect of competition by an invasive species and fire as disturbance on the performance of native species. We examined the responses of two native rhizomatous perennial grass species (Elymus repens and Brachypodium pinnatum) to competition with European invasive and American native Solidago canadensis. This was done under burned and unburned conditions, a novel disturbance type in this system. We found that competition with S. canadensis had a very strong negative effect on the performance of B. pinnatum irrespective of disturbance. In contrast, disturbance and competition had a cumulative negative influence on the performance of E. repens, with competition having greater effect than burning. Fire reduced the number of shoots of European S. canadensis individuals, but did not affect the frequently burned American populations. However, these differences did not translate into increased competitive ability of European populations compared with American ones. Thus, the competitive superiority of S. canadensis irrespective of continent of origin explained the performance loss in B. pinnatum (‘driver’ model); whereas reduced performance after burning of grass species and competitive superiority of the invasive species jointly decreased the performance of E. repens (‘back-seat driver’ model).

Can transgenerational plasticity contribute to the invasion success of annual plant species

Adaptive transgenerational plasticity (TGP), i.e., significantly higher fitness when maternal and offspring conditions match, might contribute to the population growth of non-native species in highly variable environments. However, comparative studies that directly test this hypothesis are lacking. Therefore, we performed a reciprocal split-brood experiment to compare TGP in response to N and water availability in single populations of two invasive (Amaranthus retroflexus, Galinsoga parviflora) and two congeneric non-invasive introduced species (Amaranthus albus, Galinsoga ciliata). We hypothesized that the transgenerational effect is adaptive: (1) in invasive species compared with non-invasive adventives, and (2) in stressful conditions compared with resource-rich environments. The phenotypic variation among offspring was generated, in large part, by our experimental treatments in the maternal generation; therefore, we demonstrated a direct TGP effect on the offspring’s adult fitness. We found evidence, for the first time, that invasive and non-invasive adventive species differ regarding the expression of TGP in the adult stage, as adaptive responses were found exclusively in the invasive species. The manifestation of TGP was more explicit under resource-rich conditions; therefore, it might contribute to the population dynamics of non-native species in resource-rich sites rather than to their ecological tolerance spectra.

Enhanced Fire-Related Traits May Contribute to the Invasiveness of Downy Brome (Bromus tectorum)

Although several invasive species have induced changes to the fire regime of invaded communities, potential intraspecific shifts in fire-related traits that might enhance the invasion success of these species have never been addressed. We assumed that traits conferring persistence and competitiveness in postfire conditions to downy brome, a quintessential invasive species of the Great Basin (North America), might be under selection in areas with recurrent fires. Therefore, we hypothesized that populations from frequently burned regions of the Great Basin would have (1) greater tolerance to fire at seed level, (2) higher relative seedling performance in postfire environments, and (3) greater flammability than unburned Central European populations that evolved without fire. Seeds were collected from three introduced populations from frequently burned regions in North America and three introduced populations of rarely or never burned sites from Central Europe. We performed (1) germination experiments with seeds subjected to the effect of different fire components (heat shocks, smoke, flame, ash), (2) pot experiments analyzing the effect of postfire conditions on the early growth of the seedlings, and (3) a series of flammability tests on dry biomass of plants reared in a common garden. All seeds tolerated the low-temperature treatments (40 to 100 C), but were destroyed at high heat shocks (140 and 160 C). Only the 100 C heat treatment caused a difference in reaction of seeds from different continents, as the European seeds were less tolerant to this heat shock. We found significantly increased seedling height and biomass after 4 wk of growth under postfire conditions in American populations, but not in European ones. American populations had enhanced flammability in three out of five measured parameters compared to European populations. In summary, these intraspecific differences in fire-related traits might contribute to the persistence and perhaps invasiveness of the frequently burned North American downy brome populations. Nomenclature: Downy brome, Bromus tectorum L. BROTE. Management Implications: We provide evidence that North American downy brome populations from frequently burned environments have enhanced postfire biomass accumulation and flammability compared to European populations that are rarely or never exposed to fire. Seeds of downy brome, however, do not tolerate severe fires with high heat shocks. Our results suggest that management should focus on changing the fire regime toward less frequent, but more intense, fires to stop this highly invasive species. Therefore, lengthening the fire-return interval by creating firebreaks, which would amplify the intensity and size of subsequent fires, may be one option for controlling this invasive species. However, such bands of fuel breaks, where fuels are removed by cutting several swaths to reduce fire hazard, can serve as refuge and seed source of downy brome. The association of fuel breaks with roadways might minimize the total landscape impact. To prevent the germination of downy brome, persistent cover of the ground by native vegetation must be achieved by reintroducing fire-tolerant perennial grasses and resprouting shrubs such as rabbitbrush, as closed vegetation makes for very limited alien seedling recruitment opportunities. The fire management practice should target the historical, natural fire regime that favors native species reestablishment. These native species would produce higher fuel loads and a continuous fuel bed that might also increase the intensity and decrease the frequency of fires. Although our results may suggest that the introduction of European, non–fire-adapted populations of downy brome to the United States might be an option, they would perhaps be outcompeted in short time by the fire-adapted local genotypes. The North American populations are further expected to increase flammability of stems and leaves if frequent and low-intensity fires persist in the invaded territories.

Allometric relationships between diaspore morphology and diaspore covering anatomy of herbaceous species from central-eastern Europe

Anatomical and morphological seed traits are of great ecological importance and are a main subject of, for example, seed bank or endozoochory studies. However, we observed a lack of information about the relationship between seed anatomy and seed morphology and its ecological implications. To fill this gap, we linked the anatomical features of diaspore coverings to morphological characteristics of free seeds and one-seeded fruits. We predicted that: (1) the thickness and anatomical complexity of seed coat and pericarp are related to diaspore size and shape; and (2) the presence or absence of the pericarp may influence seed-coat thickness and anatomy. In our study we investigated diaspores of 39 central-eastern European herbaceous species and recorded the thickness and anatomical complexity of their seed coverings, and we determined diaspore mass and shape. Our results indicate that diaspore mass is positively related to covering thickness, lignification degree and anatomical complexity. This might be the case because bigger diaspores tend to remain on the soil surface and are more exposed to predation risk and environmental threat than smaller diaspores. Furthermore, more round-shaped diaspores had disproportionately thicker and more lignified coverings than long or flat ones, probably because round-shaped diaspores much more frequently form seed banks and therefore persist for a long time in the soil. We also found that free seeds as diaspores have a thicker and more lignified seed coat than seeds enclosed in fruits. In one-seeded fruits, the pericarp ‘takes the protective role’, it is thick, and the seed coat is poorly developed.

Hydration status influences seed fire tolerance in temperate European herbaceous species

Prescribed burning is an important management tool in many parts of the world. While natural fires generally occur during the driest and warmest period of the year, prescribed burning is often timed out-of-season, when there is higher soil moisture and lower biomass combustibility. However, fire season may influence seedling recruitment after fire, e.g. through the effect of seed hydration status on fire tolerance. In non-fire-prone temperate regions, anthropogenic fire may occur exclusively in periods outside the growing season with higher soil moisture, which may have negative consequences on seedling recruitment. Fire tolerance of moist and dry seeds of 16 temperate European herbaceous species belonging to four families was assessed using heat treatment of 100xa0°C for 5xa0min and subsequent germination trials. Moist seeds of Asteraceae, Poaceae and Brassicaceae had a predominantly negative reaction to the heat treatment, while those of Fabaceae tolerated it or germination was even enhanced. The reaction of dry seeds was completely different, with positive responses in three species of the Fabaceae and fire tolerance in species of other families. Our results point out that hydration status may significantly influence the post-fire germination of seeds. Dry seeds were found to tolerate high heat, while moist seeds were harmed in more than half of the species. This implies that if prescribed burning is applied in temperate grasslands of Europe, it should be timed to dry periods of the dormant season in order to protect seeds from negative effects of fire.