Suppression of an invasive legume by a native grass — High impact of priority effects

From invasive species stand to species-rich grassland: Long-term changes in plant species composition during Solidago invaded site restoration

Biological invasions degrade ecosystems, negatively affecting human well-being and biodiversity. Restoration of invaded agricultural ecosystems is among specific goals of European Union Biodiversity Strategy. Successful restoration of invaded lands is a long-term process that requires monitoring to assess the effects of interventions. Here, we present the results of a long-term experiment (8 years) on restoration of semi-natural grassland on abandoned arable field overgrown by invasive Solidago species (S. gigantea and S. canadensis). We examined effect of different invaders removal methods (rototilling, turf stripping, herbicide application) and seed application practices (commercial seed mixture, fresh hay) on changes in species composition and taxonomic diversity of restored vegetation. Our results showed a positive effect of grassland restoration on taxonomic diversity and species composition, manifested by a decrease in Solidago cover and an increase in cover and richness of target graminoids and forbs characteristic of grassland. The seed source had a longer lasting and still observable effect on the vegetation composition than the Solidago removal treatments, which ceased to differ significantly in their influence after the first few years. Applying fresh hay as a seed source increased the cover of grassland species such as Arrhenatherum elatius and Poa pratensis. For commercial seed mixture, we observed the high cover of Lolium perenne and Schedonorus pratensis (introduced with seed mixture) at the beginning and the slow decrease along the experiment course. The most striking effect was the fresh hay with herbicide application, which resulted in the lowest Solidago cover and the highest cover of target graminoids. Nonetheless, with years the non-chemical methods, including no treatment, gives comparable to herbicide effectiveness of restoration. Overall, during the experiment, alpha diversity increased, while beta and gamma diversity reached a species maximum in the third year, and then decreased. In conclusion, this study gives guidance to successful restoration of species-rich grasslands on sites invaded by Solidago. It should be emphasised that short-term effect differ considerably from long-term outputs, especially highlighting the importance of seed source, as well as effectiveness of environmentally friendly methods such as regular mowing to control the invader.

Role of priority effects in invasive plant species management: Early arrival of native seeds guarantees the containment of invasion by Giant ragweed

Empirical evidence shows that early arrival of native species, which induces the priority effects, can contribute to invasive plant species containment. However, more systematic studies are required to test the applied relevance of the priority effect. This study therefore aimed at testing the priority effects generated by different sowing times of seeds of nine native species on one target invasive plant species, that is, Giant ragweed (Ambrosia trifida). This study hypothesized that, when sown earlier, some native species will be able to substantially contain A. trifida through resource preemption. An additive competition design was used to test the competitive effects of native species on A. trifida. Depending on the sowing times of native and invasive plant species, three priority treatments were conducted: all species sown at the same time (T1); native species sown 3 weeks before A. trifida (T2); and native species sown 6 weeks before A. trifida (T3). Priority effects created by all nine native species significantly affected the invasibility of A. trifida. The average value of the relative competition index (RCI_avg) of A. trifida was the highest when native seeds were sown 6 weeks early and decreased with decreasing early sowing time of native plants. The species identity effect was not significant on RCI_avg if natives were sown at the same time or 3 weeks earlier than A. trifida invasion, but it was significant (p = .0123) if they were sown 6 weeks earlier than A. trifida. Synthesis and applications. The findings of this study clearly show that native species, when sown early, provide strong competition and resist invasion through prior utilization of resources. The consideration of this knowledge might improve A. trifida invasion management practices.

Kentucky Bluegrass Invasion in the Northern Great Plains and Prospective Management Approaches to Mitigate Its Spread

Kentucky bluegrass (Poa pratensis L.) is one of the most aggressive grasses invading Northern Great Plains (NGP) grasslands, resulting in substantial native species losses. Highly diverse grasslands dominated by native species are gradually transforming into rangelands largely dominated by non-native Kentucky bluegrass. Several factors potentially associated with Kentucky bluegrass invasions, including high propagule pressure, thatch formation, climate change, and increasing nitrogen deposition, could determine the future dominance and spread of Kentucky bluegrass in the NGP. Because atmospheric CO2 is amplifying rapidly, a C3 grass like Kentucky bluegrass might be photosynthetically more efficient than native C4 grasses. As this exotic species shares similar morphological and phenological traits with many native cool-season grasses, controlling it with traditional management practices such as prescribed fire, grazing, herbicides, or combinations of these practices may also impair the growth of native species. Thus, developing effective management practices to combat Kentucky bluegrass spread while facilitating the native species cover is essential. Modifying traditional techniques and embracing science-based adaptive management tools that focus on the ecological interactions of Kentucky bluegrass with the surrounding native species could achieve these desired management goals. Enhancement of the competitiveness of surrounding native species could also be an important consideration for controlling this invasive species.

The exotic species Senecio inaequidens pays the price for arriving late in temperate European grassland communities

The exotic South African ragwort (Senecio inaequidens DC.) rapidly spread across Central Europe after its introduction, but we still do not know to what extent its timing of arrival in a plant community (i.e. before or after natives) and the composition of the native community being invaded affect (1) its capacity to invade a European grassland, (2) the performance of the native species, and (3) the direction and strength of priority effects. In a greenhouse experiment, we manipulated the timing of arrival of the exotic species (Senecio) and the composition of the native community to test the influence of these factors on the productivity and N content of exotic and native species. We also investigated if the plant species origin (native or exotic) and the native community composition affected the benefit of arriving early and the cost of arriving late in the community. The establishment success of Senecio strongly depended on its timing of arrival in a grassland community. Senecio benefited more from arriving early than did the natives. The presence of legumes in the community did not favour invasion by Senecio. When natives arrived later than Senecio, however, priority effects were weaker when legumes were part of the native community. Our results showed that inhibitory priority effects created by natives can lower the risk of invasion by Senecio. An early arrival of this species at a site with low native species abundance is a scenario that could favour invasion.

The legacy of initial sowing after 20 years of ex-arable land colonisation

Priority effects provide an advantage to early establishing species and are thought to significantly affect the course of succession. We conducted a 20-year long experiment sowing high- and low-diversity mixtures in an ex-arable field. We ask how long the effect of sowing persists and which sown species affect the course of succession. The experiment was established in the Czech Republic in five replicate blocks, each containing three random 10 × 10 m plots with three treatments: natural colonisation, sowing low- and high-diversity seed mixtures. The species cover was annually estimated in 12 permanent 1 m² quadrates within each plot. To identify the effects of sowing, we used an innovative method analysing the data separately for each year using Redundancy analysis (RDA) with identity of sown species as explanatory variables. In the first year, the effect of sowing was small; the peak of explained variability occurred between third and fifth year. The legacy of sowing was detectable in the natural colonisers for 18 years and in the sown species for the whole 20-year period. For some species, the difference between the plots where they were and were not sown remained significant for the whole 20-year period (e.g. Lathyrus pratensis) although the plots were adjacent and the area was mown with the same machine. Other ones (e.g. Trisetum flavescens) colonised all the plots evenly. The long-lasting effect of the initial sowing confirms contingency of successional pathway on the propagule pressure in the time of start of succession due to the priority effects.