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Thermally abnormal waters represent safe sites for alien invasive plants requiring warmer conditions than provided by the ambient temperatures in the temperate zone. Therefore, such safe sites are frequently inhabited by tropical and sub-tropical plants. By performing a literature review we assessed that at least 55 alien aquatic plant taxa from 21 families were found in thermally abnormal waters in Europe. The majority of these taxa are submerged or rooted macrophytes. Six taxa are listed as quarantine pests according to EPPO. Among these, Pistia stratiotes is present in seven European countries, most of the records of this presence being recent. We studied P. stratiotes in a thermally abnormal stream where a persistent population was able to survive harsh winters. Models showed that the optimum temperature for P. stratiotes biomass was 28.8 ± 3.5 °C. Here, we show that air temperatures had a higher influence on the photosynthetic efficiency of P. stratiotes, estimated by chlorophyll fluorescence measurements, than did water temperatures. Generally, growth, and consequently surface cover for free-floating plants, cannot be explained solely by thermally abnormal water temperatures. We conclude that even though the majority of thermophile alien plant occurrences resulted from deliberate introductions, thermally abnormal waters pose an invasion risk for further deliberate, accidental, or spontaneous spread, which might be more likely for free-floating macrophytes.

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Fragmentation is one of the major threats to biodiversity. In a fragmented landscape, forest specialists are losing suitable forest habitats with specific site and microclimate conditions, which results in their local extinction. Conversely, the invasion of alien species is facilitated by open forest areas and increased boundaries between forest fragments and adjacent land. We studied the effect of fragmentation in terms of fragment size impact on overall plant species richness and on selected ecologically important groups’ richness, composition, and diversity. We surveyed vegetation in the interior of 47 fragments of various sizes and one unfragmented reference forest. Our results reveal that the effect of fragmentation is complex and differs for studied plant groups. Decreasing fragment size negatively affects the overall plant richness and richness of native and ancient forest indicator plants as well as their diversity, while the effect is positive for alien plants. The highest proportion of ancient forest indicator plant species and the lowest proportion of alien plants in the unfragmented forest underline the great conservation value of forest fragments. At the same time, our results reveal that large and diverse forest ecosystems are susceptible to biological invasions as well.

Coexistence of congeneric dry grassland species presents an interesting opportunity to evaluate traits of closely related co-occurring species. Coexisting congeneric species, exhibiting common traits, are expected to experience strong niche-overlap, which can intensify competition. Mechanisms that enable coexistence should also reduce competition. We assume that plant characteristics differing among coexisting species would diminishing interspecific competition and therefore enable coexistence. We were interested in the species characteristics of three congeneric pairs found in the same sub-Mediterranean montane grassland (the Karst region in Slovenia) : Anthyllis montana var. jacquinii, Anthyllis vulneraria, Gentiana clusii, Gentiana verna ssp. tergestina, Plantago holosteum and Plantago media and an additional four accompanying common species. We compared their morphological and reproduction traits as recorded in the literature. We performed germination studies with seven-year-old seeds and compared germination rate under various conditions. Old seeds of A. montana var. jacquinii, A. vulneraria, Lotus corniculatus, Coronilla vaginalis, P. holosteum and Centaurea rupestris were able to germinate. Clustering analysis (k-means clustering) on traits and germination data resulted in two clusters, which produced ecologically interpretable patterns. Congeners were always found in separate clusters. Analysis showed the most prominent difference between clusters was germination rate at low temperatures. Results showed that, because of the high germination rate of old seeds in Anthyllis congeners, they have the potential to form longterm persistent seed banks, whereby the germination rate in the rarer congener was lower. Between Plantago congeners, the rare species exhibited the potential to be persistent in soil seed banks, however, only in the presence of unfavorable germination conditions. Seeds from both Gentiana congeners were shortlived rather than dormant. Studying coexisting species that occupy similar niches is becoming increasingly important, since the possibility for competitive, ecologically similar species to meet increases along with the range changes caused by global climate change.