Search

Horticulture is a major pathway of introduction of aquatic plants. Among traded aquatic plants, we found two species belonging to the genus Lagarosiphon Harv. (Hydrocharitaceae), native to South and Central Africa, L. major (Ridl.) Moss and L. cordofanus Casp. L. major is the main representative of the genus, having already been introduced via horticulture sale beyond its native range, and often becoming invasive. In contrast, L. cordofanus is a lesser-known congener that could be potentially sold as an alternative to L. major. It is relatively understudied, and has yet to be recorded in the wild outside its native range. Many factors can promote the invasiveness of an alien plant; among them, increased nutrient availability often facilitates opportunistic alien species such as L. major. In a horizon-scanning perspective, a manipulative experiment under controlled conditions was performed to test the response of L. cordofanus to different trophy levels using L. major as the tolerant alternative species. According to our results, the naturalization of L. cordofanus in temperate shallow waters does not seem likely, especially if considered in comparison to L. major.

The green alga Caulerpa cylindracea Sonder (Chlorophyta; Bryopsidales) is one of the most invasive alien macroalgae in the Mediterranean Sea, where it is also spreading on rhodolith beds, an important biogenic assemblage typical of deep substrates. Despite the importance of rhodoliths, data on the competitive interactions with C. cylindracea are still scarce. To deepen the knowledge on the topic, C. cylindracea occurrence on the rhodolith bed of Capo Carbonara Marine Protected Area (Italy) was explored. Quantitative analyses of videoframes obtained from Remote Operated Vehicle records in three different MPA sites, Is Piscadeddus, Santa Caterina, and Serpentara, allow for estimates of both the cover of rhodoliths (considering the main morphotypes) and of C. cylindracea, as well as their competition. All sites showed a well-developed rhodolith bed, although some differences were highlighted in their composition in terms of morphotype, shape, and dimension of rhodoliths, as well as in the C. cylindracea cover. In particular, Santa Caterina appeared to be the site with the highest mean total cover of rhodoliths (68%), and of C. cylindracea (25%). The obtained results suggest that different competitive interactions occur between C. cylindracea and rhodolith beds, in relation to the morphostructural composition of the latter and in response to environmental conditions that affect rhodolith bed composition.

Despite the importance of soil reaction for alien plant establishment, few and incomplete studies have included this key factor so far. In this study, we investigated the effects of soil pH on the germination, growth (plant height, width, dry weight, etc.) and reproductive investment (inflorescence size and n° of flowers) of Ambrosia artemisiifolia (common ragweed), an allergenic species that is highly invasive and alien in Europe, through a replicated experiment in controlled conditions. In addition, we determined if soil pH has an effect on the total pollen allergenicity of the species. After preliminary germination tests on agar at different pH (from pH4 to pH8), plants were grown in natural soils with pH values of 5 (acid), 6 (sub-acid) and 7 (neutral) obtained by modifying a natural soil by liming methods (calcium hydroxide solution). Results showed that plants grown at pH7 were shorter and developed leaves at a slower rate than those grown at pH5 and pH6; plants grown at pH7 did not produce flowers and pollen. We also observed that, at pH5 and pH6, larger plants (as assessed by the dry weight of the aerial biomass) had both larger and more numerous inflorescences and emitted pollen earlier. Finally, the IgE-binding signal was higher in pollen samples collected from plants grown at pH5 (Integrated Optical Density, IOD, range: 1.12–1.25) than in those grown at pH6 (IOD range: 0.86 −1.03). Although we acknowledge the limitations of only testing the effects of pH in controlled conditions, this study suggests that soil pH greatly affects the growth and development of A. artemisiifolia and indicates that it may have a role in limiting the distribution and hazardousness of this plant. Future field tests should therefore assess the effectiveness of liming in the management and control of ragweed and other alien species.