The germination niche of coastal dune species as related to their occurrence along a sea-inland gradient

Prepared by Silvia Del Vecchio, Edy Fantinato, Mauro Roscini, Alicia T.R. Acosta, Gianluigi Bacchetta & Gabriella Buffa

Successful seed germination and seedling survival are key factors for species establishment. Despite this important role, species distribution has been mainly explained based on the adaptation to the environmental factors of adult plants. However, plant tolerance to factors as temperature, or soil salinity, may differ between early stages of plant development (e.g. seed germination and seedling) and adult stage. In particular, during the early stages, species are more sensitive and can be more affected by limiting environmental factors than during the adult phase.

Coastal dunes, where environmental factors sharply change from the coastline to inland, are model systems to investigate the relationship between species distribution and their tolerance to environmental factors. In these systems, salinity and soil temperature decrease linearly toward inland, making the zone closer to the sea more limiting for survival than the inner zones. Nutrient levels, although generally scarce, are often abundant near the coastline, due to seagrass wrack deposits. In this context, our research aimed to investigate whether there is a relationship between seed germination responses to environmental factors and the adult species distribution. To this end, we selected three species which grow at increasing distance from the coastline (Cakile maritima, Elymus farctus, and Crucianella maritima) and analysed their germination responses to salinity, temperature, nitrogen and their interaction.

We hypothesised that germination requirements influence the sea-inland species distribution by selecting species able to tolerate high salinity and adapted to exploit nitrogen near the coastline (Cakile maritima), salt-tolerant species beyond the first belt of vegetation (Elymus farctus), and salt-sensitive species more inland (Crucianella maritima).

Seeds used for germination tests were collected in the field. They were incubated at two temperatures (20° and 25°C),  at increasing concentrations of NaCl (0 mM, 125 mM and 250 mM, that represented freshwater, and seawater dilution at 25% and 50% respectively). To test the effect of nitrogen, we used KNO₃ (0 mM and 10 mM) as a nitrogen source.

We found that Cakile maritima was the most adapted to high temperature and salinity, while Crucianella maritima showed the opposite pattern. KNO₃ slightly increased the germination percentage in Cakile maritima and Elymus farctus. When combined with NaCl, KNO₃ alleviated the negative effects of salinity only in Cakile maritima. For all the species, high temperature exacerbated the negative effect of salinity. Overall, our results suggest that the seed germination requirements contribute to determining species distribution in coastal dunes, precluding species sensitive to salt and high temperature from germinating near the coastline. By directly comparing the role of salinity and nutrients, and their interaction with temperature, our research confirms that salinity is the most limiting factor for a successful establishment, but also puts in evidence that its negative effect can be either alleviated or exacerbated, depending on the interactions with temperature and nitrogen. Moreover, our study confirms the pivotal importance of gathering information on each stage of a species life cycle.

This is a plain language summary for the paper of Del Vecchio et al. published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.12899).