How do plants colonise new territories?

The research suggests that different categories of species have dissimilar seed germination niches, which contributes to explaining their coexistence.

Have you ever paid close attention to the plants whilst walking around coastal dunes? It is likely that there is a foundation species, followed by a few, “accidental” species. All these plants are crucial to prevent the erosion of the shoreline, flooding as they trap sand and fix sediments. Seed functional traits (e.g. size, weight, germination timing) can reveal which species can colonise an environment and how the plant community is shaped. 

Dr Silvia Del Vecchio from Ca’ Foscari University of Venice and colleagues from Royal Botanic Gardens, Kew investigated the germination strategies of 19 plant species from different coastal dunes. The scientists found that there were most foundation species germinated under dark, warm conditions (e.g. deep in the sand during the spring or summer) whilst alien species germinated under light and warm conditions. Plant species were not only distributed differently along dunes but also occupied different germination niches. Dr Silvia Del Vecchio  has previously worked on how abiotic factors impact seed germination along a sea-inland gradient

The researchers used findings from previous surveys of 504 plots along the Veneto coast (Italy) to determine the frequency of different plant species at “foredunes”, “semi-fixed dunes” and “fixed dunes”. They grouped 19 species into foundation, accidental or alien species based on their distributions and frequencies and collected seeds from 100 plants. These seeds were sent to Wakehurst Place of Royal Botanic Gardens, Kew (UK) for germination tests. The seed weight, shape were measured and seeds were germinated under five temperature treatments as well as under light and complete dark conditions. 

A sandy beach
Image: Canva.

The 19 species formed four clusters according to location, germination temperature and photoperiod, suggesting specific germination strategies of different plant species at different locations along a coastal dune system. The germination strategies were “dark warm-cued” (e.g. had higher germination in the dark at warm temperature), “light warm-cued”, also characterised by spherical seeds, and overall “high-germinating” and “low-germinating”.Cluster analysis identified nine foundation (e.g. Calamagrostis arenaria), five accidental and five alien (e.g. Cenchrus longispinus) species. Whilst no species were typical for fixed dunes, species differed between foredunes and semi-fixed dunes. Most species were classified as “erect leafy” and most species were known to reproduce via seeds rather than vegetatively.

Cluster dendrogram of 19 species x 10 variables (germination at 5 temperatures x 2 photoperiods) showing three main clusters in germination strategies. Foundation species of the foredune system are in bold. Source: Del Vecchio et al., 2020

“Our study evidenced a clear differentiation in the regeneration niche, which reflects different germination requirements across species categories and allows species coexistence under current conditions”, Del Vecchio and colleagues wrote. 

Foundation species were either characterised by vegetative reproduction or being “dark warm-cued” which suggest they germinate in the spring and summer. Whilst most Mediterranean plants germinate in the autumn to avoid the seedling exposure to summer drought, the foredune species might have adapted to darkness so they germinate buried deeply in the sand. Alien species were not just location-specific but were “light warm-cued” and therefore occupied a specific germination niche. Seed germination and plant communities might change due to climate change in the future.

“Climatic events, such as rising temperature, could alter germination patterns, favoring seed regeneration of certain categories (i.e. alien and semi-fixed dune species) at the expense of others (i.e. foundation species, pivotal to ecosystem functioning), hence potentially altering the plant community structure”, the authors warned.