Arabis nemorensis: a key species in the floodplain lowland meadow

The ecology of Arabis nemorensis (Brassicaceae) has been studied since two decades by the laboratory of Norbert Hölzel (University of Münster), yet little is known about its genomic history. The species is strictly confined to large river floodplain environments in Central Europe. A. nemorensis is a biennial or short-lived perennial forb with large long-term persistent seed bank. It germinates in spring and usually initiate flowering in the late spring of the second or third year. The rosette senesces after flowering but the plant can re-sprout afterwards. Individuals grow a couple of 60cm-high flowering stalks, and produce 2-3,000 light seeds that do not present specific dispersal structures. A. nemorensis reproduces exclusively by seeds. Seed addition experiments have shown that A. nemorensis is microsite-limited in closed swards. The germination of its small seeds is strongly dependent on regular disturbances that create open patches with low competition. This specific life history, i.e. recruitment of seedlings from a persistent soil seed bank, is clearly reflected by pronounced temporal population dynamics. These fluctuations are most likely steered by disturbances such as severe flooding and drought events that create gaps and patches of bare soil in the sward of flood meadows. In certain years, the species completely disappears from aboveground vegetation, re-appearing in high densities only one or two years later. The seed bank of A. nemorensis is combined to its ability to tolerate extended periods of flooding, allowing the species to thrive in floodplain environments despite alternating flooding and drought events. It further remains confined to this environment because of its poor ability to establish in stable environments.

A. nemorensis is thus an endangered species with a unique ecology. Due to conversion to arable land and management intensification, the species–rich flood-meadows in which A. nemorensis grows have declined by more than 80 % since the 1950s. Only small remnant patches have persisted within protected areas and large efforts have been undertaken to restore its habitat. Our laboratory is now exploring its genomics, in collaboration with Prof. N. Hölzel (University of Münster) and Prof. Aurelien Tellier (Tech. University Munich). In 2015, 2016 and 2017, a total of 400 Arabis lines were collected along the Rhine and Danube. Our aim is to understand the population dynamics of these populations in both pristine and restored habitats, but also to dissect the genetic underpinning of its specific ecology.