Common gorse (Ulex europaeus L.) is a thorny evergreen shrub in the family Fabaceae, and native to the British Isles and Western Europe. Gorse is an important part of the UK landscape in terms of providing habitat (particularly for invertebrates and birds), stabilising poor or sandy soils and often used as a windbreak hedge on exposed land, and its early flowering is important for pollinators. Although most often associated with heathland or upland vegetation in the UK, gorse commonly invades neglected or disturbed land, and forests, and by forming dense impenetrable thickets can suppress plantation forests, exclude grazing animals from pastures, and increase the risk of fire in native habitats and urban areas. Due to the competitive success of gorse, it has become naturalised in many temperate countries around the world and is now regarded as a serious weed in New Zealand, Australia, Chile, North America, and Hawaii, and listed by the International Union for Conservation of Nature (IUCN) as one of the global top 100 invasive plants. Often found growing alongside gorse is the evergreen shrub common broom (Cytisus scoparius L.), which is also a successful invasive species outside of its natural range.
The competitive success of both gorse and broom is due to their large seed output and persistent soil seedbanks, the ability to modify their surrounding environment to favour their spread, and the short time to maturity and seed production. Importantly, gorse and broom are legumes, which means they can form symbiotic associations with certain soil-dwelling bacteria (called Bradyrhizobia) that form nodules on the plant roots. In these root nodules, Bradyrhizobia fix atmospheric nitrogen (N), which is then transferred to the plant and plays a key role in making gorse and broom dominant within their environment. Both plants can be replaced within plant communities through species succession (after 20-30 years); however, the occurrence of fire or other major disturbances can rejuvenate gorse and broom populations and promote further invasiveness.
It is not known whether Bradyrhizobia are ubiquitous in soils where gorse and broom are found. Importantly however, not all species of Bradyrhizobia are nodule-forming symbionts of gorse and/or broom and it is not clear how widely distributed the conserved gene for this symbiosis is within Bradyrhizobia populations. Therefore, the overarching aim of this project is to, (1) determine the spatial and temporal dynamics of the symbiotic interactions between the gorse and broom roots and natural populations of soil Bradyrhizobia, and (2) test the hypothesis that the competitive ability and invasiveness of these two plants is linked to specific strains of Bradyrhizobia.