Growth responses of seedlings along encroachment gradients of the allelopathic plant Empetrum nigrum

Empetrum nigrum is a native evergreen shrub distributed in alpine and circumpolar areas that is currently encroaching into new habitats. This is an allelopathic plant that releases secondary metabolites, which has a negative impact on seed germination and seedling growth. The species has been recorded to expand in altitude and latitude parallel to a warmer climate and is encroaching into new habitats. This study aims to test if recent encroachment by E. nigrum is causing environmental change through allelopathy using a space for time experiment. Soil samples were collected along E. nigrum encroachment gradients from 15 sites in northern Norway, with each gradient being sampled in a meadow at four locations; where the E. nigrum had been present for ~15 years, at the clone edge, and at one and two meters into the meadow. The allelopathy was assessed using a bioassay-approach. Seedlings of three local plants, a grass, a forb and a legume species were grown in the soil for 5 weeks under stable conditions in a phytotron. After five weeks, length and dry biomass of the species were measured and nodules on the roots of the legume were counted. The results showed that the three species had different growth responses in the E. nigrum encroachment gradient. The length and dry biomass growth of Phleum alpinum were negatively impacted where E. nigrum had been present for ~15 years, compared to the meadow near. Solidago virgaurea and Trifolium pratense had no growth trends related to the encroachment gradient. However, the length and dry biomass growth of these species was significantly correlated to the nitrogen content in the soils. The growth of T. pratense would also increase when nodules were present, and by the number of nodules present at each individual plant. These results indicate that E. nigrum needs to be present for more than ~15 years to influence the soil growth conditions for herbs and legumes. Grasses on the other hand are more sensitive to toxic compounds released by E. nigrum early in its encroachment. Other mechanisms of E. nigrum are therefore important for the species when it is successfully encroaching into new habitats. Management of the species as a tool to avoid biodiversity loss, would therefore be most efficient in the early phase of encroachment before the soil is strongly influenced by E. nigrum´s allelopathy.