Allelopathy is the chemical interaction between plant species whereby one organism may promote or interfere with another. Accordingly, allelopathic plant species play significant roles in shaping natural ecosystems such as affecting species distribution and diversity. While a high degree of inter-species variation in allelopathic capacities is commonly reported, variation across subspecies and between dioecious sexes is limited. Here we use the ecologically important, allelopathic dwarf shrub Empetrum nigrum to assess how allelopathic capacity varies between subspecies (ssp. nigrum and ssp. hermaphroditum) and sex (ssp. nigrum male and female plants). Specifically, we test Icelandic Empetrum, as its allelopathic nature may be an influencing factor in long-term land degradation across Icelandic highlands. Allelopathic capacity of Empetrum foliar tissues was assessed as root elongation and seed germination inhibition of the palatable grass species, Festuca richardsonii. We observed a very strong allelopathic response of Empetrum, whereby the different subspecies and sexes severely inhibited Festuca root elongation, even at very low leaf density levels. Sex and subspecies related differences in allelopathic capacity was also observed, with ssp. hermaphroditum affecting Festuca seed germination more than ssp. nigrum, and male ssp. nigrum possessing a greater germination inhibitory effect compared to female plants. Our results indicate that Empetrum may differentially impact plant interactions and ecosystem processes depending on the relative abundance of Empetrum subspecies and sexes within populations.

Land degradation due to unsustainable land use is of major concern worldwide and recovery is often slow. A potential mechanism behind slow recovery of degraded ecosystems is the retarding impacts of allelopathic plant species on the establishment of species that might facilitate the recovery process. However, the strength of the retarding impact may depend on soil type. In this study, we investigated the potential role of an abundant, evergreen and allelopathic dwarf shrub, Empetrum nigrum, in trapping tundra ecosystems in a degraded state in Iceland after centuries of unsustainable land use. We first run a series of bioassays to assess the potential allelopathic legacy effects of the Empetrum-associated volcanic soils (Andosol and Vitrosol) on seed germination and root elongation of the common grass species Festuca richardsonii in comparison with non-volcanic Histosol and Podzol soil types. Then we assessed the Empetrum leaf-soil interactions for all soil types using leaves from a degraded site in Iceland. We found no potential allelopathic legacy effects of Empetrum associated volcanic soils, whereas the non-volcanic soils negatively impacted Festuca root elongation. Empetrum leaves alone affected both seed germination and root elongation. These effects were strongly alleviated by the volcanic soils, but not by the non-volcanic soils. We conclude that abundant allelopathic plant species may significantly contribute to trapping tundra ecosystems in a degraded state, but the strength of this trapping mechanism depends on the soil environment .