Characterisation of the differential interference effects of two boreal dwarf shrub species

Our purpose was to characterize the relative competitive and phytotoxic potential of two closely related dwarf-shrub species, Empetrum nigrum and E. hermaphroditum, which form clones in a mosaic pattern in post-fire successions of the boreal forest of northern Sweden. We determined morphological and growth parameters of both species, performed bioassays and chemical analysis, and established field experiments to explore possible differing interference effects on trees by the two species. Both Empetrum species had very similar morphological and growth characteristics. E. hermaphroditum exerted considerably greater negative effects than E. nigrum against Pinus sylvestris and Populus tremula seed and seedlings. These negative effects were related to the different substitution of a bibenzyl in the two species. The effect on seed germination of the two bibenzyls isolated from E. nigrum and E. hermaphroditum was compared with that of other simple phenolics; the latter were found to be inactive, indicating a specific phytotoxic component in the bibenzyls. P. sylvestris seeds planted into clones of both species in the field revealed that E. hermaphroditum had much stronger inhibitory effects than did E. nigrum and the addition of activated carbon partially reversed these effects. Seed germination, biomass and survival of P. sylvestris after four seasons were significantly lower in E. hermaphroditum- than in E. nigrum-dominated plots. We conclude that while both Empetrum species are superficially very similar morphologically, they have vastly different effects on tree seed germination, seedling establishment and growth. These effects appear to be due, at least in part, to the different chemical profile of the two species.