INTERFERENCE MECHANISMS IN CONIFER-ERICACEAE-FEATHERMOSS COMMUNITIES

In northern boreal forests ground level species occur which may have t he capacity to suppress tree seedling regeneration substantially, part icularly in late successional stages. We investigated, through an expe riment involving sequential manipulations, the effects of three biotic components operating at the ground layer, i.e. above-ground effects o f the ericaceous shrub Empetrum hermaphroditum and of the feather-moss species Pleurozium schreberi and the below-ground effects of extramat rical mycorrhizal hyphae, on the seedling establishment, growth and nu trient acquisition of Pinus sylvestris during two growing seasons. Ger mination and seedling establishment of P. sylvestris were enhanced by reduction of the above ground components of E. hermaphroditum and P. s chreberi but were not influenced by temporary disruption of mycorrhiza l hyphae. Seedling growth and nitrogen content were, however, signifi cantly increased when hyphal connections were disrupted and above grou nd influence of P. schreberi and E. hermaphroditum were reduced. Highe r shoot:root ratios were also found in seedlings when hyphal connectio ns were disrupted. Seedlings in treatments where P. schreberi was left intact and above ground interference by E. hermaphroditum was reduced had unusually low shoot:root ratios (1.6) after two growing seasons a nd significantly lower total amounts of nutrients after one growing se ason than were initially present in the seeds, indicative of a net nut rient drain during the early establishment phase. The experiment could not explain the mechanisms behind this unexpected nutrient drain. Pos sible rhizosphere interactions immobilizing nutrients and causing seed ling nutrient losses are discussed. From the results of this study and other indirect evidence we hypothesize that the three biotic componen ts we investigated could act together to immobilize nutrients and to i nhibit tree seedling regeneration and growth in late successional stag es of boreal forests.