Pteridium aquilinum (L.) Kuhn. Ecology and interference effects on regeneration of hemiboreal forests in southern Sweden

The interference effects of Pteridium aquilinum on tree regeneration was studied in southern Sweden. Possible interference mechanisms of the species were evaluated in relation to the conifer regeneration failure reported from the area. The ecology and spread of the plant was also studied in order to interpret its pattern of interference. In bioassays, phytotoxic effects of Pteridium aquilinum were found to vary with season, with the strongest effects occurring in May, June and September. The phytotoxins were released as volatile and water-soluble inhibitors and these effects suppressed germination and early seedling growth of Populus tremula and Pinus sylvestris. The inhibition observed in May and June coincides with the start of the growing season of tree seedlings and it is proposed that this interference operates as a competitive defence strategy when P. aquilinum is still immature and vulnerable to interference from other species. When P. sylvestris was seeded in a field experiment, germination was highest on mineral soil and lowest on intact P. aquilinum litter and humus, probably due to the phytotoxic effects of P. aquilinum. Containerised planted Picea abies seedlings were not susceptible to allelochemicals which may have been released by P. aquilinum, and this was shown by the low seedling mortality and relatively high annual shoot growth, and by the fact that activated carbon did not have any effect on the seedlings. Smothering by P. aquilinum was interpreted as being a serious hindrance for P. sylvestris seedling establishment while containerised P. abies seedlings planted in dense P. aquilinum vegetation were not inhibited. Pteridium aquilinum humus from plots applied with H2SO4, glyphosate, and NH4NO3, and humus from untreated plots was found to have a low pH and a negative effect on emergence of P. tremula seedlings, probably due to phytotoxic compounds released from the humus at a lower pH. Glyphosate was found to effectively control P. aquilinum. Shoot length of P. aquilinum was positively related to shoot density after application of NH4NO3 indicating that shoot length had the potential to increase due to anthropogenic N deposition. Wood ash, lime and H2SO4 were found to have no effect on the vigour of P. aquilinum. It was found that P. aquilinum relies on vegetative propagation for expansion in southern Sweden, and this is due to low spore production and an absence of sporeling establishment. After clearcutting, rapid expansion of P. aquilinum was found to result from revegetation of an already present rhizome system while expansion in the absence of rhizomes into new areas was very slow. No increase of the species in the closed forest was observed. These results stress that P. aquilinum is not currently spreading, although increased densities of fronds after clearcutting may be important in inhibiting forest regeneration.