SEASONAL-VARIATION IN PHYTOTOXICITY OF BRACKEN (PTERIDIUM-AQUILINUM LKUHN)

Laboratory bioassays were used to test for the phytotoxicity of volati le compounds, fresh plant material as a seed bed, and water extracts f rom bracken [Pteridium aquilinum (L.) Kuhn] pinnules to germination an d seedling growth of aspen (Populus tremula L.) and Scots pine (Pinus sylvestris L.). Fronds were sampled from two bracken populations, one in the south and one in the north of Sweden. All three bioassays showe d inhibitory effects, and these varied seasonally with the most inhibi tory effects occurring in May, June, and September. The peak of inhibi tion in May and June coincides with the start of the growing season wh en bracken still is immature and vulnerable to interference from other species, The increase in inhibitory effects in September appears to b e due to transformation of natural products or an accumulation of inhi bitory compounds that are released during decomposition following fron d death. Addition of activated carbon did not remove the inhibitory ef fects.