New forestry practices are being implemented in the hope of achieving ecological sustainability, but little is known of their effects on biota. We investigated stand level effects of five different felling methods on mature boreal Picea abies forest understorey vegetation in southern Finland. We hypothesized that vegetation response depends on the level of disturbance, i.e., intensity of the felling method. There were eight one hectare sized replicates of the five treatments, which were as follows: a) control, 100 % retention; b) single-tree selection system felling, 70 % dispersed retention; c) gap felling, with and without (n=3) site preparation, 50 % aggregated retention in three patches/ha; d) retention felling, 10 % retention, three clusters of trees/ha and d) clear felling, 0 % retention. Vegetation was sampled before the treatments and during two following years. The immediate response of the understorey vegetation to the level of retention was linear. Bryophyte species richness decreased significantly in selection system, 10 % retention and clear felling, but remained at the pre-treatment level in gap fellings and control. Bryophyte cover decreased significantly with all studied retention levels except control, most prominently in clear and 10 % retention felling. Vascular plant cover decreased significantly in all studied retention levels, including control, but species richness only with 10 and 0 % retention levels. During the second post-treatment year, a significant decrease in bryophyte cover was observed in gap felling with site preparation, apparently due to the disturbance caused by the preparation, while no further decrease was observed in the other treatments. On the contrary, species number and cover of bryophytes and vascular plants increased slightly in 10% retention felling and clear felling due to the invasion of early successional species. The 10% retention and clear felling did not differ from each other in terms of vegetation response. Apparently the residual tree groups were too small to provide shelter for late successional vegetation in or outside of residuals.

Key words: bryophytes, disturbance, diversity, forestry, vascular plants