Recovery of pristine boreal forest floor community after selective removal of understorey, ground and humus layers

In boreal spruce forests that rarely experience extensive disturbances, fine-scale vegetation gaps are important for succession dynamics and species diversity. We examined the community implications of fine-scale gap disturbances by selective removal of vegetation layers in a pristine boreal spruce forest in Northern Finland. The aim was to investigate how the speed of recovery depends on the type of disturbance and the species growth form. We also wanted to know if there appeared changes in species composition after disturbance. Five different treatments were applied in the study: Control, removal of the ground layer (bryophytes and lichens), removal of the understorey layer (dwarf shrubs, herbs and graminoids), removal of both the ground and understorey layers, and complete removal of the vegetation and humus layers above the mineral soil. The vegetation recovery was monitored in terms of cover and species numbers over a 5-year period. Understorey layer cover, composed mainly of clonal dwarf shrubs, recovered completely in 4 years in treatments where the humus layer remained intact, whereas ground layer cover did not reach the control level in plots from where bryophytes and lichens were removed. Recovery was faster in terms of species number than species cover. Bryophytes, graminoids and dominant dwarf shrubs appeared in all disturbed plots quickly after disturbance. Seedlings of trees appeared exclusively in disturbed plots. Graminoids dominated after the removal of humus layer. The results indicate that the regeneration of forest floor after small gap disturbance occurs mainly by re-establishment of the dominant species. Although destruction of the humus layer leaves a long-lasting scar to the forest floor, exposing of mineral soil may enhance the sexual reproduction of dominant species and the colonization of weaker competitors.