Stream bryophyte recovery after extreme flood disturbance takes several years

Abstract

Abstract The recovery rates and assembly processes of stream bryophyte communities after severe disturbances are not well known. Breaking of an ice dam caused an extreme flood that completely removed bryophyte cover along several hundreds of metres of a boreal stream (Stream Uopajanpuro, Koillismaa, NE Finland). We monitored recolonization rates and successional processes of stream bryophytes in the disturbed stream section over 8 years. In the first two summers after the disturbance, the disturbed section remained largely unvegetated. The initial recovery of the bryophyte community resulted mainly from colonization of vegetative moss fragments of the dominant, perennial species present in the undisturbed upstream section, whereas typical early successional colonist species remained scarce. The recovery of total cover, richness and community composition of stream bryophytes took 5–6 years after the disturbance event, even though bryophyte vegetation supplying fragments was located in the immediate vicinity upstream of the disturbance site. Bryophyte species showed differences in the colonization–recovery rates. Mean spatial segregation among species showed no trend during recovery, whereas several species pairs showed both segregation and aggregation trends. Our results indicate that extreme flooding can have devastating effects on bryophyte cover, and the recovery of bryophytes is slow compared to other stream organisms, with full recovery taking up to several years. This recovery time estimate may only apply if there is an immediate upstream source of vegetative propagules from undisturbed populations. Where such populations are absent, recovery of bryophyte communities can take decades.