Ground vegetation dynamics in mountain spruce (Picea abies (L.) Karsten) forests recovering after air pollution stress impact

Our study focuses on the ground vegetation dynamics and its dependence on microsite conditions in declined climax mountain Norway spruce forests during the recovery period (1995–2006) following upon the considerable decrease of SO2 pollution. We showed that ground vegetation development shifted from prevailing mosses and vegetation-free sites covered with spruce litter to dominance by Avenella flexuosa during the earlier period of massive decline of the observed ecosystems. The expansion of Vaccinium myrtillus seems to occur mainly under the gradually defoliating tree crowns whereas larger canopy gaps and quickly deforested areas are more successfully colonized by grasses, especially Calamagrostis villosa. The gradual spruce stand decline, as well as the corresponding ground vegetation dynamics, proceeded until the end of the twentieth century. Afterwards, the ground vegetation responded to the interruption of trees dying and stopped its expansion on spruce litter microsites. Retreat of both dominant grasses accompanied by the remarkable increase in cover of mosses occurred.