Long-term succession and loss of foundation species in a temperate broadleaved forest in southern Sweden

Abstract: Dalby Söderskog is an old-growth temperate forest dominated by pedunculate oak (Querus robur), wych elm (Ulmus glabra), European ash (Fraxinus excelsior), and beech (Fagus sylvatica). After being released from a historical management regime of livestock grazing, autogenic succession began in the forest. This allowed for shade tolerant species to prevail. A steady state forest community dominated by elm was affected by Dutch elm disease (DED), while ash dieback negatively impacted on the ash population at the end of the 20th century. In this study, we report on 95 years of secondary succession in Dalby Söderskog with emphasis on the effects of DED and ash dieback on forest compositional dynamics. Additionally, future trends in forest succession were discussed in response to changing environmental conditions. In 2012, an inventory of trees > 10 cm in diameter at breast height was carried out. This was done across 74 systematically distributed plots which were reconstructed from a survey of herb layer vegetation conducted in 2010. The results were compared with similar investigations completed in 1909, 1916, 1935 and 1970. The new inventory data representing species composition and forest structure were analyzed and discussed in relation to DED, ash dieback and oak decline. For each sample tree vitality was also recorded in order to explain compositional dynamics and succession trends. The results show that oak has been retreating from the stand for a long time, while elm is declining in abundance only recently after a period of dominance. Ash has become a dominant species in 2012 due to its ability to regenerate vigorously in gaps created where elms have fallen because of DED. However, a large amount of infected ash were observed in 2012. For beech, increased recruitment was found in Dalby Söderskog during the last sampling period. Despite dramatical changes in compositional dynamics, overall structural dynamics look more stable than species level diameter structure indicates. I conclude that oak will disappear in the long run if no actions are taken in the future, as historically its presence was a result of a wood pasture regime in Dalby Söderskog. DED had a significant impact on the tree species composition and will continue to affect young elm trees, resulting in an understory population with high turnover. Ash and beech will probably increase and maintain a dominant position in the canopy. Although, changes in composional dynamics occured, the forest still proved to be relatively resistant as an ecological system as seen in its consistency pertaining to structural dynamics.