Effects of ash dieback and Dutch elm disease on forest structure in Dalby Söderskog 2012-2020

Abstract: Dalby Söderskog is a national park situated in southern Sweden. The forest is dominated by European ash (Fraxinus excelsior L.), wych elm (Ulmus glabra Huds.), pedunculate oak (Quercus robur L.) and European beech (Fagus sylvatica L.). Due to abandonment of all types of forest management, spontaneous succession of woody species had led to an increase of shade tolerant tree species during the 20th century, in particular elm. Since 1988, the forest has been affected by Dutch elm disease and more recently by ash dieback. This study reports structural and successional changes of the main tree species in the time period of 2012 – 2020. The inventory was carried out in 2020 and compared to an inventory from 2012. For results to be comparable, the same plots and methods were used to carry out the inventory i.e. all living trees were registered in two different size plots – 100 m2 and 314 m2, sharing the same plot centre. In 100 m2 all trees from size 10 to 19 cm at 1.3 m height (dbh) were measured and identified. In 314 m2 plots all trees above 20 cm in dbh were measured and identified. The results show strong impact of current disturbances on tree species populations. The overall composition of the main species has not changed, but the relative proportion of each species has. Results show that ash trees with dbh >10 cm decreased from 67 to 54 individuals and elm from 178 to 98 individuals per hectare. The density of beech trees increased from 67 to 80 individuals per hectare, while oak maintained a density of 19 individuals per hectare. Regeneration of oak has increased in Dalby Söderskog lately, even though the overall regeneration of oak in southern Sweden shows a continuous decrease. The results suggest that the changes of the main tree species are direct (mortality) and indirect (changed light conditions, competition) results of ash dieback and Dutch elm disease. Oak and beech seem to benefit most from the current conditions, and are likely to increase in the future. Unfortunately, loss of large veteran ash, elm, and oak individuals will lead to loss of organisms that are dependent on habitats that these trees provide, which probably will lead to negative effects on biodiversity.