Regeneration dynamics of pedunculate oak in natural temperate forests: a case from southern Sweden

Abstract: Oaks are some of the most wide-spread and iconic trees in human culture, yet populations of oaks are declining due to land conversion, disease and because of lack of disturbance in temperate forests. Oaks are fairly light demanding trees, requiring large-scale disturbance to create suitable environments for regeneration. After germination, light availability is a key factor for the survival and growth of oaks. This is largely determined by competing vegetation. Responses to competition can be complex, as surrounding vegetation can hamper the development of oaks, but could also benefit young oaks by improving microclimate and reducing browsing pressure. This study aimed at investigating principal threats and opportunities of advanced oak regeneration (>1.3 m height) in the national park of Dalby Söderskog, southern Sweden, after recent outbreaks of Dutch elm disease and Ash dieback. Historic records along with recent inventories provide useful information in determining the status of the oak population. Measurements on plant- and crown size of oak regeneration were compared with data on the abundance of competing trees and shrubs and to the level of encroachment by competing stems. Browsing pressure and fraying damage from ungulates was also registered. Results demonstrate that disease outbreaks can have similar effects as other large-scale disturbances such as storms, grazing or fire in terms of creating beneficial conditions for regeneration of pedunculate oak. The subsequent establishment of trees and shrubs will however influence the health and development of young oaks. The length of the green crown, crown diameter, diameter at breast height (Dbh) and total plant height of young oaks decreased with an increasing number of competing stems within a 2 meter radius. Crown encroachment also had a significant effect, as crown size and Dbh were severely affected for plants that were overtopped by competing stems. In the case of height development, plants that were completely encroached by competing stems without the disadvantage of being overtopped were taller than plants that were free growing, partially encroached, or overtopped. Both inter- and intraspecific competition had negative effects on the development of oak saplings. Beech, hawthorn and hazel had comparatively large effects, while effects of elm and oak were moderate and ash induced the least negative effects. Animal induced damages had only minor effects on the health and survival of oaks.