Exclosures can favour natural regeneration of oak after conservation-oriented thinning in mixed forests in Sweden: A 10-year study

•We studied ungulate browsing and oak regeneration in mixed oak-forests.•The density and height of both short and tall oak saplings was higher in exclosures.•Competing understory trees and shrubs were favoured more than oak by exclosures.

Oak trees are biodiversity hotspots in many landscapes and provide habitats for several organism groups, including species that are rare or threatened by extinction. Oak regeneration in the northern hemisphere is often poor, which is a serious problem in many oak-rich conservation forests. Competition from other vegetation and browsing by ungulates are two factors that limit oak regeneration. In 13 mixed oak-rich forests with high values for biodiversity we tested conservation-oriented thinning. Trees and shrubs were harvested for biofuel, but the main aim was to favour oak regeneration and biodiversity. On average, 23% of the initial basal area was removed during the winter 2002/2003. After thinning, we set up exclosures to protect oak seedlings from browsing. In 2012, after ten growing seasons, the exclosures had favoured regeneration of oak by reducing ungulate browsing, though the effect was weak. Tall oak saplings (>130 cm) were present in exclosures at five sites, and no such new stems were recorded in control plots with browsing. These five sites were characterized by higher canopy openness, and probably higher light availability, compared to the other sites. Other broadleaves, and especially shrubs, increased much more in density and height than oaks, in both exclosures and control plots. Thus, conservation-oriented thinning in combination with fences to protect oak seedlings from browsing ungulates, increased oak regeneration in the mixed conservation forests, but control of competing understory vegetation in exclosures is probably also needed.