Promoting natural regeneration of oak by manipulating disturbance

Oak regeneration is thought to be promoted by natural disturbance and past management practices that create semi-open conditions, such as periodic livestock grazing and lowintensity fire. The overall goal of this thesis was to examine whether disturbance factors can promote natural oak regeneration. The first study quantified demographic changes in the Swedish oak (Qurecus robur and Q. petraea) population and examined the role of altered land use for these changes. Despite a continuous increase in large oak trees in southern Sweden since 1953, oak saplings declined continuously after the early 1980s. This development was connected to widespread changes in land use and game management that created darker forests with more herbivores, likely reducing oak regeneration at a regional scale. Field experiments showed negative effects of present ungulate browsing pressure on oak seedlings and saplings. The combination of competing woody vegetation and browsing had greater negative effects on oak (Q. robur) survival than individual effects of either factor. Although woody vegetation provided some protection against ungulate browsing, the long-term effect on oak survival and growth was overall negative. Oak seedlings (Q. robur and Q. alba) displayed a high sprouting capacity following shoot-destructive disturbance (artificial clipping) in a greenhouse experiment. A field experiment including low-intensity fire and oak seedlings (Q. robur and Q. petraea) gave similar results. Greater light availability increased oak performance in both cases, suggesting that their tolerance to disturbance, such as browsing and low-intensity fire, is greater in high light environments. These results also indicate that fire could promote oak regeneration in temperate Europe, consistent with the North American fire-oak hypothesis. However, further research is needed to determine the long-term vitality of these oaks and their main competitors, and whether oaks gain competitive advantages after such disturbance. Oak-dominated forests are important for biodiversity and the effects of fire on species and communities also needs to be studied. In conclusion, the results suggest that in the absence of natural disturbance, active management that resemble previous disturbance regimes and restore habitats with semi-open canopies, and low to moderate browsing pressure, seem essential to promote natural oak regeneration