Influence of canopy openness, ungulate exclosure, and low-intensity fire for improved oak regeneration in temperate Europe.

Failed oak regeneration is widely reported in temperate forests and has been linked in part to changed disturbance regimes and land-use. We investigated if the North American fire-oak hypothesis could be applicable to temperate European oaks (, ) using a replicated field experiment with contrasting canopy openness, protection against ungulate browsing (fencing/no fencing), and low-intensity surface fire (burn/no burn). Survival, relative height growth (RGR), browsing damage on naturally regenerated oaks (≤300 cm tall), and changes in competing woody vegetation were monitored over three years. Greater light availability in canopy gaps increased oak RGR ( = .034) and tended to increase survival ( = .092). There was also a trend that protection from browsing positively affected RGR ( = .058) and survival ( = .059). Burning reduced survival ( < .001), nonetheless, survival rates were relatively high across treatment combinations at the end of the experiment (54%-92%). Most oaks receiving fire were top-killed and survived by producing new sprouts; therefore, RGR in burned plots became strongly negative the first year. Thereafter, RGR was greater in burned plots ( = .002). Burning altered the patterns of ungulate browsing frequency on oaks. Overall, browsing frequency was greater during winter; however, in recently burned plots summer browsing was prominent. Burning did not change relative density of oaks, but it had a clear effect on competing woody vegetation as it reduced the number of individuals ( < .001) and their heights ( < .001). Our results suggest that young, temperate European oaks may respond similarly to fire as their North American congeners. However, disturbance from a single low-intensity fire may not be sufficient to ensure a persistent competitive advantage-multiple fires and canopy thinning to increase light availability may be needed. Further research investigating long-term fire effects on oaks of various ages, species-specific response of competitors and implications for biodiversity conservation is needed.