Effects of stand structural attributes on oak recruitment in mixed temperate forests

Oak-dominated forests worldwide support high levels of biodiversity and provide many important ecosystem services. However, oak forest sustainability is challenged by unsuccessful recruitment of oaks into the overstory. It is debated whether relatively shade-intolerant oaks can maintain dominance under continuous cover forestry and examples of successful recruitment of oak into the overstory in mixed, uneven-aged forests are rare. This study, set in southern Sweden, investigated the effects of selective cutting on stand structure and oak recruits and how stand density, canopy openness, and a tree species-specific shade casting index relates to the density of oak recruits. We focused on oak recruitment from the lower and middle canopy (dbh 5–10, 10–20 cm), i.e. trees that were beyond browsing height. Our findings indicate that a lower stand density was positively related to recruitment density of the smaller diameter size class, and as an indirect effect that a higher canopy openness with a lower shade-casting index was positively (but not significantly) associated with oak recruitment. Selective cutting decreased stand basal area and stand density while it increased canopy openness, but it did not have a direct short term effect on oak recruitment. These results indicate that stand structures obtained through continuous cover forestry may benefit recruiting oaks and that stand density, canopy openness and a canopy composition with high light transmission may need to be considered when ensuring the continuity of mixed, uneven-aged oak forests.