Tree regeneration responds more to shade casting by the overstorey and competition in the understorey than to abundance per se

Manipulating the overstorey is the key tool for forest managers to steer natural regeneration. Opening up the canopy does not only create favourable light conditions for tree seedling growth, but also for (competitive) understorey species. Therefore, a thorough understanding of how changes in the abundance of the overstorey and understorey influence tree regeneration is needed to attain successful regeneration. To this end, we used the regional Flemish Forest Inventories, which contain vegetation plots that were surveyed at two times and include large variation in species composition and abundance of both overstorey and understorey layers. These plots were classified into poor and rich forest types, which differ in overstorey and understorey species composition and soil fertility. For each forest type, we first investigated the effect of overstorey abundance and shade-casting ability on the understorey herbaceous vegetation cover and its competitive nature. Then, we modelled how both these strata influence the presence-absence as well as the cover of tree regeneration, using the zero-inflated beta distribution. Our results show that the understorey cover and its competitiveness mainly increase when the abundance and shade-casting ability of the overstorey is reduced. The shade-casting ability of the overstorey and competitiveness of the understorey were more important in determining tree regeneration, especially probability of presence, than the abundance of these layers per se. This was consistent for both forest types, although directions and magnitudes of the effects differed. In predictions mimicking several thinning scenarios we found that in the poor forests, reducing overstorey abundance could lead to an increase in seedling cover, whereas in rich forests, the opposite is true and seedling cover will potentially be reduced. Finally, in a single-species analysis focusing on Quercus, we found a trade-off between sufficiently reducing overstorey abundance, while at the same retaining parent trees as potential seed sources. These findings can be used to guide forest management decisions in order to attain successful forest regeneration in temperate forests