Enhanced Natural Regeneration Potential of Sessile Oak in Northern Hungary: Role of Artificially Increased Density of Insectivorous Birds

Both artificial and natural regeneration of oaks strongly depend on the quantity and quality of the acorn crop, which show high year-to-year variation. The volume of the acorn crop is influenced by many factors including biotic and abiotic effects. The quality (i.e., germination ability, nutrient reserves) of acorns can be decreased by carpophagous insect and fungal pathogen damage or by inadequate weather conditions. Defoliating insects (caterpillars, sawfly larva, etc.) can significantly decrease the acorn crop as well. The most important predators of the defoliators are insectivorous birds during their nesting period. However, in the oak forests of the northern Hungarian mountains, there is a shortage of natural nesting holes. The main aim of our study was to artificially increase the number of the available nesting holes by nest boxes and to maximize the number of breeding insectivorous birds and register the direct (Lepidoptera densities) and indirect (acorn crop quantity and quality) effects of their presence. We found only slight effects on Lepidoptera densities, so we failed to demonstrate a direct effect in the relatively low time frame of our study. But, we could show the indirect positive effect of increased predation on the health state of the acorn crop, resulting in higher seedling densities, which may improve the natural regeneration potential of sessile oak. This result supports the outstanding importance of natural nesting holes in broadleaved forests.