Additive positive effects of canopy openness on European bilberry (Vaccinium myrtillus) fruit quantity and quality

Abstract

The European bilberry, Vaccinium myrtillus L. (Ericaceae), is an important food resource for many mammals, insects and birds in forests, including the threatened capercaillie (Tetrao urogallus). For capercaillie, a sufficient amount of bilberry fruits is crucial to store enough body fat for the winter period. Studies have shown that V. myrtillus reproduction is influenced by canopy openness, but specific effects of flower availability, light conditions and pollinators on fruit production remain unclear. We therefore assessed bud, flower and fruit abundance as well as quality traits for V. myrtillus fruits under shade, semi-shade and sun conditions in 12 50 × 50 m plots in the Black Forest National Park, Germany from April to July 2017. We further observed and identified flower-visiting insects and calculated the seed set for fruits.With greater canopy openness, flower buds and fruit abundance increased. The few fruits that developed in the shade had fewer ovules compared to fruits growing under higher light availability. In the sun, seed set was highest, suggesting higher pollinator activity on these microsites. We identified five flower-visiting insect species of the genera Andrena, Bombus and Lasioglossum. The results show that canopy openness can trigger a number of additive positive effects on the number of flowers and hence potential fruits but also their quality due to an enhanced pollinator activity. Active canopy opening could therefore be used to increase bilberry availability for capercaillie. Management plans to support capercaillie populations however, should also consider the time lag between active canopy opening and flowering response in V. myrtillus.