The short-term effects of thinning in Norway spruce stands in Sweden on bilberry flowering

Abstract

Bilberry (Vaccinium myrtillus) is a foundation species in Swedish forests, providing forage for herbivores and fulfilling many services and functions in the ecosystem. However, bilberry cover has declined in recent decades in Sweden, partly due to dense and dark timber production stands. This study examines the short-term effects of different thinning treatments in Norway spruce (Picea abies) stands on the flowering of bilberry in southern Sweden. Three thinning treatments were applied: 60% basal area removal, 30% basal area removal - imitating standard silvicultural practices in Sweden, and a control with no thinning. Bilberry flowering was assessed before the treatments were applied, and one year after thinning. Data on the number of flowers and unopened flower buds was collected from 1,286 bilberry ramets across 66 plots distributed over nine different stands. According to expectations, one year after thinning, the number of flowers in plots with 60% thinning was significantly higher compared to the other treatments. Unexpectedly however, flowering was lower in general that year compared to the previous year, likely due to weather factors and interannual cycling in bilberry reproduction. Interestingly, the number of flowers in plots with 60% thinning did not significantly change between both years, while it drastically decreased in plots with 30% thinning and control treatment. The soil C:N ratio did not influence the flowering response. Only 11.5% of the variation in flowering could be explained by basal area, suggesting that other environmental factors, like microconditions, soil moisture, or understory competition, may play a role. These results show that standard thinning practices may not be sufficient to support bilberry populations in Norway spruce stands. Higher thinning intensities may help to support bilberry on the forest floor and by that also support biodiversity, improve forage availability for herbivores, and increase ecosystem services like berry picking