Nitrogen is the main limiting nutrient in temperate and boreal forests. Large-scale nitrogen fertilization has been suggested as a potential tool to enhance production and meet the increasing demand for wood products and biofuels. Here, we test the effect of N fertilization and thinning on berry (i.e., fruit) production and incidence of fungal pathogens along a latitudinal gradient in Sweden. We used an N fertilization (100-150 kg ha-1) and thinning experiment that was established between 1970 and 1980 in 30 pine forests, covering a latitudinal gradient stretching from southern to northern Sweden. We measured fruit production and disease incidence of fungal pathogens in bilberry and cowberry in the experimental plots (30 x 30 m), over two years (2014 and 2015), when the stands were between 67-85 years old. Nitrogen fertilization reduced fruit production for both species, while thinning had a positive effect. For cowberry, treatment effects on fruit production were mainly associated with changes in plant cover, while direct treatment effects altered fruit production in bilberry. Furthermore, N application increased disease incidence of the parasitic fungus Valdensia heterodoxa in bilberry and contributed to the reduced fruit production in the N treatment. In contrast, disease incidence of the main parasitic fungus in cowberry (snow-blight disease) was negatively affected by N. Thinning decreased disease incidence in bilberry, but tended to increase incidence in cowberry. For cowberry, disease incidence increased with latitude. Overall, our results suggest that the N-induced effect on fruit production in bilberry is partly associated with presence of the parasitic fungus, and largely due to unknown direct effects. For cowberry, reduction in fruit production is correlated with N-induced negative effects on plant cover. Large-scale fertilization will have an overall negative impact on fruit production, and given that fruit production is considered highly valuable in the context of ecosystem services and functioning, this reduction should be considered when forest management scenarios that include N fertilization are evaluated. Thinning on the other hand, can promote fruit production and may be used as a management tool to generate berry-rich forests.
Berry (i.e. fruit) production was measured in nitrogen (N) and thinning experiments
N fertilization reduced, but thinning increased, berry production
Changes in plant cover explained altered fruit production only for cowberry
Low fruit production in bilberry was correlated with incidence by a parasitic fungus
Keywords: fungi, latitude, nitrogen, silviculture, thinning, Vaccinium