Forest stand characteristics drive the macronutrient composition of Vaccinium winter forage for cervids

Shrubs of the genus Vaccinium serve as foundation species in boreal ecosystems as they define much of the structure of the ground vegetation and play key roles in many ecosystem services and processes. For example, Vaccinium myrtillus (bilberry) and Vaccinium vitis-idaea (cowberry) constitute staple foods for several species of large herbivores (Cervidae, deer) in Northern Europe. However, changes to the tree layer from forestry practices have resulted in declines in habitat suitability and the abundance of these shrubs over recent decades. Here, we assess whether related changes to tree basal area and species composition also affect the macronutrient composition of these shrubs, and if so, how such alterations may influence food selection by moose (Alces alces). We sampled bilberry and cowberry twigs during wintertime in five study areas dispersed latitudinally in Sweden, using 65 forest stands dominated by Scots pine (Pinus sylvestris) or Norway spruce (Picea abies) that varied in age and site fertility, while also taking into account soil C:N, pH, and moisture. We found that the macronutrient composition of bilberry and cowberry forage was significantly altered by forest density and tree species composition. In denser and more spruce-dominated forests (i.e., lower understory light), forage contained less nonstructural carbohydrates, but more protein and lignin, compared to shrubs growing in more open and pine-dominated forests. We also found that the forage available in such shaded environments was closer to the presumed nutritional target balance of moose. Our results illustrate that management decisions influence the macronutrient composition of understory shrubs in a way that may be important for herbivore foraging choices. We suggest that a larger variation in forest structure, both within and among stands across the landscape, will provide cervids with greater variation in forage qualities, since even small differences in forest structure can increase the nutritional variation of the forage. We discuss our results in the context of plant resource allocation, herbivore nutritional balancing and game and forest management.