The puzzle of forestry and cervid interactions : a missing piece is in the understory

Abstract: Shrubs of the family Ericaceae, including bilberry (Vaccinium myrtillus), cowberry (V. vitisidaea) and heather (Calluna vulgaris), often dominate the understory in Eurasian boreal and cold-temperate forests. These ericaceous shrubs play crucial roles in forest ecosystems, acting as foundation species for forest communities, as well as influencing carbon cycles and the delivery of other ecosystem services. They also provide a key food resource for various wildlife, including cervids. However, over recent decades, forestry induced changes to the tree layer is thought to have reduced habitat suitability and thus the abundance of these shrubs in Sweden. Nevertheless, the precise nature of how the overstory impacts ericaceous shrubs remains largely unquantified. This thesis investigated the impact of the forest overstory on three species of ericaceous shrubs, focusing on the tree species composition and density of forest stands distributed along a large latitudinal gradient in Sweden. We carried out biomass collections and measurements, surveys, and a three-year browsing exclosure experiment. We quantified how forest stand characteristics shaped the above-ground biomass, growth, morphology, and macro-nutritional composition of these shrubs. We determined that Norway spruce (Picea abies) dominated stands had comparatively less above-ground biomass of the three focal species than Scots pine (Pinus sylvestris) stands; their plants were also shorter, and a larger proportion of their biomass provided suitable forage for cervids (I). These shorter shrubs also provided a larger proportion of new shoot biomass (II) and had a different macronutrient composition (IV) compared to shrubs in more open stands. These findings indicate that forest owners’ decisions regarding tree species composition and density have a strong influence on the biomass, morphology, and nutritional composition of these plant species in Sweden. Three years of exclusion to large cervids’ browsing did not reveal differences in the plants’ growth and morphology in pine dominated stands (III). In addition to providing new insights into the ecology of ericaceous shrubs, we also provide biometric functions to predict the above-ground and forage biomass of the three shrubs (I), and models to predict their production of annual shoot biomass (II). In summary, our results provide a better understanding of canopy influences on ericaceous shrubs growing in conifer forests. This thesis provides tools that can help improve the management of these shrub species, and greatly increase our ability to determine stand and landscape scale availability of this food resource for cervids. We consider our findings within the larger context of landscape scale management, and highlight the importance of ensuring the continued maintenance of these ericaceous shrubs and the diverse range of ecosystem services they provide.