How do forest characteristics relate to brown bear (Ursus arctos) density?

Brown bear (Ursus arctos) is one of our most common large carnivores. The historic population in Sweden has been reduced due to conflicts with humans. The diet of the brown bear varies with the season and location. In the USA, brown bears feed more on salmon (Salmo salar) and trout (Salmo trutta), while in Sweden it is more reindeer (Rangifer tarandus) and moose (Alces alces) during the spring which interferes with reindeer husbandry. Bilberry (Vaccinium myrtilus), and lingonberry (Vaccinium vitis ideae) is the most important food resource during the late summer and fall, occasionally the bears also feed on ants. Bears occur in boreal forests and females select home ranges that provide good food resources. Before entering hibernation, bilberries are one of the most important food resources and are heavily affected by forest management, the thinning phase, and mature forests that are ready to be clear-cut. After a clear-cut, the bilberry is greatly decreased but starts to recover as the forest grows and the canopy closes. When the forest becomes too dense the bilberry stops growing but it starts to increase again after thinning. This study investigated how the forest characteristics are related to the bear density since the bilberry abundance changes during one management cycle of the forest and bilberry is one of the most important food recourses. Two linear mixed models (lme) were created with bear density as a response variable. Model one contained mean height, basal area, and age. The second model contained the seven groups of field layers (bilberry, lingonberry, poor, grass, herbs, crowberry, and no field layer) as explanatory variables. The bear density data was estimated from spatial capture-recapture surveys based on DNA from feces collected during fall, the forest data were obtained through the NFI (National Forest Inventory). The results show that age was positively correlated with the bear density. This could be since bilberry is heavily affected by clear-cutting and takes years to recover. The basal area and mean height, on the other hand, had a negative correlation with bear density. This could be since bilberry is favored by forests that are not too dense but have a basal area of around 30-40 m2 /ha and a canopy openness of 50%. Furthermore, both bilberry and lingonberry abundance peak at a lower forest height, bilberries around 15 meters and lingonberries around 0 meters. There was no difference between the field layers, except the No field layer which gave a lower bear density compared to bilberry