Winter diet overlap among moose, roe deer and red deer in coniferous and mixed forests depends on snow cover depth

Several sympatric deer species that utilize the same, limited resources during the lean season may compete for food resources and thus strongly affect forest regeneration and increase wildlife-forestry conflicts. Using fecal DNA metabarcoding, we studied winter diet compositions, food preferences, and dietary niche partitioning among moose, roe deer, and red deer in NE Poland during harsh (snowy) and mild winters to test the competition avoidance hypothesis. In coniferous and mixed forests, these three species exhibited browsing strategy, as their winter diets were dominated by a high proportion of woody browse (>99% of plant DNA sequences in moose, >90% in roe deer, and >79% in red deer fecal samples) and contained almost no grasses. Dietary richness per fecal sample was the lowest for moose (9.8), higher for roe deer (19.6), and the highest for red deer (26.3), as it was also composed of different forbs and ferns. The studied cervids were selective and strongly preferred Scots pine, Maleae (incl. rowan tree), Salix sp. and the ericaceous dwarf shrubs, and avoided Norway spruce, silver birch, oak, bramble, and alder buckthorn. During mild winters, increased browsing by roe deer and red deer on the ericaceous dwarf shrubs resulted in the overlap of their diets. When the snow cover depth exceeded 10 cm, these two deer species effectively partitioned their diets, in line with the hypothesis of competition avoidance for food resources. During harsh winter, however, the diets of roe deer and red deer overlapped to a larger extent with the moose 'conifer diet', resulting in possible competition for food. In effect, the joint browsing of native European deer species on young coniferous trees during harsh winter was larger than that during mild winters. The same could be the case in managed forests where available forage is usually scarce. These findings may have profound consequences for future forest and ungulate management under ongoing climate change.