Summer nitrogen availability as a potential nutritional constraint on moose in south-central Alaska

Conventional wisdom predicts that winter is the nutritionally limiting season for northern ungulates, but recent studies suggest the importance of summer diet. While N content of summer browse is high, tannins may reduce its availability. We tested the hypothesis that digestible protein (DP) of summer browse is positively associated with animal condition, fecundity, and recruitment in predator-rich habitat of moose (Alces alces). We compared DP of the primary browse species, Salix pulchra, between two regions in South-Central Alaska: the Nelchina Basin, a region characterized by poor reproductive performance, and Denali National Park, a more productive region, for three years. We then modeled summer net protein intake (NPI) for cow moose feeding on S. pulchra, compared NPI between regions, and compared NPI with recruitment the following year in Nelchina. We found 23.5% more DP in Denali leaves than Nelchina leaves and this difference was significant all three years (P < 0.05). We also found large differences in DP between years within each region (P < 0.01 Denali, P < 0.05 Nelchina). Tannins reduced N availability 69% during the summer, with reductions of 44% in June, 60% in July, and 94% in August. The NPI model shows that Denali cows are in positive protein balance 8-9 days longer than Nelchina cows, accumulating 13.4 kg more lean body mass over the summer on average. Furthermore, inter-annual differences in predicted NPI were positively associated with recruitment the following year in Nelchina. These results suggest the importance of summer nutritional quality for northern ungulates.