Winter Foraging Ecology of Woodland Caribou in Northeastern Washington

Abstract

To better understand late winter foraging ecology of woodland caribou (Rangifer tarandus caribou) feeding on arboreal lichens, we used bottle-raised caribou in experimental arena trials with artificial trees, and in field trials within historical late-winter habitat. Factors with the greatest influence on intake rate differed between experimental arena and field trials. Bite size was the most important variable in experimental arena trials; bite rate was the most important in field trials. During late winter field trials, caribou foraged on lichen primarily on standing subalpine fir (Abies lasiocarpa) and dead trees. Bite size, bite rate, intake rate, tree resident time, and amount of lichen eaten per tree were included in a general linear model with tree species, tree size class, and tree lichen class ( average) as the independent variables. Tree lichen class was the most important variable in the model and 76% of all bites occurred on >average lichen class trees. Compared to theoretical maximums, intake rate was low on all lichen class trees (range = 1.4-2.1 g/min). At these intake rates caribou would have to forage 14-21 hours to meet predicted daily requirements. Tree resident time and time between trees varied inversely with tree density. In cafeteria style preference trials with the 2 primary arboreal lichen genera, caribou strongly preferred Bryoria spp. (92%) compared to Alectoria sarmentosa (8%). Apparent dry matter digestibility of this diet was 82%. Timber stands must be substantially older than traditional harvest rotation lengths to provide the high lichen biomass found on >average lichen class trees. Caribou remained in habitats where Bryoria was the predominant genus of arboreal lichen and would not forage in A. sarmentosa dominated valley bottom habitat.