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The significance of snow and arboreal lichen in the winter ecology of mountain caribou (Rangifer tarandus caribou) in the North Thompson Watershed of British Columbia Antifeau, Theodore Danial


The winter ecology of mountain caribou (Rangifer tarandus caribou) in the North Thompson watershed of British Columbia was investigated over winters 1978-79 and 1979-80. The main objective of the study was to evaluate caribou movements and habitat use in relation to indices of energy cost of locomotion in snow and to forage availability, especially arboreal lichens. These data were collected in habitats from valley bottom to alpine throughout winter. Largely because of their high arboreal lichen productivity, mature forests are regarded by wildlife managers as essential winter habitat of caribou, leading to conflicts with forest harvesting. Data were compared between mature forests and other habitat types, to evaluate their importance to caribou. An index of caribou locomotion cost in snow was caribou track depth in snow. A significant regression between caribou track depth and human sinking depth in snow permitted an estimate of caribou locomotion costs in all habitats. Locomotion costs often were greater in cutovers than in mature forests, and broadly increased with elevation; while temporal trends were cyclical, due to alternating accumulations of fresh, soft snow followed by settlement and maturation of the surface snow. Analysis of fecal and rumen samples, and feeding-site inspections were used to determine caribou winter food habits. Arboreal lichens (Alectoria sp. and Bryoria spp.) dominated the diet by mid winter because terrestrial forage availability declined due to deep and crusted snowpacks. For each habitat, the absolute abundance of arboreal lichen was inventoried, and then this data together with snowpack measurements were used to estimate the relative availability of arboreal lichen over winter. Arboreal lichen availability was greatest in mature forests, and generally increased with elevation; it also increased within habitats as snow deepened and elevated caribou to higher forest canopy levels where greater quantities of lichen occurred. For the first time, radio telemetry was used to determine mountain caribou movements and habitat use. Observations of non-radiocollared caribou were also used in some analyses. In both used and unused habitats, estimated energy costs of locomotion and the availability of arboreal lichen were treated as indices of energy expenditure and of energy intake of foraging. These indices were qualitatively integrated in a net energy balance relationship to evaluate caribou movements and habitat use. Caribou appeared to follow a general optimizing strategy, balancing their energy expenditure for locomotion in snow against the energy available from forage, when both terrestrial and arboreal forages are considered. Throughout winter, caribou preferably used mature forests, which offered much greater energetic benefits than cutovers and immature forests. As snow in subalpine (Engelmann Spruce - Subalpine Fir Zone) and alpine summer habitats deepened over early winter, caribou migrated to lower subalpine and lower slope and valley (Interior Cedar - Hemlock Zone), mature forest habitats. Caribou locomotion conditions and forage availability, primarily of terrestrial forages, were most favourable at these lower elevations, despite lower arboreal lichen availability, because of snowfall interception by the forest canopy and lower snowfall. Firmer, mid-winter snowpack conditions allowed caribou to reascend to late winter range in higher elevation subalpine forests, which, because of greater arboreal lichen availabilities combined with moderated locomotion conditions, became the most favourable habitats. Minor elevational shifts during this period occurred in response to fluctuations in locomotion conditions caused by cycles of snow accumulation and snow settlement. This study confirmed that mature forests are required habitat for caribou throughout winter, by providing critical arboreal lichen forage, and compared to cutovers, having lower locomotion costs and greater availability of terrestrial forage. Proposed forest reserves above 1680 m elevation in the upper subalpine are insufficient therefore to ensure essential caribou winter habitat. Mature forests from valley bottoms to the lower subalpine must also be reserved.

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