Forest habitats and the nutritional ecology of Sitka black-tailed deer: a research synthesis with implications for forest management.

Abstract

Research on forest habitats and the nutritional ecology of Sitka black-tailed deer conducted during 1981 through 1986 is reviewed and synthesized. The research approach was based on the assumption that foraging efficiency is the best single measure of habitat quality for an individual deer. Overstory-understory relations and the influence of forest overstory on snow depth and density, forage availability, and forage quality were studied in the western hemlock-Sitka spruce forests of southeastern Alaska. The effects of forest management were analyzed in terms of their consequences of changing the historic disturbance regime of old-growth forests from one of high-frequency, low-magnitude disturbance to the low-frequency, high-magnitude disturbance regime of even-aged forests. Old-growth and even-aged forests differ greatly in their production of forage, protein digestibility of sun- and shade-grown leaves, and relative carrying capacities for deer. Forest overstories reduce snow depths significantly, but only at high crown closures (>95 percent). Analyses of species composition and quality of the diet of black-tailed deer and nutritional quality of forages indicated digestible energy and digestible protein are probably the potentially greatest nutritional limiting factors for deer in Alaska. Digestible protein probably is not limiting in shaded habitats but may be the greatest limitation to deer productivity and carrying capacity in clearcuts during summer. Digestible energy is probably the most limiting factor in forests during summer and all habitats during winter. Modeling of foraging energetics indicated snow, even at low depths, is a critical factor affecting foraging efficiency and carrying capacity of habitats. Its greatest effect is on reducing energy intake by changing forage availability and diet composition rather than by increasing energy costs of locomotion. Foraging efficiency and carrying capacity are shown to be related but very different concepts: for black-tailed deer, forage biomass is a relatively minor factor affecting foraging efficiency but a major factor affecting carrying capacity. It is suggested that habitats be evaluated primarily on the basis of nutritionally based estimates of carrying capacity and that greater emphasis be placed on summer and spring range than is currently the practice. Retention of old-growth forests for winter range during periods of snow will remain an important feature of habitat management for deer while techniques for increasing the carrying capacity of even-aged stands are sought.

Citation

Hanley, T.A.; Robbins, C.T.; Spalinger, D.E. 1989. Forest habitats and the nutritional ecology of Sitka black-tailed deer: a research synthesis with implications for forest management. Gen. Tech. Rep. PNW-GTR-230. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 52 p.