Wood bison population recovery and forage availability in northwestern Canada

Abstract

Forage availability was assessed to determine sustainable stocking rates for eight broadly defined vegetation types (Treed Uplands, Treed Lowlands, Mixed Tall Shrub/Sedge, Closed-canopied Willow, and Open-canopied Willow, Meadow, Wetland Grass, Wetland Sedge) for use by wood bison ( Bison bison athabascae), a threatened subspecies, in the Canadian boreal forest of northern Alberta. Clip plots ( n = 108) were used to sample peak availability of herbs and current annual growth of Salix spp. in late summer. Graminoid wetlands dominated by Carex atherodes, Carex aquatilis, Carex utriculata, Scolochloa festucacea, or Calamagrostis stricta produced 1975–4575 kg ha −1 of fair to good quality forage, whereas treed stands produced <250 kg ha −1 of forb-dominated forage (>85% content), which was below a published 25% foraging efficiency threshold of 263 kg ha −1 for bison. Upland forests that dominate the region produced ≤1 animal unit day (AUD) of forage per hectare in summer. Most forest understory plants were of poor forage value, suggesting the potential sustainable stocking rate of such areas was actually ≤0.3 AUD ha −1, with even lower rates during winter due to snow cover. Herbaceous wetlands contained ∼78 AUD ha −1 of forage, but were considered largely unavailable in summer because of flooding and soft organic soils that make access difficult. Conversion of prime foraging habitat to agricultural land, forest expansion due to fire control, and a warmer and wetter climatic regime after the mid-1900s likely contributed to a regional reduction in carrying capacity. It is hypothesized that substantial recovery of the wood bison population toward historical levels will be constrained in northern Alberta by the availability of summer forage, and the limited extent of graminoid wetlands that provide winter foraging habitat.