Browse Availability, Bite Size, and Effects of Stand Age on Species Composition and Browse Density for Moose in Northeastern Minnesota

Abstract

Large ruminant herbivores like moose spend most of their time foraging and ruminating to acquire and process enough plant biomass to meet energy and nutrient requirements. In northeastern Minnesota, moose forage in a mosaic of forest stands with ages shaped by harvest and other disturbances. Distribution and abundance of browse species varies across the landscape and each browse species has unique growth patterns and a patchy distribution within and among different stand types. To estimate available and consumed biomass, we collected available twigs and created diameter-at-point-of-browsing-biomass regressions for each browse species. These relationships varied by canopy closure and were used to estimate biomass consumed on foraging paths. We also measured browse availability and use along foraging paths of GPS radio-collared moose and within randomly selected regenerating stands in northeastern Minnesota. We measured all sites using traditional methods and a method that simulates moose foraging behavior by measuring large feeding stations. We tested the hypotheses that (1) browse density is higher at large feeding stations than at random locations along a foraging path, (2) browse density is higher at large feeding stations than at randomly chosen feeding stations along a foraging path, and (3) browse density is higher at large feeding stations than along a straight transect. At each site we measured available species composition, available browse density, diet composition, and browse species selection. Combined with the use of GPS collars this method allowed us to compare the foraging path diet composition and browse selection of individual free-ranging moose. Paper birch, willow, and quaking aspen were common in young stands while hazel, mountain maple, and balsam fir (winter) or juneberry (summer) were common in older stands. Browse density also changed with stand age, but the changes in species composition and browse density were similar along foraging paths and within randomly selected regenerating stands indicating that moose habitat restoration projects can effectively create forage for moose. In areas with and without collared moose the simulated browsing method was an effective tool for measuring browse availability and use. We also provide evidence from the field that moose, and possibly other large herbivores, obtain most of their energy intake from small patches of high density browse.