Influence of repeated fertilization on forage production for native mammalian herbivores in young lodgepole pine forests

Stand thinning and fertilization are silvicultural practices designed to enhance wood and biomass production. Applications of nitrogen-based fertilizers make nutrients potentially available to all trees, plants, and wildlife in a given forest ecosystem, and therefore may affect productivity of forage plants for native mammalian herbivores. Species associated with areas of forest fertilization in temperate and boreal zones of North America include mule deer (Odocoileus hemionus), elk (Cervus elaphus), moose (Alces alces), and woodland caribou (Rangifer tarandus), snowshoe hare (Lepus americanus), and several species of voles. Impacts of fertilization on forest plant species may have important consequences for the nutrition, cover, and consequent survival of these herbivores, particularly in winter. This study tested the hypothesis (H₁) that large-scale repeated fertilization, up to 20 years after the onset of treatments, would enhance the biomass production of forage plants, particularly grass, forb, shrub and tree species for native mammalian herbivores. A secondary hypothesis (H₂) predicted that mosses and terrestrial lichens would decline as part of the ground vegetation in fertilized stands. Study areas were located in lodgepole pine (Pinus contorta var. latifolia) stands near Summerland, Kelowna, and Williams Lake in south-central British Columbia, Canada. Each study area had eight replicate stands: four unfertilized, and four fertilized five times at 2-year intervals.Mean biomass of total grasses responded dramatically starting in the first year after fertilization. Total forbs and herbs also followed this pattern, although not to a significant degree until after the second and third applications of fertilizer. All of the dominant grasses and forbs serve as summer forage for mule deer, moose, elk, and woodland caribou. Fireweed (Epilobium angustifolium), a preferred forage for mule deer, increased in fertilized stands. Grasses and dominant herbs in fertilized stands provide excellent forage and cover habitat for snowshoe hares and Microtus voles.Mean biomass of total shrubs was not affected by fertilization. However, saskatoon berry (Amelanchier alnifolia), prickly rose (Rosa acicularis), and red raspberry (Rubus idaeus) increased significantly in biomass in fertilized stands. Willow (Salix spp.) also increased in biomass, but was variable across treatment stands. Snowshoe hares respond favourably to enhanced shrub growth for food and cover in fertilized stands. All of these shrubs are readily eaten by deer, moose, and elk, and their structural attributes provide security and thermal cover. Mean biomass of understory Douglas-fir (Pseudotsuga menziesii) responded positively to fertilization, but subalpine fir (Abies lasiocarpa) and the three deciduous tree species did not. Dwarf shrubs such as kinnikinnick (Arctostaphylos uva-ursi), twinflower (Linnaea borealis), and dwarf blueberry (Vaccinium caespitosum) declined in fertilized stands. Thus, H₁ was partially supported for some species. Increasing cover of grasses and nitrophilous herbs, and canopy cover from rapidly growing crop trees, in fertilized stands may have contributed to the decline of some dwarf shrubs. Mean crown volume index of total mosses and terrestrial lichens declined significantly in fertilized stands, thereby supporting H₂.Mean biomass of total grasses (increase), R. acicularis (increase), and V. caespitosum (decline) were significantly affected after one application of fertilizer. Repeated applications of fertilizer may enhance biomass of some additional forage forbs and shrubs, but reduce biomass of some dwarf shrubs, mosses, and lichens.