UBC Theses and Dissertations
Herbivore-plant-soil interactions in the boreal forest : selective winter feeding by spruce grouse Mueller, Fritz Paul
This thesis examines the unusual winter forage selection of spruce grouse (Dendragapus canadensis) in the Yukon. The winter diet of spruce grouse consists entirely of conifer needles. Spruce grouse feed selectively on individual trees (feeding trees) of a single species, white spruce (Picea glauca), leaving adjacent trees of similar size and age uneaten. These feeding trees are used throughout the winter and some are used repeatedly for several years. Accumulations of droppings on the ground and defoliated branches indicate preferred trees. Between 0.3 to 5.0 kg dry weight of spruce grouse faeces accumulate annually under feeding trees and between 25 to 90 % (mean 40 %) of the needles on preferred trees are removed by feeding grouse. Why do spruce grouse feed so selectively? Chapter 2 describes the role of foliar chemistry in the selection of winter forage by spruce grouse. During feeding trials, captive spruce grouse had a marked preference for needles from feeding trees over control trees. Chemical analyses of needles also support the hypothesis that needle chemistry accounts for the winter forage selection of spruce grouse. Concentrations of two monoterpene antifeedants, camphor and bornyl acetate, and the ratio of resin to nitrogen, were inversely related to grouse forage preferences and may explain the selection by spruce grouse of individual trees for winter feeding. Feeding trees are exposed during winter to recurring high levels of herbivory by spruce grouse. This herbivory may affect growth rate, architecture, reproductive output, and chemical defence of selected trees (Chapter 3). Feeding trees have higher lateral (twig) growth rates, longer needles, and longer, more highly branched limbs and more rounded crowns than control trees. Also, cone production is significantly lower in feeding trees than in adjacent control trees. The relatively high growth rates, the low reproductive output, and the low secondary chemical content of feeding trees suggests a within-tree trade-off in allocation of limited carbon resources. Large amounts of spruce grouse faeces accumulate annually under feeding trees. Decomposition of these faeces is rapid relative to spruce litter. In the nutrient limited boreal forest, spruce grouse faecal inputs under feeding trees may locally increase soil nutrient availability and nutrient cycling rates (Chapter 4). The nitrogen content of grouse faeces may account for the relatively high growth rates of feeding trees and may lead to the higher forage quality of feeding trees compared with control trees. Experimental defoliation and fertilization of white spruce trees suggests that the observed differences between feeding trees and adjacent control trees result, in part, from the effects of selective feeding by spruce grouse. The growth rates of experimentally fertilized trees increased significantly over control tree growth rates suggesting faecal input is critical to regrowth. The growth rates of clipped trees did not change in response to simulated grouse herbivory. The combined effects of defoliation and nutrient return by spruce grouse may lead to regrowth that is more palatable than forage on uneaten plants. Spruce grouse feeding may result in patches of highly palatable forage that attract further feeding, generating a feeding-regrowth feedback loop (Chapter 5). Positive effects on forage palatability and quantity may account for the unusual and prolonged use by spruce grouse of individual trees for winter feeding. Spruce grouse may be farming their food plants.
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