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The increased soil cohesion or reinforcement provided by roots varies among plant species and soil types. This study compared the contribution of paper birch (Betula papyrifera Marsh.) and lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) roots to soil shear resistance in two controlled environment experiments. Roots of paper birch and lodgepole pine trees contributed to a significant increase in soil shear resistance, regardless of soil type. At a shear depth of 0.20 to 0.44 m, paper birch contributed greater reinforcement than lodgepole pine. Both birch and pine provided the most root reinforcement in coarse-textured soil and the least reinforcement in medium-textured silt soil. Soil texture, porosity, and shear strength may have affected root branching of the two species, which in turn may have affected root reinforcement. At less than 1 year of age, paper birch trees planted as plugs can substantially increase soil shear strength compared with lodgepole pine in all soil types.

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Management of paper birch in mixedwood stands is a sustainable forest management practice. In addition to the ecological and economical benefits of mixedwoods, paper birch trees can maintain or enhance slope stability. This thesis attempted to quantify the contribution of birch root reinforcement in BC to slope shear resistance. The objectives of this thesis were to determine the: 1) genetic variation in paper birch root reinforcement, 2) environmental variation in root reinforcement between birch and pine, and 3) differences in root reinforcement between birch and pine. The first study compared the contribution of birch and pine roots (from different populations growing in three soil types) to soil shear resistance using two controlled environment shear tests (Sonotube and Polytube Experiments). The second study (Tree Uprooting Experiment) compared the vertical uprooting resistance of birch and pine growing in different soil types at three field study sites. The third study (Genecology Experiment) determined the variation of four birch populations growing at one location. Results from the tube experiments found that the roots of birch and pine trees contributed to a significant increase in shear strength, regardless of soil type. At a depth of 20-44 em, paper birch increased shear strength by as much 88%, while pine increased strength by as much as 61%. There was little variation in root reinforcement among the six birch populations in the Sonotube Experiment, which suggested that these trees were from one generalist population rather than six specialist populations. Soil texture affected the root reinforcement of birch and pine in the Polytube Experiment~ both species had the highest root reinforcement in coarse textured sand and the least root reinforcement in medium textured silt. The limiting factor in root reinforcement, in this case, was attributed to a lack of water and nutrients in the silt soil. In the Tree Uprooting Experiment, birch trees had 50% greater resistance to uprooting than did pine trees. Small diameter birch and pine at Aleza Lake had greater uprooting resistance than birch and pine at other field sites. However, larger diameter trees at Gregg Creek and Red Rock had greater uprooting resistance than similar size trees at Aleza Lake. Soil strength and moisture content may have accounted for the uprooting resistance differences among diameter classes at Aleza Lake. Results from the Genecology Experiment showed that the Skeena population had the greatest uprooting resistance, and the greatest height, diameter, and root biomass compared to the other three populations. The results from companion trials, and from the uprooting tests suggested that Skeena trees represent a generalist population. In the same experiment, the nursery where the trees were grown impacted uprooting resistance, even after five years growing in the field. The results from this study reconfirmed the significant length of time nursery can affect field performance. Significant findings arising from this thesis were ...