Graduate Thesis Or Dissertation
 

Response of Canada milkvetch (Astragalus canadensis var. mortonii (Nutt.) Wats.) to range and forest improvement practices in Northeastern Oregon

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  • The response of Canada milkvetch (Astragalus canadensis var. mortonii) to range and forest improvement practices was studied on a stand of mixed-coniferous forest which was clear -cut and burned in 1963 and 1964, respectively. The field studies were conducted on the Hall Ranch of the Eastern Oregon Experiment Station during the summers of 1968, 1969, and 1970. Canada milkvetch, which was virtually absent at the time of treatment, became one of the most important herbaceous species. The cover, frequency, and density of this species rapidly increased from 1965 to 1967; thereafter, these parameters have decreased, even to the levels of 1965 in some cases. The influence of forest canopy and physical site factor s, viz., soil moisture, soil temperature, and hydrogen ion concentration of the soil were studied. Phenology of seedling, root systems, shoot and flower development, seed production and germination were investigated. Studies were also made on the effects of grazing as they influence flowering, plant height, yield and number s of plants. The population parameters which were studied included cover, frequency, and density. The effects of calcium, copper, and cobalt each alone and in all combinations were studied by growing plants in the greenhouse. In addition, nitrogen fixation and the effect of nitrogen upon root nodulation were examined. The environmental parameters that influence Canada milkvetch are soil moisture and temperature for root nodulation and soil moisture for pod ripening. Soil from areas dominated by Canada milkvetch has higher pH values than soils in the forest and lower values than soils dominated by introduced grass species. In the experimental area, the amount of organic materials on the soil can be correlated with the intensity of grazing. Canada milkvetch begins growth shortly after snow melts, flowers in late June and July, and pods ripen in August. Flowering may begin in early June and continue to September, but the ovaries of the flowers produced early or late abort, because pollination occur s only when a medium sized bumble bee (Bombus sp. ) is the most active. This is the only species which trips the flowers of Canada milkvetch.The greatest amount of damage to the reproductive potential of Canada milkvetch is done by a weevil (Apion sp. ) which lives within the developing pod. At least 50 percent of the pods were destroyed by this insect during the summers of 1968 and 1969. Untreated seeds germinate and produce seedlings at the rate of approximately five percent per month. The rate of germination can be greatly increased by moist heat, but the amount of hard seeds remaining decreases and the amount decayed seed increases. The effects of grazing on this species can be evaluated by plant height, dry weight yield per plant, total yield per unit area, and the number of blossoms per flower stalk. Cattle prefer the inflorescences, but graze the leaves and stems along with the associated species at approximately the same intensity of use. On the other hand, big cattle preferentially graze this species from the time it begins growth in early spring until the animals leave the area. Canada milkvetch was always more productive under a grazing regime of cattle only than under a regime of domestic plus wild animals. Because the vegetation of the experimental area is rather low in the successional hierarchy, rapid changes in the values of cover, frequency, and density have occurred. The importance of Canada milkvetch has steadily decreased since 1967 and the relative importance of grasses has increased. Calcium seems to be required for nutrition and nodulation rather than as an amendment to alter the pH of the soil. Low levels of copper and cobalt also increase growth. Further research is required to adequately define the effects of these elements alone and in combination. Perhaps the successional role of Canada milkvetch can be prolonged by adequate fertility. This research provides an example demonstrating that range and forestry improvement must be coordinated and management intensified in order to maximize the biological potential of a given area.
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